Test Your Sysadmin Skills

"A great Admin doesn't need to know everything, but they should be able to come up with amazing solutions to impossible projects." - cwheeler33 (ServerFault)

"My skills are making things work, not knowing a billion facts. [...] If I need to fix a system I’ll identify the problem, check the logs and look up the errors. If I need to implement a solution I’ll research the right solution, implement and document it, the later on only really have a general idea of how it works unless I interact with it frequently... it’s why it’s documented." - Sparcrypt (Reddit)

This project contains 254 test questions and answers that can be used as a test your knowledge or during an interview/exam for position such as *nix System Administrator.

The answers are only examples and do not exhaust the whole topic. Most of them contains useful resources for a deeper understanding.

Questions marked * don’t have answer yet or answer is incomplete

All suggestions are welcome!

Table of Contents

Basic Knowledge
Simple Questions 14 questions
General Knowledge
Junior Sysadmin 55 questions
Regular Sysadmin 86 questions
Senior Sysadmin 88 questions
Secret Knowledge
Guru Sysadmin 11 questions

Basic Knowledge

Simple Questions

  • What did you learn this week?
  • What excites or interests you about the sysadmin world?
  • What is a recent technical challenge you experienced and how did you solve it?
  • Tell me about the last major project you finished.
  • Do you contribute to any open source projects?
  • Describe the setup of your homelab.
  • What personal achievement are you most proud of?
  • Tell me about the biggest mistake you’ve made.
  • Tell me about your favorite UNIX-like system.
  • What software tools are you going to install the first day at a new job?
  • Tell me about how you manage your knowledge database (e.g. wikis).
  • What news sources do you check daily? (sysadmin, security-related or other)
  • How do you interact with developers: us vs. them or all pulling together with a different approach?
  • Which question would you ask, if you were interviewing me, to know, how good I’m with non-standard situations?

General Knowledge

Junior Sysadmin

System Questions (31)

Give some examples of Linux distribution names. What is your favorite Linux distro and why?

  • Red Hat Enterprise Linux
  • Fedora
  • CentOS
  • Debian
  • Ubuntu
  • Mint
  • SUSE Linux Enterprise Server (SLES)
  • SUSE Linux Enterprise Desktop (SLED)
  • Slackware
  • Arch
  • Kali
  • Backbox

My favorite Linux distribution:

  • Arch Linux, which offers a nice minimalist base system on which one can build a custom operating system. The beauty of it too is that it has the Arch User Repository (AUR), which when combined with its official binary repositories allows it to probably have the largest repositories of any distribution. Its packaging process is also very simple, which means if one wants a package not in its official repositories or the AUR, it should be easy to make it for oneself.
  • Linux Mint, which is also built from Ubuntu LTS releases, but features editions featuring a few different desktop environments, including Cinnamon, MATE and Xfce. Mint is quite polished and its aesthetics are rather appealing, I especially like its new icon theme, although I do quite dislike its GTK+ theme (too bland to my taste). I’ve also found a bug in its latest release Mint 19, that is getting quite irritating as I asked for help with it over a fortnight ago on their forums and I have received no replies so far and it is a bug that makes my life on it more difficult.
  • Kali Linux, is a Debian-based Linux distribution aimed at advanced Penetration Testing and Security Auditing. Kali contains several hundred tools which are geared towards various information security tasks, such as Penetration Testing, Security research, Computer Forensics and Reverse Engineering.

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What are the differences between Unix, Linux, BSD and GNU?

GNU isn’t really an OS. It’s more of a set of rules or philosophies that govern free software, that at the same time gave birth to a bunch of tools while trying to create an OS. So GNU tools are basically open versions of tools that already existed, but were reimplemented to conform to principals of open software. GNU/Linux is a mesh of those tools and the Linux kernel to form a complete OS, but there are other GNUs, e.g. GNU/Hurd.

Unix and BSD are “older” implementations of POSIX that are various levels of “closed source”. Unix is usually totally closed source, but there are as many flavors of Unix as there are Linux (if not more). BSD is not usually considered “open”, but it was considered to be very open when it was released. Its licensing also allowed for commercial use with far fewer restrictions than the more “open” licenses of the time allowed.

Linux is the newest of the four. Strictly speaking, it’s “just a kernel”; however, in general, it’s thought of as a full OS when combined with GNU Tools and several other core components.

The main governing differences between these are their ideals. Unix, Linux, and BSD have different ideals that they implement. They are all POSIX, and are all basically interchangeable. They do solve some of the same problems in different ways. So other then ideals and how they choose to implement POSIX standards, there is little difference.

For more info I suggest your read a brief article on the creation of GNU, OSS, Linux, BSD, and UNIX. They will be slanted towards their individual ideas, but those articles should give you a better idea of the differences.

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What is a CLI? Tell me about your favorite CLI tools, tips and hacks.

CLI is an acronym for Command Line Interface or Command Language Interpreter. The command line is one of the most powerful ways to control your system/computer.

In Unix like systems, CLI is the interface by which a user can type commands for the system to execute. The CLI is very powerful, but is not very error-tolerant.

My favorite tools

  • screen - free terminal multiplexer, I can start a session and My terminals will be saved even when you connection is lost, so you can resume later or from home
  • ssh - the most valuable over-all command to learn, I can use it to do some amazing things:
    • mount a file system over the internet with sshfs
    • forward commands: runs against a rsync server with no rsync deamon by starting one itself via ssh
    • run in batch files: I can redirect the output from the remote command and use it within local batch file
  • vi/vim - is the most popular and powerful text editor, it’s universal, it’s work very fast, even on large files
  • bash-completion - contains a number of predefined completion rules for shell

Tips & Hacks

  • searches the command history with CTRL + R
  • popd/pushd and other shell builtins which allow you manipulate the directory stack
  • editing keyboard shortcuts like a CTRL + U, CTRL + E
  • combinations will be auto-expanded:
    • !* - all arguments of last command
    • !! - the whole of last command
    • !ssh - last command starting with ssh

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What is your favourite shell and why?

BASH is my favorite. It’s really a preferential kind of thing, where I love the syntax and it just “clicks” for me. The input/output redirection syntax (>>, << 2>&1, 2>, 1>, etc) is similar to C++ which makes it easier for me to recognize.

I also like the ZSH shell, because is much more customizable than BASH. It has the Oh-My-Zsh framework, powerful context based tab completion, pattern matching/globbing on steroids, loadable modules and more.

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Your first 5 commands on a *nix server after login.

  • w - a lot of great information in there with the server uptime
  • top - you can see all running processes, then order them by CPU, memory utilization and more
  • netstat - to know on what port and IP your server is listening on and what processes are using those
  • df - reports the amount of available disk space being used by file systems
  • history - tell you what was previously run by the user you are currently connected to

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What is your experience with the production environments? *

To be completed.

What do the fields in ls -al output mean?

In the order of output:

-rwxrw-r--    1    root   root 2048    Jan 13 07:11 db.dump
  • file permissions,
  • number of links,
  • owner name,
  • owner group,
  • file size,
  • time of last modification,
  • file/directory name

File permissions is displayed as following:

  • first character is - or l or d, d indicates a directory, a line represents a file, l is a symlink (or soft link) - special type of file
  • three sets of characters, three times, indicating permissions for owner, group and other:
    • r = readable
    • w = writable
    • x = executable

In your example -rwxrw-r--, this means the line displayed is:

  • a regular file (displayed as -)
  • readable, writable and executable by owner (rwx)
  • readable, writable, but not executable by group (rw-)
  • readable but not writable or executable by other (r--)

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How do you get a list of logged-in users?

For a summary of logged-in users, including each login of a username, the terminal users are attached to, the date/time they logged in, and possibly the computer from which they are making the connection, enter:

# It uses /var/run/utmp and /var/log/wtmp files to get the details.

For extensive information, including username, terminal, IP number of the source computer, the time the login began, any idle time, process CPU cycles, job CPU cycles, and the currently running command, enter:

# It uses /var/run/utmp, and their processes /proc.

Also important for displays a list of last logged in users, enter:

# It uses /var/log/wtmp.

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What is the advantage of executing the running processes in the background? How can you do that?

The most significant advantage of executing the running process in the background is that you can do any other task simultaneously while other processes are running in the background. So, more processes can be completed in the background while you are working on different processes. It can be achieved by adding a special character & at the end of the command.

Generally applications that take too long to execute and doesn’t require user interaction are sent to background so that we can continue our work in terminal.

For example if you want to download something in background, you can:

wget https://url-to-download.com/download.tar.gz &

When you run the above command you get the following output:

[1] 2203

Here 1 is the serial number of job and 2203 is PID of the job.

You can see the jobs running in background using the following command:


When you execute job in background it give you a PID of job, you can kill the job running in background using the following command:

kill PID

Replace the PID with the PID of the job. If you have only one job running you can bring it to foreground using:


If you have multiple jobs running in background you can bring any job in foreground using:

fg %#

Replace the # with serial number of the job.

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Why is it bad to run commands as root user?

Running (everything) as root is bad because:

  • Stupidity: nothing prevents you from making a careless mistake. If you try to change the system in any potentially harmful way, you need to use sudo, which ensures a pause (while you’re entering the password) to ensure that you aren’t about to make a mistake.

  • Security: harder to hack if you dont know the admin user’s login account. root means you already have one half of the working set of admin credentials.

  • You don’t really need it: if you need to run several commands as root, and you’re annoyed by having to enter your password several times when sudo has expired, all you need to do is sudo -i and you are now root. Want to run some commands using pipes? Then use sudo sh -c "command1 | command2".

  • You can always use it in the recovery console: the recovery console allows you to recover from a major mistake, or fix a problem caused by an app (which you still had to run as sudo). Ubuntu doesn’t have a password for the root account in this case, but you can search online for changing that - this will make it harder for anyone that has physical access to your box to be able to do harm.

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How to check memory stats and CPU stats?

You’d use top/htop for both. Using free and vmstat command we can display the physical and virtual memory statistics respectively. With the help of sar command we see the CPU utilization & other stats (but sar isn’t even installed in most systems).

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What is load average?

Linux load averages are “system load averages” that show the running thread (task) demand on the system as an average number of running plus waiting threads. This measures demand, which can be greater than what the system is currently processing. Most tools show three averages, for 1, 5, and 15 minutes.

Some interpretations:

  • if the averages are 0.0, then your system is idle.
  • if the 1 minute average is higher than the 5 or 15 minute averages, then load is increasing.
  • if the 1 minute average is lower than the 5 or 15 minute averages, then load is decreasing.
  • if they are higher than your CPU count, then you might have a performance problem (it depends).

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Where is password file located in Linux/UNIX?

The passwords are not stored anywhere on the system at all. What is stored in /etc/shadow are so called hashes of the passwords.

A hash of some text is created by performing a so called one way function on the text (password), thus creating a string to check against. By design it is “impossible” (computationally infeasible) to reverse that process.

Older UNIX variants stored the encrypted passwords in /etc/passwd along with other information about each account.

Newer ones simply have a * in the relevant field in /etc/passwd and use /etc/shadow to store the password, in part to ensure nobody gets read access to the passwords when they only need the other stuff (shadow is usually protected more strongly than passwd).

For more info consult man crypt, man shadow, man passwd.

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How to recursively change permissions for all directories except files and for all files except directories?

To change all the directories e.g. to 755 (drwxr-xr-x):

find /opt/data -type d -exec chmod 755 {} \;

To change all the files e.g. to 644 (-rw-r–r–):

find /opt/data -type f -exec chmod 644 {} \;

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Every command fails with command not found. How to trace the source of the error and resolve it?

It looks that at one point or another are overwriting the default PATH environment variable. The type of errors you have, indicates that PATH does not contain e.g. /bin, where the commands (including bash) reside.

One way to begin debugging your bash script or command would be to start a subshell with the -x option:

bash --login -x

This will show you every command, and its arguments, which is executed when starting that shell.

Also very helpful is show PATH variable values:

echo $PATH

If you run this:


most commands should start working - and then you can edit ~/.bash_profile instead of ~/.bashrc and fix whatever is resetting PATH there. Default PATH variable values for root and other users is in /etc/profile file.

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You typing CTRL + C but your script still running. How do you stop it? *

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What is grep command? How to match multiple strings in the same line?

The grep utilities are a family of Unix tools, including egrep and fgrep.

grep searches file patterns. If you are looking for a specific pattern in the output of another command, grep highlights the relevant lines. Use this grep command for searching log files, specific processes, and more.

For match multiple strings:

grep -E "string1|string2" filename


grep -e "string1" -e "string2" filename

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Explain file content commands along with the description.

  • head: to check the starting of a file.
  • tail: to check the ending of the file. It is the reverse of head command.
  • cat: used to view, create, concatenate the files.
  • more: used to display the text in the terminal window in pager form.
  • less: used to view the text in the backward direction and also provides single line movement.

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Explain SIGHUP, SIGINT, SIGKILL and SIGTERM Posix signals.

  • SIGHUP - is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop (a hangup). Many daemons will reload their configuration files and reopen their logfiles instead of exiting when receiving this signal.
  • SIGINT - is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing Ctrl+C, but on some systems, the “delete” character or “break” key can be used.
  • SIGKILL - is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal.
  • SIGTERM - is sent to a process to request its termination. Unlike the SIGKILL signal, it can be caught and interpreted or ignored by the process. This allows the process to perform nice termination releasing resources and saving state if appropriate. SIGINT is nearly identical to SIGTERM.

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What does a kill command do?

In Unix and Unix-like operating systems, kill is a command used to send a signal to a process. By default, the message sent is the termination signal, which requests that the process exit. But kill is something of a misnomer; the signal sent may have nothing to do with process killing.

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What is the difference between rm and rm -rf?

rm only deletes the named files (and not directories). With -rf as you say:

  • -r, -R, --recursive recursively deletes content of a directory, including hidden files and sub directories
  • -f, --force ignore nonexistent files, never prompt

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How do you list contents of archive.tgz and extract only one file?

# list of conent
tar tf archive.tgz

# extract file
tar xf archive.tgz filename

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How can I sync two local directories?

To sync the contents of dir1 to dir2 on the same system, type:

rsync -av --progress --delete dir1/ dir2
  • -a, --archive - archive mode
  • --delete - delete extraneous files from dest dirs
  • -v, --verbose - verbose mode (increase verbosity)
  • --progress - show progress during transfer

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Many basic maintenance tasks require you to edit config files. Explain ways to undo the changes you make.

  • manually backup of a file before editing (with brace expansion like this: cp filename{,.orig})
  • manual copy of the directory structure where file is stored (e.g. cp, rsync or tar)
  • make a backup of original file in your editor (e.g. set rules in your editor configuration file)
  • the best solution is to use git (or any other version control) to keep track of configuration files (e.g. etckeeper for /etc directory)

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You have to find all files larger than 20M. How will you do this?

find / -type f -size +20M

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Why do we use sudo su - and not just sudo su?

sudo is in most modern Linux distributions where (but not always) the root user is disabled and has no password set. Therefore you cannot switch to the root user with su (you can try). You have to call sudo with root privileges: sudo su.

su just switches the user, providing a normal shell with an environment nearly the same as with the old user.

su - invokes a login shell after switching the user. A login shell resets most environment variables, providing a clean base.

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How to find files that have been modified on your system in the past 60 minutes?

find / -mmin -60 -type f

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What are the main reasons for keeping old log files?

They are essential to investigate issues on the system. Log management is absolutely critical for IT security.

Servers, firewalls, and other IT equipment keep log files that record important events and transactions. This information can provide important clues about hostile activity affecting your network from within and without. Log data can also provide information for identifying and troubleshooting equipment problems including configuration problems and hardware failure.

It’s your server’s record of who’s come to your site, when, and exactly what they looked at. It’s incredibly detailed, showing:

  • where folks came from
  • what browser they were using
  • exactly which files they looked at
  • how long it took to load each file
  • and a whole bunch of other nerdy stuff

Factors to consider:

  • legal requirements for retention or destruction
  • company policies for retention and destruction
  • how long the logs are useful
  • what questions you’re hoping to answer from the logs
  • how much space they take up

By collecting and analyzing logs, you can understand what transpires within your network. Each log file contains many pieces of information that can be invaluable, especially if you know how to read them and analyze them.

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What is an incremental backup?

An incremental backup is a type of backup that only copies files that have changed since the previous backup.

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What is RAID? What is RAID0, RAID1, RAID5, RAID6, RAID10?

A RAID (Redundant Array of Inexpensive Disks) is a technology that is used to increase the performance and/or reliability of data storage.

