2003-08-15 11:11:54

by Ricardo Galli

[permalink] [raw]
Subject: 2.6.0: Bad udp checksum in loopback interface

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/


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

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/
> -
> To unsubscribe from this list: send the line "unsubscribe linux-net" in
> the body of a message to [email protected]
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>

---
Catalin(ux) BOIE
[email protected]

2003-08-15 16:25:49

by Stephen Hemminger

[permalink] [raw]
Subject: Re: 2.6.0: Bad udp checksum in loopback interface

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