Using ssh-agent with ssh

http://mah.everybody.org/docs/ssh

Using ssh-agent with ssh

    p::docs
    Using ssh-agent with ssh

    by Mark A. Hershberger (weblog)

    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.

    Related Pages

        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 "[email protected]"
                  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] [email protected]
                  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 [email protected]
                  Enter passphrase for DSA key '[email protected]': ^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 ([email protected]).
                  Enter passphrase:
                  $
               

        Now, you should test it:

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

        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.