The SSH plays most important role when perform any remote login to any Linux server. That’s why it is the first targeted method by any hacker to compromise a Linux server. First step that is performed by any sysadmins or Devops engineer is to harden SSH to secure it from external attacks and hackers. This tutorial explains you how to secure Linux SSH servers using SSH Hardening Tips.

Tip 1 : Enable Firewall To Protect the SSH Ports.

First of all You need to integrate the firewall to protect your SSH Port using UFW firewall and Firewalld firewall.

Tip 2 : Enable SSH Key based authentication instead of password based authentication.

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Secondly you need to enable the key based SSH authentication to protect your SSH servers.

Tip 3 : Harden your sshd configuration file.

Then Edit your SSH configuration file. 

# vim /etc/ssh/sshd_config

After that Find the given lines 

PermitRootLogin yes
PasswordAuthentication yes
AllowTcpForwarding YES
ClientAliveCountMax 3
Compression DELAYED
LogLevel INFO
MaxAuthTries 6
MaxSessions 10
Port 22
TCPKeepAlive YES
UsePrivilegeSeparation YES
X11Forwarding YES
AllowAgentForwarding YES

As soon as Change it to 

PermitRootLogin no
PasswordAuthentication no
AllowTcpForwarding NO
ClientAliveCountMax 2
Compression NO
LogLevel VERBOSE
MaxAuthTries 2
MaxSessions 2
Port 3526
TCPKeepAlive NO
UsePrivilegeSeparation SANDBOX
X11Forwarding NO
AllowAgentForwarding NO

Finally Restart the SSH services to apply the changes. 

# systemctl restart sshd

Tip 4 : Integrate Advanced Diffie-Hellman Key based algorithm to secure SSH.

Firstly Integrate Advanced Diffie-Hellman key based algorithm to enhance the security of SSH server.

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Tip 5 : Limit SSH Brute-force attack using Fail2ban.

Secondly Limit SSH Brute-force attack using Fail2ban.
Install Fail2ban on Ubuntu / Debian based distribution, 

# apt install fail2ban -y

Install Fail2ban on RHEL / CentOS based distribution.

Note: you need to install EPEL repository first before proceeding to the given steps.

# yum install fail2ban -y

Enable the fail2ban on Linux distribution. 

# cp -rv /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

Start the fail2ban services to apply the changes. 

# systemctl start fail2ban

Congratulations now you have harden and secure the SSH on Ubuntu and CentOS.