GPG Agent Forwarding

18 May 2023 6:48 PM    nixos gnupg ssh macos
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As I’ve mentioned before, I have a bunch of Mac Minis running NixOS hosting various services. I have SSH access set up to them all with my SSH key, but I don’t have access to my GPG keys on those machines. And since these are the only NixOS machines that I have, things like updating my flake lockfile become annoyingly complicated:

[sam](~/nixos) $ ssh blinky
[sam@blinky](~) $ cd nixos
[sam@blinky](~/nixos) $ nix flake update
[sam@blinky](~/nixos) $ exit
[sam](~/nixos) $ scp blinky:nixos/flake.lock .
[sam](~/nixos) $ git commit -m "Update lock file" flake.lock
[sam](~/nixos) $ git push
[sam](~/nixos) $ ssh blinky
[sam@blinky](~) $ cd nixos
[sam@blinky](~) $ NIX_SSHOPTS="-t" nixos-rebuild boot --target-host pinky --flake .#pinky --use-remote-sudo

since I want to sign all the commits, which can only be done with access to my GPG keys, but I also can only update the lockfile and perform NixOS rebuilds on a NixOS machine.

Fortunately, GnuPG supports forwarding gpg-agent over SSH connections, just like you can forward your SSH agent with ForwardAgent. That’s what we’re doing today.

Following the instructions on the linked GnuPG wiki page, we first ensure that my public keys are on the remote machines:

[sam] $ gpg --export "Samasaur1" "OTHER_KEY_NAME" --armor > keys.gpg
[sam] $ scp keys.gpg blinky:
[sam@blinky](~) $ gpg --import keys.gpg

Then, take note of the location of the “extra” socket on the local machine (the machine you are SSHing from, which has the GPG secret keys on it):

[sam] $ gpgconf --list-dir agent-extra-socket

For me on my Mac, this is /Users/sam/.gnupg/S.gpg-agent.extra.

Then, on the remote machine (that which you are SSHing into, which does not have the GPG keys):

[sam@blinky] $ gpgconf --list-dir agent-socket

For me on blinky, pinky, and inky (the three NixOS Mac Minis), this is /run/user/1000/gnupg/S.gpg-agent. I believe this is true (with your user ID instead of 1000 — find it with id -u) of any systemd machine.

Next, modify your SSH config. I didn’t have an explicit config for any of these three machines, so I created an entry for each of them in my ~/.ssh/config file:

Host blinky
    RemoteForward /run/user/1000/gnupg/S.gpg-agent /Users/sam/.gnupg/S.gpg-agent.extra
Host pinky
    RemoteForward /run/user/1000/gnupg/S.gpg-agent /Users/sam/.gnupg/S.gpg-agent.extra
Host inky
    RemoteForward /run/user/1000/gnupg/S.gpg-agent /Users/sam/.gnupg/S.gpg-agent.extra

This is enough for it to work given some specific preconditions. If the directory /run/user/1000/gnupg/ exists on the remote machine but there is no S.gpg-agent socket even though that is the location that the GPG agent will look for a socket (which means that the agent is not running on the remote machine), connecting via SSH should forward the GPG agent and let you use your private keys.

To make it so that /run/user/1000/gnupg/ always exists, I added gpgconf --create-socketdir to my bashrc. I don’t have my NixOS machines set up with home-manager yet, and I haven’t started messing with the programs.bach.*shellInit options, so I just put that as the last line in my actual ~/.bashrc file.

So that we succeed even if S.gpg-agent exists, you can set StreamLocalBindUnlink yes in the sshd config. On NixOS, I simply modified my common.nix file in my flake:

  services.openssh = {
    enable = true;
    settings = {
      PasswordAuthentication = false;
      KbdInteractiveAuthentication = false;
    };
    extraConfig = ''
      StreamLocalBindUnlink yes
    '';
  };

I was also running into issues when I had multiple SSH connections to the same machine. If I SSHed in with connection 1, the agent would be correctly forwarded and I could use secret keys. If I then SSHed in with connection 2, I could use the agent on both connections. But if I closed connection 1 before connection 2¹, then the forwarded agent would no longer be accessible on the remote machine, and attempting to use gpg would start a local agent. This was solved by enabling SSH multiplexing (post to come).

Finally, there is an issue that I’m still having where if gpg is used on the remote machine when the agent is not forwarded (e.g., when physically logging in, or when SSHing in from a machine that does not forward its agent), so that the agent starts locally, it takes multiple connect-disconnects and some indeterminate amount of time for the forwarded agent to properly take priority.

Useful sources:

• https://wiki.gnupg.org/AgentForwarding (linked above as well)
• https://gist.github.com/TimJDFletcher/85fafd023c81aabfad57454111c1564d