For more security change ssh port for a custom port, but a risk to sshd broken and you can lost server ssh access. To test a custom settings before to the production.

  1. How to do this in safe mode?
  2. How to avoid to lost ssh connection?

Enviromment

Linux Ubuntu Server 20.04 64
Required root access / sudo

Make backup of sshd_config file

cp /etc/ssh/sshd_config /etc/ssh/sshd_config.port22
cp /etc/ssh/sshd_config /etc/ssh/sshd_config.port5000

You can run a second instance of openssh-server using a custom configuration, make all change you need.

vim /etc/ssh/sshd_config.port5000
change port to 5000
Port 5000

To test the syntax of custom config file

sshd -t -f /etc/ssh/sshd_config.port5000

Run a new server using new sshd_config, do not forget to open port at firewall.

-4d: Ipv4 + debug
-f : config file

/usr/sbin/sshd -4d -f /etc/ssh/sshd_config.port5000

ctrl+c to stop

Open a new terminal and try a ssh connection using port 5000 from authorized client.

ssh user@server -p 5000

If you have success, then copy ssh_config.port5000 to sshd_config (default) and restart the service. Try lot of times new connection before leaving your server!

cp /etc/ssh/sshd_config.port5000 /etc/ssh/sshd_config
sudo service ssh restart