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To this case,
if I properly disable ipv6, according to:
https://access.redhat.com/solutions/8709
respectively in /etc/sysctl.conf:
> net.ipv6.conf.eth0.disable_ipv6 = 1
It works for me without any problems.


If I leave ipv6 enabled and remove ipv6 addresses from all inteusing 
# /sbin/ifconfig <interface> inet6 del <ipv6address>/<prefixlength>
I'm also unable to reproduce your problem.


Using current version of openssh:
openssh-5.3p1-104.el6.x86_64

Can you specify your reproducer how to get to your state or retest it with current version?

I can confirm this very same problem on Fedora21 with openssh-6.6.1p1-11.1.fc21.x86_64 -- I had to add "AddressFamily inet" into sshd_config after disabling IPv6 via the /etc/sysctl.conf method.

This same openssh problem has already been around at least five years, also has Debian and Ubuntu bugs issued to it (the latter also has a suggested patch):

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=595014
https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/882878

Now I can reproduce this behaviour on my Fedora 21, but still not on RHEL 6

# echo "net.ipv6.conf.eth0.disable_ipv6 = 1" >> /etc/sysctl.conf
# service sshd restart

and from different shell
# ssh -X f21
X11 forwarding request failed on channel 0
# rpm -q openssh
openssh-6.6.1p1-11.1.fc21.x86_64


Yes, there is more information about this problem even in upstream bugzilla [1], [2] and [3], but no final resolution.

It doesn't look like there will be some resolution in upstream. The solution in your linked patch [4] has some issues, namely CVE-2008-1483, as stated in [3], so I think the solution will be one of these:
1) Configure your IPv6 address or disable it (correctly).
2) Configure your openssh (to use or not to use IPv6) according to your network configuration.


[1] https://bugzilla.mindrot.org/show_bug.cgi?id=1457
[2] https://bugzilla.mindrot.org/show_bug.cgi?id=2143
[3] https://bugzilla.mindrot.org/show_bug.cgi?id=1356
[4] https://build.opensuse.org/package/view_file/openSUSE:Factory/openssh/openssh-6.5p1-X_forward_with_disabled_ipv6.patch?rev=1c09c84b8dda320105cf7b59928951c4

Development Management has reviewed and declined this request.
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