  • RAID0: Also known as disk striping, is a technique that breaks up a file and spreads the data across all the disk drives in a RAID group. There are no safeguards against failure
  • RAID1: A popular disk subsystem that increases safety by writing the same data on two drives. Called “mirroring,” RAID 1 does not increase write performance, but read performance may equal up to the sum of each disks’ performance. However, if one drive fails, the second drive is used, and the failed drive is manually replaced. After replacement, the RAID controller duplicates the contents of the working drive onto the new one
  • RAID5: It is disk subsystem that increases safety by computing parity data and increasing speed by interleaving data across three or more drives (striping). Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost
  • RAID6: RAID 6 extends RAID 5 by adding another parity block. It requires a minimum of four disks and can continue to execute read and write of any two concurrent disk failures. RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations because of the overhead associated with parity calculations
  • RAID10: Also known as RAID 1+0, is a RAID configuration that combines disk mirroring and disk striping to protect data. It requires a minimum of four disks, and stripes data across mirrored pairs. As long as one disk in each mirrored pair is functional, data can be retrieved. If two disks in the same mirrored pair fail, all data will be lost because there is no parity in the striped sets

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How is a user’s default group determined? How would you change it?

useradd -m -g initial_group username

-g/--gid: defines the group name or number of the user’s initial login group. If specified, the group name must exist; if a group number is provided, it must refer to an already existing group.

If not specified, the behaviour of useradd will depend on the USERGROUPS_ENAB variable contained in /etc/login.defs. The default behaviour (USERGROUPS_ENAB yes) is to create a group with the same name as the username, with GID equal to UID.

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Why would you want to mount servers in a rack?

  • Protecting Hardware
  • Proper Cooling
  • Organized Workspace
  • Better Power Management
  • Cleaner Environment

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Network Questions (19)

Draw me a simple network diagram: you have 20 systems, 1 router, 4 switches, 5 servers, and a small IP block. *

To be completed.

What are the most important things to understand about the OSI (or any other) model?

The most important things to understand about the OSI (or any other) model are:

  • we can divide up the protocols into layers
  • layers provide encapsulation
  • layers provide abstraction
  • layers decouple functions from others

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What is the difference between a VLAN and a subnet? Do you need a VLAN to setup a subnet?

VLANs and subnets solve different problems. VLANs work at Layer 2, thereby altering broadcast domains (for instance). Whereas subnets are Layer 3 in the current context.

Subnet - is a range of IP addresses determined by part of an address (often called the network address) and a subnet mask (netmask). For example, if the netmask is (or /24 for short), and the network address is, then that defines a range of IP addresses through Shorthand for writing that is

VLAN - a good way to think of this is “switch partitioning.” Let’s say you have an 8 port switch that is VLAN-able. You can assign 4 ports to one VLAN (say VLAN 1) and 4 ports to another VLAN (say VLAN 2). VLAN 1 won’t see any of VLAN 2’s traffic and vice versa, logically, you now have two separate switches. Normally on a switch, if the switch hasn’t seen a MAC address it will “flood” the traffic to all other ports. VLANs prevent this.

Subnet is nothing more than an IP address range of IP addresses that help hosts communicate over layer 2 and 3. Each subnet does not require its own VLAN. VLANs are implemented for isolation (are sandbox for layer two communication, no 2 systems of 2 different VLANs may communicate but it can be done through Inter VLAN routing), ease of management and security.

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List 5 common network ports you should know.

FTP 20 for data transfer and 21 for connection established
DNS 53
DHCP 67/UDP for DHCP server, 68/UDP for DHCP client
SSH 22

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How to check default route and routing table?

Using the commands netstat -nr, route -n or ip route show we can see the default route and routing tables.

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What is the difference between and localhost?

Well, the most likely difference is that you still have to do an actual lookup of localhost somewhere.

If you use, then (intelligent) software will just turn that directly into an IP address and use it. Some implementations of gethostbyname will detect the dotted format (and presumably the equivalent IPv6 format) and not do a lookup at all.

Otherwise, the name has to be resolved. And there’s no guarantee that your hosts file will actually be used for that resolution (first, or at all) so localhost may become a totally different IP address.

By that I mean that, on some systems, a local hosts file can be bypassed. The host.conf file controls this on Linux (and many other Unices).

If you use a unix domain socket it’ll be slightly faster than using TCP/IP (because of the less overhead you have). Windows is using TCP/IP as a default, whereas Linux tries to use a Unix Domain Socket if you choose localhost and TCP/IP if you take

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Which port is used for ping command?

ping uses ICMP, specifically ICMP echo request and ICMP echo reply packets. There is no ‘port’ associated with ICMP. Ports are associated with the two IP transport layer protocols, TCP and UDP. ICMP, TCP, and UDP are “siblings”; they are not based on each other, but are three separate protocols that run on top of IP.

ICMP packets are identified by the ‘protocol’ field in the IP datagram header. ICMP does not use either UDP or TCP communications services, it uses raw IP communications services. This means that the ICMP message is carried directly in an IP datagram data field. (‘raw’ comes from how this is implemented in software, to create and send an ICMP message, one opens a ‘raw’ socket, builds a buffer containing the ICMP message, and then writes the buffer containing the message to the raw socket.)

The IP protocol value for ICMP is 1. (The protocol field is part of the IP header and identifies what is in the data portion of the IP datagram.)

However, you could use nmap to see whether ports are open or not:

nmap -p 80 example.com

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Server A can’t talk to Server B. Describe possible reasons in a few steps.

To troubleshoot communication problems between servers, it is better to ideally follow the TCP/IP stack:

  1. Application Layer: are the services up and running on both servers? Are they correctly configured (eg. bind the correct IP and correct port)? Do application and system logs show meaningful errors?

  2. Transport Layer: are the ports used by the application open (try telnet!)? Is it possible to ping the server?

  3. Network Layer: Is there a firewall on the network or on the OS correctly configured? Is the IP stack correctly configured (IP, routes, dns, etc.)? Are switches and routers working (check the ARP table!)?

  4. Physical Layer: Are the servers connected to a network? Are packets being lost?

How to resolve the domain name (using external dns) with CLI? Can IPs be resolved to domain names?

Examples for resolve IP address to domain name:

# with host command:
host domain.com

# with dig command:
dig @ google.com

# with nslookup command:
nslookup domain.com

You can (sometimes) resolve an IP Address back to a hostname. IP Address can be stored against a PTR record. You can then do:

dig A <hostname>

To lookup the IPv4 address for a host, or:

dig AAAA <hostname>

To lookup the IPv6 address for a host, or:

dig PTR ZZZ.YYY.XXX.WWW.in-addr.arpa.

To lookup the hostname for IPv4 address WWW.XXX.YYY.ZZZ (note the octets are reversed), or:

dig PTR b.a.

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How to test port connectivity with telnet or nc?

# with telnet command:
telnet code42.example.com 5432

# with nc (netcat) command:
nc -vz code42.example.com 5432

Why should you avoid telnet to administer a system remotely?

Modern operating systems have turned off all potentially insecure services by default. On the other hand, some verndors of network devices still allow to establish communication using the telnet protocol.

Telnet uses most insecure method for communication. It sends data across the network in plain text format and anybody can easily find out the password using the network tool.

In the case of Telnet, these include the passing of login credentials in plain text, which means anyone running a sniffer on your network can find the information he needs to take control of a device in a few seconds by eavesdropping on a Telnet login session.

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What is the difference between wget and curl?

The main differences are: wget’s major strong side compared to curl is its ability to download recursively. Wget is command line only. Curl supports FTP, FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, LDAP, LDAPS, FILE, POP3, IMAP, SMTP, RTMP and RTSP.

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What is SSH and how does it work?

SSH stands for Secure Shell. It is a protocol that lets you drop from a server “A” into a shell session to a server “B”. It allows you interact with your server “B”.

An SSH connection to be established, the remote machine (server A) must be running a piece of software called an SSH daemon and the user’s computer (server B) must have an SSH client.

The SSH daemon and SSH client listen for connections on a specific network port (default 22), authenticates connection requests, and spawns the appropriate environment if the user provides the correct credentials.

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What is a packet filter and how does it work?

Packet filtering is a firewall technique used to control network access by monitoring outgoing and incoming packets and allowing them to pass or halt based on the source and destination Internet Protocol (IP) addresses, protocols and ports.

What are the advantages of using a reverse proxy server?

Hide the topology and characteristics of your back-end servers

The reverse proxy server can hide the presence and characteristics of the origin server. It acts as an intermediate between internet cloud and web server. It is good for security reason especially when you are using web hosting services.

Allows transparent maintenance of backend servers

Changes you make to servers running behind a reverse proxy are going to be completely transparent to your end users.

Load Balancing

The reverse proxy will then enforce a load balancing algorithm like round robin, weighted round robin, least connections, weighted least connections, or random, to distribute the load among the servers in the cluster.

When a server goes down, the system will automatically failover to the next server up and users can continue with their secure file transfer activities.

SSL offloading/termination

Handles incoming HTTPS connections, decrypting the requests and passing unencrypted requests on to the web servers.

IP masking

Using a single ip but different URLs to route to different back end servers.

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What is the difference between a router and a gateway? What is the default gateway?

Router describes the general technical function (layer-3 forwarding) or a hardware device intended for that purpose, while gateway describes the function for the local segment (providing connectivity to elsewhere). You could also state that “you set up a router as gateway”. Another term is hop which describes the forwarding in between subnets.

It’s just a matter of perspective, the device is the same.

The term default gateway is used to mean the router on your LAN which has the responsibility of being the first point of contact for traffic to computers outside the LAN.

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Explain the function of each of the following DNS records: SOA, PTR, A, MX, and CNAME.

DNS records are basically mapping files that tell the DNS server which IP address each domain is associated with, and how to handle requests sent to each domain. Some DNS records syntax that are commonly used in nearly all DNS record configurations are A, AAAA, CNAME, MX, PTR, NS, SOA, SRV, TXT, and NAPTR.

  • SOA - A Start Of Authority
  • A - Address Mapping records
  • AAAA - IP Version 6 Address records
  • CNAME - Canonical Name records
  • MX - Mail exchanger record
  • NS - Name Server records
  • PTR - Reverse-lookup Pointer records

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What is the smallest IPv4 subnet mask that can be applied to a network containing up to 30 devices?

Whether you have a standard /24 VLAN for end users, a /30 for point-to-point links, or something in between and subnet that must contain up to 30 devices works out to be a /27 - or a subnet mask of

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What are some common HTTP status codes?

  • 1xx - Informational responses - communicates transfer protocol-level information
  • 2xx - Success - indicates that the client’s request was accepted successfully
  • 3xx - Redirection - indicates that the client must take some additional action in order to complete their request
  • 4xx - Client side error - this category of error status codes points the finger at clients
  • 5xx - Server side error - the server takes responsibility for these error status codes

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Devops Questions (4)

What is version control? Are your commit messages good looking?

It is a system that records changes to a file or set of files over time so that you can recall specific versions later. Version control systems consist of a central shared repository where teammates can commit changes to a file or set of file. Then you can mention the uses of version control.

Version control allows you to:

  • revert files back to a previous state
  • revert the entire project back to a previous state
  • compare changes over time
  • see who last modified something that might be causing a problem
  • who introduced an issue and when

The seven rules of a great commit message:

  • separate subject from body with a blank line
  • limit the subject line to 50 characters
  • capitalize the subject line
  • do not end the subject line with a period
  • use the imperative mood in the subject line
  • wrap the body at 72 characters
  • use the body to explain what and why vs. how

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Explain some basic git commands.

  • git init - create a new local repository
  • git commit -m "message" - commit changes to head
  • git status - list the files you’ve added with git add and also commit any files you’ve changed since then
  • git push origin master - send changes to the master branch of your remote repository

Explain a simple Countinuos Integration pipeline.

  • clone repository
  • deploy stage (QA)
  • testing environment (QA)
  • deploy stage (PROD)

Explain some basic docker commands.

  • docker ps - show running containers
  • docker ps -a - show all containers
  • docker images - show docker images
  • docker logs <container-id|container-name> - get logs from container
  • docker network ls - show all docker networks
  • docker volumes ls - show all docker volumes
  • docker exec -it <container-id|container-name> bash - execute bash in container with interative shell

Cyber Security Questions (1)

What is a Security Misconfiguration?

Security misconfiguration is a vulnerability when a device/application/network is configured in a way which can be exploited by an attacker to take advantage of it. This can be as simple as leaving the default username/password unchanged or too simple for device accounts etc.

Regular Sysadmin

System Questions (50)

Explain the boot process of the Linux system.

BIOS: Full form of BIOS is Basic Input or Output System that performs integrity checks and it will search and load and then it will execute the bootloader.

Bootloader: Since the earlier phases are not specific to the operating system, the BIOS-based boot process for x86 and x86-64 architectures is considered to start when the master boot record (MBR) code is executed in real mode and the first-stage boot loader is loaded. In UEFI systems, a payload, such as the Linux kernel, can be executed directly. Thus no boot loader is necessary. Some popular bootloaders: GRUB, Syslinux/Isolinux or Lilo.

Kernel: The kernel in Linux handles all operating system processes, such as memory management, task scheduling, I/O, interprocess communication, and overall system control. This is loaded in two stages - in the first stage, the kernel (as a compressed image file) is loaded into memory and decompressed, and a few fundamental functions such as basic memory management are set up.

Init: Is the parent of all processes on the system, it is executed by the kernel and is responsible for starting all other processes.

  • SysV init - init’s job is “to get everything running the way it should be once the kernel is fully running. Essentially it establishes and operates the entire user space. This includes checking and mounting file systems, starting up necessary user services, and ultimately switching to a user-environment when system startup is completed.
  • systemd - the developers of systemd aimed to replace the Linux init system inherited from UNIX System V. Like init, systemd is a daemon that manages other daemons. All daemons, including systemd, are background processes. Systemd is the first daemon to start (during booting) and the last daemon to terminate (during shutdown).
  • runint - runit is an init scheme for Unix-like operating systems that initializes, supervises, and ends processes throughout the operating system. It is a reimplementation of the daemontools[13] process supervision toolkit that runs on the Linux, Mac OS X, *BSD, and Solaris operating systems.

Useful resources:

Why is a load of 1.00 not ideal on a single-core machine?

The problem with a load of 1.00 is that you have no headroom. In practice, many sysadmins will draw a line at 0.70.

The “Need to Look into it” Rule of Thumb: 0.70 If your load average is staying above > 0.70, it’s time to investigate before things get worse.

The “Fix this now” Rule of Thumb: 1.00. If your load average stays above 1.00, find the problem and fix it now. Otherwise, you’re going to get woken up in the middle of the night, and it’s not going to be fun.

Rule of Thumb: 5.0. If your load average is above 5.00, you could be in serious trouble, your box is either hanging or slowing way down, and this will (inexplicably) happen in the worst possible time like in the middle of the night or when you’re presenting at a conference. Don’t let it get there.

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What does it mean when the effective user is root, but the real user ID is still your name?

The real user ID is who you really are (the user who owns the process), and the effective user ID is what the operating system looks at to make a decision whether or not you are allowed to do something (most of the time, there are some exceptions).

When you log in, the login shell sets both the real and effective user ID to the same value (your real user id) as supplied by the password file.

If, for instance, you execute setuid, and besides running as another user (e.g. root) the setuid program is also supposed to do something on your behalf:

After executing setuid, it will have your real id (since you’re the process owner) and the effective user id of the file owner (for example root) since it is setuid.

Let’s use the case of passwd:

-rwsr-xr-x 1 root root 45396 may 25  2012 /usr/bin/passwd

When user2 wants to change their password, they execute /usr/bin/passwd.

The RUID will be user2 but the EUID of that process will be root.

user2 can use only passwd to change their own password, because internally passwd checks the RUID and, if it is not root, its actions will be limited to real user’s password.

It’s necessary that the EUID becomes root in the case of passwd because the process needs to write to /etc/passwd and/or /etc/shadow.

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Developer added cron job which generate massive log files. How do you prevent them from getting so big?

Using logrotate is the usual way of dealing with logfiles. But instead of adding content to /etc/logrotate.conf you should add your own job to /etc/logrotate.d/, otherwise you would have to look at more diffs of configuration files during release upgrades.

If it’s actively being written to you don’t really have much you can do by way of truncate. Your only options are to truncate the file:

: >/var/log/massive-logfile

It’s very helpful, because it’s truncate the file without disrupting the processes.

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How the Linux kernel creates, manages and deletes the processes in the system? *

To be completed.

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How would you recognize a process that is hogging resources?

top works reasonably well, as long as you look at the right numbers. - M Sorts by current resident memory usage - T Sorts by total ( or cummulaative) CPU usage - P Sorts by current CPU usage (this is the default refresh) - ? Displays a usage summary for all top commands

This is very important information to obtain when problem solving why a computer process is running slowly and making decisions on what processes to kill / software to uninstall.

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When your server is booting up some errors appears on the console. How to examine boot messages and where are they stored?

Your console has two types of messages:

  • generated by the kernel (via printk)
  • generated by userspace (usually your init system)

Kernel messages are always stored in the kmsg buffer, visible via dmesg command. They’re also often copied to your syslog. This also applies to userspace messages written to /dev/kmsg, but those are fairly rare.

Meanwhile, when userspace writes its fancy boot status text to /dev/console or /dev/tty1, it’s not stored anywhere at all. It just goes to the screen and that’s it.

dmesg is used to review boot messages contained in the kernel ring buffer. A ring buffer is a buffer of fixed size for which any new data added to it overwrites the oldest data in it.

It shows operations once the boot process has completed, such as command line options passed to the kernel; hardware components detected, events when a new USB device is added, or errors like NIC (Network Interface Card) failure and the drivers report no link activity detected on the network and so much more.

