SSH agents

Public key authentication has a lot of advantages for connecting to servers, particularly if it’s the only allowed means of authentication, reducing the chances of a brute force password attack to zero. However, it doesn’t solve the problem of having to type in a password or passphrase on each connection, unless you’re using a private key with no passphrase, which is quite risky if the private key is compromised.

Thankfully, there’s a nice supplement to a well-secured SSH key setup in the use of agents on trusted boxes to securely store decrypted keys per-session, per-user. Judicious use of an SSH agent program on a trusted machine allows you to connect to any server for which your public key is authorised by typing your passphrase to decrypt your private key only once.

SSH agent setup

The ssh-agent program is designed as a wrapper for a shell. If you have a private and public key setup ready, and you have remote machines for which your key is authorised, you can get an idea of how the agent works by typing:

$ ssh-agent bash

This will prompt you for your passphrase, and once entered, within the context of that subshell, you will be able to connect to authorised remote servers without typing in the passphrase again. Once loaded, you can examine the identities you have by using ssh-add -l to see the fingerprints, and ssh-add -L for the public keys:

$ ssh-agent bash
Enter passphrase for /home/user/.ssh/id_rsa:
Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)
$ ssh-add -l
2048 07:1e:7d:c4:8a:0e:bc:b0:74:40:71:49:7c:70:9c /home/user/.ssh/id_rsa (RSA)
$ ssh-add -L

You can set up your .bashrc file to automatically search for accessible SSH agents to use for the credentials for new connections, and to prompt you for a passphrase to open a new one if it need be. There are very workable instructions on GitHub for setting this up.

If you want to shut down the agent at any time, you can use ssh-agent -k.

$ ssh-agent -k
echo Agent pid 790 killed;

SSH agent forwarding

Where the configuration of the remote machine allows it, you can forward authentication requests made from the remote machine back to the agent on your workstation. This is handy for working with semi-trusted gateway machines that you trust to forward your authentication requests correctly, but on which you’d prefer not to put your private key.

This means that if you connect to a remote machine from your workstation running an SSH agent with the following, using the -A parameter:

user@workstation:~$ ssh -A

You can then connect to another machine from using your private key on workstation:

user@remote:~$ ssh

SSH agent authentication via PAM

It’s also possible to use SSH agent authentication as a PAM method for general authentication, such as for sudo, using pam_ssh_agent_auth.

Shortcut for adding SSH keys

If you’ve dabbled with SSH much, for example by following the excellent tutorial a few years ago, you’ll know about adding keys to allow passwordless login (or, if you prefer, a passphrase) using public key authentication. Specifically, you copy the public key ~/.ssh/ or ~/.ssh/ off the machine from which you wish to connect into the /.ssh/authorized_keys file on the target machine. That will allow you to open an SSH session with the machine from the user account on the local machine to the one on the remote machine, without having to type in a password.

tom@conan:~$ scp ~/.ssh/ crom:.ssh/conan.pubkey
tom@conan:~$ ssh crom
tom@crom:~$ cd .ssh
tom@crom:~$ cat .ssh/conan.pubkey >>~/.ssh/authorized_keys

However, there’s a nice shortcut that I didn’t know about when I first learned how to do this, which has since been added to that tutorial too — specifically, the ssh-copy-id tool, which is available in most modern OpenSSH distributions and combines this all into one less error-prone step. If you have it available to you, it’s definitely a much better way to add authorized keys onto a remote machine.

tom@conan:~$ ssh-copy-id crom

Incidentally, this isn’t just good for convenience or for automated processes; strong security policies for publically accessible servers might disallow logging in via passwords completely, as usernames and passwords can be guessed. It’s a lot harder to guess an entire SSH key, so forcing this login method will reduce your risk of script kiddies or automated attacks brute-forcing your OpenSSH server to zero. You can arrange this by setting ChallengeResponseAuthentication to no in your sshd_config, but if that’s a remote server, be careful not to lock yourself out!