Using ssh-agent with ssh

I went searching for a good, general page that would explain how to do passwordless logins using ssh-agent and didn’t find much at the time (now there is much more out there). So I wrote this page.

Goals

Get a secure, encrypted connection to a remote machine without typing in a password.

Executive Summary

Create a key pair on the local machine.
Put the public key on any remote machines.
Run ssh-agent to cache login credentials for the session. ssh-agent requires the user to “unlock” the private key first.

Alternate agent startup scripts — Working with KDE, Cygwin, or csh-derived shell? Some scripts to help
Troubleshooting — Can’t connect? Here’s some ideas to help you troubleshoot the problem.
Automatic ssh — Daemons, long-lived processes and ssh.

Methods

Use OpenSSH to handle the authentication.

For Windows users, the methods I describe here will work with the OpenSSH that is part of the CygWin toolset.

Anyway, here is how to set up a pair of keys for passwordless authentication via ssh-agent.

Generate the keys. Do this on the host that you want to connect from.
Note: Older versions of OpenSSH (1.2.xx) and, perhaps, commercial SSH may require that you have to use RSA keys. In this case substitute "RSA" for "DSA" after "-t" and "identity" for "id_dsa". Continue to substitute "RSA" where you see "DSA" throughout. Everything else should be the same.

Also Note: On Windows machines, the command prompt doesn’t understand the ~ which on Unix machines means "the home directory". Instead use %HOME% wherever you see the tilde.
```
      $ ssh-keygen -t dsa -f ~/.ssh/id_dsa -C "you@example.com"
      Generating DSA keys:  Key generation complete.
      Enter passphrase (empty for no passphrase): USE-A-PASSPHRASE
      Enter same passphrase again: USE-A-PASSPHRASE
      Your identification has been saved in ~/.ssh/id_dsa
      Your public key is:
      1024 35 [really long string] you@example.com
      Your public key has been saved in ~/.ssh/id_dsa.pub
      $
    
```

To use the key on other hosts you will be connecting from, copy the ~/.ssh/id_dsa key to the other hosts:

	  $ scp ~/.ssh/id_dsa you@another-box:.ssh/

However, it is probably better just to generate new keys for those hosts.

Make sure the public key is in the ~/.ssh/authorized_keys file on the hosts you wish to connect to. You can use a password authenticated connection to do this:
```
	  $ cat ~/.ssh/id_dsa.pub | ssh you@other-host 'cat - >> ~/.ssh/authorized_keys'
	  you@other-host's password: 
	  $
	
```
Note: If an older version of ssh is running on the remote host, you may have to use the ~/.ssh/authorized_keys2 file.

Note: If your local machine is Windows, try
```
	  C:\> type %HOME%/.ssh/id_dsa.pub | ssh you@other-host "cat - >> ~/.ssh/authorized_keys"
	  you@other-host's password: 
	  C:\>
	
```
Also note: If the remote server is Windows, you will probably want to use type instead of cat for the second half of your command.

Verify that DSA authentication works:

      $ ssh you@example.com
      Enter passphrase for DSA key 'you@example.com': ^D
      $

If you don’t get the prompt for your DSA key, then something has gone wrong. (One thing to check: verify that sshd_config on the server has been configured to do DSA authentication. Look for DSAAuthentication yes or get your system administrator to add it if necessary.)

Now that that works, you will want the passwordless part, right?

Start up ssh-agent. You can have it create a subprocess which inherits the SSH_AUTH_SOCK environment variable, or you can run it as a daemon.
Since I run gdm on Debian, ssh-agent is started automatically when I log in. If you don’t have this benefit, you can get it by putting the following line at the end of your .xsession file (You can substitute your window manager for gnome-session if that is what you use):
```
      ssh-agent gnome-session
      
```
Which basically means that ssh-agent starts up, creates a socket, sets up a couple of environment variables and then starts up gnome-session. That way all of the programs run in Gnome have access to the agent.

The above solution is the best one if you are logging in via GDM or another graphical login manager under *nix. However, if you login at the console, or want to use ssh-agent under Cygwin, you’ll have to use one of the following solutions.

If you want to, say, put it in your .profile, then you might try the following setup. In my .bash_profile, I have
```
	  SSHAGENT=/usr/bin/ssh-agent
	  SSHAGENTARGS="-s"
	  if [ -z "$SSH_AUTH_SOCK" -a -x "$SSHAGENT" ]; then
            eval `$SSHAGENT $SSHAGENTARGS`
	    trap "kill $SSH_AGENT_PID" 0
	  fi
	
```
(If you use csh or tcsh, see this note for the equivilent piece of code for your .login shell.)

This brings SSH_AUTH_SOCK and SSH_AGENT_PID as environment variables into the current shell.

The trap should kill off any remaining ssh-agent process. If it doesn’t, you won’t want the ssh-agent daemons sitting around, so you might want the following in your .logout:
```
	  kill $SSH_AGENT_PID
	
```
An alternative, provided by John Buttery, is
```
        if [ ${SSH_AGENT_PID+1} == 1 ]; then
          ssh-add -D
          ssh-agent -k > /dev/null 2>&1
          unset SSH_AGENT_PID
	  unset SSH_AUTH_SOCK
	fi
	
```
Finally, this solution from Joseph M. Reagle by way of Daniel Starin:
```
SSH_ENV="$HOME/.ssh/environment"

function start_agent {
     echo "Initialising new SSH agent..."
     /usr/bin/ssh-agent | sed 's/^echo/#echo/' > "${SSH_ENV}"
     echo succeeded
     chmod 600 "${SSH_ENV}"
     . "${SSH_ENV}" > /dev/null
     /usr/bin/ssh-add;
}

# Source SSH settings, if applicable

if [ -f "${SSH_ENV}" ]; then
     . "${SSH_ENV}" > /dev/null
     #ps ${SSH_AGENT_PID} doesn’t work under cywgin
     ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
         start_agent;
     }
else
     start_agent;
fi 
	
```
This last version is especially nice since it will see if you’ve already started ssh-agent and, if it can’t find it, will start it up and store the settings so that they’ll be usable the next time you start up a shell.

(Update 25 Sep 2007: Adam Piper pointed out that quoting anything that uses $HOME is necessary on Cygwin.)

Finally, time to type a password. The last one of this session, maybe.

      $ ssh-add ~/.ssh/id_dsa
      Need passphrase for /home/mah/.ssh/id_dsa (you@example.com).
      Enter passphrase:
      $

Now, you should test it:

      $ ssh you@example.com
      Last login: Tue Apr 25 13:40:21 1492 from europe.com
      Sun Microsystems Inc.   SunOS 5.7       Generic October 1998
      No mail.
      [you@example.com]$

Jubilation! It worked! Go forth and conquer! (If it doesn’t work, try chmod -R go-rw ~/.ssh on the server and try again.)

Ok, so, did it work or no? Let me know.

If you want to use this setup for editing remote files in emacs under Windows, check out my Tramp-on-NT page.

If you want to understand a little bit more about how all this works, read An Illustrated Guide to SSH Agent Forwarding.

Acknowlegements

27 Aug 2002: David Previti offered the working Windows-client to Windows-server key copy command as well as other Window-isms.

9 Aug 2002: Lloyd Smith suggested verifying that sshd_config is set up to do DSA authentication if it fails initially.

3 Jul 2002: David Newcomb offered the trick of putting trap 0 in the login/profile script.

18 May 2002: A more complete logout script from John Buttery. Also added tips on using RSA rather than DSA keys.

Using ssh-agent with ssh

Goals

Executive Summary

Related Pages

Methods

Acknowlegements