If system logging is done via the journal component you should use journalctl. It shows messages include kernel and boot messages; messages from syslog or various services.

Boot issues/errors calls for a system administrator to look into certain important files in conjunction with particular commands (handled differently by different versions of Linux):

  • /var/log/boot.log - system boot log, it contains all that unfolded during the system boot
  • /var/log/messages - stores global system messages, including the messages that are logged during system boot
  • /var/log/dmesg - contains kernel ring buffer information

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Swap usage too high. What are the reasons for this and how to resolve swapping problems?

Swap space is a restricted amount of physical memory that is allocated for use by the operating system when available memory has been fully utilized. It is memory management that involves swapping sections of memory to and from physical storage.

If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.

Workload increases your RAM demand. You are running a workload that requires more memory. Usage of the entire swap indicates that. Also, changing swappiness to 1 might not be a wise decision. Setting swappiness to 1 does not indicate that swapping will not be done. It just indicates how aggressive kernel will be in respect of swapping, it does not eliminate swapping. Swapping will happen if needs to be done.

  • Increasing the size of the swap space - firstly, you’d have increased disk use. If your disks arn’t fast enough to keep up, then your system might end up thrashing, and you’d experience slowdowns as data is swapped in and out of memory. This would result in a bottleneck.

  • Adding more RAM - the real solution is to add more memory. There’s no substitute for RAM, and if you have enough memory, you’ll swap less.

For monitoring swap space usage:

  • cat /proc/swaps - to see total and used swap size
  • grep SwapTotal /proc/meminfo - to show total swap space
  • free - to display the amount of free and used system memory (also swap)
  • vmstat - to check swapping statistics
  • top, htop- to check swap space usage
  • atop - to show is that your system is overcommitting memory
  • or use one-liner shell command to list all applications with how much swap space search is using in kilobytes: bash for _fd in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $_fd done | sort -k 2 -n -r | less

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What is umask? How to set it permanently for a user?

On Linux and other Unix-like operating systems, new files are created with a default set of permissions. Specifically, a new file’s permissions may be restricted in a specific way by applying a permissions “mask” called the umask. The umask command is used to set this mask, or to show you its current value.

Permanently change (set e.g. umask 02):

  • ~/.profile
  • ~/.bashrc
  • ~/.zshrc
  • ~/.cshrc

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Explain the differences among the following umask values: 000, 002, 022, 027, 077 and 277.

Umask FIle result Directory result
000 666 rw- rw- rw- 777 rwx rwx rwx
002 664 rw- rw- r-- 775 rwx rwx r-x
022 644 rw- r-- r-- 755 rwx r-x r-x
027 640 rw- r-- --- 750 rwx r-x ---
077 600 rw---- --- 700 rwx --- ---
277 400 r-- --- --- 500 r-x --- ---

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What is the difference between a symbolic link and a hard link?

Underneath the file system files are represented by inodes (or is it multiple inodes not sure)

  • a file in the file system is basically a link to an inode
  • a hard link then just creates another file with a link to the same underlying inode

When you delete a file it removes one link to the underlying inode. The inode is only deleted (or deletable/over-writable) when all links to the inode have been deleted.

  • a symbolic link is a link to another name in the file system.

Once a hard link has been made the link is to the inode. deleting renaming or moving the original file will not affect the hard link as it links to the underlying inode. Any changes to the data on the inode is reflected in all files that refer to that inode.

Note: Hard links are only valid within the same File System. Symbolic links can span file systems as they are simply the name of another file.


  • Hardlink cannot be created for directories. Hard link can only be created for a file
  • Softlink also termed a symbolic links or symlinks can link to a directory

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How does the sticky bit work? The SUID/GUID is the same?

This is probably one of my most irksome things that people mess up all the time. The SUID/GUID bit and the sticky-bit are 2 completely different things.

If you do a man chmod you can read about the SUID and sticky-bits.


What the above man page is trying to say is that the position that the x bit takes in the rwxrwxrwx for the user octal (1st group of rwx) and the group octal (2nd group of rwx) can take an additional state where the x becomes an s. When this occurs this file when executed (if it’s a program and not just a shell script) will run with the permissions of the owner or the group of the file.

So if the file is owned by root and the SUID bit is turned on, the program will run as root. Even if you execute it as a regular user. The same thing applies to the GUID bit.


no suid/guid - just the bits rwxr-xr-x are set.

ls -lt b.pl
-rwxr-xr-x 1 root root 179 Jan  9 01:01 b.pl

suid & user’s executable bit enabled (lowercase s) - the bits rwsr-x-r-x are set.

chmod u+s b.pl
ls -lt b.pl
-rwsr-xr-x 1 root root 179 Jan  9 01:01 b.pl

suid enabled & executable bit disabled (uppercase S) - the bits rwSr-xr-x are set.

chmod u-x b.pl
ls -lt b.pl
-rwSr-xr-x 1 root root 179 Jan  9 01:01 b.pl

guid & group’s executable bit enabled (lowercase s) - the bits rwxr-sr-x are set.

chmod g+s b.pl
ls -lt b.pl
-rwxr-sr-x 1 root root 179 Jan  9 01:01 b.pl

guid enabled & executable bit disabled (uppercase S) - the bits rwxr-Sr-x are set.

chmod g-x b.pl
ls -lt b.pl
-rwxr-Sr-x 1 root root 179 Jan  9 01:01 b.pl

sticky bit

The sticky bit on the other hand is denoted as t, such as with the /tmp directory:

ls -l /|grep tmp
drwxrwxrwt. 168 root root 28672 Jun 14 08:36 tmp

This bit should have always been called the “restricted deletion bit” given that’s what it really connotes. When this mode bit is enabled, it makes a directory such that users can only delete files & directories within it that they are the owners of.

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What does LC_ALL=C before command do? In what cases, it will be useful?

LC_ALL is the environment variable that overrides all the other localisation settings. This sets all LC_ type variables at once to a specified locale.

The main reason to set LC_ALL=C before command is that fine to simply get English output (general change the locale used by the command).

On the other hand, also important is to increase the speed of command execution with LC_ALL=C e.g. grep or fgrep. Using the LC_ALL=C locale increased our performance and brought command execution time down.

For example, if you set LC_ALL=en_US.utf8 your system opened multiple files from the /usr/lib/locale directory. For LC_ALL=C a minimum amount of open and read operations is performed.

If you want to restore all your normal (original) locale settings for the session:


If LC_ALL does not work, try using LANG (if that still does not work, try LANGUAGE):

LANG=C date +%A

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During server configuration one of the steps is setting the permissions to the apps directories. What steps will you take and what mistakes to avoid?

1) Main requirements - remember about this

  • which users have access to the app filesystem
  • permissions for web servers, e.g. apache and app servers e.g. uwsgi
  • permissions for specific directories like a uploads, cache and main app directory like a /var/www/app01/html
  • correct umask value for users and suid/sgid (only for specific situations)
  • permissions for all future files and directories
  • permissions for cron jobs and scripts

2) Application directories

/var/www contains a directory for each website (isolation of the apps), e.g. /var/www/app01, /var/www/app02

mkdir /var/www/{app01,app02}

3) Application owner and group

Each application has a designated owner (e.g. u01-prod, u02-prod) and group (e.g. g01-prod, g02-prod) which are set as the owner of all files and directories in the website’s directory:

chown -R u01-prod:g01-prod /var/www/app01
chown -R u02-prod:g02-prod /var/www/app02

4) Developers owner and group

All of the users that maintain the website have own groups and they’re attach to application group:

id alice
uid=2000(alice) gid=4000(alice) groups=8000(g01-prod)
id bob
uid=2001(bob) gid=4001(bob) groups=8000(g01-prod),8001(g02-prod)

So alice user has standard privileges for /var/www/app01 and bob user has standard privileges for /var/www/app01 and /var/www/app02.

5) Web server owner and group

Any files or directories that need to be written by the webserver have their owner. If the web servers is Apache, default owner/group are apache:apache or www-data:www-data and for Nginx it will be nginx:nginx. Don’t change these settings.

If applications works with app servers like a uwsgi or php-fpm should set the appropriate user and group (e.g. for app01 it will be u01-prod:g01-prod) in specific config files.

6) Permissions

Set properly permissions with Access Control Lists:

# For web server
setfacl -Rdm "g:apache:rwx" /var/www/app01
setfacl -Rm "g:apache:rwx" /var/www/app01

# For developers
setfacl -Rdm "g:g01-prod:rwx" /var/www/app01
setfacl -Rm "g:g01-prod:rwx" /var/www/app01

If you use SELinux remember about security context:

chcon -R system_u:object_r:httpd_sys_content_t /var/www/app01

7) Security mistakes

  • root owner for files and directories
  • root never executes any files in website directory, and shouldn’t be creating files in there
  • to wide permissions like a 777 so some critical files may be world-writable and world-readable
  • avoid creating maintenance scripts or other critical files with suid root

If you allow your site to modify the files which form the code running your site, you make it much easier for someone to take over your server.

A file upload tool allows users to upload a file with any name and any contents. This allows a user to upload a mail relay PHP script to your site, which they can place wherever they want to turn your server into a machine to forward unsolicited commercial email. This script could also be used to read every email address out of your database, or other personal information.

If the malicious user can upload a file with any name but not control the contents, then they could easily upload a file which overwrites your index.php (or another critical file) and breaks your site.

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What steps will be taken by init when you run telinit 1 from run level 3? What will be the final result of this? *

To be completed.

I have forgotten the root password! What do I do in BSD?

Restart the system, type boot -s at the Boot: prompt to enter single-user mode.

At the question about the shell to use, hit Enter which will display a # prompt.

Enter mount -urw / to remount the root file system read/write, then run mount -a to remount all the file systems.

Run passwd root to change the root password then run exit to continue booting.

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How could you modify a text file without invoking a text editor?

For example:

# cat  >filename ... - overwrite file
# cat >>filename ... - append to file
cat > filename << __EOF__
data data data

How to add & change the kernel parameters?

To set the kernel parameters in UNIX-like, first edit the file /etc/sysctl.conf after making the changes save the file and run the command sysctl -p, this command will make the changes permanently without rebooting the machine.

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Explain /proc filesystem.

/proc is a virtual file system that provides detailed information about kernel, hardware and running processes.

Since /proc contains virtual files, it is called virtual file system. These virtual files have unique qualities. Most of them are listed as zero bytes in size.

Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts and /proc/partitions provide an up-to-the-moment glimpse of the system’s hardware. Others: /proc/filesystems file and the /proc/sys/ directory provide system configuration information and interfaces.

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Describe your data backup process. Do you test your backups on a regular basis? *

To be completed.

Explain three types of journaling in ext3/ext4.

There are three types of journaling available in ext3/ext4 file systems:

  • Journal - metadata and content are saved in the journal
  • Ordered - only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default
  • Writeback - only metadata is saved in the journal. Metadata might be journaled either before or after the content is written to the disk

What is an inode?

An inode is a data structure on a filesystem on Linux and other Unix-like operating systems that stores all the information about a file except its name and its actual data. A data structure is a way of storing data so that it can be used efficiently.

A Unix file is “stored” in two different parts of the disk - the data blocks and the inodes. (I won’t get into superblocks and other esoteric information.) The data blocks contain the “contents” of the file. The information about the file is stored elsewhere - in the inode.

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How to increase the size of LVM partition?

Use the lvextend command for resize LVM partition.

  • extending the size by 500MB:
lvextend -L +500M /dev/vgroup/lvolume
  • extending all available free space:
lvextend -l +100%FREE /dev/vgroup/lvolume

and resize2fs or xfs_growfs to resize filesystem:

  • for ext filesystems:
resize2fs /dev/vgroup/lvolume
  • for xfs filesystem:
xfs_growfs mountpoint_for_/dev/vgroup/lvolume

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Describe a process to create partition, LVM partition and filesystem.

  1. Create partition
fdisk /dev/sdb
  1. Create LVM
pvcreate /dev/sdb1
vgcreate vg0 /dev/sdb1
lvcreate --name datastore --size 50G vg0
  1. Create filesystem
mkfs -t xfs /dev/mapper/vg0-datastore

What is a zombie/defunct process?

Is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the “Terminated state”.

Processes marked are dead processes (so-called “zombies”) that remain because their parent has not destroyed them properly. These processes will be destroyed by init if the parent process exits.

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Present and explain the good ways of using the kill command.

Speaking of killing processes never use kill -9/SIGKILL unless absolutely mandatory. This kill can cause problems because of its brute force.

Always try to use the following simple procedure:

  • first, send SIGTERM (kill -15) signal first which tells the process to shutdown and is generally accepted as the signal to use when shutting down cleanly (but remember that this signal can be ignored).
  • next try to send SIGHUP (kill -1) signal which is commonly used to tell a process to shutdown and restart, this signal can also be caught and ignored by a process.

The far majority of the time, this is all you need - and is much cleaner.

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What is strace command? How should strace be used?

strace is a powerful command line tool for debugging and trouble shooting programs in Unix-like operating systems such as Linux. It captures and records all system calls made by a process and the signals received by the process.

Strace Overview

strace can be seen as a light weight debugger. It allows a programmer/user to quickly find out how a program is interacting with the OS. It does this by monitoring system calls and signals.


Good for when you don’t have source code or don’t want to be bothered to really go through it. Also, useful for your own code if you don’t feel like opening up GDB, but are just interested in understanding external interaction.

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What was getfacl and setfacl commands do?

The command setfacl refers to Set File Access Control Lists and getfacl refers to Get File Access Control List. Each file and directory in a Linux filesystem is created with a specific set of file permissions for its access. In order to know the access permissions of a file or directory we use getfacl.

Which algorithms are supported in /etc/shadow file?

Typical current algorithms are:

  • MD5
  • SHA-1 (also called SHA)

both should not be used for cryptographic/security purposes any more!!

  • SHA-256
  • SHA-512
  • SHA-3 (KECCAK was announced the winner in the competition for a new federal approved hash algorithm in October 2012)

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What is key-based authentication? Explain.

Key-based authentication is a kind of authentication that may be used as an alternative to password authentication. Instead of requiring a user’s password, it is possible to confirm the client’s identity by using asymmetric cryptography algorithms, with public and private keys.

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What is the use of ulimit in Unix-like systems?

Most UNIX-like operating systems, including Linux and BSD, provide ways to limit and control the usage of system resources such as threads, files, and network connections on a per-process and per-user basis. These “ulimits” prevent single users from using too many system resources.

What are soft limits and hard limits?

Hard limit is the maximum allowed to a user, set by the superuser or root. This value is set in the file /etc/security/limits.conf. The user can increase the soft limit on their own in times of needing more resources, but cannot set the soft limit higher than the hard limit.

You have configured an RSA key login but your server show Server refused our key as expected. Where will you look for the cause of the problem?

Server side

Setting LogLevel VERBOSE in file /etc/ssh/sshd_config is probably what you need, although there are higher levels:

SSH auth failures are logged in /var/log/auth.log, /var/log/secure or /var/log/audit/audit.log.

The following should give you only ssh related log lines (for example):

grep 'sshd' /var/log/auth.log

Next, the most simple command to list all failed SSH logins is the one shown below:

grep "Failed password" /var/log/auth.log

also useful is:

grep "Failed\|Failure" /var/log/auth.log

On newer Linux distributions you can query the runtime log file maintained by Systemd daemon via journalctl command (ssh.service or sshd.service). For example:

journalctl _SYSTEMD_UNIT=ssh.service | egrep "Failed|Failure"

Client side

Also you should run SSH client with -v|--verbose - it is in first level of verbosity. Next, you can enable additional (level 2 and 3) verbosity for even more debugging messages as shown with e.g. -vv.

Useful resources:

What is the difference between cron and anacron?

  • one of the main difference between cron and anacron jobs is that cron works on the system that are running continuously that means it is designed for the system that is running 24*7. While anacron is used for the systems that are not running continuously
  • other difference between the two is cron jobs can run every minute, but anacron jobs can be run only once a day
  • any normal user can do the scheduling of cron jobs, but the scheduling of anacron jobs can be done by the superuser only
  • cron should be used when you need to execute the job at a specific time as per the given time in cron, but anacron should be used in when there is no any restriction for the timing and can be executed at any time

A project manager needs a new SQL Server. What do you ask her/his? *

I want the DBA to ask questions like:

  • How big will the database be? (whether we can add the database to an existing server)
  • How critical is the database? (about clustering, disaster recovery, high availability)

Create a file with 100 lines with random values.

For example:

cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 100 > /path/to/file

How to run script as another user without password?

For example (with visudo command):

user1 ALL=(user2) NOPASSWD: /opt/scripts/bin/generate.sh

The command paths must be absolute! Then call sudo -u user2 /opt/scripts/bin/generate.sh from a user1 shell.

How to check if running as root in a bash script?

In a bash script, you have several ways to check if the running user is root.

As a warning, do not check if a user is root by using the root username. Nothing guarantees that the user with ID 0 is called root. It’s a very strong convention that is broadly followed but anybody could rename the superuser another name.

