Re: [2.6.24.3][net] bug: TCP 3rd handshake abnormal timeouts

On Sat, Mar 15, 2008 at 09:47:24AM +0100, Gabriel Barazer wrote:
Hi

Thanks for the netdev Cc, I didn't know where to write to the "network
guys".

except I just noticed I got it wrong: it's netdev@xxxxxxxxxxxxxxx, and
I omitted the "vger" part. That's what is expected when posting before
caffeine :-)

Feel free to repost the whole issue overthere (along with your new tests)
if you don't get useful replies in a few days.

By the way thanks for replying. It's hard to explain and describe a
problem when you know people will ask you hundreds of questions related
to application-level problems, or not reply because web/mysql problems
are so common and generally not related to any kernel issue.

What caught my attention was the usual "3s delay", which is purely TCP
and application-independant.

On 03/15/2008 7:58:49 AM +0100, Willy Tarreau <w@xxxxxx> wrote:

You should carefully check the the SYN-ACK received by the client has a
correct checksum ("cksum OK" in tcpdump output). It would be possible
that for some reason, something on the network randomly corrupts it.

I used to use TCP offloading one time, and by the way never had a
problem with it. Besides just to be sure, I have been able to reproduce
the problem without any offload engine enabled (= not compiled into the
kernel, mainly because it seems to hang the kernel at boot in 2.6.24.3).
So I assume that is not the problem

OK

I use wireshark to analyse my pcap files and it says the checksum is
correct on all packets.

OK

Also, you say you have netfilter with conntrack. Is this on the client ?
If so, you should try disabling it to rule out any possible bug in the
connection tracking.

I have the conntrack on both the client and server, and unfortunately
can't disable it now on the client (I use it only for the REDIRECT
target on a precise destination address and port, not MySQL related),
however I will test today and disable it on the server, after I get some
sleep (although I think the issue is on the client).

I'm sure it's a client issue too, that's why it would be reasonable to
be able to try without conntrack. Can't you use a TCP proxy instead of
REDIRECT ? Also, you said that you also noticed the same behaviour in
other environments, maybe there you can disable conntrack ?

Willy

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