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?
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.
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 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.
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 [email protected]_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 [email protected] < 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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