Subject: tcpdump reports invalid checksums when using ex(4) hardware checksums
To: None <>
From: Rui Paulo <>
List: tech-net
Date: 10/04/2004 23:14:29

When I enable ex(4) hardware checksums, tcpdump (and all programs that
use bpfilter) will report bad checksums:
23:07:59.772771 > icmp: echo request seq 0 (ttl
255, id 51117, len 84, bad cksum 0 (->ffff0721)!)
23:07:59.773806 > icmp: echo reply seq 0 (ttl 255,
id 15820, len 84)

I wonder, is this normal ? It should be normal if the hardware
checksum routines performs checksum calculation after the layer
bpfilter works on. But I just wanted to make sure. 

BTW, is this ex(4) specific or every driver that performs hardware
checksums has this minor problem ?

Thanks in advance.

Rui Paulo                          "Simplicity is the ultimate sophistication."
                                      -- Leonardo da Vinci