* UDP checksum broken since 2.6.18?
@ 2007-05-22 21:47 "Thomas B. Rücker"
2007-05-22 23:12 ` Stephen Hemminger
0 siblings, 1 reply; 6+ messages in thread
From: "Thomas B. Rücker" @ 2007-05-22 21:47 UTC (permalink / raw)
To: netdev; +Cc: Philipp Gühring
hi,
a friend of mine recently contacted me about what he at first thought
were IPv6 issues with some java software.
As it turns out it probably is a general IP issue with the Linux kernel:
He wrote this piece of c which sends an UDP packet to 127.28.50.50 -
http://www2.futureware.at/~philipp/udp-problem.c
Packets generated by this code were captured by tcpdump and wireshark.
When feeding the dump into wireshark it says:
"Checksum: 0x62fd [incorrect, should be 0xe4f3]" for the udp packet.
We've tested this on several kernel versions.
Wireshark reports checksum broken:
Linux version 2.6.18-4-vserver-686 (Debian 2.6.18.dfsg.1-12)
(waldi@debian.org) (gcc version 4.1.2 20061115 (prerelease) (Debian
4.1.1-21)) #1 SMP Mon Mar 26 19:55:22 UTC 2007
Linux version 2.6.19-dm8tbr-1 (root@oberon) (gcc version 4.1.2 20061028
(prerelease) (Debian 4.1.1-19)) #3 SMP PREEMPT Sun Dec 3 18:31:00 CET
2006 - (that's vanilla)
Linux version 2.6.21.1-dm8tbr-1 (root@oberon) (gcc version 4.1.2
20061115 (prerelease) (Debian 4.1.1-21)) #3 SMP Fri May 18 09:04:55 CEST
2007 - (that's vanilla + dscape patch)
Wireshark reports checksum ok:
Linux version 2.6.16.13-4-default (geeko@buildhost) (gcc version 4.1.0
(SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006
Linux version 2.6.17-11-386 (root@terranova) (gcc version 4.1.2 20060928
(prerelease) (Ubuntu 4.1.1-13ubuntu5)) #2 Tue Mar 13 23:30:30 UTC 2007
(Ubuntu 2.6.17-11.37-386)
So my guess is something between 2.6.17 and 2.6.18 broke.
Second option is: The way you are supposed to send UDP packets changed
in 2.6.18 and sun javavm and that piece of c are broken for the same reason.
Third option: everything is perfectly ok, the UDP checksum is computed
in a different way since 2.6.18 - due to some reason I don't know - and
Wireshark is broken.
We'd be grateful for some enlightment.
Cheers
Thomas
PS: please keep me CCed - I'm not subscribed to the netdev ml.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: UDP checksum broken since 2.6.18?
2007-05-22 21:47 UDP checksum broken since 2.6.18? "Thomas B. Rücker"
@ 2007-05-22 23:12 ` Stephen Hemminger
2007-05-22 23:33 ` "Thomas B. Rücker"
0 siblings, 1 reply; 6+ messages in thread
From: Stephen Hemminger @ 2007-05-22 23:12 UTC (permalink / raw)
To: "Thomas B. =?UTF-8?B?UsO8Y2tlciI=?= <dm8tbr
Cc: netdev, Philipp Gühring
On Tue, 22 May 2007 21:47:22 +0000
"Thomas B. Rücker" <dm8tbr@openpma.org> wrote:
> hi,
>
> a friend of mine recently contacted me about what he at first thought
> were IPv6 issues with some java software.
>
> As it turns out it probably is a general IP issue with the Linux kernel:
>
> He wrote this piece of c which sends an UDP packet to 127.28.50.50 -
> http://www2.futureware.at/~philipp/udp-problem.c
> Packets generated by this code were captured by tcpdump and wireshark.
> When feeding the dump into wireshark it says:
> "Checksum: 0x62fd [incorrect, should be 0xe4f3]" for the udp packet.
>
> We've tested this on several kernel versions.
> Wireshark reports checksum broken:
> Linux version 2.6.18-4-vserver-686 (Debian 2.6.18.dfsg.1-12)
> (waldi@debian.org) (gcc version 4.1.2 20061115 (prerelease) (Debian
> 4.1.1-21)) #1 SMP Mon Mar 26 19:55:22 UTC 2007
> Linux version 2.6.19-dm8tbr-1 (root@oberon) (gcc version 4.1.2 20061028
> (prerelease) (Debian 4.1.1-19)) #3 SMP PREEMPT Sun Dec 3 18:31:00 CET
> 2006 - (that's vanilla)
> Linux version 2.6.21.1-dm8tbr-1 (root@oberon) (gcc version 4.1.2
> 20061115 (prerelease) (Debian 4.1.1-21)) #3 SMP Fri May 18 09:04:55 CEST
> 2007 - (that's vanilla + dscape patch)
>
> Wireshark reports checksum ok:
> Linux version 2.6.16.13-4-default (geeko@buildhost) (gcc version 4.1.0
> (SUSE Linux)) #1 Wed May 3 04:53:23 UTC 2006
> Linux version 2.6.17-11-386 (root@terranova) (gcc version 4.1.2 20060928
> (prerelease) (Ubuntu 4.1.1-13ubuntu5)) #2 Tue Mar 13 23:30:30 UTC 2007
> (Ubuntu 2.6.17-11.37-386)
>
> So my guess is something between 2.6.17 and 2.6.18 broke.
