X11 forwarding to IPv6 enabled host not working.

[Mark Ennis]

I have been experiencing a problem with using X11 forwarding on an IPv6
enabled host (both CentOS 4.3 and Feddora Core 5 in x86) when
X11UseLocalhost is off.

Having looked at the code and the previous discussion regarding
http://bugzilla.mindrot.org/show_bug.cgi?id=164 I think the problem is
due to the IPV6_V6ONLY code interacting poorly with the
DONT_TRY_OTHER_AF hack. Basically, on current Linux systems which
support the IPV6_V6ONLY socket option, the forwarded socket is created
for IPV6 only and then the DONT_TRY_OTHER_AF prevents an IPV4 listen
socket from also being created. Thus, only forwarding X11 connections
over IPV6 are then supported.

I would recommend addressing this by changing the DONT_TRY_OTHER_AF hack
to apply only where the the IPV6_V6ONLY socket option is not supported.
For example:

diff -r openssh-4.3p2/channels.c openssh-4.3p2-modified/channels.c
2816c2816
< #ifndef DONT_TRY_OTHER_AF
---
> #if defined(IPV6_V6ONLY) || !defined(DONT_TRY_OTHER_AF)

- Mark.