I think the best way when using bash is to use $EUID because $UID could be changed and not reflect the real user running the script.

if (( $EUID != 0 )); then
  echo "Please run as root"

Is there a way to redirect output to a file and have it display on stdout?

The command you want is named tee:

foo | tee output.file

For example, if you only care about stdout:

ls -a | tee output.file

If you want to include stderr, do:

program [arguments...] 2>&1 | tee outfile

2>&1 redirects channel 2 (stderr/standard error) into channel 1 (stdout/standard output), such that both is written as stdout. It is also directed to the given output file as of the tee command.

Furthermore, if you want to append to the log file, use tee -a as:

program [arguments...] 2>&1 | tee -a outfile

What is the preferred bash shebang and why? What is the difference between executing a file using ./program or bash program?

You should use #!/usr/bin/env bash for portability: different *nixes put bash in different places, and using /usr/bin/env is a workaround to run the first bash found on the PATH.

Running ./program does exactly that, and requires execute permission on the file, but is agnostic to what type of a program it is. It might be a bash script, an sh script, or a Perl, Python, awk, or expect script, or an actual binary executable. Running bash program would force it to be run under sh, instead of anything else.

Useful resources:

You must run command that will be performed for a very long time. How to prevent killing this process after the ssh session drops?

Use nohup to make your process ignore the hangup signal:

nohup long-running-process &

or you want to be using GNU Screen:

screen -d -m long-running-process

Useful resources:

What is the main purpose of the intermediate certification authorities?

To find out the main purpose of an intermediate CA, you should first learn about Root CAs, Intermediate CAs, and the SSL Certificate Chain Trust.

Root CAs are primary CAs which typically don’t directly sign end entity/server certificates. They issue Root certificates which are usually pre-installed within all browsers, mobiles, and applications. The private key of these certificates is used to sign other subsequent certificates called intermediate certificates. Root CAs are usually kept “offline” and in a highly secure environment with stringently limited access.

Intermediates CAs are CAs that subordinate to the Root CA by one or more levels, being trusted by these to sign certificates on their behalf. The purpose of creating and using Intermediate CAs is primarily for security because if the intermediate private key is compromised, then the Root CA can revoke the intermediate certificate and create a new one with a new cryptographic key pair.

SSL Certificate Chain Trust is the list of SSL certificates, from the root certificate to the end entity/server certificate. For an SSL Certificate to be trusted, it must be issued by a trusted CAs which is included in the trusted CA list of the connecting device (browser, mobile, and application). Therefore, the connecting device will test the trustworthiness of each SSL Certificate in the Chain Trust until it matches the one issued by a trusted CA.

The Root-Intermediate CA structure is created by each major CA to protect against the disastrous effects of a root key compromise. If a root key is compromised, it would render the root and all subordinated certificates untrustworthy. For this reason, creating an Intermediate CA is a best practice to ensure a rigorous protection of the primary root key.

Useful resources:

How to reload PostgreSQL after configuration changes?

Solution 1:

systemctl reload postgresql

Solution 2:

su - postgres
/usr/bin/pg_ctl reload

Solution 3:

SELECT pg_reload_conf();

How to restore config file in Debian GNU/Linux?

Will recreate any missing configuration files, e.g. /etc/mysql/my.cnf in your case:

dpkg -i --force-confmiss mysql-common.deb

You have added several aliases to .profile. How to reload shell without exit?

The best way is exec $SHELL -l because exec replaces the current process with a new one. Also good (but other) solution is . ~/.profile.

Useful resources:

How to exit without saving shell history?

kill -9 $$


unset HISTFILE && exit

Useful resources:

What is this UID 0 toor account? Have I been compromised?

toor is an alternative superuser account, where toor is root spelled backwards. It is intended to be used with a non-standard shell so the default shell for root does not need to change.

This is important as shells which are not part of the base distribution, but are instead installed from ports or packages, are installed in /usr/local/bin which, by default, resides on a different file system. If root’s shell is located in /usr/local/bin and the file system containing /usr/local/bin) is not mounted, root will not be able to log in to fix a problem and will have to reboot into single-user mode in order to enter the path to a shell.

Some people use toor for day-to-day root tasks with a non-standard shell, leaving root, with a standard shell, for single-user mode or emergencies. By default, a user cannot log in using toor as it does not have a password, so log in as root and set a password for toor before using it to login.

Useful resources:

Is there an easy way to search inside 1000s of files in a complex directory structure to find files which contain a specific string within the file?

For example use fgrep:

fgrep * -R "string"


grep -insr "pattern" *
  • -i ignore case distinctions in both the PATTERN and the input files
  • -n prefix each line of output with the 1-based line number within its input file
  • -s suppress error messages about nonexistent or unreadable files.
  • -r read all files under each directory, recursively.

Useful resources:

How to find out the dynamic libraries executables loads when run?

You can do this with ldd command:

ldd /bin/ls

You have the task of sync the testing and production environments. What steps will you take?

It’s easy to get dragged down into bikeshedding about cloning environments and miss the real point:

  • only production is production

and every time you deploy there you are testing a unique combination of deploy code + software + environment.

Every once in a while a good solution is regular cloning of the production servers to create testing servers. You can create instances with an exact copy of your production environment under a dev/test with snapshots, for example:

  • generate a snapshot of production
  • copy the snapshot to staging (or other)
  • create a new disk using this snapshot

Sure, you can spin up clones of various system components or entire systems, and capture real traffic to replay offline (the gold standard of systems testing). But many systems are too big, complex, and cost-prohibitive to clone.

Before environment synchronization a good way is keeping track of every change that you make to the testing environment and provide a way for propagating this to the production environment, so that you do not skip any step and do it as smoothly as possible.

Also structure comparison tool or deploy scripts that update the testing environment from production environment is a good solution.

Presync tasks

First of all is informing developers and clients about not making changes on the test environment (if possible, disabling test domains that target this environment or set static pages with information about synchronization).

It is also important to make backup/snapshots of both environments.

Database servers

  • sync/update system version (e.g. packages)
  • create dump file from database on production db server
  • import dump file on testing db server
  • if necessary, syncs login permissions, roles, database permissions, open connections to the database and other

Web/App servers

  • sync/update system version (e.g. packages)
  • if necessary, updated kernel parametres, firewall rules and other
  • sync/update configuration files of all running/important services
  • sync/update user accounts (e.g. permissions) and their home directories
  • deploy project from git/svn repository
  • sync/update important directories existing in project, e.g. static, asset and other
  • sync/update permissions for project directory
  • remove/update all webhooks
  • update cron jobs

Others tasks

  • updated configurations of load balancers for testing domains and specific urls
  • updated configurations of queues, session and storage instances

Useful resources:

Network Questions (25)

Configure a virtual interface on your workstation. *

To be completed.

According to an HTTP monitor, a website is down. You’re able to telnet to the port, so how do you resolve it?

If you can telnet to the port, this means that the service listening on the port is running and you can connect to it (it’s not a networking problem). It is good to check this way for the IP address to which the domain is resolved and using the same domain to test connection.

First of all check if your site is online from a other location. It then lets you know if the site is down everywhere, or if only your network is unable to view it. It is also a good idea to check what the web browser returns.

If only IP connection working

  • you can use whois to see what DNS servers serve up the hostname to the site: whois www.example.com
  • you can use tools like dig or host to test DNS to see if the host name is resolving: host www.example.org dns.example.org
  • you can also check global public dns servers: host www.example.com

If domain not resolved it’s probably probem with DNS servers.

If domain resolved properly

  • investigate the log files and resolve the issue regarding to the logs, it’s the best way to show what’s wrong
  • check the http status code, usually it will be the response with the 5xx, maybe server is overload because clients making lot’s of connection to the website or specific url? maybe your caching rules not working properly?
  • check web/proxy server configuration (e.g. nginx -t -c </path/to/nginx.conf>), maybe another sysadmin has made some changes to the domain configuration?
  • maybe something on the server has crashed? maybe run out of space or run out of memory?
  • maybe it’s a programming error on the website?

Load balancing can dramatically impact server performance. Discuss several load balancing mechanisms. *

To be completed.

List examples of network troubleshooting tools that can degrade during DNS issues. *

To be completed.

Why won’t the hostnames resolve on your server? Fix this issue. *

To be completed.

Explain difference between HTTP 1.1 and HTTP 2.0.

HTTP/2 supports queries multiplexing, headers compression, priority and more intelligent packet streaming management. This results in reduced latency and accelerates content download on modern web pages.

Key differences with HTTP/1.1:

  • it is binary, instead of textual
  • fully multiplexed, instead of ordered and blocking
  • can therefore use one connection for parallelism
  • uses header compression to reduce overhead
  • allows servers to “push” responses proactively into client caches

Useful resources:

Dev team reports an error: POST http://ws.int/api/v1/Submit/ resulted in a 413 Request Entity Too Large. What’s wrong?

Modify NGINX configuration file for domain

Set correct client_max_body_size variable value:

client_max_body_size 20M;

Restart Nginx to apply the changes.

Modify php.ini file for upload limits

It’s not needed on all configurations, but you may also have to modify the PHP upload settings as well to ensure that nothing is going out of limit by php configurations.

Now find following directives one by one:


and increase its limit to 20M, by default they are 8M and 2M:

upload_max_filesize = 20M
post_max_size = 20M

Finally save it and restart PHP.

Useful resources:

What is handshake mechanism and why do we need 3 way handshake?

Handshaking begins when one device sends a message to another device indicating that it wants to establish a communications channel. The two devices then send several messages back and forth that enable them to agree on a communications protocol.

A three-way handshake is a method used in a TCP/IP network to create a connection between a local host/client and server. It is a three-step method that requires both the client and server to exchange SYN and ACK (SYN, SYN-ACK, ACK) packets before actual data communication begins.

Useful resources:

Why is UDP faster than TCP?

UDP is faster than TCP, and the simple reason is because its nonexistent acknowledge packet (ACK) that permits a continuous packet stream, instead of TCP that acknowledges a set of packets, calculated by using the TCP window size and round-trip time (RTT).

Useful resources:

What is NAT? What is it used for?

It enables private IP networks that use unregistered IP addresses to connect to the Internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses, before packets are forwarded to another network.

Workstations or other computers requiring special access outside the network can be assigned specific external IPs using NAT, allowing them to communicate with computers and applications that require a unique public IP address. NAT is also a very important aspect of firewall security.

Useful resources:

What is the purpose of Spanning Tree?

This protocol operates at layer 2 of the OSI model with the purpose of preventing loops on the network. Without STP, a redundant switch deployment would create broadcast storms that cripple even the most robust networks. There are several iterations based on the original IEEE 802.1D standard; each operates slightly different than the others while largely accomplishing the same loop-free goal.

How to check which ports are listening in my Linux Server?

Use the:

  • lsof -i
  • ss -l
  • netstat -atn - for tcp
  • netstat -aun - for udp
  • netstat -tulapn

What mean Host key verification failed when you connect to the remote host? Do you accept it automatically?

Host key verification failed means that the host key of the remote host was changed. This can easily happen when connecting to a computer who’s host keys in /etc/ssh have changed if that computer was upgraded without copying its old host keys. The host keys here are proof when you reconnect to a remote computer with ssh that you are talking to the same computer you connected to the first time you accessed it.

Whenever you connect to a server via SSH, that server’s public key is stored in your home directory (or possibly in your local account settings if using a Mac or Windows desktop) file called known_hosts. When you reconnect to the same server, the SSH connection will verify the current public key matches the one you have saved in your known_hosts file. If the server’s key has changed since the last time you connected to it, you will receive the above error.

Don’t delete the entire known_hosts file as recommended by some people, this totally voids the point of the warning. It’s a security feature to warn you that a man in the middle attack may have happened.

Before accepting the new host key, contact your/other system administrator for verification.

Usefule resources:

How to send an HTTP request using telnet?

For example:

telnet example.com 80
Connected to example.com.
Escape character is '^]'.
GET /questions HTTP/1.0
Host: example.com

HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8

How do you kill program using e.g. 80 port in Linux?

To list any process listening to the port 80:

# with lsof
lsof -i:80

# with fuser
fuser 80/tcp

To kill any process listening to the port 80:

kill $(lsof -t -i:80)

or more violently:

kill -9 $(lsof -t -i:80)

or with fuser command:

fuser -k 80/tcp

Useful resources:

You get curl: (56) TCP connection reset by peer. What steps will you take to solve this problem?

  • check if the URL is correct, maybe you should add www or set correctly Host: header? Check also scheme (http or https)
  • check the domain is resolving into a correct IP address
  • enable debug tracing with --trace-ascii curl.dump. “Recv failure” is a really generic error so its hard for more info
  • use external proxy with --proxy for debug connection from external ip
  • use network sniffer (e.g. tcpdump) for debug connection in the lower TCP/IP layers
  • check firewall rules on the production environment and on the exit point of your network, also check your NAT rules
  • check MTU size of packets travelling over your network
  • check SSL version with ssl/tls curl params if you connecting to https protocol
  • it may be a problem on the client side e.g. the netfilter drop or limit connections from your IP address to the domain

Useful resources:

How to allow traffic to/from specific IP with iptables?

For example:

/sbin/iptables -A INPUT -p tcp -s XXX.XXX.XXX.XXX -j ACCEPT
/sbin/iptables -A OUTPUT -p tcp -d  XXX.XXX.XXX.XXX -j ACCEPT

How to block abusive IP addresses with pf in OpenBSD?

The best way to do this is to define a table and create a rule to block the hosts, in pf.conf:

table <badhosts> persist
block on fxp0 from <badhosts> to any

And then dynamically add/delete IP addresses from it:

pfctl -t badhosts -T add
pfctl -t badhosts -T delete

How to disable cache for specific domain in Varnish?

For example:

sub vcl_recv {
   # don't cache foo.com or bar.com - optional www
   if (req.host ~ "(www)?(foo|bar).com") {


sub vcl_recv {
  # don't cache foo.com or bar.com - optional www
  if (req.http.host ~ "(www\.)?(foo|bar)\.com") {
  # cache foobar.com - optional www
  if (req.http.host ~ "(www\.)?foobar\.com") {

Useful resources:

When does the web server like Apache or Nginx write info to log file? Before or after serving the request?

Both servers provides very comprehensive and flexible logging capabilities - for logging everything that happens on your server, from the initial request, through the URL mapping process, to the final resolution of the connection, including any errors that may have occurred in the process.


The Apache server access log records all requests processed by the server (after the request has been completed).


NGINX writes information about client requests in the access log right after the request is processed.

Useful resources:

Analyse web server log and show only 5xx http codes. What external tools do you use?

tail -n 100 -f /path/to/logfile | grep "HTTP/[1-2].[0-1]\" [5]"

Examples of http/https log management tools:

  • goaccess - is an open source real-time web log analyzer and interactive viewer that runs in a terminal in *nix systems or through your browser
  • graylog - is a free and open-source log management platform that supports in-depth log collection and analysis

Useful resources:

Developer uses private key on the server to deploy app through ssh. Why it is incorrect behavior and what is the better (but not ideal) solution in such situations?

You have the private key for your personal account. The server needs your public key so that it can verify that your private key for the account you are trying to use is authorized.

The whole point with private keys is that they are private, meaning only you have your private key. If someone takes over your private key, it will be able to impersonate you any time he wants.

A better solutions is the use of ssh key forwarding. An essence, you need to create a ~/.ssh/config file, if it doesn’t exist. Then, add the hosts (either domain name or IP address in the file and set ForwardAgent yes). Example:

Host git.example.com
    User john
    PreferredAuthentications publickey
    IdentityFile ~/.ssh/id_rsa.git.example.com
    ForwardAgent yes

Your remote server must allow SSH agent forwarding on inbound connections and your local ssh-agent must be running.

Forwarding an ssh agent carries its own security risk. If someone on the remote machine can gain access to your forwarded ssh agent connection, they can still make use of your keys. However, this is better than storing keys on remote machines: the attacker can only use the ssh agent connection, not the key itself. Thus, only while you’re logged into the remote machine can they do anything. If you store the key on the remote machine, they can make a copy of it and use it whenever they want.

If you use ssh keys remember about passphrases which is strongly recommended to reduce risk of keys accidentally leaking.

Useful resources:

What is the difference between CORS and CSPs?

CORS allows the Same Origin Policy to be relaxed for a domain.

e.g. normally if the user logs into both example.com and example.org, the Same Origin Policy prevents example.com from making an AJAX request to example.org/current_user/full_user_details and gaining access to the response.

This is the default policy of the web and prevents the user’s data from being leaked when logged into multiple sites at the same time.

Now with CORS, example.org could set a policy to say it will allow the origin https://example.com to read responses made by AJAX. This would be done if both example.com and example.org are ran by the same company and data sharing between the origins is to be allowed in the user’s browser. It only affects the client-side of things, not the server-side.

CSPs on the other hand set a policy of what content can run on the current site. For example, if JavaScript can be executed inline, or which domains .js files can be loaded from. This can be beneficial to act as another line of defence against XSS attacks, where the attacker will try and inject script into the HTML page. Normally output would be encoded, however say the developer had forgotten only on one output field. Because the policy is preventing in-line script from executing, the attack is thwarted.

