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#1 May 28 2017


Exercice: Linux System Administrator/DevOps Interview Questions

Hello people, this is the Linux System Administrator/DevOps Interview Questions , as seen here:
https://github.com/chassing/linux-sysad … -questions

I think it would be nice to try to collaboratively solve these questions. I know answers to a few, obviously, and learned new things by reading the questions, for examples the commands tac and tr, which I never used. tac can be useful. tr is a simpler option to awk and sed (or others) when you need to do a  very simple replace.

Some are quite interesting, for example
"A careless sysadmin executes the following command: chmod 444 /bin/chmod- what do you do to fix this?"

Or this, I didn't know it was possible:
"Explain how could you ssh login into a Linux system that DROPs all new incoming packets using a SSH tunnel"

Anyway, here it is.

General Questions:

* What did you learn yesterday/this week?
* Talk about your preferred development/administration environment. (OS, Editor, Browsers, Tools etc.)
* Tell me about the last major Linux project you finished.
* Tell me about the biggest mistake you've made in [some recent time period] and how you would do it differently today. What did you learn from this experience?
* Why we must choose you?
* What function does DNS play on a network?
* What is HTTP?
* What is an HTTP proxy and how does it work?
* Describe briefly how HTTPS works.
* What is SMTP? Give the basic scenario of how a mail message is delivered via SMTP.
* What is RAID? What is RAID0, RAID1, RAID5, RAID10?
* What is a level 0 backup? What is an incremental backup?
* Describe the general file system hierarchy of a Linux system.

Simple Linux Questions:

* What is the name and the UID of the administrator user?
* How to list all files, including hidden ones, in a directory?
* What is the Unix/Linux command to remove a directory and its contents?
* Which command will show you free/used memory? Does free memory exist on Linux?
* How to search for the string "my konfi is the best" in files of a directory recursively?
* How to connect to a remote server or what is SSH?
* How to get all environment variables and how can you use them?
* I get "command not found" when I run ifconfig -a. What can be wrong?
* What happens if I type TAB-TAB?
* What command will show the available disk space on the Unix/Linux system?
* What commands do you know that can be used to check DNS records?
* What Unix/Linux commands will alter a files ownership, files permissions?
* What does chmod +x FILENAMEdo?
* What does the permission 0750 on a file mean?
* What does the permission 0750 on a directory mean?
* How to add a new system user without login permissions?
* How to add/remove a group from a user?
* What is a bash alias?
* How do you set the mail address of the root/a user?
* What does CTRL-c do?
* What is in /etc/services?
* How to redirect STDOUT and STDERR in bash? (> /dev/null 2>&1)
* What is the difference between UNIX and Linux.
* What is the difference between Telnet and SSH?
* Explain the three load averages and what do they indicate. What command can be used to view the load averages?
* Can you name a lower-case letter that is not a valid option for GNU ls?

Medium Linux Questions:

