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From: larsi@ifi.uio.no (Lars Magne Ingebrigtsen)
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Subject: Re: Connecting to a newsserver with nntp.
Date: 22 Feb 1996 19:03:29 +0100
Organization: Dept. of Informatics, University of Oslo, Norway
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pp@pfawww.pp.se (Per Persson) writes:

> ffap.el which comes with 19.30 does this and has some variables for
> it, it's pretty much spread out so I guess you have to poke around in
> it a little yuorself. :-)

Hmn.  Here's the relevant code:

	(condition-case error
	    (progn
	      (delete-process
	       (open-network-stream
		"ffap-machine-p" nil host (or service "discard")))
	      t)
	  (error
	   (let ((mesg (car (cdr error))))
	     (cond
	      ;; v18:
	      ((string-match "^Unknown host" mesg) nil)
	      ((string-match "not responding$" mesg) mesg)
	      ((equal mesg "connection failed")
	       (if (equal (nth 2 error) "permission denied")
		   nil			; host does not exist
		 ;; Other errors mean host exists:
		 (nth 2 error)))
	      ;; Could be "Unknown service":
	      (t (signal (car error) (cdr error)))))))

So it doesn't really ping at all -- it just tries to connect to the
"discard" port on the machine.  If that fails, then it assumes the
machine is down.

Anyways, Gnus could do something similar.  

-- 
  "Yes.  The journey through the human heart 
     would have to wait until some other time."