netdev - Re: net-2.6.22 UDP stalls/hangs

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Message-Id: <20070423.133730.115642229.davem@davemloft.net>
Date:	Mon, 23 Apr 2007 13:37:30 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	akpm@...ux-foundation.org
Cc:	netdev@...r.kernel.org
Subject: Re: net-2.6.22 UDP stalls/hangs

From: Andrew Morton <akpm@...ux-foundation.org>
Date: Mon, 23 Apr 2007 13:27:19 -0700

> On Mon, 23 Apr 2007 13:18:10 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > From: Andrew Morton <akpm@...ux-foundation.org>
> > Date: Mon, 23 Apr 2007 13:07:34 -0700
> > 
> > > The interesting bit is:
> >  ...
> > > I think I saw the same problem maybe 1.5 weeks ago on this machine, but I
> > > didn't have time to investigate further.  So it's not some recent thing.
> > 
> > My initial reaction is that DNS responses are being lost or dropped
> > for some reason.
> 
> Plausible.   I'll try booting it with the ethernet unplugged.

That won't test the same scenerio.

If the network cable is unplugged, ARP responses won't arrive and
therefore sendmsg() calls will return with a host unreachable error.

The situation you need to recreate is specifically UDP packets getting
dropped.

The reason I wanted the tcpdump trace is so that we can see whether
the problem is UDP packets going out or going in which are being
mangled/dropped.

You don't need a hub to get a dump.  Instead you can run a caching
named on some other system, configure your FC6 box to use that system
for DNS via /etc/resolv.conf, then run tcpdump on the caching named
machine.
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