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.
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?
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?
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?
4. Is it possible to login to SSH server without password? How
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?
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?
# 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.
7. SSH has two protocols? Justify this statement.
Again, we need to open the SSH configuration file and add/edit the lines as shown below.
# protocol 2,1 to 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.
# 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?
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?
# 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.
which you provide information regarding ssh is nice, it ‘s very useful to sysadmins in linux.
Hello.
I hope you can help me.
What is the -f flag in scp?
Thank you
@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.”
http://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch03_08.htm
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):”
https://blogs.oracle.com/janp/entry/how_the_scp_protocol_works
@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..
I face most of the problems in Linux Network commands. What are the main questions related to Networking in ssh ?
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
Seems you have not installed nano editor.
try
# vi /etc/issue
[Note there is gap between vi and /etc]
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!
good one!
Thanks for the recognition.
keep Connected for more such posts.
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!
They always say something like “love to research nix” but they seem to never do that before posting articles.
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.
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.
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!