* What do the following commands do and how would you use them?
  * tee
  * awk
  * tr
  * cut
  * tac
  * curl
  * wget
  * watch
  * head
  * tail
* What does an & after a command do?
* What does & disown after a command do?
* What is a packet filter and how does it work?
* What is Virtual Memory?
* What is swap and what is it used for?
* What is an A record, an NS record, a PTR record, a CNAME record, an MX record?
* Are there any other RRs and what are they used for?
* What is a Split-Horizon DNS?
* What is the sticky bit?
* What does the immutable bit do to a file?
* What is the difference between hardlinks and symlinks? What happens when you remove the source to a symlink/hardlink?
* What is an inode and what fields are stored in an inode?
* How to force/trigger a file system check on next reboot?
* What is SNMP and what is it used for?
* What is a runlevel and how to get the current runlevel?
* What is SSH port forwarding?
* What is the difference between local and remote port forwarding?
* What are the steps to add a user to a system without using useradd/adduser?
* What is MAJOR and MINOR numbers of special files?
* Describe the mknod command and when you'd use it.
* Describe a scenario when you get a "filesystem is full" error, but 'df' shows there is free space.
* Describe a scenario when deleting a file, but 'df' not showing the space being freed.
* Describe how 'ps' works.
* What happens to a child process that dies and has no parent process to wait for it and what’s bad about this?
* Explain briefly each one of the process states.
* How to know which process listens on a specific port?
* What is a zombie process and what could be the cause of it?
* You run a bash script and you want to see its output on your terminal and save it to a file at the same time. How could you do it?
* Explain what echo "1" > /proc/sys/net/ipv4/ip_forward does.
* Describe briefly the steps you need to take in order to create and install a valid certificate for the site https://foo.example.com.
* Can you have several HTTPS virtual hosts sharing the same IP?
* What is a wildcard certificate?
* Which Linux file types do you know?
* What is the difference between a process and a thread? And parent and child processes after a fork system call?
* What is the difference between exec and fork?
* What is "nohup" used for?
* What is the difference between these two commands?
  * myvar=hello
  * export myvar=hello
* How many NTP servers would you configure in your local ntp.conf?
* What does the column 'reach' mean in ntpq -p output?
* You need to upgrade kernel at 100-1000 servers, how you would do this?
* How can you get Host, Channel, ID, LUN of SCSI disk?
* How can you limit process memory usage?
* What is bash quick substitution/caret replace(^x^y)?
* Do you know of any alternative shells? If so, have you used any?
* What is a tarpipe (or, how would you go about copying everything, including hardlinks and special files, from one server to another)?
* How can you tell if the httpd package was already installed?
* How can you list the contents of a package?
* How can you determine which package is better: openssh-server-5.3p1-118.1.el6_8.x86_64 or openssh-server-6.6p1-1.el6.x86_64 ?

Hard Linux Questions:

* What is a tunnel and how you can bypass a http proxy?
* What is the difference between IDS and IPS?
* What shortcuts do you use on a regular basis?
* What is the Linux Standard Base?
* What is an atomic operation?
* Your freshly configured http server is not running after a restart, what can you do?
* What kind of keys are in ~/.ssh/authorized_keys and what it is this file used for?
* I've added my public ssh key into authorized_keys but I'm still getting a password prompt, what can be wrong?
* Did you ever create RPM's, DEB's or solaris pkg's?
* What does :(){ :|:& };: do on your system?
* How do you catch a Linux signal on a script?
* Can you catch a SIGKILL?
* What's happening when the Linux kernel is starting the OOM killer and how does it choose which process to kill first?
* Describe the linux boot process with as much detail as possible, starting from when the system is powered on and ending when you get a prompt.
* What's a chroot jail?
* When trying to umount a directory it says it's busy, how to find out which PID holds the directory?
* What's LD_PRELOAD and when it's used?
* You ran a binary and nothing happened. How would you debug this?
* What are cgroups? Can you specify a scenario where you could use them?
* How can you remove/delete a file with file-name consisting of only non-printable/non-type-able characters?
* How can you increase or decrease the priority of a process in Linux?
* What are run-levels in Linux?

Expert Linux Questions:

* A running process gets EAGAIN: Resource temporarily unavailable on reading a socket. How can you close this bad socket/file descriptor without killing the process?

Networking Questions:

