2.6.0: Bad udp checksum in loopback interface

2.6.0: Bad udp checksum in loopback interface

[Ricardo Galli]

tcpdump -v shows bad udp checksum in the loopback interface. But I'm not sure 
if the packets is discarded or the error is ignored.

ponti:~# tcpdump -i lo -n -v

ponti:~# dig @192.168.0.3 gallimedina.net

13:02:12.360728 192.168.0.3.32827 > 192.168.0.3.53: [bad udp cksum 5e6d!]  
35220+ A? gallimedina.net. (33) (DF) [ttl 0] (id 0, len 61)
13:02:12.364470 192.168.0.3.53 > 192.168.0.3.32827:  35220* 1/1/1 
gallimedina.net. A 192.168.0.10 (85) (DF) [ttl 0] (id 12492, len 113)

ponti:~# dig @127.0.0.1 gallimedina.net


13:03:53.299883 127.0.0.1.32827 > 127.0.0.1.53: [bad udp cksum 167d!]  32901+ 
A? gallimedina.net. (33) (DF) [ttl 0] (id 0, len 61)
13:03:53.303448 127.0.0.1.53 > 127.0.0.1.32827:  32901* 1/1/1 gallimedina.net. 
A 192.168.0.10 (85) (DF) [ttl 0] (id 7725, len 113)



This machine has a bridge, but I also tried in another one with netcat (nc -l 
-u localhost -p 2000 and nc -u localhost 2000)

antoli:~# tcpdump -i lo -n -v
tcpdump: listening on lo
13:09:04.125385 127.0.0.1.32890 > 127.0.0.1.2000: [bad udp cksum 5a0!] udp 14 
(DF) [ttl 0] (id 32006, len 42)
13:09:12.472940 127.0.0.1.2000 > 127.0.0.1.32890: [bad udp cksum 837!] udp 7 
(DF) [ttl 0] (id 34086, len 35)


I saw the short message does arrive to the listening netcat, but tcpdump still 
gives the bad udp error message.

Regards,

-- 
  ricardo galli       GPG id C8114D34
  http://mnm.uib.es/~gallir/

Re: 2.6.0: Bad udp checksum in loopback interface (me too!)

[Catalin BOIE]

Hi!

I have almost the same program.
cups sends on udp browse informations.
It sends 3 + 1 udps.
The last one has bad checksum.


> tcpdump -v shows bad udp checksum in the loopback interface. But I'm not sure
> if the packets is discarded or the error is ignored.
>
> ponti:~# tcpdump -i lo -n -v
>
> ponti:~# dig @192.168.0.3 gallimedina.net
>
> 13:02:12.360728 192.168.0.3.32827 > 192.168.0.3.53: [bad udp cksum 5e6d!]
> 35220+ A? gallimedina.net. (33) (DF) [ttl 0] (id 0, len 61)
> 13:02:12.364470 192.168.0.3.53 > 192.168.0.3.32827:  35220* 1/1/1
> gallimedina.net. A 192.168.0.10 (85) (DF) [ttl 0] (id 12492, len 113)
>
> ponti:~# dig @127.0.0.1 gallimedina.net
>
>
> 13:03:53.299883 127.0.0.1.32827 > 127.0.0.1.53: [bad udp cksum 167d!]  32901+
> A? gallimedina.net. (33) (DF) [ttl 0] (id 0, len 61)
> 13:03:53.303448 127.0.0.1.53 > 127.0.0.1.32827:  32901* 1/1/1 gallimedina.net.
> A 192.168.0.10 (85) (DF) [ttl 0] (id 7725, len 113)
>
>
>
> This machine has a bridge, but I also tried in another one with netcat (nc -l
> -u localhost -p 2000 and nc -u localhost 2000)
>
> antoli:~# tcpdump -i lo -n -v
> tcpdump: listening on lo
> 13:09:04.125385 127.0.0.1.32890 > 127.0.0.1.2000: [bad udp cksum 5a0!] udp 14
> (DF) [ttl 0] (id 32006, len 42)
> 13:09:12.472940 127.0.0.1.2000 > 127.0.0.1.32890: [bad udp cksum 837!] udp 7
> (DF) [ttl 0] (id 34086, len 35)
>
>
> I saw the short message does arrive to the listening netcat, but tcpdump still
> gives the bad udp error message.
>
> Regards,
>
> --
>   ricardo galli       GPG id C8114D34
>   http://mnm.uib.es/~gallir/
> -
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>

---
Catalin(ux) BOIE
catab@deuroconsult.ro

Re: 2.6.0: Bad udp checksum in loopback interface

[Stephen Hemminger]

Linux turns off IP checksumming on the loopback device for performance
(unless you specifically turn on #define for LOOPBACK_MUST_CHECKSUM).