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2.6.0: Bad udp checksum in loopback interface

DISCUSSION Ricardo Galli 3 participants 3 messages Aug 15, 2003

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Ricardo Galli

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#0 lore raw

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,

Signature · 2 lines
  ricardo galli       GPG id C8114D34
  http://mnm.uib.es/~gallir/
#1 lore raw

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.

Quoted 38 lines

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,

Quoted 8 lines

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