netdev - Re: IPv6 path MTU discovery broken

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Date:	Sat, 28 Sep 2013 23:19:49 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Steinar H. Gunderson" <sgunderson@...foot.com>
Cc:	netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: IPv6 path MTU discovery broken

On Sat, Sep 28, 2013 at 10:51:31PM +0200, Steinar H. Gunderson wrote:
> On Sat, Sep 28, 2013 at 10:33:18PM +0200, Hannes Frederic Sowa wrote:
> >> Could this be related somehow to the packets coming from 2001:67c:29f4::31,
> >> while the default route is to a link-local address? (An RPF issue?) This used
> >> to work (although it was often flaky for me) in 3.10 and before. I can't
> >> easily bisect, though, as I don't boot this machine too often.
> > This looks like a bug and should definitely get fixed. There should be
> > no RPF issue. May I have a look at your /proc/net/ipv6_route?
> 
> Hi,
> 
> I removed all the “weird” routes, and confirmed it fixed the problem.
> However, upon adding them back again, the problem was still gone
> (despite flushing the route cache).
> 
> This means that the issue has gone back to being intermittent, which is of
> course the worst kind of bug to trace down. :-) I'll dump
> /proc/net/ipv6_route and send you once I see the bug manifest itself again,
> OK?

Yes, that would be very helpful.

Also, you can try to churn up your bgp connection a bit so that the fib
serial numbers get incremented a lot (drop and install new routes). When
tcp_ipv6 processes the icmp errors it will drop the in-socket cached
routing entry then and will reinstall a relookuped one.  This is my only
suspect currently. If that would help to reproduce the problem the suspects
would be the changes in the next-hop selection. Sorry, no other idea
currently.

Thanks,

  Hannes

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