ssh-agent-helper

ssh-agent-helper

Overview

Challenge: You have a script/program that needs to scp/sftp/etc. files from one server to another, and since you're a responsible admin, you want to do it securely.

Problems:

1. You do not want to have an ssh key around that doesn't have a password.
2. You cannot type in a password each time the program runs.

Solution: ssh-agent-helper.

How it works

ssh-agent-helper consists of three components:

1. the ssh-agent-helper script.
2. a wrapper for running your program with the agent that you started with ssh-agent-helper.
3. a nagios check to ensure that the agent is running and has a key loaded.

ssh-agent-helper

ssh-agent-helper takes two optional arguments: a keyfile and a directory for the agent socket. If the options are omitted, the script will use ssh-add's defaults for keyfiles and will set /var/run/<user> as the socket directory. When run, you will be asked for the key's password, and if entered successfully, the key will be loaded into the agent.

  myhost(~) % ssh-agent-helper -f /home/me/.ssh/id_rsa.git
  Enter passphrase for /home/me/.ssh/id_rsa.git:
  Identity added: /home/me/.ssh/id_rsa.git (/home/me/.ssh/id_rsa.git)
  myhost(~) %

run-with-agent

Use this script to run a program using the agent that you started with ssh-agent-helper. run-with-agent takes one optional argument and one required argument. The optional argument, -s, is the full path to the ssh-agent socket. This option is necessary if you set a custom socket directory in ssh-agent-helper. If you used the default socket directory in ssh-agent-helper, this option can be omitted. The required argument is the program you wish to run with the ssh-agent.

  myhost(~) % run-with-agent ssh [email protected]
  Hi me! You've successfully authenticated, but GitHub does not provide shell access.
  Connection to github.com closed.

check_ssh_agent

Since your program is now relying on ssh-agent-helper's ssh-agent, you will need to know if there is no key in the agent -- or if the agent isn't running -- so that you can restart it. Otherwise, your program will fail. check_ssh_key takes one required argument: the full path to the agent socket.

  myhost(~) % /usr/local/libexec/nagios/check_ssh_key /var/run/me/ssh-agent.sock
  OK: key is loaded
  myhost(~) %

Packaging

A build script is included to build packages with these three utilites for FreeBSD and Linux.

Things to remember

1. If the server where your program runs reboots, you will need to login to the server to start ssh-agent-help.
2. YOUR SSH KEYS SHOULD ALWAYS HAVE PASSWORDS.
3. Whenever possible, use purpose-built keys that use commands= in authorized_keys.