10 Useful SSH (Secure Shell) Interview Questions and Answers

SSH stands for Secure Shell is a network protocol, used to access remote machine in order to execute command-line network services and other commands over a Network. SSH is Known for its high security, cryptographic behavior and it is most widely used by Network Admins to control remote web servers primarily.

SSH Interview Questions
10 SSH Interview Questions

Here in this Interview Questions series article, we are presenting some useful 10 SSH (Secure Shell) Questions and their Answers.

1. SSH is configured on what Port Number, by default? How to change the port of SSH?
Answer : SSH is configured on port 22, by default. We can change or set custom port number for SSH in configuration file.

We can check port number of SSH by running the below one liner script, directly on terminal.

# grep Port /etc/ssh/sshd_config		[On Red Hat based systems]

# grep Port /etc/ssh/ssh_config		        [On Debian based systems]

To change the port of SSH, we need to modify the configuration file of SSH which is located at ‘/etc/ssh/sshd_config‘ or ‘/etc/ssh/ssh_config‘.

# nano /etc/ssh/sshd_config	[On Red Hat based systems]

# nano /etc/ssh/ssh_config		[On Debian based systems]

Searh for the Line.

Port 22

And replace ‘22‘ with any UN-engaged port Number say ‘1080‘. Save the file and restart the SSH service to take the changes into effect.

# service sshd restart					[On Red Hat based systems]

# service ssh restart					[On Debian based systems]
2. As a security implementation, you need to disable root Login on SSH Server, in Linux. What would you suggest?
Answer : The above action can be implemented in the configuration file. We need to change the parameter ‘PermitRootLogin’ to ‘no’ in the configuration file to disable direct root login.

To disable SSH root login, open the configuration file located at ‘/etc/ssh/sshd_config‘ or ‘/etc/ssh/ssh_config‘.

# nano /etc/ssh/sshd_config			[On Red Hat based systems]

# nano Port /etc/ssh/ssh_config			[On Debian based systems]

Change the parameter ‘PermitRootLogin‘ to ‘no‘ and restart the SSH service as show above.

3. SSH or Telnet? Why?
Answer : Both SSH and Telnet are network Protocol. Both the services are used in order to connect and communicate to another machine over Network. SSH uses Port 22 and Telnet uses port 23 by default. Telnet send data in plain text and non-encrypted format everyone can understand whereas SSH sends data in encrypted format. Not to mention SSH is more secure than Telnet and hence SSH is preferred over Telnet.
4. Is it possible to login to SSH server without password? How
Answer : Yes! It is possible to login to a remote SSH server without entering password. We need to use ssh-keygen technology to create public and private keys.

Create ssh-keygen using the command below.

$ ssh-keygen

Copy public keys to remote host using the command below.

$ ssh-copy-id -i /home/USER/.ssh/id_rsa.pub REMOTE-SERVER

Note: Replace USER with user name and REMOTE-SERVER by remote server address.

The next time we try to login to SSH server, it will allow login without asking password, using the keygen. For more detailed instructions, read how to login remote SSH server without password.

5. How will you allows users and groups to have access to SSH Sever?
Answer : Yes! It is possible to allow users and groups to have access to SSH server.

Here again we need to edit the configuration file of SSH service. Open the configuration file and add users and groups at the bottom as show below and then, restart the service.

AllowUsers Tecmint Tecmint1 Tecmint2
AllowGroups group_1 group_2 group_3
6. How to add welcome/warning message as soon as a user login to SSH Server?
Answer : In order to add a welcome/warning message as soon as a user logged into SSH server, we need to edit file called ‘/etc/issue’ and add message there.
# nano /etc/issue

And add your custom message in this file. See, below a screen grab that shows a custom message as soon as user logged into server.

SSH Login Banner
SSH Login Message
7. SSH has two protocols? Justify this statement.
Answer : SSH uses two protocols – Protocol 1 and Protocol 2. Protocol 1 is older than protocol 2. Protocol 1 is less secure than protocol 2 and should be disabled in the config file.

Again, we need to open the SSH configuration file and add/edit the lines as shown below.

# protocol 2,1


Protocol 2

Save the configuration file and restart the service.

8. Is it possible to trace unauthorized login attempts to SSH Server with date of Intrusion along with their corresponding IP.
Answer : Yes! we can find the failed login attempts in the log file created at location ‘/var/log/secure’. We can make a filter using the grep command as shown below.
# cat /var/log/secure | grep “Failed password for”

Note: The grep command can be tweaked in any other way to produce the same result.

9. Is it possible to copy files over SSH? How?
Answer : Yes! We can copy files over SSH using command SCP, stands for ‘Secure CopY’. SCP copies file using SSH and is very secure in functioning.

