[Dnsmasq-discuss] bad udp cksum

[Simon Kelley]

On 15/02/14 08:39, e9hack wrote:
> Hi,
>
> I capture the DNS traffic of dnsmasq with tcpdump:
>
> root at WLAN-DSL6a:~# /tmp/tcpdump -i br-lan -vv -s 0 -l -n port 53
> tcpdump: listening on br-lan, link-type EN10MB (Ethernet), capture size 65535 bytes
> 09:30:32.218876 IP (tos 0x0, ttl 128, id 16449, offset 0, flags [none], proto UDP (17),
> length 72)
>      192.168.100.2.55225 > 192.168.100.1.53: [udp sum ok] 1+ PTR?
> 1.100.168.192.in-addr.arpa. (44)
> 09:30:32.911458 IP (tos 0x0, ttl 64, id 55975, offset 0, flags [DF], proto UDP (17),
> length 100)
>      192.168.100.1.53 > 192.168.100.2.55225: [bad udp cksum 0x49b6 -> 0x69fe!] 1* q: PTR?
> 1.100.168.192.in-addr.arpa. 1/0/0 1.100.168.192.in-addr.arpa. PTR WLAN-DSL6a.lan. (72)
>
> All udp packets from dnsmasq have a wrong udp check sum. Is this a problem of dnsmasq or
> the linux itself? Dnsmasq runs on OpenWRT. The version is 2.69test8-5-ge3ec15a.
>

Probably neither. Modern network interfaces implement checksum 
generation in hardware, so it's quite possible that the packets being 
handed to the network interface (which is what tcpdump sees) don't have 
valid checksums, as they're being added later.

Simon.