Linux Security

SSH Keys
Firewalls
SELinux

Backup sshd_config and ssh_config (RHEL systems seem to use sshd_config)

cp /etc/ssh/ssh_config /etc/ssh/ssh_config~

cp /etc/ssh/sshd_config /etc/ssh/sshd_config~

Disabling passwords (prevents brute force attacks)

IMPORTANT: Make sure SSH Keys are working otherwise root will be impossible to access without reinstall of OS
In /etc/ssh/sshd_config:

PasswordAuthentication no

Restart sshd to apply changes:

systemctl restart sshd