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Troubles with NFS & ip_conntrack: packets go to wrong mac

To: netdev@xxxxxxxxxxx
Subject: Troubles with NFS & ip_conntrack: packets go to wrong mac
From: Patrick McHardy <kaber@xxxxxxxxx>
Date: Wed, 12 Feb 2003 18:05:15 +0100
Cc: Netfilter Development Mailinglist <netfilter-devel@xxxxxxxxxxxxxxxxxxx>
Sender: netdev-bounce@xxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.2.1) Gecko/20021226 Debian/1.2.1-9
I've been experiencing strange problems with nfs and ip_conntrack for a while now, unfortunately noone so far was able to help. The problem occurs when ip_conntrack
is loaded on the nfs server. nfs reads hang and the clients start logging

UDP: short packet: 0/120 to
UDP: short packet: 28562/120 to

There are two ways to make it work: 1. remove ip_conntrack or 2. set mtu to 1484 on the nfs server. One suspicion was ip_conntrack breaking udp path mtu discovery since it seems to defragment packets with DF|MF and refragment them (with possibly different mtu) at POSTROUTING. This doesn't seem to be the problem, but i noted the
nfs server sends out fragments with wrong destination mac.

This is a packet captured on
0:e0:7d:74:ab:cc 0:e0:7d:74:ab:cd 0800 1514: > (frag 44777:1480@4440+) (ttl 64, len 1500, bad cksum 2294!)

This happens every 1-30 seconds. I never saw more than one misdirected fragment per packet.

Relevent parts from neighbour table: dev eth0 lladdr 00:e0:29:3c:c1:c9 nud reachable dev eth0 lladdr 00:e0:7d:74:ab:cd nud reachable

On packets for show up. Both clients time out during reassembly. I placed some printks though the netfilter code and ip_output.c but couldn't find any further pointers. I looked for broken checksums (something seems to alter the ip after checksumming) in ip_finish_output2, but everything is ok there. I can't see anything netfilter related touching packets after that.
Any help and/or pointers where to look further would be appreciated.


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