emacs daemon and ssh agent forwarding

Question

In order to use ssh agent forwarding with emacs in daemon mode, running on a remote server, I've come-up with the following.

I have an alias for the client like so:

alias emacsclient="update_agent;/usr/local/bin/emacsclient -nw"

Which is calling the following which will update a soft-link to the current agent socket each time the emacs client is started (it is not used in any other circumstance):

alias update_agent='if [ -S "$SSH_AUTH_SOCK" ] && [ ! -h "$SSH_AUTH_SOCK" ]; then ln -sfv "$SSH_AUTH_SOCK" ~/.ssh/ssh_auth_sock; fi'

Providing I don't run 2 emacs clients simultaneously, and then kill the 2nd client before the 1st, this works fine.

In order to make the daemon use the updated socket I must set SSH_AUTH_SOCK to the soft link at daemon creation time.

I came up with 2 ways of doing this.

This way works:

 alias emacs="SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock /usr/local/bin/emacs --daemon"

This way does not work:

alias emacs="/usr/local/bin/emacs --daemon --eval '(setenv \"SSH_AUTH_SOCK\" \"~/.ssh/ssh_auth_sock\")'"

Both return the same value when getenv is called on SSH_AUTH_SOCK from within an emacs client.

But any attempt to use Dired on a remote directory eg C-x d /ssh:servername: or to ssh from M-x shell returns the following using the 2nd alias:

Failing with `Permission denied (publickey).`

My question is why doesn't the 2nd alias for emacs daemon allow ssh agent to be used for authentication?

Note - the use of the soft-link is necessary to make sure that not only the emacs daemon environment is updated, but all spawned M-x shells within the daemon are also updated. Otherwise it would be a trivial case of just updatng the daemon's SSH_AUTH_SOCK environment variable each time you connect to the daemon with setenv to the value of the current terminal's socket? Although if anyone has a better/different way of using ssh agent forwarding with emacs daemon, then I'm keen to hear it too!

References:

https://superuser.com/questions/180148/how-do-you-get-screen-to-automatically-connect-to-the-current-ssh-agent-when-re/424588#424588

https://chrisdown.name/2013/08/02/fixing-stale-ssh-sockets-in-tmux.html

http://ergoemacs.org/emacs/emacs_env_var_paths.html

Answer 1

Both return the same value when getenv is called on SSH_AUTH_SOCK from within an emacs client.

That looks unlikely. One would return ~/.ssh/ssh_auth_sock and the other would return /path/to/HOME/.ssh/ssh_auth_sock, no?

Have you tested whether SSH_AUTH_SOCK can actually contain ~ as a substitute for your HOME directory? Usually that would be expanded by a shell.