[Bug 160206] [gif] gifX stops working after a while (IPv6 tunnel)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=160206

--- Comment #8 from Paul Scott <pscott at skycoast.us> ---
It may not be specific to IPFW, after all, although IPFW does seem to
exacerbate the problem.

Recently, I've noticed that the FreeBSD 12.2-RELEASE-p1 GENERIC (and64) droplet
has had issues with both ICMP and ssh over IPv6.

The following occurs even with IPFW disabled:

ICMPv6 and ssh -6 setup are working fine, and then suddenly they stop working.
When this happens, a ping6 will never succeed no longer how long it runs, and
an ssh -6 will ultimately fail. If I interrupt (Ctrl-C) the process and try
again it MAY immediately start working and continue to work indefinitely.
Curiously, I can have two ping6 processes running simultaneously and one will
indefinitely fail and the other will indefinitely succeed.

When ssh -6 setup succeeds the session will remain responsive indefinitely
without any failure even if another ssh -6 setup fails simultaneously.

Note that IPv4 experienced none of these issues. Further, this isn't a DNS
failure since the name is properly resolved, and using an IP address instead of
a name produces the same results.

CentOS linux systems in the same data center location experience none of these
problems and IPv6 is 100% reliable there. The FreeBSD droplets all experience
the problem, however.

Given that a non-FreeBSD OS in the same data center has none of these problems
and that other people -- witnessed by this PR and quite a few google search
results -- have had similar IPv6 issues on FreeBSD over various network
adapters, it's extremely likely this is a FreeBSD or FreeBSD subsystem defect.

I was hoping to move all my systems from CentOS to FreeBSD but these IPv6
issues will, sadly, prevent me from making that transition.

If there is anything I can do to help bring about a resolution quickly, please
let me know. Unfortunately, as this problem has a long history, on the order of
years, I'm not hopeful it will get solved soon.

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