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
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC+WvWXmVPx6UYB/uf+HTh1Y5zEVOmSeFfj6IC0fwN
lELVoFco9qdM4cuh6E6UaDURezjLSiayKt237DFHMgK9Hp4QPgN3ZJ7f7mesH7EHRnpLcvt0Rl3k1I4
C6gConwmkPZj3ax/cr6DAI9v7Ggeo7YPdKYhntB4TCEZfXlfihF5Vh5A2Od8cCNqy5KFKsFaLoI8Gwr
+ZC0CoxIoW6t5t6C/ZNRK2ojVwRWvp3nxcZsOzSdZJu3jcNHGSr0fxpdythRrOjzdDHgCiBuH+7mGKa
tLewbchdj8AgdeCE410xDJkov+tQuGYXZQAOx+JzWgiDI0VzWZsaV2QuyEF4NyG/
/home/user/.ssh/id_rsa

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
unset SSH_AUTH_SOCK;
unset SSH_AGENT_PID;
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 remote.example.com

You can then connect to another machine from remote.example.com using your private key on workstation:

user@remote:~$ ssh another.example.com

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.

One thought on “SSH agents

  1. The GitHub guide mentions that on Mac OS X ssh-agent is automatically started but in order to have Keychain Access.app manage your SSH key you have to add the -K option to ssh-add.

    $ ssh-add –help 2>&1 | grep ‘-K’ -K Store passphrases in your keychain.

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