OpenSSH server¶

OpenSSH is a powerful collection of tools for remotely controlling networked computers and transferring data between them. Here we’ll describe some of the configuration settings possible with the OpenSSH server application and how to change them on your Ubuntu system.

OpenSSH is a freely available version of the Secure Shell (SSH) protocol family of tools. Traditional tools, such as telnet or rcp, are insecure and transmit the user’s password in cleartext when used. OpenSSH provides a server daemon and client tools to facilitate secure, encrypted, remote control and file transfer operations, effectively replacing the legacy tools.

The OpenSSH server component, sshd, listens continuously for client connections from any of the client tools. When a connection request occurs, sshd sets up the correct connection depending on the type of client tool connecting. For example, if the remote computer is connecting with the SSH client application, the OpenSSH server sets up a remote control session after authentication. If a remote user connects to an OpenSSH server with scp, the OpenSSH server daemon initiates a secure copy of files between the server and client after authentication.

OpenSSH can use many authentication methods, including plain password, public key, and Kerberos tickets.

Install OpenSSH¶

To install the OpenSSH client applications on your Ubuntu system, use this command at a terminal prompt:

sudo apt install openssh-client

To install the OpenSSH server application, and related support files, use this command at a terminal prompt:

sudo apt install openssh-server

Configure OpenSSH¶

To configure the default behavior of the OpenSSH server application, sshd, edit the file /etc/ssh/sshd_config. For information about the configuration directives used in this file, refer to the online manpage or run man sshd_config at a terminal prompt.

There are many directives in the sshd configuration file, which control things like communication settings and authentication modes. The following are examples of configuration directives that can be changed by editing the /etc/ssh/sshd_config file.

Tip: Before editing the configuration file, you should make a copy of the original /etc/ssh/sshd_config file and protect it from writing so you will have the original settings as a reference and to reuse as necessary. You can do this with the following commands:
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.original
sudo chmod a-w /etc/ssh/sshd_config.original

Since losing an SSH server might mean losing your way to reach a server, check the configuration after changing it and before restarting the server:

sudo sshd -t -f /etc/ssh/sshd_config

Example configuration directive¶

Let’s take a look at an example of a configuration directive change. To make your OpenSSH server display the contents of the /etc/issue.net file as a pre-login banner, you can add or modify this line in the /etc/ssh/sshd_config file:

Banner /etc/issue.net

After making changes to the /etc/ssh/sshd_config file, save the file. Then, restart the sshd server application to effect the changes using the following command:

sudo systemctl restart ssh.service

Warning: 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 when 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 particularly careful when editing this file on a remote server.

SSH keys¶

SSH allows authentication between two hosts without the need of a password. SSH key authentication uses a private key and a public key.

To generate the keys, run the following command:

ssh-keygen -t rsa

This will generate the keys using the RSA Algorithm. At the time of this writing, the generated keys will have 3072 bits. You can modify the number of bits by using the -b option. For example, to generate keys with 4096 bits, you can use:

ssh-keygen -t rsa -b 4096

During the process you will be prompted for a password. Simply hit Enter when prompted to create the key.

By default, the public key is saved in the file ~/.ssh/id_rsa.pub, while ~/.ssh/id_rsa is the private key. Now copy the id_rsa.pub file to the remote host and append it to ~/.ssh/authorized_keys by running:

ssh-copy-id username@remotehost

Finally, double check the permissions on the authorized_keys file – only the authenticated user should have read and write permissions. If the permissions are not correct then change them by:

chmod 600 .ssh/authorized_keys

You should now be able to SSH to the host without being prompted for a password.

Import keys from public keyservers¶

These days many users have already SSH keys registered with services like Launchpad or GitHub. Those can be imported with:

ssh-import-id <username-on-remote-service>

The prefix lp: is implied and means fetching from Launchpad. The alternative gh: will make the tool fetch from GitHub instead.

Two factor authentication¶

You can add an extra layer of security to the default key-based authentication using two factor authentication. You can add two factor authentication using U2F/FIDO hardware authentication devices. Alternatively, in cases U2F/FIDO hardware authentication devices are unavailable or impractical for your use case you can add it using HMAC/Time based One Time Passwords (HOTP/TOTP).