The service had stopped working for me since it did not have access to
the environment variable and thus was not reaching out to my yubi key.

howto/cumin.md

@@ -86,8 +86,13 @@ After=network.target
 
 [Service]
 ExecStart=/usr/bin/ssh -nNT -o ExitOnForwardFailure=yes -o BatchMode=yes -L 8080:localhost:8080 puppetdb-01.torproject.org
+Environment=SSH_AUTH_SOCK=/run/user/1003/gnupg/S.gpg-agent.ssh
 ```
 
+In the file above, change the path given to `SSH_AUTH_SOCK` so that it matches
+your user ID. This environment variable is necessary for the ssh command to be
+able to request the key from your yubi key.
+
 With this in place, make sure that systemd has loaded this unit file:
 
     systemd --user daemon-reload