How do you get screen to automatically connect to the current ssh-agent when re-attaching to an existing screen?

Question

If you start a screen session while ssh-agent is running (from ssh -A agent forwarding), accessing ssh-agent works fine. However, if you detach from that session, log out, log in again (with ssh-agent forwarding), and re-attach to your screen session, ssh-agent access doesn't work.

How can this be fixed?

Answer 1

1) In your SSH rc script (~/.ssh/rc) you will set up a symbolic link from a canonical location to the "current" SSH_AUTH_SOCK. Here's how I do it in bash (content of ~/.ssh/rc):

#!/bin/bash
if test "$SSH_AUTH_SOCK" ; then
    ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
fi

(and make sure to chmod 755 ~/.ssh/rc). The "test" is just to prevent an error from displaying if you aren't running ssh-agent (ie you ssh without -A). The second half of that command sets up a symlink in a canonical location that updates itself to the "real" SSH_AUTH_SOCK at login time. This is independent of using a shell in ssh or calling a command directly, works also with "ssh -t screen -RRD".

Note: the existence of ~/.ssh/rc changes the behaviour of sshd. Notably, it will not call xauth. See man sshd for more information, and how to fix this.

Also, you should not use "-v" with ln as soon as it will break rsync-over-ssh with following diagnostics:

$ rsync -n addr.maps.dev.yandex.net: .
protocol version mismatch -- is your shell clean?
(see the rsync man page for an explanation)
rsync error: protocol incompatibility (code 2) at compat.c(173) [Receiver=3.0.7]

2) In your .screenrc, you just need to override the SSH_AUTH_SOCK to the canonical location:

setenv SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock

Note that you use setenv no matter what shell you use; I think that setenv is screen syntax, not the shell.

Solution originally adapted from this post, which doesn't work, but has the right idea.

Answer 2

"ssh -t some.machine screen -R" won't run bash and therefore won't run the .bash_profile script where the symlink is created.

You could try: ssh -t some.machine bash -c "screen -R"

(assuming you are using bash as your shell of course)

Edit: That "answer" is actually a comment on the first answer given above :)

Answer 3

I think you need autossh. I've been using it for years now, and combined with screen it makes all my terminal sessions completely portable and transparent. I simply close lappy, move to new location, open lappy and all my screens and nested screens connect automatically. I don't even think about it anymore.

http://www.linux.com/archive/feature/134133

is the basics... I ruby'd up a lil script to automate the process in my .screenrc for a given host. ( also does my ssh forwarding, so in all these different places i can tunnel my connection through my servers )

in the autossh distro there should be a program called rscreen ( and.. there is! )

#!/bin/sh                                                                       
#
# sample script to use autossh to open up a remote screen
# session, or reconnect to an existing one. 
#
# $Id: rscreen,v 1.4 2002/05/07 17:54:13 harding Exp $
#
if [ "X$1" = "X" ]; then
    echo "usage: `basename $0` <host>"
    exit 1
fi

if [ "X$SSH_AUTH_SOCK" = "X" ]; then
    eval `ssh-agent -s`
    ssh-add $HOME/.ssh/id_rsa
fi

#AUTOSSH_POLL=600
#AUTOSSH_PORT=20000
#AUTOSSH_GATETIME=30
#AUTOSSH_LOGFILE=$HOST.log
#AUTOSSH_DEBUG=yes 
#AUTOSSH_PATH=/usr/local/bin/ssh
export AUTOSSH_POLL AUTOSSH_LOGFILE AUTOSSH_DEBUG AUTOSSH_PATH AUTOSSH_GATETIME 

autossh -M 20004 -t $1 "screen -e^Zz -D -R"

This should help with ssh/screen problems

Finally, in order to keep my ssh-agent running , I use keychain, since I am kind of a shell head... I think OSX has something available to keep your agent around ...

Answer 4

I think this works as a simplification of @sandip-bhattacharya's answer. Put this in your ~/.bashrc file, and run the export command in any currently running screen sessions.

if [ -S "$SSH_AUTH_SOCK" ] && [ ! -h "$SSH_AUTH_SOCK" ]; then
    ln -sf "$SSH_AUTH_SOCK" ~/.ssh/ssh_auth_sock
fi
export SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock

That reads "if $SSH_AUTH_SOCK is a socket (-S) and not a symbolic link (! -h), create a new symbolic link at the known path. In all cases, redefine SSH_AUTH_SOCK to point to the known path.

The ! -h avoids creating a circular reference if you run this multiple times.

Also, if you use byobu, it does this automatically, without needing to edit any config files.

The only bug I've found in this (byobu has it too) is if you open a second ssh -A or ForwardAgent connection it will overwrite the first socket, and if you close the second connection before the first, you will lose your only good socket.