Useful resources:

Explain four types of responses from firewall when scanning with nmap.

There might be four types of responses:

  • Open port - few ports in the case of the firewall
  • Closed port - most ports are closed because of the firewall
  • Filtered - Nmap is not sure whether the port is open or not
  • Unfiltered - Nmap can access the port but is still confused about the open status of the port

What does a tcpdump do? How to capture only incoming traffic to your interface?

tcpdump is a most powerful and widely used command-line packets sniffer or package analyzer tool which is used to capture or filter TCP/IP packets that received or transferred over a network on a specific interface.

tcpdump puts your network card into promiscuous mode, which basically tells it to accept every packet it receives. It allows the user to see all traffic being passed over the network. Wireshark uses pcap to capture packets.

If you want to view only packets that come to your interface you should:

  • -Q in - for Linux tcpdump version
  • -D in - for BSD tcpdump version

Both params set send/receive direction direction for which packets should be captured.

tcpdump -nei eth0 -Q in host and port 8080

Devops Questions (7)

Which are the top DevOps tools? Which tools have you worked on?

The most popular DevOps tools are mentioned below:

  • Git : Version Control System tool
  • Jenkins : Continuous Integration tool
  • Selenium : Continuous Testing tool
  • Puppet, Chef, Ansible : Configuration Management and Deployment tools
  • Nagios : Continuous Monitoring tool
  • Docker : Containerization tool

How do all these tools work together?

The most popular DevOps tools are mentioned below:

  • Developers develop the code and this source code is managed by Version Control System tools like Git etc.
  • Developers send this code to the Git repository and any changes made in the code is committed to this Repository
  • Jenkins pulls this code from the repository using the Git plugin and build it using tools like Ant or Maven
  • Configuration management tools like puppet deploys & provisions testing environment and then Jenkins releases this code on the test environment on which testing is done using tools like selenium
  • Once the code is tested, Jenkins send it for deployment on the production server (even production server is provisioned & maintained by tools like puppet)
  • After deployment It is continuously monitored by tools like Nagios
  • Docker containers provides testing environment to test the build features

What are playbooks in Ansible?

Playbooks are Ansible’s configuration, deployment, and orchestration language.

They can describe a policy you want your remote systems to enforce, or a set of steps in a general IT process. Playbooks are designed to be human-readable and are developed in a basic text language.

At a basic level, playbooks can be used to manage configurations of and deployments to remote machines.

What is NRPE (Nagios Remote Plugin Executor) in Nagios?

The NRPE addon is designed to allow you to execute Nagios plugins on remote Linux/Unix machines. The main reason for doing this is to allow Nagios to monitor “local” resources (like CPU load, memory usage, etc.) on remote machines.

Since these public resources are not usually exposed to external machines, an agent like NRPE must be installed on the remote Linux/Unix machines.

What is the difference between Active and Passive check in Nagios?

The major difference between Active and Passive checks is that Active checks are initiated and performed by Nagios, while passive checks are performed by external applications.

Passive checks are useful for monitoring services that are:

  • asynchronous in nature and cannot be monitored effectively by polling their status on a regularly scheduled basis.
  • located behind a firewall and cannot be checked actively from the monitoring host.

The main features of Actives checks are as follows:

  • active checks are initiated by the Nagios process.
  • active checks are run on a regularly scheduled basis.

How to git clone including submodules?

For example:

# With -j8 - performance optimization
git clone --recurse-submodules -j8 git://github.com/foo/bar.git

# For already cloned repos or older Git versions
git clone git://github.com/foo/bar.git
cd bar
git submodule update --init --recursive

Mention what are the advantages of using Redis? What is redis-cli?

  • it provides high speed (exceptionally faster than others)
  • it supports a server-side locking
  • it has got lots of client lib
  • it has got command level Atomic Operation (tx operation)
  • supports for rich data types like hashes, sets, bitmaps

redis-cli is the Redis command line interface, a simple program that allows to send commands to Redis, and read the replies sent by the server, directly from the terminal.

Useful resources:

Cyber Security Questions (4)

What is XSS, how will you mitigate it?

Cross Site Scripting is a JavaScript vulnerability in the web applications. The easiest way to explain this is a case when a user enters a script in the client side input fields and that input gets processed without getting validated. This leads to untrusted data getting saved and executed on the client side.

Countermeasures of XSS are input validation, implementing a CSP (Content security policy) etc.

HIDS vs NIDS and which one is better and why?

HIDS is host intrusion detection system and NIDS is network intrusion detection system. Both the systems work on the similar lines. It’s just that the placement in different. HIDS is placed on each host whereas NIDS is placed in the network. For an enterprise, NIDS is preferred as HIDS is difficult to manage, plus it consumes processing power of the host as well.

What is compliance?

Abiding by a set of standards set by a government/Independent party/organisation, e.g. an industry which stores, processes or transmits Payment related information needs to be complied with PCI DSS (Payment card Industry Data Security Standard). Other compliance examples can be an organisation complying with its own policies.

What is a WAF and what are its types?

WAF stands for web application firewall. It is used to protect the application by filtering legitimate traffic from malicious traffic. WAF can be either a box type or cloud based.

Senior Sysadmin

System Questions (59)

Explain the current architecture you’re responsible for and point out where it’s scalable or fault-tolerant. *

To be completed.

Tell me how code gets deployed in your current production. *

To be completed.

What are the different types of kernels? Explain.

Monolithic Kernels

Earlier in this type of kernel architecture, all the basic system services like a process and memory management, interrupt handling etc were packaged into a single module in kernel space. This type of architecture led to some serious drawbacks like:

  • the size of the kernel, which was huge
  • poor maintainability, which means bug fixing or addition of new features resulted in recompilation of the whole kernel which could consume hours

In a modern day approach to monolithic architecture, the kernel consists of different modules which can be dynamically loaded and unloaded. This modular approach allows easy extension of OS’s capabilities. With this approach, maintainability of kernel became very easy as only the concerned module needs to be loaded and unloaded every time there is a change or bug fix in a particular module.

Linux follows the monolithic modular approach.


This architecture majorly caters to the problem of ever growing size of kernel code which we could not control in the monolithic approach. This architecture allows some basic services like device driver management, protocol stack, file system etc to run in user space.

In this architecture, all the basic OS services which are made part of user space are made to run as servers which are used by other programs in the system through inter process communication (IPC).

e.g. We have servers for device drivers, network protocol stacks, file systems, graphics, etc. Microkernel servers are essentially daemon programs like any others, except that the kernel grants some of them privileges to interact with parts of physical memory that are otherwise off limits to most programs.

Hybrid Kernels (Modular Kernels)

This is a combination of the above two, where the key idea is that Operating System services are in Kernel Space, and and there is no message passing, no performance overhead and no reliability benefits, of having services in user space.

This is used by Microsoft’s NT kernels, all the way up to the latest Windows version.

Useful resources:

The program returns the error of the missing library. How to provide dynamically linkable libraries?

Environment variable LD_LIBRARY_PATH is a colon-separated set of directories where libraries should be searched for first, before the standard set of directories; this is useful when debugging a new library or using a nonstandard library for special purposes.

The best way to use LD_LIBRARY_PATH is to set it on the command line or script immediately before executing the program. This way the new LD_LIBRARY_PATH isolated from the rest of your system.

Example of use:

export LD_LIBRARY_PATH="/list/of/library/paths:/another/path" ./program

Useful resources:

Explain all the fields in the /etc/passwd file.

  • Username: First field is the username that contains the username which is 1 to 32 length characters
  • Password: This field does not show the actual password as the password is encrypted. Here, x character shows that password is encrypted that is located in /etc/shadow file
  • User ID (UID): All the users created in Linux is given a user ID whenever the user is created. UID 0 is fixed and reserved for the root user
  • Group ID (GID): This field specifies the name of the group to which the user belongs. The group information is also stored in a file /etc/group
  • User ID Info: Here you can add comments and you can add any extra information related to the users like full name, contact number, etc
  • Home directory: This field provides the path where the user is directed after the login. For example, /home/smith
  • Command/shell: This field provides the path of a command/shell and denotes that user has access to this shell i.e. /bin/bash

Useful resources:

What is the advantage of synchronizing UID/GID across multiple systems?

There are several principle reasons why you want to co-ordinate the user/UID and group/GID management across your network.

The first is relatively obvious - it has to do with user and administrative convenience.

If each of your users are expected to have relatively uniform access to the systems throughout the network, then they’ll expect the same username and password to work on each system that they are supposed to use. If they change their password they will expect that change to be global.

It also has a relationship with names and group names in Unix and Linux. They are mapped into numeric forms (UID’s and GID’s respectively). All file ownership (inodes) and processes use these numerics for all access and identity determination throughout the kernel and drivers. These numeric values are reverse mapped back to their corresponding principle symbolic representations (the names) by the utilities that display or process that information.

It is also recommended that you adopt a policy that UID’s are not re-used. When a user leaves your organization you “retire” their UID (disabling their access by *‘ing out their passwd, removing them from the groups maps, setting their “shell” to some /bin/denied binary and their “home” directory to a secured “graveyard” - I use /home/.graveyard on my systems).

The reason for this may not be obvious. However, if you are maintaining archival backups for several years (or indefinitely) you’ll want to avoid any ambiguities and confusion that might result from restoring one (long gone) user’s files and finding them owned by one of your new users.

Useful resources:

What principles to follow for successful system performance tuning? *

To be completed.

Useful resources:

Describe start-up configuration files and directory in BSD systems.

In BSD the primary start-up configuration file is /etc/defaults/rc.conf. System startup scripts such as /etc/rc and /etc/rc.d just include this file.

If you want to add other programs to system startup you need to change /etc/rc.conf file instead of /etc/defaults/rc.conf.

The Junior dev accidentally destroyed production database. How can you prevent such situations?

Create disaster recovery plan

Disaster recovery and business continuity planning are integral parts of the overall risk management for an organization. Is a documented process or set of procedures to recover and protect a business IT infrastructure.

If you don’t have a recovery solution, then your restoration efforts will become rebuilding efforts, starting from scratch to recreate whatever was lost.

You should use commonly occurring real life data disaster scenarios to simulate what your backups will and won’t do in a crisis.

Create disaster recovery center

As a result, in the event of unplanned interruptions in the functioning of the primary location, service and all operational activities are switched to the backup center and therefore the unavailability of services is limited to the absolute minimum.

Does the facility have sufficient bandwidth options and power to scale and deal with the increased load during a major disaster? Are resources available to periodically test failover?

Create regular backups and tested it!

Backups are a way to protect the investment in data. By having several copies of the data, it does not matter as much if one is destroyed (the cost is only that of the restoration of the lost data from the backup).

When you lose data, one thing is certain: downtime.

To assure the validity and integrity of any backup, it’s essential to carry out regular restoration tests. Ideally, a test should be conducted after every backup completes to ensure data can be successfully secured and recovered. However, this often isn’t practical due to a lack of available resources or time constraints.

Make backups of entire virtual machines and important components in the middle of them.

Create snapshots: vm, disks or lvm

Snapshots are perfect if you want to recover a server from a previous state but it’s only a “quick method”, it cannot restore the system after too many items changed.

Create them always before making changes on production environments (and not only).

Disk snapshots are used to generate a snapshot of an entire disk. These snapshots don’t make it easy to restore individual chunks of data (e.g. a lost user account), though it’s possible. The primary purpose is to restore entire disks in case of disk failure.

The LVM snapshots can be primarily used to easily copy data from production environment to staging environment.

Remember: Snapshots are not backups!

Development and testing environments

A production environment is the real instance of the application and its database used by the company or the clients. The production database has all the real data.

Setting up development environments based directly on the production database, instead of using a backup for this (removing the need for the above). Dev and test environment that your engineers can get to and a prod environment that only a few people can push updates to following an approved change.

All environments such as prod, dev and test should have one major difference: authorization data for services. For example postgres database instance on testing environment should be consistent (if possible) with the production base, however, in order to eliminate errors of database names and logins and passwords for authorization should be different.

Single point of failure

The general method to avoid single points of failures is to provide redundant components for each necessary resource, so service can continue if a component fails.

Synchronization and replication process for databases

The replication procedure is super fragile and prone to error.

A good idea is also slightly longer delay of data replication (e.g. for DRC). As in replicas, the data changes will usually be replicated within minutes, so the lost data won’t be on the replica database either once that happens.

Create database model with users, roles and rights, use different methods of protection

Only very advanced devs have permissions for db admin access. The other really don’t need write access to clone a database. On the other hand just don’t give a developer write access to prod.

The production database should refuse connections from any server and pc which isn’t the one running the production application, even if it provides a valid username/password.

How the hell development machines can access a production database right like that? How about a simple firewall rule to just let the servers needing the DB data access the database?

Create summmary/postmortem documents after failures

The post-mortem audience includes customers, direct reports, peers, the company’s executive team and often investors.

Explain what caused the outage on a timeline. Every incident begins with a specific trigger at a specific time, which often causes some unexpected behavior. For example, our servers were rebooted and we expected them to come back up intact, which didn’t happen.

Furthermore, every incident has a root cause: the reboot itself was trigger, however a bug in the driver caused the actual outage. Finally, there’re consequences to every incident, the most obvious one is that the site goes down.

The post-mortem answers the single most important question of what could have prevented the outage.

Despite how painful an outage may have been, the worst thing you can do is to bury it and never properly close the incident in a clear and transparent way.

If you also made a big mistake…

Humans are just apes with bigger computers.” - african_cheetah (Reddit)

I’ve come to appreciate not having access to things I don’t absolutely need.” - warm_vanilla_sugar (Reddit)

Document whatever happened somewhere. Write setup guides. Failure is instructive.

Useful resources:

How to scan newly assigned luns on Linux box without rebooting?

Run the command: echo "---" >/sys/class/scsi_host/hostX/scan

You need to move ext4 journal on another disk/partition. What are the reasons for this? *

To be completed.

Explain system calls used for process management.

There are some system calls for process management. These are as follows:

  • Fork(): it is used to create a new process
  • Exec(): it is used to execute a new process
  • Wait(): it is used to make the process to wait
  • Exit(): it is used to exit or terminate the process
  • Getpid(): it is used to find the unique process ID
  • Getppid(): it is used to check the parent process ID
  • Nice(): it is used to bias the currently running process property

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Can’t mount the root file system. Why? *

To be completed.

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You have to delete 100GB files. Which method will be the most optimal? *

To be completed.

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Explain interrupts and interrupt handlers in Linux.

Here’s a high-level view of the low-level processing. I’m describing a simple typical architecture, real architectures can be more complex or differ in ways that don’t matter at this level of detail.

When an interrupt occurs, the processor looks if interrupts are masked. If they are, nothing happens until they are unmasked. When interrupts become unmasked, if there are any pending interrupts, the processor picks one.

Then the processor executes the interrupt by branching to a particular address in memory. The code at that address is called the interrupt handler. When the processor branches there, it masks interrupts (so the interrupt handler has exclusive control) and saves the contents of some registers in some place (typically other registers).

The interrupt handler does what it must do, typically by communicating with the peripheral that triggered the interrupt to send or receive data. If the interrupt was raised by the timer, the handler might trigger the OS scheduler, to switch to a different thread. When the handler finishes executing, it executes a special return-from-interrupt instruction that restores the saved registers and unmasks interrupts.

The interrupt handler must run quickly, because it’s preventing any other interrupt from running. In the Linux kernel, interrupt processing is divided in two parts:

  • The “top half” is the interrupt handler. It does the minimum necessary, typically communicate with the hardware and set a flag somewhere in kernel memory.
  • The “bottom half” does any other necessary processing, for example copying data into process memory, updating kernel data structures, etc. It can take its time and even block waiting for some other part of the system since it runs with interrupts enabled.

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What considerations come into play when designing a highly available application, both at the architecture level and the application level? *

To be completed.

What fields are stored in an inode?

Within a POSIX system, a file has the following attributes which may be retrieved by the stat system call:

  • Device ID (this identifies the device containing the file; that is, the scope of uniqueness of the serial number). File serial numbers
  • The file mode which determines the file type and how the file’s owner, its group, and others can access the file
  • A link count telling how many hard links point to the inode
  • The User ID of the file’s owner
  • The Group ID of the file
  • The device ID of the file if it is a device file.
  • The size of the file in bytes
  • Timestamps telling when the inode itself was last modified (ctime, inode change time), the file content last modified (mtime, modification time), and last accessed (atime, access time)
  • The preferred I/O block size
  • The number of blocks allocated to this file

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Ordinary users are able to read /etc/passwd, is this a security hole? You know alternative password shadowing scheme?

Typically, the hashed passwords are stored in /etc/shadow on most Linux systems:

-rw-r----- 1 root shadow 1349 2011-07-03 03:54 /etc/shadow

They are stored in /etc/master.passwd on BSD systems.

Programs that need to perform authentication still need to run with root privileges:

-rwsr-xr-x 1 root root 42792 2011-02-14 14:13 /usr/bin/passwd

If you dislike the setuid root programs and one single file containing all the hashed passwords on your system, you can replace it with the Openwall TCB PAM module. This provides every single user with their own file for storing their hashed password - as a result the number of setuid root programs on the system can be drastically reduced.

