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Agent Configuration: Linux

In order to use an OpenPGP smart card for SSH, the SSH_AUTH_SOCK environment variable needs to point at a GnuPG agent (gpg-agent) socket. Getting this right can be tricky.

Debian 9 (stretch) and Xfce

If you use Xfce on Debian stretch or newer (including buster), getting this working is actually quite straightforward.

TL;DR (quick setup):

xfconf-query -c xfce4-session -p /startup/ssh-agent/enabled -n -t bool -s false
echo use-standard-socket >> ~/.gnupg/gpg-agent.conf
echo enable-ssh-support >> ~/.gnupg/gpg-agent.conf

Xfce starts both gpg-agent and ssh-agent instances for you at login, but you want to avoid the standalone ssh-agent and instead configure just gpg-agent for SSH. In fact, the gpg-agent is likely started for you as a systemd user service: you probably want to keep that setup.

To make sure Xfce doesn't start ssh-agent for you, you need to disable it in your settings. Unfortunately this isn't presented anywhere in the GUI that I can find, but can be easily disabled on the command-line:

xfconf-query -c xfce4-session -p /startup/ssh-agent/enabled -n -t bool -s false

Then you need to tell gpg-agent to always enable its SSH support, and ideally for it to use a standard (stable) socket path rather than something random in /tmp. The latter is particularly useful if you need to restart gpg-agent for any reason, as the environment variables get baked into your desktop session, and is the default in GnuPG 2.2 which is in buster.

To set this up, you need to add the following two lines to your ~/.gnupg/gpg-agent.conf file, creating it if it doesn't exist:

use-standard-socket
enable-ssh-support

You can omit the first line on Debian buster as it is the default there.

You then need to logout and log back into your session for the changes to take effect.

If your Xfce setup uses a systemd user session, as is the default on Debian stretch, you should see something like the following:

$ echo $SSH_AUTH_SOCK
/run/user/1000/gnupg/S.gpg-agent.ssh
$ gpgconf --list-dirs agent-ssh-socket
/run/user/1000/gnupg/S.gpg-agent.ssh
$ systemctl --user show-environment | grep SSH_AUTH_SOCK
SSH_AUTH_SOCK=/run/user/1000/gnupg/S.gpg-agent.ssh

If so, you're all set!