For a long time I’ve wanted an ssh-agent setup that would ask me before every use, so I could slightly more comfortably forward authentication over SSH without worrying that my session might get hijacked somewhere at the remote end (I often find myself wanting to pull authenticated git repos on remote hosts). I’m at DebConf this week, which is an ideal time to dig further into these things, so I did so today. As is often the case it turns out this is already possible, if you know how.
I began with a setup that was using GNOME Keyring to manage my SSH keys. This isn’t quite what I want (eventually I want to get to the point that I can sometimes forward a GPG agent to remote hosts for signing purposes as well), so I set about setting up gpg-agent. I used Chris’ excellent guide to GnuPG/SSH Agent setup as a starting point and ended up doing the following:
$ echo use-agent >> ~/.gnupg/options $ echo enable-ssh-support >> ~/.gnupg/gpg-agent.conf $ sudo sed -i.bak "s/^use-ssh-agent/# use-ssh-agent/" /etc/X11/Xsession.options $ sudo rm /etc/xdg/autostart/gnome-keyring-ssh.desktop
The first 2 commands setup my local agent, and told it to do SSH agent foo. The next stopped X from firing up ssh-agent, and the final one prevents GNOME Keyring from being configured to be the SSH agent, without having to remove
libpam-gnome-keyring as Chris did. After the above I logged out of and into X again, and could see
~/.gnupg/S.gpg-agent.ssh getting created and
env | grep SSH showing
SSH_AUTH_SOCK pointing to it (if GNOME Keyring is still handling things it ends up pointing to something like
[Update: Luca Capello emailed to point out this was a bad approach; there’s thankfully no need to do the last 2 commands that require root. #767341 removed the need to edit Xsession.options and you can prevent GNOME Keyring starting on a per user basis with:
(cat /etc/xdg/autostart/gnome-keyring-ssh.desktop ; echo 'X-GNOME-Autostart-enabled=false') > \ ~/.config/autostart/gnome-keyring-ssh.desktop
After this it turned out all I need to do was
ssh-add -c <ssh keyfile>. The
-c says “confirm use” and results in the confirm flag being appended to the end of
~/.gnupg/sshcontrol (so if you’ve already done the ssh-add you can go and add the confirm if that’s the behaviour you’d like).
Simple when you know how, but I’ve had conversations with several people in the past who wanted the same thing and hadn’t figured out how, so hopefully this is helpful to others.