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1.3. Configuration
You may configure the default behavior of the OpenSSH server application, sshd, by editing the file /etc/ ssh/sshd_config. For information about the configuration directives used in this file, you may view the appropriate manual page with the following command, issued at a terminal prompt:
man sshd_config
There are many directives in the sshd configuration file controlling such things as communication settings, and authentication modes. The following are examples of configuration directives that can be changed by editing the /etc/ssh/sshd_config file.
Prior to editing the configuration file, you should make a copy of the original file and protect it from writing so you will have the original settings as a reference and to reuse as necessary.
Copy the /etc/ssh/sshd_config file and protect it from writing with the following commands, issued at a terminal prompt:
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.original sudo chmod a-w /etc/ssh/sshd_config.original
The following are examples of configuration directives you may change:
• To set your OpenSSH to listen on TCP port 2222 instead of the default TCP port 22, change the Port directive as such:
Port 2222
• To have sshd allow public key-based login credentials, simply add or modify the line: PubkeyAuthentication yes
If the line is already present, then ensure it is not commented out.
• To make your OpenSSH server display the contents of the /etc/issue.net file as a pre-login banner, simply add or modify the line:
Banner /etc/issue.net
In the /etc/ssh/sshd_config file.
After making changes to the /etc/ssh/sshd_config file, save the file, and restart the sshd server application to effect the changes using the following command at a terminal prompt:
sudo systemctl restart sshd.service
Many other configuration directives for sshd are available to change the server application's behavior to fit your needs. Be advised, however, if your only method of access to a server is ssh, and you make a mistake in configuring sshd via the /etc/ssh/sshd_config file, you may find you are locked out of the server upon restarting it. Additionally, if an incorrect configuration directive is supplied, the sshd server may refuse to start, so be extra careful when editing this file on a remote server.