Using ssh-agent with ssh

Back in 2002, 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 from your machine (local) to a remote machine (remote) without typing in a password.

Executive Summary

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

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.

1. Generate the keys. Do this on the host that you want to connect from — your local computer. Do not do this over the internet. 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.

   you@local$ 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
   you@local$
   

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

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

   However, it is probably better just to generate new keys for those hosts.
3. 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:

   you@local$ cat ~/.ssh/id_dsa.pub | ssh you@remote 'cat - >> ~/.ssh/authorized_keys'
   you@remote's password:
   you@local$
   

   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.
4. Verify that DSA authentication works:

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

   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?

1. 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. Kyle Amon has provided the following bit for a .bash_profile:

   #
   # setup ssh-agent
   #
   
   
   # set environment variables if user's agent already exists
   [ -z "$SSH_AUTH_SOCK" ] && SSH_AUTH_SOCK=$(ls -l /tmp/ssh-*/agent.* 2> /dev/null | grep $(whoami) | awk '{print $9}')
   [ -z "$SSH_AGENT_PID" -a -z `echo $SSH_AUTH_SOCK | cut -d. -f2` ] && SSH_AGENT_PID=$((`echo $SSH_AUTH_SOCK | cut -d. -f2` + 1))
   [ -n "$SSH_AUTH_SOCK" ] && export SSH_AUTH_SOCK
   [ -n "$SSH_AGENT_PID" ] && export SSH_AGENT_PID
   
   # start agent if necessary
   if [ -z $SSH_AGENT_PID ] && [ -z $SSH_TTY ]; then  # if no agent & not in ssh
     eval `ssh-agent -s` > /dev/null
   fi
   
   # setup addition of keys when needed
   if [ -z "$SSH_TTY" ] ; then                     # if not using ssh
     ssh-add -l > /dev/null                        # check for keys
     if [ $? -ne 0 ] ; then
       alias ssh='ssh-add -l > /dev/null || ssh-add && unalias ssh ; ssh'
       if [ -f "/usr/lib/ssh/x11-ssh-askpass" ] ; then
         SSH_ASKPASS="/usr/lib/ssh/x11-ssh-askpass" ; export SSH_ASKPASS
       fi
     fi
   fi
   

   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:

   ssh-add -D
   ssh-agent -k
   

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

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

3. Now, you should test it:

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

   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.