Comment 4 for bug 31273

I've seen the same problem and can reproduce it. It
occurs when UDP packets are transmittted. I used
tcpdump's -X option to analyze the packets and
all fields are correct except for the UDP checksum.
In one test, I sent a single UDP packet between two
machines and found that only the checksum differed
(being wrong in the trace on the transmitting machine).

While the problem may be in tcpdump or a library it
uses, it could also be a kernel bug that results in a
incorrect copy of the packet being sent to tcpdump:
I verified that all the fields in a transmitted packet are
correct except for the UDP checksum - I compared
ithe traces for the transmitted packet versus the
received packet.

I've enclosed the software I used in the attachment:
a couple of very short programs (I'm running ubuntu
dapper).