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What are some of the benefits of using systemd over SysV init? *

To be completed.

How do you run command every time a file is modified?

For example:

while inotifywait -e close_write filename ; do

  echo "changed" >> /var/log/changed


You need to copy a large amount of data. Explain the most effective way. *

To be completed.

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Tell me about the dangers and caveats of LVM.

Risks of using LVM

  • Vulnerable to write caching issues with SSD or VM hypervisor
  • Harder to recover data due to more complex on-disk structures
  • Harder to resize filesystems correctly
  • Snapshots are hard to use, slow and buggy
  • Requires some skill to configure correctly given these issues

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Python dev team in your company have a dilemma what to choose: uwsgi or gunicorn. What are the pros/cons of each of the solutions from the admin’s perspective? *

To be completed.

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What if kill -9 does not work? Describe exceptions for which the use of SIGKILL is insufficient.

kill -9 (SIGKILL) always works, provided you have the permission to kill the process. Basically either the process must be started by you and not be setuid or setgid, or you must be root. There is one exception: even root cannot send a fatal signal to PID 1 (the init process).

However kill -9 is not guaranteed to work immediately. All signals, including SIGKILL, are delivered asynchronously: the kernel may take its time to deliver them. Usually, delivering a signal takes at most a few microseconds, just the time it takes for the target to get a time slice. However, if the target has blocked the signal, the signal will be queued until the target unblocks it.

Normally, processes cannot block SIGKILL. But kernel code can, and processes execute kernel code when they call system calls.

A process blocked in a system call is in uninterruptible sleep. The ps or top command will (on most unices) show it in state D.

To remove a D State Process, since it is uninterruptible, only a machine reboot can solve the problem in case its not automatically handled by the system.

Usually there is a very few chance that a process stays in D State for long. And if it does then there is something not properly being handled in the system. This can be a potential bug as well.

A classical case of long uninterruptible sleep is processes accessing files over NFS when the server is not responding; modern implementations tend not to impose uninterruptible sleep (e.g. under Linux, the intr mount option allows a signal to interrupt NFS file accesses).

You may sometimes see entries marked Z (or H under Linux) in the ps or top output. These are technically not processes, they are zombie processes, which are nothing more than an entry in the process table, kept around so that the parent process can be notified of the death of its child. They will go away when the parent process pays attention (or dies).

Summary exceptions:

  • Zombie processes cannot be killed since they are already dead and waiting for their parent processes to reap them
  • Processes that are in the blocked state will not die until they wake up again
  • The init process is special: It does not get signals that it does not want to handle, and thus it can ignore SIGKILL. An exception from this exception is while init is ptraced on Linux
  • An uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem

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Difference between nohup, disown and &. What happens when using all together?

  • & puts the job in the background, that is, makes it block on attempting to read input, and makes the shell not wait for its completion
  • disown removes the process from the shell’s job control, but it still leaves it connected to the terminal. One of the results is that the shell won’t send it a SIGHUP. Obviously, it can only be applied to background jobs, because you cannot enter it when a foreground job is running
  • nohup disconnects the process from the terminal, redirects its output to nohup.out and shields it from SIGHUP. One of the effects (the naming one) is that the process won’t receive any sent SIGHUP. It is completely independent from job control and could in principle be used also for foreground jobs (although that’s not very useful)

If you use all three together, the process is running in the background, is removed from the shell’s job control and is effectively disconnected from the terminal.

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What is the main advantage of using chroot? When and why is it used? What is the purpose of the mount dev, proc, sys in a chroot environment?

An advantage of having a chroot environment is the file-system is totally isolated from the physical host. chroot has a separate file-system inside the file-system, the difference is its uses a newly created root(/) as its root directory.

A chroot jail is a way to isolate a process and its children from the rest of the system. It should only be used for processes that don’t run as root, as root users can break out of the jail very easily.

The idea is that you create a directory tree where you copy or link in all the system files needed for a process to run. You then use the chroot() system call to change the root directory to be at the base of this new tree and start the process running in that chroot’d environment. Since it can’t actually reference paths outside the modified root, it can’t perform operations (read/write etc.) maliciously on those locations.

On Linux, using a bind mounts is a great way to populate the chroot tree. Using that, you can pull in folders like /lib and /usr/lib while not pulling in /usr, for example. Just bind the directory trees you want to directories you create in the jail directory.

Chroot environment is useful for:

  • reinstall bootloader
  • reset a forgotten password
  • perform a kernel upgrade (or downgrade)
  • rebuild your initramdisk
  • fix your /etc/fstab
  • reinstall packages using your package manager
  • whatever

When working in a chrooted environment, there is a few special file systems that needs to be mounted so all programs behave properly.

Limitation is that /dev, /sys and /proc are not mounted by default but needed for many tasks.

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What are segmentation faults (segfaults), and how can identify what’s causing them?

A segmentation fault (aka segfault) is a common condition that causes programs to crash. Segfaults are caused by a program trying to read or write an illegal memory location.

Program memory is divided into different segments:

  • a text segment for program instructions
  • a data segment for variables and arrays defined at compile time
  • a stack segment for temporary (or automatic) variables defined in subroutines and functions
  • a heap segment for variables allocated during runtime by functions, such as malloc (in C)

In practice, segfaults are almost always due to trying to read or write a non-existent array element, not properly defining a pointer before using it, or (in C programs) accidentally using a variable’s value as an address. Thus, when Process A reads memory location 0x877, it reads information residing at a different physical location in RAM than when Process B reads its own 0x877.

All modern operating systems support and use segmentation, and so all can produce a segmentation fault.

Segmentation fault can also occur under following circumstances:

  • a buggy program/command, which can be only fixed by applying patch
  • it can also appear when you try to access an array beyond the end of an array under C programming
  • inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing
  • sometime hardware or faulty memory or driver can also create problem
  • maintain suggested environment for all computer equipment (overheating can also generate this problem)

To debug this kind of error try one or all of the following techniques:

  • enable core files: $ ulimit -c unlimited
  • reproduce the crash: $ ./<program>
  • debug crash with gdb: $ gdb <program> [core file]
  • or run LD_PRELOAD=...path-to.../libSegFault.so <program> to get a report with backtrace, loaded libs, etc


  • make sure correct hardware installed and configured
  • always apply all patches and use updated system
  • make sure all dependencies installed inside jail
  • turn on core dumping for supported services such as Apache
  • use strace which is a useful diagnostic, instructional, and debugging tool

Sometimes segmentation faults are not caused by bugs in the program but are caused instead by system memory limits being set too low. Usually it is the limit on stack size that causes this kind of problem (stack overflows). To check memory limits, use the ulimit command in bash.

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One of the processes runs slowly. How to check how long has been running and which tools will you use?

To be completed.

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What is a file descriptor in Linux?

In Unix and related computer operating systems, a file descriptor (FD, less frequently fildes) is an abstract indicator (handle) used to access a file or other input/output resource, such as a pipe or network socket. File descriptors form part of the POSIX application programming interface.

What is an open file table?

The process table entry (aka process control block) contains a table, the file descriptor table that gives the mapping between the descriptor the process uses to refer to a file connection and the data structure inside the kernel that represents the actual file connection.

What is the difference between /sbin/nologin, /bin/false and /bin/true?

When /sbin/nologin is set as the shell, if user with that shell logs in, they’ll get a polite message saying ‘This account is currently not available’.

/bin/false is just a binary that immediately exits, returning false, when it’s called, so when someone who has false as shell logs in, they’re immediately logged out when false exits. Setting the shell to /bin/true has the same effect of not allowing someone to log in but false is probably used as a convention over true since it’s much better at conveying the concept that person doesn’t have a shell.

/bin/nologin is the more user-friendly option, with a customizable message given to the user trying to log in, so you would theoretically want to use that; but both nologin and false will have the same end result of someone not having a shell and not being able to ssh in.

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What is the meaning of the error maxproc limit exceeded by uid %i … in FreeBSD?

The FreeBSD kernel will only allow a certain number of processes to exist at one time. The number is based on the kern.maxusers variable.

kern.maxusers also affects various other in-kernel limits, such as network buffers. If the machine is heavily loaded, increase kern.maxusers. This will increase these other system limits in addition to the maximum number of processes.

To adjust the kern.maxusers value, see the File/Process Limits section of the Handbook. While that section refers to open files, the same limits apply to processes.

If the machine is lightly loaded but running a very large number of processes, adjust the kern.maxproc tunable by defining it in /boot/loader.conf.

How to read a file line by line and assigning the value to a variable.

For example:

while IFS='' read -r line || [[ -n "$line" ]] ; do
  echo "Text read from file: $line"
done < "/path/to/filename"


  • IFS='' (or IFS=) prevents leading/trailing whitespace from being trimmed.
  • -r prevents backslash escapes from being interpreted.
  • || [[ -n $line ]] prevents the last line from being ignored if it doesn’t end with a \n (since read returns a non-zero exit code when it encounters EOF).

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The client reports that his site received a grade B in the ssllabs scanner. Prepare a checklist of best practice for ssl configuration. *

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What does “CPU jumps” mean?

An OS is a very busy thing, particularly so when you have it doing something (and even when you aren’t). And when we are looking at an active enterprise environment, something is always going on.

Most of this activity is “bursty”, meaning processes are typically quiescent with short periods of intense activity. This is certainly true of any type of network-based activity (e.g. processing PHP requests), but also applies to OS maintenance (e.g. file system maintenance, page reclamation, disk I/O requests).

If you take a situation where you have a lot of such bursty processes, you get a very irregular and spiky CPU usage plot.

As “500 - Internal Server Error” says, the high number of context switches are going to make the situation even worse.

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How does strace connect to an already running process?

strace -p <PID> - to attach a process to strace.

strace -e trace=read,write -p <PID> - by this you can also trace a process/program for an event, like read and write (in this example). So here it will print all such events that include read and write system calls by the process.

Other such examples

  • -e trace= network - trace all the network related system calls.
  • -e trace=signal - trace all signal related system calls.
  • -e trace=ipc - trace all IPC related system calls.
  • -e trace=desc - trace all file descriptor related system calls.
  • -e trace=memory - trace all memory mapping related system calls.

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How to remove all files except some from a directory?

Solution 1 - with extglob:

shopt -s extglob
rm !(textfile.txt|backup.tar.gz|script.php|database.sql|info.txt)

Solution 2 - with find:

find . -type f -not -name '*txt' -print0 | xargs -0 rm --

How to check if a string contains a substring in Bash?

You can use * (wildcards) outside a case statement, too, if you use double brackets:

string='some text'
if [[ $string = *"My long"* ]] ; then

Explain differences between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1.

  • a number 1 = standard out (i.e. STDOUT)
  • a number 2 = standard error (i.e. STDERR)
  • if a number isn’t explicitly given, then number 1 is assumed by the shell (bash)

First let’s tackle the function of these.


The general form of this one is M>&-, where “M” is a file descriptor number. This will close output for whichever file descriptor is referenced, i.e. “M”.


The general form of this one is M>/dev/null, where “M” is a file descriptor number. This will redirect the file descriptor, “M”, to /dev/null.


The general form of this one is M>&N, where “M” & “N” are file descriptor numbers. It combines the output of file descriptors “M” and “N” into a single stream.


This is just an abbreviation for 2>&1 |. It was added in Bash 4.


This is just an abbreviation for >/dev/null 2>&1. It redirects file descriptor 2 (STDERR) and descriptor 1 (STDOUT) to /dev/null.


This is just an abbreviation for 1>/dev/null. It redirects file descriptor 1 (STDOUT) to /dev/null.

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How to redirect stderr and stdout to different files in the same line?

Just add them in one line command 2>> error 1>> output.

However, note that >> is for appending if the file already has data. Whereas, `> will overwrite any existing data in the file.

So, command 2> error 1> output if you do not want to append.

Just for completion’s sake, you can write 1> as just > since the default file descriptor is the output. so 1> and > is the same thing.

So, command 2> error 1> output becomes, command 2> error > output.

Load averages are above 30 on a server with 24 cores but CPU shows around 70 percent idle. One of the common causes of this condition is? How to debug and fixed?

Requests which involve disk I/O can be slowed greatly if cpu(s) needs to wait on the disk to read or write data. I/O Wait, is the percentage of time the CPU has to wait on disk.

Lets looks at how we can confirm if disk I/O is slowing down application performance by using a few terminal command line tools (top, atop and iotop).

Example of debug:

  • answering whether or not I/O is causing system slowness
  • finding which disk is being written to
  • finding the processes that are causing high I/O
  • process list state
  • finding what files are being written too heavily
  • do you see your copy process put in D state waiting for I/O work to be done by pdflush?
  • do you see heavy synchronous write activity on your disks?


  • using top command - load averages and wa (wait time)
  • using atop command to monitor DSK (disk) I/O stats
  • using iotop command for real-time insight on disk read/writes

For improvement performance:

  • check drive array configuration
  • check disk queuing algorithms and tuning them
  • tuning general block I/O parameters
  • tuning virtual memory management to improve I/O performance
  • check and tuning mount options and filesystem params (also responsible for cache)

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How to enforce authorization methods in SSH?

Force login with a password:

ssh -o PreferredAuthentications=password -o PubkeyAuthentication=no user@remote_host

Force login using the key:

ssh -o PreferredAuthentications=publickey -o PubkeyAuthentication=yes -i id_rsa user@remote_host

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Getting Too many Open files error for Postgres. How to resolve it?

Fixed the issue by reducing max_files_per_process e.g. to 200 from default 1000. This parameter is in postgresql.conf file and this sets the maximum number of simultaneously open files allowed to each server subprocess.

Usually people start to edit /etc/security/limits.conf file, but forget that this file only apply to the actively logged in users through the pam system.

In what circumstance can df and du disagree on available disk space? How do you solve it?

du checks usage of directories, but df checks free’d inodes, and files can be held open and take space after they’re deleted.

Solution 1

Check for files on located under mount points. Frequently if you mount a directory (say a sambafs) onto a filesystem that already had a file or directories under it, you lose the ability to see those files, but they’re still consuming space on the underlying disk.

I’ve had file copies while in single user mode dump files into directories that I couldn’t see except in single usermode (due to other directory systems being mounted on top of them).

Solution 2

On the other hand df -h and du -sh could mismatched by about 50% of the hard disk size. This was caused by e.g. apache (httpd) keeping large log files in memory which had been deleted from disk.

This was tracked down by running lsof | grep "/var" | grep deleted where /var was the partition I needed to clean up.

The output showed lines like this:

httpd     32617    nobody  106w      REG        9,4 1835222944     688166 /var/log/apache/awstats_log (deleted)

The situation was then resolved by restarting apache (service httpd restart) and cleared of disk space, by allowing the locks on deleted files to be cleared.

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What is the difference between encryption and hashing?

Hashing: Finally, hashing is a form of cryptographic security which differs from encryption whereas encryption is a two step process used to first encrypt and then decrypt a message, hashing condenses a message into an irreversible fixed-length value, or hash.

Should the root certificate go on the server?

Self-signed root certificates need not/should not be included in web server configuration. They serve no purpose (clients will always ignore them) and they incur a slight performance (latency) penalty because they increase the size of the SSL handshake.

If the client does not have the root in their trust store, then it won’t trust the web site, and there is no way to work around that problem. Having the web server send the root certificate will not help - the root certificate has to come from a trusted 3rd party (in most cases the browser vendor).

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How to log all commands run by root on production servers?

auditd is the correct tool for the job here:

  1. Add these 2 lines to /etc/audit/audit.rules:
-a exit,always -F arch=b64 -F euid=0 -S execve
-a exit,always -F arch=b32 -F euid=0 -S execve

These will track all commands run by root (euid=0). Why two rules? The execve syscall must be tracked in both 32 and 64 bit code.

  1. To get rid of auid=4294967295 messages in logs, add audit=1 to the kernel’s cmdline (by editing /etc/default/grub)

  2. Place the line

session  required                pam_loginuid.so

in all PAM config files that are relevant to login (/etc/pam.d/{login,kdm,sshd}), but not in the files that are relevant to su or sudo. This will allow auditd to get the calling user’s uid correctly when calling sudo or su.

Restart your system now.

Let’s login and run some commands:

$ id -u
$ sudo ls /
bin  boot  data  dev  etc  home  initrd.img  initrd.img.old  lib  lib32  lib64  lost+found  media  mnt  opt  proc  root  run  sbin  scratch  seLinux  srv  sys  tmp  usr  var  vmlinuz  vmlinuz.old
$ sudo su -
# ls /etc

Now read /var/log/audit/auditd.log for show what has been logged in.

Useful resources:

How to prevent dd from freezing system?

Try using ionice:

ionice -c3 dd if=/dev/zero of=z

This start the dd process with the “idle” IO priority: it only gets disk time when no other process is using disk IO for a certain amount of time.

Of course this can still flood the buffer cache and cause freezes while the system flushes out the cache to disk. There are tunables under /proc/sys/vm/ to influence this, particularly the dirty_* entries.

