tramp and ssh-agent / ssh-add

[Roland Winkler <Roland.Winkler@physik.uni-erlangen.de>]

Usually when I have emacs running on a remote machine, I do this as
a subprocess of ssh-agent. Then when I establish ssh connections
via tramp, I want to make sure that I type my password only once via
ssh-add. What is the cleanest way to do that? Is there some support
from tramp for that? Right now, my home-made solution uses a
defadvice for tramp-open-connection-rsh, see the code below.
However, if tramp-open-connection-rsh is the right entry point for
what I want to do, it seems to me that a hook might be helpful. Or
am I missing something else?

Thanks,

Roland


(defun ssh-add-p ()
  "Return t if ssh identities known."
  (with-temp-buffer
    (call-process "/usr/bin/ssh-add" nil t nil "-l")
    (goto-char (point-min))
    (not (search-forward "The agent has no identities." nil t))))

(defun ssh-add (&optional password)
  "Add ssh passphrase."
  (interactive)
  (if (ssh-add-p)
      (if (interactive-p) (message "Passphrase already entered."))
    (with-temp-buffer
      (insert (or password (read-passwd "Passphrase: ")) "\n")
      (let ((process-environment (copy-alist process-environment)))
        (setenv "DISPLAY") ;; unset DISPLAY
        (call-process-region (point-min) (point-max)
                             "/usr/bin/ssh-add" t t nil))
      ;; Massage output
      (goto-char (point-min))
      ;; suppress "Enter passphrase for ...: "
      (search-forward ": " nil t)
      (let ((beg (point)))
        (goto-char (point-max))
        (skip-chars-backward " \t\n")
        (message "%s" (buffer-substring-no-properties beg (point)))))))

(defadvice tramp-open-connection-rsh (before ssh-add activate)
  "First call `ssh-add'."
  (if (string= "ssh" (ad-get-arg 1))
      (ssh-add)))