X forwarding: trying to forward to busy local port

[Lars Kruse]

Hi to all of you,

I would like to summarize the current state of the problem as described
in http://permalink.gmane.org/gmane.network.openssh.devel/13345.

If the openssh server is running in ipv4/ipv6 mode ("AddressFamily
Any"), then pseudo-random "unable-to-connect-to-display" errors occour
for clients connecting via ssh for X-forwarded remote sessions.

For now the only workaround would be, to disable ipv6 support for
openssh daemons used for X-forwarding.

From my point of view, there are two ways to solve the root of this
problem:

1) improved "is this port usable on all interfaces?"-detection
ipv4/ipv6 mixed openssh daemons should behave like pure ipv4 daemons:
unusable DISPLAY settings may never be offered to clients


2) avoid to randomly allocate critical ports
the openssh daemon may never allocate ports for running X-sessions which
are in the range, that is used for new X-forwarding connections (maybe
6000..6100).


From my point of view, this issue is a highly irritating one, as it is
very hard to track down the source of this seemingly random
"unable-to-connect-to-display" problem. If the previously described
short-term-workaround would not be available, then our current
X-session-setup would have to be replaced by a more reliable, but less
preferable solution. 
So I am very glad, that you helped me to find this workaround ...

But how can this issue be solved without loosing ipv6 compatibility?

thanks and regards,
Lars