What does #ffff## in xauth do (apart from breaking my X forwarding...)

Matthijs Kooijman matthijs at stdin.nl
Wed May 20 07:14:23 PDT 2009

Hi all,

I've been struggling with getting my X forwarding over SSH working. After some
time, I found that there were two entries in my .Xauthority file that have
#ffff## as a hostname, which break forwarding.

The relevant entries in my .Xauthority files were:
  #ffff##:  MIT-MAGIC-COOKIE-1  <snipped>
  #ffff##:  XDM-AUTHORIZATION-1  <snipped>
  localhost/unix:10  MIT-MAGIC-COOKIE-1  <snipped>

From the strace output, it seems that xlib always sends the
XDM-AUTHORIZATION-1 entry, even when DISPLAY=localhost:10.0.

From the xauth manpage, I couldn't find what these #ffff## entries are
supposed to do, let alone why they break my X/ssh forwarding. I'm apparently
not the first to spot this, since I've spotted a script [1] which deliberately
strips these entries from a .Xauthority file for this very reason.

Can anyone shed some light on what happens here?



(Please CC me, I'm not subscribed)
[1]: http://isg.ee.ethz.ch/tools/isgtc/index.cgi?page=module_source;module=xauthclean;source=xauthclean

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