A dummy SCP command in action is depicted below:

$ scp text_file_to_be_copied Your_username@Remote_Host_server:/Path/To/Remote/Directory

For more practical examples on how to copy files/folders using scp command, read the 10 SCP Commands to Copy Files/Folders in Linux.

10. Is it possible to pass input to SSH from a local file? If Yes! How?
Answer : Yes! We can pass input to SSH from a local file. We can do this simply as we do in scripting Language. Here is a simple one liner command, which will pass input from local files to SSH.
# ssh username@servername < local_file.txt

SSH is a very hot topic from interview point, of all times. The above questions would have surely added to your knowledge.

That’s all for now. I’ll soon be here with another interesting article. Till then Stay Tuned and connected to Tecmint. Don’t forget to provide us with your valuable feedback in our comment section.

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  1. In Redhat OS, there are 2 files under /etc/ssh i.e sshd_config and ssh_config? What is the difference and what is the use of both files and for what purpose??

    • sshd_config is for daemon – ssh _server_ process working on host you connect to.

      ssh_config is used to set up system-wide default settings for ssh command (ssh-client)

  2. when given three departments how can write policy document for the security system for the bank that ensures integrity authentication in the bank.

  3. Oh-oh-oh. There is no difference in config file names between RH, Debian, Gentoo and so on. I.e.
    root@seafile:~# lsb_release -d
    Description: Debian GNU/Linux 8.6 (jessie)
    root@seafile:~# ls /etc/ssh/*_config
    /etc/ssh/ssh_config /etc/ssh/sshd_config
    root@seafile:~# head -n 3 /etc/ssh/ssh_config

    # This is the ssh client system-wide configuration file. See
    # ssh_config(5) for more information. This file provides defaults for
    root@seafile:~# head -n 5 /etc/ssh/sshd_config
    # Package generated configuration file
    # See the sshd_config(5) manpage for details

    # What ports, IPs and protocols we listen for
    Port 22
    [root@aloe ~]# cat /etc/redhat-release
    CentOS release 4.9 (Final)
    [root@aloe ~]# ls /etc/ssh/*_config
    /etc/ssh/ssh_config /etc/ssh/sshd_config
    [root@aloe ~]# head -n 3 /etc/ssh/ssh_config
    # $OpenBSD: ssh_config,v 1.19 2003/08/13 08:46:31 markus Exp $

    # This is the ssh client system-wide configuration file. See
    [root@aloe ~]# head -n 5 /etc/ssh/sshd_config
    # $OpenBSD: sshd_config,v 1.69 2004/05/23 23:59:53 dtucker Exp $

    # This is the sshd server system-wide configuration file. See
    # sshd_config(5) for more information.
    isa ~ # cat /etc/gentoo-release
    Gentoo Base System release 2.3
    isa ~ # ls /etc/ssh/*_config
    /etc/ssh/ssh_config /etc/ssh/sshd_config
    isa ~ # head -n 3 /etc/ssh/ssh_config
    # $OpenBSD: ssh_config,v 1.30 2016/02/20 23:06:23 sobrado Exp $

    # This is the ssh client system-wide configuration file. See
    isa ~ # head -n 5 /etc/ssh/sshd_config
    # $OpenBSD: sshd_config,v 1.99 2016/07/11 03:19:44 tedu Exp $

    # This is the sshd server system-wide configuration file. See
    # sshd_config(5) for more information.

    Ubuntu, Mint, Calculate all of them have two config files ssh_config for ssh client and sshd_config for daemon.
    So seriously mistakes… :( Very bad!

  4. Hi Ravi,

    How does ssh work, mean to say what is the functionality of public and private key ,how the communication takes place b/w client and server.
    It was asked to me in vmware interview.
    Can you please help

  5. Can SSH is used for making http requests to a web server, checking network path to a remote, and writing a web server, accessing a remote computer. please tell me the answer

    • @Alan,

      There isn’t any -f switch in scp man pages, yes there is capital -F option, which is used to specify per-user configuration file for ssh.

      • I found that is a undocumented option to, specified “from” (-f) and “to” (-t)

        “In all cases aside from remote-to-remote scenario the scp command processes command line options and then starts an SSH connection to the remote host. Another scp command is run on the remote side through that connection in either source or sink mode. Source mode reads files and sends them over to the other side, sink mode accepts them. Source and sink modes are triggered using -f (from) and -t (to) options, respectively. These options are for internal usage only and aren’t documented. There is also the 3rd hidden option, -d, when the target is expected to be a directory.”
        And this:

        “So, how does the transfer protocol actually works? If you forget about ssh, sshd and the connection between them and concentrate only on interaction between scp in “normal” mode and scp in the sink mode, you can see the scenario like this (if you copied from remote to local the remote scp command would have been run with -f option instead of -t, denoting the source mode):”

        • @Alan,

          Thanks for the detailed information about background process of ssh and scp, very helpful, let me go through it and come back to you..

