Understanding SSH and Hardening SSH

Introduction

When I first started learning Linux administration, I quickly realized that almost every server is managed remotely. SSH (Secure Shell) provides encrypted remote access to Linux servers.

What is SSH?

SSH is a secure network protocol used to remotely connect to servers, execute commands, transfer files (SCP/SFTP), and create encrypted tunnels.

Example:

ssh username@server-ip 

SSH Configuration

Main configuration file:

/etc/ssh/sshd_config

Common settings:

PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
MaxAuthTries 3

Restart service:

sudo systemctl reload sshd

Hardening SSH

• Disable root login
• Use SSH keys
• Disable password authentication
• Limit login attempts
• Change default port (optional)
• Allow only approved users

Commands

systemctl status sshd
sshd -t
ss -tulpn | grep ssh

Mistakes I Made

• Edited sshd_config without validating it.
• Disabled passwords before confirming SSH key login worked.
• Forgot to back up the configuration file.

What I Learned

• Always test configuration with sshd -t.
• Always use systemctl reload instead of systemctl restart to preserve active sessions.
• Keep one SSH session open while testing another.
• Security should never be an afterthought.

Conclusion

SSH is the foundation of Linux remote administration. Learning how to secure it has made me more confident in managing production servers.