How to limit processes to not exceed more than X% of CPU usage?


nice is a great tool for ‘one off’ tweaks to a system:



cpulimit if you need to run a CPU intensive job and having free CPU time is essential for the responsiveness of a system:

cpulimit -l 50 COMMAND


cgroups apply limits to a set of processes, rather than to just one:

cgcreate -g cpu:/cpulimited
cgset -r cpu.shares=512 cpulimited
cgexec -g cpu:cpulimited COMMAND_1
cgexec -g cpu:cpulimited COMMAND_2
cgexec -g cpu:cpulimited COMMAND_3

How mount a temporary ram partition?

# -t - filesystem type
# -o - mount options
mount -t tmpfs tmpfs /mnt -o size=64M

How to kills a process that is locking a file?

fuser -k filename

Other admin trying to debug a server accidentally typed: chmod -x /bin/chmod. How to reset permissions back to default?

# 1:
cp /bin/ls chmod.01
cp /bin/chmod chmod.01
./chmod.01 700 file

# 2:
/bin/busybox chmod 0700 /bin/chmod

# 3:
setfacl --set u::rwx,g::---,o::--- /bin/chmod

# 4:
/usr/lib/ld*.so /bin/chmod 0700 /bin/chmod

Useful resources:

grub> vs grub-rescue>. Explain.

  • grub> - this is the mode to which it passes if you find everything you need to run the system in addition to the configuration file. With this mode, we have access to most (if not all) modules and commands. This mode can be called from the menu by pressing the ‘c’ key
  • grub-rescue - this is the mode to which it passes if it is impossible to find its own directory (especially the directory with modules and additional commands, e.g. directory /boot/grub/i386-pc), if its contents are damaged or if no normal module is found, contains only basic commands

How to check whether the private key and the certificate match?

(openssl rsa -noout -modulus -in private.key | openssl md5 ; openssl x509 -noout -modulus -in certificate.crt | openssl md5) | uniq

How to create user without useradd command in Linux?

  1. Add an entry of user details in /etc/passwd
# username:password:UID:GID:Comments:Home_Directory:Login Shell
user:x:501:501:test user:/home/user:/bin/bash
  1. You will have to create a group with same name in /etc/group
  1. Assign a password to the user
passwd user

Why do we need mktemp command? Present an example of use.

mktemp randomizes the name. It is very important from the security point of view.

Just imagine that you do something like:

echo "random_string" > /tmp/temp-file

in your root-running script. And someone (who has read your script) does

ln -s /etc/passwd /tmp/temp-file

The mktemp command could help you in this situation:

TEMP=$(mktemp /tmp/temp-file.XXXXXXXX)
echo "random_string" > ${TEMP}

Now this ln /etc/passwd attack will not work.

Is it safe to attach the strace to a running process on the production? What are the consequences?

strace is the system call tracer for Linux. It currently uses the arcane ptrace() (process trace) debugging interface, which operates in a violent manner: pausing the target process for each syscall so that the debugger can read state. And doing this twice: when the syscall begins, and when it ends.

This means strace pauses your application twice for each syscall, and context-switches each time between the application and strace. It’s like putting traffic metering lights on your application.


  • can cause significant and sometimes massive performance overhead, in the worst case, slowing the target application by over 100x. This may not only make it unsuitable for production use, but any timing information may also be so distorted as to be misleading
  • can’t trace multiple processes simultaneously (with the exception of followed children)
  • visibility is limited to the system call interface

Useful resources:

What is the easiest, safest and most portable way to remove -rf directory entry?

They’re effective but not optimally portable:

  • rm – -fr
  • perl -le ‘unlink(“-fr”);’

People who go on about shell command line quoting and character escaping are almost as dangerous as those who simply don’t even recognize why a file name like that poses any problem at all.

The most portable solution:

rm ./-fr

Write a simple bash script (or pair of scripts) to backup and restore your system. *

To be completed.

Network Questions (21)

Create SPF records for your site to help control spam. *

To be completed.

What is the difference between an authoritative and a nonauthoritative answer to a DNS query? *

To be completed.

If you try resolve hostname you get NXDOMAIN from host command. Your resolv.conf stores two nameservers but only second of this store this domain name. Why did not the local resolver check the second nameserver?

NXDOMAIN is nothing but non-existent Internet or Intranet domain name. If domain name is unable to resolved using the DNS, a condition called the NXDOMAIN occurred.

The default behavior for resolv.conf and the resolver is to try the servers in the order listed. The resolver will only try the next nameserver if the first nameserver times out.

The algorithm used is to try a name server, and if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of retries are made.

If a nameserver responds with SERVFAIL or a referral (nofail) or terminate query (fail) also only the first dns server will be used.


nameserver   # it's not a dns
nameserver          # not store gate.test.int
nameserver        # store gate.test.int

so if you check:

host -v -t a gate.test.int
Trying "gate.test.int"                        # trying first dns ( but response is time out, so try the next nameserver
Host gate.test.int not found: 3(NXDOMAIN)     # ok but response is NXDOMAIN (not found this domain name)
Received 88 bytes from in 43 ms
Received 88 bytes from in 43 ms
                                              # so the last server in the list was not asked

To avoid this you can use e.g. nslookup command which will use the second nameserver if it receives a SERVFAIL from the first nameserver.

Useful resources:

Explore the current MTA configuration at your site. What are some of the special features of the MTA that are in use? *

To be completed.

How to find a domain based on the IP address? What techniques/tools can you use? *

To be completed.

Is it possible to have SSL certificate for IP address, not domain name?

It is possible (but rarely used) as long as it is a public IP address.

An SSL certificate is typically issued to a Fully Qualified Domain Name (FQDN) such as https://www.domain.com. However, some organizations need an SSL certificate issued to a public IP address. This option allows you to specify a public IP address as the Common Name in your Certificate Signing Request (CSR). The issued certificate can then be used to secure connections directly with the public IP address (e.g.

According to the CA Browser forum, there may be compatibility issues with certificates for IP addresses unless the IP address is in both the commonName and subjectAltName fields. This is due to legacy SSL implementations which are not aligned with RFC 5280, notably, Windows OS prior to Windows 10.

Useful resources:

How do you do load testing and capacity planning for web sites? *

To be completed.

Useful resources:

Does having Varnish in front of your website/app mean you don’t need to care about load balancing or redundancy?

It depends. Varnish is a cache server, so its purpose is to cache contents and to act as a reverse proxy, to speed up retrieval of data and to lessen the load on the webserver. Varnish can be also configured as a load-balancer for multiple web servers, but if we use just one Varnish server, this will become our single point of failure on our infrastructure.

A better solution to ensure load-balancing or redundacy will be a cluster of at least two Varnish istances, in active-active mode or active-passive mode.

What are hits, misses, and hit-for-pass in Varnish Cache?

A hit is a request which is successfully served from the cache, a miss is a request that goes through the cache but finds an empty cache and therefore has to be fetched from the origin, the hit-for-pass comes in when Varnish Cache realizes that one of the objects it has requested is uncachable and will result in a pass.

Useful resources:

What is a reasonable TTL for cached content given the following parameters? *

To be completed.

Developer says: “htaccess is full of magic and it should be used”. What is your opinion about using htaccess files? What are the consequences for web application?

.htaccess files were born out of an era when shared host­ing was com­mon­ place:

  • sysad­mins need­ed a way to allow mul­ti­ple clients to access their serv­er under dif­fer­ent accounts, with dif­fer­ent con­fig­u­ra­tions for their web­sites.

The .htaccess file allowed them to mod­i­fy how Apache works with­out hav­ing access to the entire server. These files can reside in any and every directory in the directory tree of the website and provide features to the directory and the files and folders inside it.

It’s horrible for performance

For .htaccess to work Apache needs to check EVERY directory in the requested path for the existence of a .htaccess file and if it exists it reads EVERY one of them and parses it. This happens for EVERY request. Remember that the second you change that file, it’s effective. This is because Apache reads it every time.

Every sin­gle request the web­serv­er han­dles - even for the lowli­est .png or .css file - caus­es Apache to:

  • look for a .htaccess file in the direc­to­ry of the cur­rent request
  • then look for a .htaccess file in every direc­to­ry from there up to the serv­er root
  • coa­lesce all of these .htaccess files together
  • recon­fig­ure the web­serv­er using the new settings
  • final­ly, deliv­er the file

Every web­page can gen­er­ate dozens of requests. This is over­head you don’t need, and what’s more, it’s com­plete­ly unnec­es­sary.

Security and permission loss

Allowing individual users to modify the configuration of a server using .htaccess can cause security concerns if not taken care properly. If you add any directive in the .htaccess file, it will be considered as they are added to Apache configuration file.

This means it may be possible for non-admins to write these files and thus ‘undo’ all of your security. If you need to do something that is temporary, .htaccess is a good place to do it, if you need to do something more permanent, just put it in your /etc/apache/sites-avaliable/site.conf (or httpd.conf or whatever your server calls.


You should avoid using .htaccess files completely if you have access to httpd main server config file. If it worked in .htaccess, it will work in your virtual host .conf file as well.

If you cannot avoid using .htaccess files, you should follow these rules.

  • use only one .htaccess file or as few as possible
  • place the .htaccess file in the site root directory
  • keep your .htaccess file short and simple

Useful resources:

How to test connection to remote host with and without SNI?

With OpenSSL:

# Testing connection to remote host (with SNI support)
echo | openssl s_client -showcerts -servername google.com -connect google.com:443
# Testing connection to remote host (without SNI support)
echo | openssl s_client -connect google.com:443 -showcerts

With GnuTLS:

# Testing connection to remote host (with SNI support)
gnutls-cli -p 443 google.com
# Testing connection to remote host (without SNI support)
gnutls-cli --disable-sni -p 443 google.com

How are cookies passed in the HTTP protocol?

The server sends the following in its response header to set a cookie field:


If there is a cookie set, then the browser sends the following in its request header:


What is the proper way to test NFS performance?

The best benchmark is always “the application(s) that you normally use”. The load on a NFS system when you have 20 people simultaneously compiling a Linux kernel is comletely different from a bunch of people logging in at the same time or the accounts uses as “home directories for the local web-server”.

But we have some good tools for testing this.

  • boonie - a classical performances evaluation tool tests. The main program tests database type access to a single file (or a set of files if you wish to test more than 1G of storage), and it tests creation, reading, and deleting of small files which can simulate the usage of programs such as Squid, INN, or Maildir format email.
  • DBench - was written to allow independent developers to debug and test SAMBA. It is heavily inspired of the original SAMBA tool.
  • IOZone - performance tests suite. POSIX and 64 bits compliant. This tests is the file system test from the L.S.E. Main features: POSIX async I/O, Mmap() file I/O, Normal file I/O Single stream measurement, Multiple stream measurement, Distributed file server measurements (Cluster) POSIX pthreads, Multi-process measurement Selectable measurements with fsync, O_SYNC Latency plots.

How to run scp with a second remote host?

With ssh:

ssh user1@remote1 'ssh user2@remote2 "cat file"' > file

With tar (with compression):

ssh user1@remote1 'ssh user2@remote2 "cd path2; tar cj file"' | tar xj

With ssh and port forwarding tunnel:

# First, open the tunnel
ssh -L 1234:remote2:22 -p 45678 user1@remote1

# Then, use the tunnel to copy the file directly from remote2
scp -P 1234 user2@localhost:file .

How can you reduce load time of a dynamic website?

  • webpage optimization
  • cached web pages
  • quality web hosting
  • compressed text files
  • apache/nginx tuning

What types of dns cache working when you type api.example.com in your browser and press return?

Browser checks if the domain is in its cache (to see the DNS Cache in Chrome, go to chrome://net-internals/#dns). When this cache fails, it simply asks the OS to resolve the domain.

The OS resolver has it’s own cache which it will check. If it fails this, it resorts to asking the OS configured DNS servers.

The OS configured DNS servers will typically be configured by DHCP from the router where the DNS servers are likely to be the ISP’s DNS servers configured by DHCP from the internet gateway to the router.

In the event the router has it’s own DNS servers, it may have it’s own cache otherwise you should be directed straight to your ISP’s DNS servers most typically as soon as the OS cache was found to be empty.

Useful resources:

What is the difference between Cache-Control: max-age=0 and Cache-Control: no-cache?

When sent by the origin server:

max-age=0 simply tells caches (and user agents) the response is stale from the get-go and so they SHOULD revalidate the response (e.g. with the If-Not-Modified header) before using a cached copy, whereas, no-cache tells them they MUST revalidate before using a cached copy.

In other words, caches may sometimes choose to use a stale response (although I believe they have to then add a Warning header), but no-cache says they’re not allowed to use a stale response no matter what. Maybe you’d want the SHOULD-revalidate behavior when baseball stats are generated in a page, but you’d want the MUST-revalidate behavior when you’ve generated the response to an e-commerce purchase.

When sent by the user agent:

If a user agent sends a request with Cache-Control: max-age=0 (aka. “end-to-end revalidation”), then each cache along the way will revalidate its cache entry (e.g. with the If-Not-Modified header) all the way to the origin server. If the reply is then 304 (Not Modified), the cached entity can be used.

On the other hand, sending a request with Cache-Control: no-cache (aka. “end-to-end reload”) doesn’t revalidate and the server MUST NOT use a cached copy when responding.

What are the security risks of setting Access-Control-Allow-Origin?

By responding with Access-Control-Allow-Origin: *, the requested resource allows sharing with every origin. This basically means that any site can send an XHR request to your site and access the server’s response which would not be the case if you hadn’t implemented this CORS response.

So any site can make a request to your site on behalf of their visitors and process its response. If you have something implemented like an authentication or authorization scheme that is based on something that is automatically provided by the browser (cookies, cookie-based sessions, etc.), the requests triggered by the third party sites will use them too.

Create a single-use TCP or UDP proxy with netcat.

### TCP -> TCP
nc -l -p 2000 -c "nc [ip|hostname] 3000"

### TCP -> UDP
nc -l -p 2000 -c "nc -u [ip|hostname] 3000"

### UDP -> UDP
nc -l -u -p 2000 -c "nc -u [ip|hostname] 3000"

### UDP -> TCP
nc -l -u -p 2000 -c "nc [ip|hostname] 3000"

Explain 3 techniques for avoiding firewalls with nmap.

Use Decoy addresses

# Generates a random number of decoys.
nmap -D RND:10 [target]

# Manually specify the IP addresses of the decoys.
nmap -D decoy1,decoy2,decoy3

In this type of scan you can instruct Nmap to spoof packets from other hosts.In the firewall logs it will be not only our IP address but also and the IP addresses of the decoys so it will be much harder to determine from which system the scan started.

Source port number specification

nmap --source-port 53 [target]

A common error that many administrators are doing when configuring firewalls is to set up a rule to allow all incoming traffic that comes from a specific port number.The –source-port option of Nmap can be used to exploit this misconfiguration.Common ports that you can use for this type of scan are: 20, 53 and 67.

Append Random Data

nmap --data-length 25 [target]

Many firewalls are inspecting packets by looking at their size in order to identify a potential port scan.This is because many scanners are sending packets that have specific size.In order to avoid that kind of detection you can use the command –data-length to add additional data and to send packets with different size than the default.


nmap -sA [target]

It is always good to send the ACK packets rather than the SYN packets because if there is any active firewall working on the remote computer then because of the ACK packets the firewall cannot create the log, since firewalls treat ACK packet as the response of the SYN packet.

Useful resources:

Devops Questions (3)

Explain how Flap Detection works in Nagios?

Flapping occurs when a service or host changes state too frequently, this causes lot of problem and recovery notifications.

Once you have defined Flapping, explain how Nagios detects Flapping. Whenever Nagios checks the status of a host or service, it will check to see if it has started or stopped flapping.

Nagios follows the below given procedure to do that:

  • storing the results of the last 21 checks of the host or service analyzing the historical check results and determine where state changes/transitions occur
  • using the state transitions to determine a percent state change value (a measure of change) for the host or service
  • comparing the percent state change value against low and high flapping thresholds

What are the advantages that Containerization provides over Virtualization?

Below are the advantages of containerization over virtualization:

  • containers provide real-time provisioning and scalability but VMs provide slow provisioning
  • containers are lightweight when compared to VMs
  • VMs have limited performance when compared to containers
  • containers have better resource utilization compared to VMs

You have to prepare a redis cluster. How will you ensure security?

  • protect a given Redis instance from outside accesses via firewall
  • binding it to if only local clients are accessing it
  • sandboxed environment
  • enabling AUTH
  • enabling Protected Mode
  • data encryption support (e.g. spiped)
  • disabling of specific commands
  • users ACLs

Useful resources:

Cyber Security Questions (5)

What is OWASP Application Security Verification Standard? Explain in a few points. *

To be completed.

What is CSRF?

Cross Site Request Forgery is a web application vulnerability in which the server does not check whether the request came from a trusted client or not. The request is just processed directly. It can be further followed by the ways to detect this, examples and countermeasures.

What is the difference between policies, processes and guidelines?

As security policy defines the security objectives and the security framework of an organisation. A process is a detailed step by step how to document that specifies the exact action which will be necessary to implement important security mechanism. Guidelines are recommendations which can be customised and used in the creation of procedures.