  6. in #6. How to add welcome/warning message as soon as a user login to SSH Server?
    the command nano/etc/issue is not working, any other solution for this…
    but i am using /etc/motd for welcome/warning message

  7. At Debian systems the SSH config could be found at “/etc/ssh/sshd_config” not “/etc/ssh/ssh_config” this is the SSH-Client config!

  8. On Debian and most Debian-based systems like, Ubuntu the ssh server configuration is in /etc/ssh/sshd_config
    There are a lot of differences between Debian and Red Hat, but this is identical!

  9. Ref #6
    I would also like a message when i login, but I only get a message, maybe my configuration is wrong ;-)

    Ref #8
    A better way is
    grep “Failed password for” /var/log/secure
    because pipe will fork another process.

  10. On #1 and #2, as has been said by others, the /etc/ssh/ssh_config file is the client configuration while /etc/ssh/sshd_config is the server configuration file. All distributions should have both. Debian is not special here.

    On #2 I prefer the without-password setting for PermitRootLogin. This disables password authentication for the root account but still allows root access via more secure authentication methods. Normally that means key authentication but it can also mean any other non-password authentication you have configured (such as the also previously mentioned Kerberos).

    On #4 you would only need to specify a key file to ssh-copy-id if the file is not one of the standard file names and is not loaded into an agent. Otherwise ssh-copy-id will authorize every key it can find. You should also mention how to manually authorize a key since servers often have password authentication disabled (something that should be an additional question) and therefore ssh-copy-id can’t get in to authorize your key(s).

    On #9 both sftp and rsync should also be mentioned. Both are actually superior to scp unless you need the rarely used scp -3 functionality.

  11. Thank you Kumar, excellent article. One quick question, I was checking for the “/var/log/secure” file on my linux mint 17 Qiana and was not there. Is there a command or comment change I need to do to get SSH to start keeping logs about fail log in attempts.? Thank you in advance!

    • Actually, I found out that in Debian based systems the “/var/log/secure” file is in “/var/log/auth.log” ^.^ Thank you!

    • @halcyon,
      How you changing? are you changing in sshd_config file? or any other way.. The proper way is to edit the sshd_config file and replace the port 22 with your choice of port…and restart ssh to work on new port..

  12. Thank you !!!!!!!!!!!!!!!!!!Nice topics and also comment of Roberto C. Sanchez is also very good .

    thanks for sharing info.

  13. Somebody forgot to proof read this article to the point that it’s factually incorrect.


    “Protocol 1 is less secure than protocol 1 and should be disabled in the config file.”

  14. Several notes about your article (I numbered them to correspond to your questions):

    1. The /etc/ssh/ssh_config file is NOT the correct file. That file is for the system-wide ssh client configuration. The daemon is configured by /etc/ssh/sshd_config (as in other distros). Also, it is valid to specify the port directive with any sort of capitalization (e.g., “port” or “PORT” or even “pOrT”), so the grep command should be ‘grep -i port /etc/ssh/sshd_config’. Additionally, that command will only tell you what port has been configured, not what port the running daemon is actually listening on. That is, if the port specification in the configuration is changed but the daemon is not restarted, you will be misled. To see what port the daemon is currently listening on, you can run ‘netstat -nplt |grep sshd’. There is no ‘service’ command on Debian. You can restart ssh by running ‘/etc/init.d/ssh restart’.

    2. Same as with #1, the file in Debian is /etc/ssh/sshd_config. Also, the command ‘nano Port /etc/ssh/ssh_config’ does not make sense.

    3. In a Kerberos environment with a Kerberos-enabled telnet, it can use encryption. Of course, SSH can also take advantage of Kerberos authentication, so it is still likely a better choice these days than even encrypted telnet.

    4. It is also possible to login via SSH without a password when using Kerberos for authentication.

    6. /etc/issue is just the default, you can use any file with the directive ‘Banner /path/to/message’ in sshd_config. This can be used if you need to display different welcome messages to remote users than to local users.

    8. First, it is pointless and unnecessary to ‘cat’ a file only to pipe it into ‘grep’. If the file is very large, it can be unnecessarily slow. You can achieve exactly the same by running ‘grep “Failed password for” /var/log/secure’. Also, there is no /var/log/secure on Debian systems. On a Debian system you would look in /var/log/auth.log. Also, the string “Failed password for” will only show you when someone tries to log in as an authorized user. If the user is not authorized (because you have used the AllowedUsers and/or AllowedGroups), then you should also look for “Invalid user”.

    9. You make no mention of the sftp command.

    10. You left out mention of some very useful features of ssh, including using the -f option to background ssh before executing a command on the remote host, as well as setting up secure tunnels with ssh.

    • You are doing the lord’s work, Roberto Sanchez. You covered everything I was going to gripe about and added in some nuggets I was unaware of. Bravo.


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