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Re: Remote TCP server through ssh tunnel [Was: Re: Server port]

From: Ken Raeburn
Subject: Re: Remote TCP server through ssh tunnel [Was: Re: Server port]
Date: Sun, 24 Oct 2010 17:50:31 -0400

On Oct 24, 2010, at 12:48, Lluís wrote:
> Which reminds me... can I use emacsclient to connect to a server behind
> a firewall?

This would probably be a more appropriate question for emacs-help, but....

> I've tried with this:
> server$ emacs --daemon --eval '(setq server-use-tcp t server-host "")'
> server$ netstat -nap | grep emacs
> tcp        0      0 *               LISTEN    
>   12078/emacs     
> server$ ssh -R 13502:localhost:13501 firewall
> client$ ssh -L 13501:localhost:13502 address@hidden
> # I already have an existing tunnel for ssh connections to server
> client$ scp server:.emacs.d/server/server /tmp/server
> client$ sed -i -e s/ /tmp/server
> client$ emacsclient -f /tmp/server -c
> Waiting for Emacs...
> *ERROR*: Display :0.0 can't be opened
> [Exit 1 ]

":0.0" is the name you generally use for connecting to the local X11 display on 
the machine running the X program.  That's what you've got on the client, and 
what emacsclient passes over to the emacs server process.  Unfortunately since 
that's running on a different machine, the local X display *there* -- if there 
even is one -- is not the one on your client machine.

If your SSH path through is forwarding X11 as well as TCP connections, you need 
to find the server-side name for that forwarded "display" and give it to 
emacsclient to pass through.  I think that'll work; I haven't tried it.  (I 
usually just run emacsclient on the server machine, over an SSH session, and 
let it pop up a window over that same SSH session.)

> client$ emacsclient -f /tmp/server -nw
> *ERROR*: Could not open file: /dev/pts/5

This can't work.  Emacs running on the server can't access the local tty device 
on your client machine.

> Do you have any clue of why is this happenning? According to the
> documentation, this should work flawlessly (at least with hosts on the
> same network):
> http://www.gnu.org/software/emacs/manual/html_node/emacs/emacsclient-Options.html

It's a bit misleading, yeah... evaluating an expression or editing a file in 
shared NFS space might make sense to trigger remotely like this, and the 
documentation was probably fine when multi-tty and daemon support hadn't come 
along, but now it probably needs to highlight these problems.

Please file a bug report (M-x report-emacs-bug RET) and/or submit a fix...

(And of course the Emacs driving the new window will be running on the server, 
so it can't access files on the client machine without jumping through hoops.  
If you really want to be able to do something on the local machine that 
triggers editing of files that are on the server machine, you might also look 
at the Tramp package, which will let you use your local Emacs, which may be 
faster.  But if you want to run subprocesses and such over there, or share an 
Emacs session there across multiple clients, yeah, getting a window popped up 
remotely may be best.)


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