What is a false positive and false negative in case of IDS?

When the device generated an alert for an intrusion which has actually not happened: this is false positive and if the device has not generated any alert and the intrusion has actually happened, this is the case of a false negative.

5 quick points on web server hardening.

Web server hardening is filtering of unnecessary services running on various ports and removal of default test scripts from the servers. Although web server hardening is a lot more than this and usually organisations have a customised checklist for hardening the servers. Any server getting created has to be hardened and hardening has to be re-confirmed on a yearly basis. Even the hardening checklist has to be reviewed on a yearly basis for new add-ons.


  • if machine is a new install, protect it from hostile network traffic, until the operating system is installed and hardened
  • create a separate partition with the nodev, nosuid, and noexec options set for /tmp
  • create separate partitions for /var, /var/log, /var/log/audit, and /home
  • enable randomized virtual memory region placement
  • remove legacy services (e.g., telnet-server; rsh, rlogin, rcp; ypserv, ypbind; tftp, tftp-server; talk, talk-server).
  • limit connections to services running on the host to authorized users of the service via firewalls and other access control technologies
  • disable source routed packet acceptance
  • enable TCP/SYN cookies
  • disable SSH root login
  • install and configure AIDE
  • install and configure OSsec HIDS
  • configure SELinux
  • all administrator or root access must be logged
  • integrity checking of system accounts, group memberships, and their associated privileges should be enabled and tested
  • set password creation requirements

Secret Knowledge

Guru Sysadmin

How to profile app in Linux environment?

Ideally, I need an app that will attach to a process and log periodic snapshots of: memory usage number of threads CPU usage.

  1. You can use top in batch mode. It runs in the batch mode either until it is killed or until N iterations is done:
top -b -p `pidof a.out`


top -b -p `pidof a.out` -n 100
  1. You can use ps (for instance in a shell script):
ps --format pid,pcpu,cputime,etime,size,vsz,cmd -p `pidof a.out`

I need some means of recording the performance of an application on a Linux machine.

  1. To record performance data:
perf record -p `pidof a.out`

or to record for 10 secs:

perf record -p `pidof a.out` sleep 10

or to record with call graph ():

perf record -g -p `pidof a.out`

2) To analyze the recorded data

perf report --stdio
perf report --stdio --sort=dso -g none
perf report --stdio -g none
perf report --stdio -g

This is an example of profiling a test program

  1. I run my test program (c++):
./my_test 100000000
  1. Then I record performance data of a running process:
perf record -g  -p `pidof my_test` -o ./my_test.perf.data sleep 30
  1. Then I analyze load per module:
perf report --stdio -g none --sort comm,dso -i ./my_test.perf.data

# Overhead  Command                 Shared Object
# ........  .......  ............................
    70.06%  my_test  my_test
    28.33%  my_test  libtcmalloc_minimal.so.0.1.0
     1.61%  my_test  [kernel.kallsyms]
  1. Then load per function is analyzed:
perf report --stdio -g none -i ./my_test.perf.data | c++filt

# Overhead  Command                 Shared Object                       Symbol
# ........  .......  ............................  ...........................
    29.30%  my_test  my_test                       [.] f2(long)
    29.14%  my_test  my_test                       [.] f1(long)
    15.17%  my_test  libtcmalloc_minimal.so.0.1.0  [.] operator new(unsigned long)
    13.16%  my_test  libtcmalloc_minimal.so.0.1.0  [.] operator delete(void*)
     9.44%  my_test  my_test                       [.] process_request(long)
     1.01%  my_test  my_test                       [.] operator delete(void*)@plt
     0.97%  my_test  my_test                       [.] operator new(unsigned long)@plt
     0.20%  my_test  my_test                       [.] main
     0.19%  my_test  [kernel.kallsyms]             [k] apic_timer_interrupt
     0.16%  my_test  [kernel.kallsyms]             [k] _spin_lock
     0.13%  my_test  [kernel.kallsyms]             [k] native_write_msr_safe

  1. Then call chains are analyzed:
perf report --stdio -g graph -i ./my_test.perf.data | c++filt

# Overhead  Command                 Shared Object                       Symbol
# ........  .......  ............................  ...........................
    29.30%  my_test  my_test                       [.] f2(long)
            --- f2(long)
                --29.01%-- process_request(long)

    29.14%  my_test  my_test                       [.] f1(long)
            --- f1(long)
               |--15.05%-- process_request(long)
               |          main
               |          __libc_start_main
                --13.79%-- f2(long)


So at this point you know where your program spends time.

Also the simple way to do app profile is to use the pstack utility or lsstack.

Other tool is Valgrind. So this is what I recommend. Run program first:

valgrind --tool=callgrind --dump-instr=yes -v --instr-atstart=no ./binary > tmp

Now when it works and we want to start profiling we should run in another window:

callgrind_control -i on

This turns profiling on. To turn it off and stop whole task we might use:

callgrind_control -k

Now we have some files named callgrind.out.* in current directory. To see profiling results use:

kcachegrind callgrind.out.*

I recommend in next window to click on “Self” column header, otherwise it shows that “main()” is most time consuming task.

Useful resources:

Use sysfs virtual filesystem to test connection on all interfaces (without loopback).

For example:


for iface in $(ls /sys/class/net/ | grep -v lo) ; do

  if [[ $(cat /sys/class/net/$iface/carrier) = 1 ]] ; then state=1 ; fi


if [[ $state -ne 0 ]] ; then echo "not connection" > /dev/stderr ; exit ; fi

How to rewrite POST data with payload in Nginx?

You just need to write a Nginx rewrite rule with HTTP status code 307 or 308:

location / {
    proxy_pass              http://localhost:80;
    client_max_body_size    10m;

location /api {
    # HTTP 307 only for POST method.
    if ($request_method = POST) {
        return 307 https://api.example.com?request_uri;

    # You can keep this for non-POST requests.
    rewrite ^ https://api.example.com?request_uri permanent;

    client_max_body_size    10m;

HTTP Status code 307 or 308 should be used instead of 301 because it changes the request method from POST to GET.

Developer reports a problem with connectivity to the remote service. Use /dev for troubleshooting.

# <host> - set remote host
# <port> - set destination port

# 1
timeout 1 bash -c "</dev/tcp/<host>/<port>" >/dev/null 2>&1 ; echo $?

# 2
timeout 1 bash -c 'cat < /dev/null > </dev/tcp/<host>/<port>' ; echo $?

# 2
&> echo > "</dev/tcp/<host>/<port>"

Useful resources:

How do I measure request and response times at once using cURL?

cURL supports formatted output for the details of the request (see the cURL manpage for details, under -w| -write-out ‘format’). For our purposes we’ll focus just on the timing details that are provided.

  1. Create a new file, curl-format.txt, and paste in:
    time_namelookup:  %{time_namelookup}\n
       time_connect:  %{time_connect}\n
    time_appconnect:  %{time_appconnect}\n
   time_pretransfer:  %{time_pretransfer}\n
      time_redirect:  %{time_redirect}\n
 time_starttransfer:  %{time_starttransfer}\n
         time_total:  %{time_total}\n
  1. Make a request:
curl -w "@curl-format.txt" -o /dev/null -s "http://example.com/"

What this does:

  • -w “@curl-format.txt” - tells cURL to use our format file
  • -o /dev/null - redirects the output of the request to /dev/null
  • -s - tells cURL not to show a progress meter http://example.com/ is the URL we are requesting. Use quotes particularly if your URL has “&” query string parameters

Is there a way to allow multiple cross-domains using the Access-Control-Allow-Origin header in Nginx?

To match a list of domain and subdomain this regex make it ease to work with fonts:

location ~* \.(?:ttf|ttc|otf|eot|woff|woff2)$ {
   if ( $http_origin ~* (https?://(.+\.)?(domain1|domain2|domain3)\.(?:me|co|com)$) ) {
      add_header "Access-Control-Allow-Origin" "$http_origin";

More sligtly configuration:

location / {

    if ($http_origin ~* (^https?://([^/]+\.)*(domainone|domaintwo)\.com$)) {
        set $cors "true";

    # Nginx doesn't support nested If statements. This is where things get slightly nasty.
    # Determine the HTTP request method used
    if ($request_method = 'GET') {
        set $cors "${cors}get";
    if ($request_method = 'POST') {
        set $cors "${cors}post";

    if ($cors = "true") {
        # Catch all incase there's a request method we're not dealing with properly
        add_header 'Access-Control-Allow-Origin' "$http_origin";

    if ($cors = "trueget") {
        add_header 'Access-Control-Allow-Origin' "$http_origin";
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';

    if ($cors = "truepost") {
        add_header 'Access-Control-Allow-Origin' "$http_origin";
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';


Explain :(){ :|:& };: and how stop this code if you are already logged into a system?

It’s a fork bomb.

  • :() - this defines the function. “:” is the function name and the empty parenthesis shows that it will not accept any arguments
  • { } - these characters shows the beginning and end of function definition
  • :|: - it loads a copy of the function “:” into memory and pipe its output to another copy of the “:” function, which has to be loaded into memory
  • & - this will make the process as a background process, so that the child processes will not get killed eventhough the parent gets auto-killed
  • : - final “:” will execute the function again and hence the chain reaction begins

The best way to protect a multi-user system is to use PAM to limit the number of processes a user can use. We know the biggest problem with a fork bomb is the fact it takes up so many processes.

So we have two ways of attempting to fix this, if you are already logged into the system: - execute a SIGSTOP command to stop the process: killall -STOP -u user1 - if you can’t run at the command line you will have to use exec to force it to run (due to processes all being used): exec killall -STOP -u user1

With fork bombs your best method for this is preventing from being to big of an issue in the first place.

How to recover deleted file held open by Apache?

If a file has been deleted but is still open, that means the file still exists in the filesystem (it has an inode) but has a hard link count of 0. Since there is no link to the file, you cannot open it by name. There is no facility to open a file by inode either.

Linux exposes open files through special symbolic links under /proc. These links are called /proc/12345/fd/42 where 12345 is the PID of a process and 42 is the number of a file descriptor in that process. A program running as the same user as that process can access the file (the read/write/execute permissions are the same you had as when the file was deleted).

The name under which the file was opened is still visible in the target of the symbolic link: if the file was /var/log/apache/foo.log, then the target of the link is /var/log/apache/foo.log (deleted).

Thus you can recover the content of an open deleted file given the PID of a process that has it open and the descriptor that it’s opened on like this:

recover_open_deleted_file () {
  old_name=$(readlink "$1")
  case "$old_name" in
    *' (deleted)')
      old_name=${old_name%' (deleted)'}
      if [ -e "$old_name" ]; then
        new_name=$(TMPDIR=${old_name%/*} mktemp)
        echo "$oldname has been replaced, recovering content to $new_name"
      cat <"$1" >"$new_name";;
    *) echo "File is not deleted, doing nothing";;
recover_open_deleted_file "/proc/$pid/fd/$fd"

If you only know the process ID but not the descriptor, you can recover all files with:

for x in /proc/$pid/fd/* ; do
  recover_open_deleted_file "$x"

If you don’t know the process ID either, you can search among all processes:

for x in /proc/[1-9]*/fd/* ; do
  case $(readlink "$x") in
    /var/log/apache/*) recover_open_deleted_file "$x";;

You can also obtain this list by parsing the output of lsof, but it isn’t simpler nor more reliable nor more portable (this is Linux-specific anyhow).

How to install Linux on disk, from and where other Linux exist and running?

It is possible that the question should be: “System installation from the level and in place of already other system working”.

On the example of the Debian GNU/Linux distribution.

  1. Creating a working directory and downloading the system using the debootstrap tool.
mkdir $_working_directory
debootstrap --verbose --arch amd64 {wheezy|jessie} . http://ftp.en.debian.org/debian
  1. Mounting sub-systems: proc, sys, dev and dev/pts.
for i in proc sys dev dev/pts ; do mount -o bind $i $_working_directory/$i ; done
  1. Copy system backup for restore.
cp system_backup_22012015.tgz $_working_directory/mnt

However, it is better not to waste space and do it in a different way (assuming that the copy is in /mnt/backup):

mkdir $_backup_directory && mount --bind /mnt/backup $_backup_directory
  1. Chroot to “new” system.
chroot $_working_directory /bin/bash
  1. Updating information about mounted devices.
grep -v rootfs /proc/mounts > /etc/mtab
  1. In the “new” system, the next thing to do is mount the disk on which the “old” system is located (e.g. /dev/sda1).
mkdir $_working_directory && mount /dev/sda1 $_working_directory
  1. Remove all files of the old system.
for i in $(ls | awk '!(/proc/ || /dev/ || /sys/ || /mnt/)') ; do rm -fr $i ; done
  1. The next step is to restore the system from a backup.
tar xzvfp $_backup_directory/system_backup_22012015.tgz -C $_working_directory
  1. And mount proc, sys, dev and dev/pts in a new working directory.
for i in proc sys dev dev/pts ; do mount -o bind $i $_working_directory/$i ; done
  1. Install and update grub configuration.
chroot $_working_directory /bin/bash -c "grub-install --no-floppy --root-directory=/ /dev/sda"
chroot $_working_directory /bin/bash -c "update-grub"
  1. Unmount proc, sys, dev and dev/pts filesystems.
grep $_working_directory /proc/mounts | cut -f2 -d " " | sort -r | xargs umount -n

None of the available commands, i.e. halt, shutdown or reboot, will work. You need to reload the system configuration - to do this, use the kernel debugger (without the ‘b’ option):

echo 1 > /proc/sys/kernel/sysrq
echo reisu > /proc/sysrq-trigger

Of course, it is recommended to fully restart the machine in order to completely load the current system. To do this:

sync ; reboot -f

How does the OOM killer decide which process to kill first? How to control this?

Major distribution kernels set the default value of /proc/sys/vm/overcommit_memory to zero, which means that processes can request more memory than is currently free in the system.

If memory is exhaustively used up by processes, to the extent which can possibly threaten the stability of the system, then the OOM killer comes into the picture.

NOTE: It is the task of the OOM Killer to continue killing processes until enough memory is freed for the smooth functioning of the rest of the process that the Kernel is attempting to run.

The OOM Killer has to select the best process(es) to kill. Best here refers to that process which will free up the maximum memory upon killing and is also the least important to the system.

The primary goal is to kill the least number of processes that minimizes the damage done and at the same time maximizing the amount of memory freed.

To facilitate this, the kernel maintains an oom_score for each of the processes. You can see the oom_score of each of the processes in the /proc filesystem under the pid directory.

cat /proc/10292/oom_score

The higher the value of oom_score of any process, the higher is its likelihood of getting killed by the OOM Killer in an out-of-memory situation.

If you want to create a special control group containing the list of processes which should be the first to receive the OOM killer’s attention, create a directory under /mnt/oom-killer to represent it:

mkdir lambs

Set oom.priority to a value high enough:

echo 256 > /mnt/oom-killer/lambs/oom.priority

oom.priority is a 64-bit unsigned integer, and can have a maximum value an nsigned 64-bit number can hold. While scanning for the process to be killed, the OOM-killer selects a process from the list of tasks with the highest oom.priority value.

Add the PID of the process to be added to the list of tasks:

echo <pid> > /mnt/oom-killer/lambs/tasks

To create a list of processes, which will not be killed by the OOM-killer, make a directory to contain the processes:

mkdir invincibles

Setting oom.priority to zero makes all the process in this cgroup to be excluded from the list of target processes to be killed.

echo 0 > /mnt/oom-killer/invincibles/oom.priority

To add more processes to this group, add the pid of the task to the list of tasks in the invincible group:

echo <pid> > /mnt/oom-killer/invincibles/tasks

What are salted hashes? Generate the password with salt for the /etc/shadow file.

Salt at its most fundamental level is random data. When a properly protected password system receives a new password, it will create a hashed value for that password, create a new random salt value, and then store that combined value in its database. This helps defend against dictionary attacks and known hash attacks.

For example, if a user uses the same password on two different systems, if they used the same hashing algorithm, they could end up with the same hash value. However, if even one of the systems uses salt with its hashes, the values will be different.

The encrypted passwords in /etc/shadow file are stored in the following format:


The $ID indicates the type of encryption, the $SALT is a random (up to 16 characters) string and $ENCRYPTED is a password’s hash.

Hash Type ID Hash Length
MD5 $1 22 characters
SHA-256 $5 43 characters
SHA-512 $6 86 characters

Use the below commands from the Linux shell to generate hashed password for /etc/shadow with the random salt:

  • Generate MD5 password hash
python -c "import random,string,crypt; randomsalt = ''.join(random.sample(string.ascii_letters,8)); print crypt.crypt('MySecretPassword', '\$1\$%s\$' % randomsalt)"
  • Generate SHA-256 password hash
python -c "import random,string,crypt; randomsalt = ''.join(random.sample(string.ascii_letters,8)); print crypt.crypt('MySecretPassword', '\$5\$%s\$' % randomsalt)"
  • Generate SHA-512 password hash
python -c "import random,string,crypt; randomsalt = ''.join(random.sample(string.ascii_letters,8)); print crypt.crypt('MySecretPassword', '\$6\$%s\$' % randomsalt)"