PuTTY bug x11-ipv6-breakage

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We've had a couple of reports of X forwarding breakage related to IPv6.

One report implied that X forwarding in PuTTY broke right after we integrated the IPv6 code. Presumably the IPv6 changes have had some sort of side effect on the local network connection between PuTTY and the X server, but it isn't clear what since we haven't been able to reproduce it.

We received this report by email (<41E6F012.2040105@ressukka.net>). Details include:

• The symptom at the remote end is that X applications report "X connection to localhost:10.0 broken (explicit kill or server shutdown)".
• The X display in the PuTTY configuration is the PuTTY default (equivalent to localhost:0). The reporter found that requesting 127.0.0.1:0 instead worked, although not consistently. (This leads me to wonder whether "localhost" might have been resolving to an IPv6 address on the system in question.)
• The X server was xorg 6.8.1 from Cygwin.
• Windows version was XP SP2.

Another report (<4E2CCFB2C8CC5244939D17374DC28E0003444974@ing.com>):

• Windows XP Pro SP1
• Connecting via an HTTP proxy
• Same symptom ("explicit kill or server shutdown")
• Using 127.0.0.1:0 doesn't help at all
• X server doesn't matter (tried Exceed, Cygwin)
• Still happens with "latest development snapshot" (2006-04)
• Uninstalling IPv6 made the problem go away
• PuTTY Event Log entries (approximate):
  

  2006-04-12 12:44:08 Received X11 connect request from 127.0.0.1:39241
  2006-04-12 12:44:08 Opening X11 forward connection succeeded
  2006-04-12 12:44:08 Forwarded X11 connection terminated

If anyone is able to reproduce this or shed some light on possible causes, we'd be grateful.