Mastering the Art of SSH: 30+ Interview Questions to Ace Your Next Interview

Embarking on a new job hunt can be an exhilarating yet daunting experience, especially when it comes to technical interviews. One area that often catches candidates off guard is the realm of Secure Shell (SSH) – a critical protocol for secure remote access and data transfer. Whether you’re a seasoned Linux administrator or a budding DevOps engineer, having a solid grasp of SSH is an invaluable asset.

In this comprehensive guide, we’ll explore 30+ SSH interview questions that can help you prepare for your next big opportunity. From understanding the fundamentals to diving into advanced configurations, we’ve got you covered. So, buckle up and get ready to impress your interviewers with your SSH prowess!

Understanding the Basics

Before we delve into the nitty-gritty of SSH, let’s ensure you have a solid foundation.

1. What is SSH?

   • Secure Shell (SSH) is a cryptographic network protocol that enables secure remote access and file transfer over unsecured networks.

2. What is the default port number for SSH?

   • The default port number for SSH is 22.

3. What is the configuration file for the SSH server?

   • The configuration file for the SSH server is /etc/ssh/sshd_config.

4. What is the configuration file for the SSH client?

   • The configuration file for the SSH client is /etc/ssh/ssh_config.

5. Is it possible to change the default SSH port number?

   • Yes, it is possible to change the default SSH port number for added security. This can be done by modifying the Port directive in the /etc/ssh/sshd_config file.

Access Control and Security

Ensuring secure access to your servers is a top priority, and SSH provides various mechanisms to achieve this.

1. How can you access your server over SSH without the actual root password?

   • You can enable key-based authentication by generating an SSH key pair on your local machine and adding the public key to the authorized_keys file on the remote server.

2. For security reasons, you need to disable direct root login to your server. What would you do?

   • You can disable direct root login by setting PermitRootLogin to no in the /etc/ssh/sshd_config file.

3. How can you limit SSH access to specific users or groups?

   • You can use the AllowUsers and AllowGroups directives in the /etc/ssh/sshd_config file to specify which users or groups are allowed to access the server over SSH.

4. How can you deny SSH access to specific users or groups?

   • Conversely, you can use the DenyUsers and DenyGroups directives to block specific users or groups from accessing the server over SSH.

5. What is the difference between SSH and Telnet, and which one is preferred for security reasons?

   • SSH is a secure protocol that encrypts data during transmission, while Telnet transmits data in plain text, making it vulnerable to eavesdropping and man-in-the-middle attacks. SSH is the preferred choice for security reasons.

Advanced Configuration and Troubleshooting

As you gain more experience, you’ll encounter more complex scenarios that require advanced SSH configurations and troubleshooting skills.

1. How can you enable debugging in the SSH command?

   • You can enable debugging in the SSH command by using the -v (verbose) option. Increasing the number of vs (e.g., -vvv) increases the verbosity level.

2. What are the different protocols used for SSH communication, and which one is more secure?

   • SSH supports two protocols: Protocol 1 and Protocol 2. Protocol 2 is more secure and is the recommended protocol to use.

3. How can you check the SSH server’s version details from the command line?

   • You can check the SSH server’s version details by running the ssh -V command on the server.

4. How can you copy files using SSH, and what command is used for this purpose?

   • You can copy files securely over an SSH connection using the scp (Secure Copy) command. For example, scp file.txt user@remote_host:/path/to/destination.

5. If your SSH port is non-default, how can you use the SCP command to copy files?

   • If your SSH port is non-default, you can specify the port number using the -P option with the scp command. For example, scp -P 2022 file.txt user@remote_host:/path/to/destination.

6. What is the purpose of the blowfish cipher in the SCP command?

   • The blowfish cipher can be used with the scp command to increase the transfer speed. By default, scp uses the Triple-DES cipher, which is slower than blowfish.

7. Can you briefly explain the working principle of the SCP command?

   • The SCP command initiates an SSH connection to the remote host and requests an SCP process to be started. The remote SCP process can operate in two modes: source mode (reading files from the remote host) or sink mode (writing files to the remote host).

