OpenSSH is a powerful collection of tools for the remote control of, and transfer of data between, networked computers. You will also learn about 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 for remotely controlling, or transferring files between, computers. Traditional tools used to accomplish these functions, 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.
Installation of the OpenSSH client and server applications is simple. 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
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:
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
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.
/etc/ssh/sshd_configfile 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
Furthermore 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
The following is an example of a configuration directive you may change:
- To make your OpenSSH server display the contents of the
/etc/issue.netfile as a pre-login banner, simply add or modify this line in the
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_configfile, 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.
SSH allow 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, from a terminal prompt enter:
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 do:
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 is the private key. Now copy the
id_rsa.pub file to the remote host and append it to
~/.ssh/authorized_keys by entering:
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 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 easily imported with:
lp: is implied and means fetching from launchpad, the alternative
gh: will make the tool fetch from github instead.
Two factor authentication with U2F/FIDO
OpenSSH 8.2 added support for U2F/FIDO hardware authentication devices. These devices are used to provide an extra layer of security on top of the existing key-based authentication, as the hardware token needs to be present to finish the authentication.
It’s very simple to use and setup. The only extra step is generate a new keypair that can be used with the hardware device. For that, there are two key types that can be used:
ed25519-sk. The former has broader hardware support, while the latter might need a more recent device.
Once the keypair is generated, it can be used as you would normally use any other type of key in openssh. The only requirement is that in order to use the private key, the U2F device has to be present on the host.
For example, plug the U2F device in and generate a keypair to use with it:
$ ssh-keygen -t ecdsa-sk Generating public/private ecdsa-sk key pair. You may need to touch your authenticator to authorize key generation. <-- touch device Enter file in which to save the key (/home/ubuntu/.ssh/id_ecdsa_sk): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/ubuntu/.ssh/id_ecdsa_sk Your public key has been saved in /home/ubuntu/.ssh/id_ecdsa_sk.pub The key fingerprint is: SHA256:V9PQ1MqaU8FODXdHqDiH9Mxb8XK3o5aVYDQLVl9IFRo ubuntu@focal
Now just transfer the public part to the server to
~/.ssh/authorized_keys and you are ready to go:
$ ssh -i .ssh/id_ecdsa_sk firstname.lastname@example.org Confirm user presence for key ECDSA-SK SHA256:V9PQ1MqaU8FODXdHqDiH9Mxb8XK3o5aVYDQLVl9IFRo <-- touch device Welcome to Ubuntu Focal Fossa (GNU/Linux 5.4.0-21-generic x86_64) (...) email@example.com:~$
FIDO2 resident keys
FIDO2 private keys consist of two parts: a “key handle” part stored in the private key file on disk, and a per-device key that is unique to each FIDO2 token and that cannot be exported from the
token hardware. These are combined by the hardware at authentication time to derive the real key that is used to sign authentication challenges.
For tokens that are required to move between computers, it can be cumbersome to have to move the private key file first. To avoid this, tokens implementing the newer FIDO2 standard support resident keys, where it is possible to retrieve the key handle part of the key from the hardware.
Using resident keys increases the likelihood of an attacker being able to use a stolen token device. For this reason, tokens normally enforce PIN authentication before allowing download of keys, and users should set a PIN on their tokens before creating any resident keys. This is done via the hardware token management software.
OpenSSH allows resident keys to be generated using the ssh-keygen
-O resident flag at key generation time. This will produce a public/private key pair as usual, but it will be possible to retrieve the private key part (the key handle) from the token later. This is done by running
$ ssh-keygen -K
which will download the resident key from the token and write public/private key files for them. It is also possible to download and add resident keys directly to ssh-agent by running
$ ssh-add -K
In this case no file is written, and the public key can be printed by running