Answer 5

Here's the method I use:

SOCK=$(sudo cat /proc/$(pgrep -f "screen -(r|DR)")/environ | tr "\0" "\n" | grep SSH_AUTH_SOCK) ; eval $SOCK ; export SSH_AUTH_SOCK
DISP=$(sudo cat /proc/$(pgrep -f "screen -(r|DR)")/environ | tr "\0" "\n" | grep DISPLAY) ; eval $DISP ; export DISP

I usually set up an alias or a shell function with theses commands:

function ssh-screen-auth() {
  SOCK=$(sudo cat /proc/$(pgrep -f "screen -(r|DR)")/environ | tr "\0" "\n" | grep SSH_AUTH_SOCK)
  eval $SOCK
  export SSH_AUTH_SOCK
  DISP=$(sudo cat /proc/$(pgrep -f "screen -(r|DR)")/environ | tr "\0" "\n" | grep DISPLAY)
  eval $DISP
  export DISPLAY
}

You may have to adapt the regular expression 'screen -(r|DR)' to the exact commands you use to reattach your screen.

• The first line reads the SSH_AUTH_SOCK environment variable in the process space of the "screen -r" command you just typed and updates the value in your current shell.
• The second line is necessary if you use "ssh -X" to forward X11 connections: it updates the DISPLAY variable in the same way.

A caveat with my method: things may go wrong if there is another "screen" command running on the computer.

Answer 6

I commonly keep long term (6+ months) sessions running at my workplace on different servers. So repeatedly reattaching and having a viable ssh forwarding agent has been problematic. This is what I set up on my systems:

if [ -z "${STY}" -a -t 0 -a X${USER} = Xmyusername ]; then
    reattach () {
        if [ -n "${SSH_AUTH_SOCK}" ]; then
            ln -snf "${SSH_AUTH_SOCK}" "${HOME}/.ssh/agent-screen"
            SSH_AUTH_SOCK="${HOME}/.ssh/agent-screen" export SSH_AUTH_SOCK
        fi
        exec screen -A -D -RR ${1:+"$@"} ;
    }

    screen -wipe
    echo 'starting screen... (type Cntl-C to abort)'
    sleep 5 && reattach
fi

If I just log in to the remote server without starting/reattaching screen, then there will be two "sockets", one in use by screen and another by the new shell. There shouldn't be two "startup" sessions, but a second session could still be started using reattach -S new; in this situation, the agent would be shared with the ~/.ssh/agent-screen value. To get a working forwarding agent back, then I would detach, log back in. The X${USER} = Xmyusername ensures that the code won't be called through sudo on the same server.

Answer 7

I am using a variation of what @apinstein is using for my .bashrc.

case "$TERM" in
    screen)
           export SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock
        ;;
         *)
           if [[ -n "$SSH_AUTH_SOCK" ]]; then
               ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
           fi
        ;;
esac

This works for all the apps running in my screen session. This would work for all new shells in your screen session. For existing shells you need to run export SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock on the host shell to make it work.

P.S. Sorry for adding this as an independent answer, whereas it just built upon @apinstein's answer. Had to do this as comments in stackoverflow do not support code blocks.

Answer 8

I tried this simple one liner as suggested on Let's make screen and ssh-agent friends and it works for me.

First Time login to Target.Needs to be done only once.

ssh -o StrictHostKeyChecking=no -C <userid>@<server>

Launch Screen for the First time..Needs to be done only once.

eval `ssh-agent`; /usr/bin/screen -D -R -h 10000
ssh-add

If detached or disconnected, use this command to login subsequently to connect to the exiting screen.

ssh -o StrictHostKeyChecking=no -C -t <userid>@<server> ssh-agent /usr/bin/screen -D -R -h 10000

Answer 9

These are all really good answers I do it slightly differently. After I start a new ssh session and reattach screen, I reset the SSH_AUTH_SOCK environment variable based on the contents of the root bash environment. I only require ssh-agent access occasionaly when I am using svn so I just reset the SSH_AUTH_SOCK as required in these shells.

This uses the proc file system so is linux specific. I have only tested this on a headless linux box that is only accessed by me, it might take some tweaking to get it to work on other environments.

To reset SSH_AUTH_SOCK (this could be made an alias).

$ . ~/bin/screen_auth.sh

screen_auth.sh looks like this

# Find the pid of putty's bash shell
tty=`who | awk '/[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/ { print substr($2, 5) }'`
pid=`ps -t $tty | grep bash | awk '{print $1}'`
# Find the SSH_AUTH_SOCK variable in its enviornment
auth_sock=`xargs --null --max-args=1 echo < /proc/$pid/environ | grep SSH_AUTH_SOCK`
eval "export $auth_sock"