8. What are some commonly used SCP switches, and what do they do?

   • Some commonly used SCP switches include:
     • -p: Preserves modification times, access times, and modes from the original file.
     • -r: Recursively copies entire directories.
     • -U: Removes source files after copying them to the destination.

9. How can you limit SSH access to a specific subnet using the AddressFamily directive?

   • You can use the AddressFamily directive in the /etc/ssh/sshd_config file to specify the allowed subnet. For example, AddressFamily 192.168.1.0/24.

10. What is the purpose of the sshpass command?

    • The sshpass command allows you to supply the SSH password on the command line, which can be useful for automation scripts.

11. How can you limit SSH access to specific IP addresses using the ListenAddress directive?

    • You can use the ListenAddress directive in the /etc/ssh/sshd_config file to specify the IP addresses on which the SSH server should listen. For example, ListenAddress 192.168.1.100.

12. What is the purpose of the LoginGraceTime directive in the SSH configuration?

    • The LoginGraceTime directive specifies the time (in seconds) that the SSH server will wait for a successful login before disconnecting the client.

13. What is the purpose of the MaxAuthTries directive in the SSH configuration?

    • The MaxAuthTries directive specifies the maximum number of authentication attempts allowed before the SSH server disconnects the client.

14. How can you restart the SSH daemon on different Linux distributions?

    • On older distributions (e.g., CentOS/RHEL 6), you can restart the SSH daemon with the command /etc/init.d/sshd restart or service sshd restart.
    • On newer distributions (e.g., CentOS/RHEL 7 and later), you can use the command systemctl restart sshd.

15. How can you check the status of the SSH daemon on different Linux distributions?

    • On older distributions (e.g., CentOS/RHEL 6), you can check the SSH daemon status with the command /etc/init.d/sshd status or service sshd status.
    • On newer distributions (e.g., CentOS/RHEL 7 and later), you can use the command systemctl status sshd.

16. How can you limit the bandwidth used by the SCP command?

    • You can limit the bandwidth used by the scp command by using the -l option followed by the desired bandwidth limit. For example, scp -l 100k file.txt user@remote_host:/path/to/destination.

17. How can you create a banner to display a message when logging into the server via SSH?

    • You can create a banner by editing the /etc/ssh/sshd_config file and adding the Banner directive with the path to a file containing the desired message.

18. What do you mean by SSH ciphers, and what are some common ciphers used in SSH?

    • SSH ciphers are cryptographic algorithms used to encrypt data during SSH communication. Some common ciphers used in SSH include blowfish, DES, and 3DES.

19. How can you access a remote GUI application using an SSH connection?

    • You can access a remote GUI application using an SSH connection by enabling X11 forwarding with the -X or -Y option when initiating the SSH connection.

Bonus: Real-world Scenarios

To truly test your SSH prowess, let’s explore a few real-world scenarios that interviewers might present.

1. Your team is tasked with setting up a new server, and you need to ensure secure access while adhering to industry best practices. Describe the steps you would take to configure SSH on the server.

   • This question evaluates your ability to apply SSH concepts in a practical setting. Your answer should cover steps such as changing the default port, enabling key-based authentication, disabling root login, and configuring access controls.

2. You’ve been asked to troubleshoot an issue where a specific user is unable to connect to a server via SSH. What steps would you take to diagnose and resolve the problem?

   • This scenario tests your troubleshooting skills and your understanding of SSH access controls. Your response should outline a systematic approach, such as checking the user’s SSH keys, verifying their presence in the authorized_keys file, reviewing relevant log files, and examining the server’s SSH configuration.

3. Your organization has recently implemented a new security policy that requires all SSH connections to use a specific cipher suite. How would you ensure that all SSH clients and servers are configured to comply with this policy?

   • This question assesses your ability to understand and configure SSH ciphers, as well as your knowledge of implementing organizational policies across multiple systems.

By mastering the concepts and techniques covered in these 30+ SSH interview questions, you’ll be well-prepared to tackle even the most challenging SSH-related scenarios during your next job interview. Remember, practice makes perfect, so don’t hesitate to review these questions repeatedly until you feel confident in your responses.

Good luck on your job hunt, and may the SSH force be with you!