* What is localhost and why would ping localhost fail?
* What is the similarity between "ping" & "traceroute" ? How is traceroute able to find the hops.
* What is the command used to show all open ports and/or socket connections on a machine?
* Is 300.168.0.123 a valid IPv4 address?
* Which IP ranges/subnets are "private" or "non-routable" (RFC 1918)?
* What is a VLAN?
* What is ARP and what is it used for?
* What is the difference between TCP and UDP?
* What is the purpose of a default gateway?
* What is command used to show the routing table on a Linux box?
* A TCP connection on a network can be uniquely defined by 4 things. What are those things?
* When a client running a web browser connects to a web server, what is the source port and what is the destination port of the connection?
* How do you add an IPv6 address to a specific interface?
* You have added an IPv4 and IPv6 address to interface eth0. A ping to the v4 address is working but a ping to the v6 address gives yout the response sendmsg: operation not permitted. What could be wrong?
* What is SNAT and when should it be used?
* Explain how could you ssh login into a Linux system that DROPs all new incoming packets using a SSH tunnel.
* How do you stop a DDoS attack?
* How can you see content of an ip packet?
* What is IPoAC (RFC 1149)?

MySQL questions:

* How do you create a user?
* How do you provide privileges to a user?
* What is the difference between a "left" and a "right" join?
* Explain briefly the differences between InnoDB and MyISAM.
* Describe briefly the steps you need to follow in order to create a simple master/slave cluster.
* Why should you run "mysql_secure_installation" after installing MySQL?
* How do you check which jobs are running?

DevOps Questions:

* Can you describe your workflow when you create a script?
* What is GIT?
* What is a dynamically/statically linked file?
* What does "./configure && make && make install" do?
* What is puppet/chef/ansible used for?
* What is Nagios/Zenoss/NewRelic used for?
* What is Jenkins/TeamCity/GoCI used for?
* What is the difference between Containers and VMs?
* How do you create a new postgres user?
* What is a virtual IP address? What is a cluster?
* How do you print all strings of printable characters present in a file?
* How do you find shared library dependencies?
* What is Automake and Autoconf?
* ./configure shows an error that libfoobar is missing on your system, how could you fix this, what could be wrong?
* What are the advantages/disadvantages of script vs compiled program?
* What's the relationship between continuous delivery and DevOps?
* What are the important aspects of a system of continuous integration and deployment?

Fun Questions:

* A careless sysadmin executes the following command: chmod 444 /bin/chmod- what do you do to fix this?
* I've lost my root password, what can I do?
* I've rebooted a remote server but after 10 minutes I'm still not able to ssh into it, what can be wrong?
* If you were stuck on a desert island with only 5 command-line utilities, which would you choose?
* You come across a random computer and it appears to be a command console for the universe. What is the first thing you type?
* Tell me about a creative way that you've used SSH?
* You have deleted by error a running script, what could you do to restore it?
* What will happen on 19 January 2038?
* How to reboot server when reboot command is not responding?

Demo Time:

* Unpack test.tar.gz without man pages or google.
* Remove all "*.pyc" files from testdir recursively?
* Search for "my konfu is the best" in all *.py files.
* Replace the occurrence of "my konfu is the best" with "I'm a linux jedi master" in all *.txt files.
* Test if port 443 on a machine with IP address X.X.X.X is reachable.
* Get http://myinternal.webserver.local/test.html via telnet.
* How to send an email without a mail client, just on the command line?
* Write a get_prim method in python/perl/bash/pseudo.
* Find all files which have been accessed within the last 30 days.
* Explain the following command (date ; ps -ef | awk '{print $1}' | sort | uniq | wc -l ) >> Activity.log
* Write a script to list all the differences between two directories.
* In a log file with contents as <TIME> : [MESSAGE] : [ERROR_NO] - Human readable text display summary/count of specific error numbers that occurred every hour or a specific hour.

Other Great References:

Some questions are 'borrowed' from other great references like:

* https://github.com/darcyclarke/Front-en … -Questions
* https://github.com/kylejohnson/linux-sy … er/test.md
* http://slideshare.net/kavyasri790693/li … -questions


#2 May 28 2017


Re: Exercice: Linux System Administrator/DevOps Interview Questions

Regarding this question about how to access a system where the firewall is dropping all packets, I think this could be an answer - the system has to be readied, though. I don't think that it's possible to access a system that has only a simple firewall blocking everything, without some special service to open ports on request such as this one.



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