UDP checksum calculated incorrectly

[kosma]

The following sample program generates incorrect UDP checksum.

import umit.umpa
import umit.umpa.protocols

ethernet = umit.umpa.protocols.Ethernet(
    src="00:11:09:81:ec:dc",
    dst="00:1e:e5:6d:7b:42",
)
ip = umit.umpa.protocols.IP(
        src="192.168.1.234",
        dst="156.17.70.157",
)
udp = umit.umpa.protocols.UDP(
        srcport=1234,
        dstport=1234,
)
payload = umit.umpa.protocols.Payload(
        data="UMPAPA\n"
)

packet = umit.umpa.Packet(ethernet, ip, udp, payload)
socket = umit.umpa.SocketL2()
socket.send(packet)

Wireshark complains about:

User Datagram Protocol, Src Port: 1234 (1234), Dst Port: 1234 (1234)
(...)
Checksum: 0x5129 [incorrect, should be 0x50f2 (maybe caused by "UDP checksum offload"?)]

[getxsick]

What if you do not use Ethernet protocol? How about the trunk? How about TCP?

[kosma]

Replying to getxsick:

What if you do not use Ethernet protocol? How about the trunk? How about TCP?

UDP with trunk - checksum incorrect. TCP with trunk - checksum correct.

Will investigate further.

[getxsick]

If so, please change in description to trunk.

How about the milestone. 0.3 is not released yet. Please do not mess it up.

[kosma]

Apparently the bug was caused by a typo. _checksum belongs to post_raw; _length to pre_raw. Even the comment mentions Length.

[getxsick]

Why unit tests don't catch this bug? If it wasn't covered, please provide tests for this case along with patch.

[luis]

This is a bug from trunk.

I tested in both branches and it happens when UDP have payload. If UDP doesn't contain payload it will work fine.

Note: tested under GNU/Linux.

[getxsick]

luis, does it happen after the patch is applied? do i understand you correctly?

[luis]

Ok.

I tested the patch. It is working fine for me.

I just noticed that the bug doesn't exists with a empty UDP Packet (without payload).

[kosma]

Yes, that is correct. This is because without payload the protocol_bits value is zero and the assignment:

self.get_field('_checksum')._tmp_value = protocol_bits

forces the checksum to be 0x0000 which means 'checksum not calculated'.

[getxsick]

Can anyone test it under Windows?

Also, I would like to see appropriate unit tests for this case.

[luis]

Tested under Windows XP SP2. It works fine.

Just missing tests.

[kosma]

Attached a fix with proper unit tests. Please review.

[luis]

works for me.

[getxsick]

so, kosma, please apply the patch and close the ticket.

[kosma]

Fixed in r5615.