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Forwarding gpg-agent Over SSH

2021-12-27 | #git #gpg #unix #linux #ssh

Intro

Basic notes from setting up gpg-agent forwarding[1] between a MacOS and Debian Linux system.

1: https://wiki.gnupg.org/AgentForwarding

Client System

Assuming system is running MacOS install gpg[2] and pinentry (GPG password prompt tool) using Homebrew[3],

2: https://gnupg.org/

3: https://brew.sh/

brew install gnupg pinentry-mac

Edit `~/.gnupg/gpg-agent.conf` and make sure that ${HOME} is the full path to your users home dir, eg `/Users/$USER`, this must match what's setup in `RemoteForward` later,

default-cache-ttl 600
max-cache-ttl 7200
enable-ssh-support
pinentry-program /usr/local/bin/pinentry-mac
extra-socket $HOME/.gnupg/S.gpg-agent.extra

`~/.gnupg/gpg.conf` (where `$GPG_KEY` is the fingerprint of the GPG key to use)

default-key $GPG_KEY
personal-digest-preferences SHA512
cert-digest-algo SHA512
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
no-emit-version

Edit `~/.ssh/config` and setup a `RemoteForward`, this is important since the forward is required in order to communicate over a socket. Make sure the `$UID` and `$USER` on the remote and local system are set accordingly, as these will differ depending on the OS and login.

Also double check the socket names that they match up from `~/.gnupg/gpg-agent.conf` and `~/.ssh/config`.

`~/.ssh/config`

Host $REMOTE_IP
   ForwardAgent yes
   Compression yes
   ForwardX11 yes
   RemoteForward /run/user/$UID/gnupg/S.gpg-agent /Users/${USER}/.gnupg/S.gpg-agent.extra

Remote System

Update `/etc/ssh/sshd_config` (or wherever it is depending on your OS) configuration to include `StreamLocalBindUnlink yes` and restart `sshd` This option will,

> Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and StreamLocalBindUnlink is not enabled, ssh will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file.

Troubleshooting gpg-agent

Sometimes `StreamLocalBindUnlink yes` doesn't work and the client agent needs restarted and sockets removed from the remote system,

Restart the agent on the client system,

gpg-connect-agent reloadagent /bye

Remove the socket on the remote system,

rm /run/user/1000/gnupg/S.gpg-agent

ssh back into the remote system and gpg-agent forwarding should work again.

References

Remote gpg-agent Via ssh Forwarding[4]

4: https://web.archive.org/web/20190423113837/https://www.isi.edu/~calvin/gpgagent.htm