>
> Second option is: The way you are supposed to send UDP packets changed
> in 2.6.18 and sun javavm and that piece of c are broken for the same reason.
>
> Third option: everything is perfectly ok, the UDP checksum is computed
> in a different way since 2.6.18 - due to some reason I don't know - and
> Wireshark is broken.
>
> We'd be grateful for some enlightment.
>
> Cheers
>
> Thomas
>
The packet passed to packet capture programs may not have a valid checksum
if you have checksum offload configured on the device. What kind of hardware
do you have on sender and receiver? Try disabling checksum offload with
ethtool.
If you are getting bad UDP checksums then the counters in 'netstat -s'
will be increasing.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: UDP checksum broken since 2.6.18?
2007-05-22 23:12 ` Stephen Hemminger
@ 2007-05-22 23:33 ` "Thomas B. Rücker"
2007-05-23 10:01 ` Herbert Xu
0 siblings, 1 reply; 6+ messages in thread
From: "Thomas B. Rücker" @ 2007-05-22 23:33 UTC (permalink / raw)
To: Stephen Hemminger, Philipp Gühring, netdev
Stephen Hemminger wrote:
> The packet passed to packet capture programs may not have a valid checksum
> if you have checksum offload configured on the device. What kind of hardware
> do you have on sender and receiver?
The c-snippet uses 127.x.x.x --> loopback
I think Philipp tested this on real NICs too.
I just proxied this information to this list because I don't have the
indepth knowledge about IP, UDP and the kernel. I had a Chat with Ralf
Baechle and he recommended sending a report to netdev@. Maybe Philipp
can give some more information. He discovered this.
Cheers
Thomas
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: UDP checksum broken since 2.6.18?
2007-05-22 23:33 ` "Thomas B. Rücker"
@ 2007-05-23 10:01 ` Herbert Xu
2007-05-23 10:33 ` "Thomas B. Rücker"
0 siblings, 1 reply; 6+ messages in thread
From: Herbert Xu @ 2007-05-23 10:01 UTC (permalink / raw)
To: "Thomas B. R??cker"; +Cc: shemminger, pg, netdev
"Thomas B. R??cker" <dm8tbr@openpma.org> wrote:
> Stephen Hemminger wrote:
>> The packet passed to packet capture programs may not have a valid checksum
>> if you have checksum offload configured on the device. What kind of hardware
>> do you have on sender and receiver?
> The c-snippet uses 127.x.x.x --> loopback
We never compute the complete checksum for loopback so that explains
the bogus checksum.
Cheers,
--
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: UDP checksum broken since 2.6.18?
2007-05-23 10:01 ` Herbert Xu
@ 2007-05-23 10:33 ` "Thomas B. Rücker"
2007-05-23 11:50 ` Herbert Xu
0 siblings, 1 reply; 6+ messages in thread
From: "Thomas B. Rücker" @ 2007-05-23 10:33 UTC (permalink / raw)
To: Herbert Xu; +Cc: shemminger, pg, netdev
Herbert Xu wrote:
> "Thomas B. R??cker" <dm8tbr@openpma.org> wrote:
>
>> Stephen Hemminger wrote:
>>
>>> The packet passed to packet capture programs may not have a valid checksum
>>> if you have checksum offload configured on the device. What kind of hardware
>>> do you have on sender and receiver?
>>>
>> The c-snippet uses 127.x.x.x --> loopback
>>
>
> We never compute the complete checksum for loopback so that explains
> the bogus checksum.
>
Does this also explain why wireshark thinks the checksum is right when
tested on <=2.6.17?
Cheers
Thomas
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: UDP checksum broken since 2.6.18?
2007-05-23 10:33 ` "Thomas B. Rücker"
@ 2007-05-23 11:50 ` Herbert Xu
0 siblings, 0 replies; 6+ messages in thread
From: Herbert Xu @ 2007-05-23 11:50 UTC (permalink / raw)
To: "Thomas B. Rücker"; +Cc: shemminger, pg, netdev
On Wed, May 23, 2007 at 10:33:16AM +0000, "Thomas B. Rücker" wrote:
>
> Does this also explain why wireshark thinks the checksum is right when
> tested on <=2.6.17?
Probably not. It would help if you use the same tool between the kernels.
Cheers,
--
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
^ permalink raw reply [flat|nested] 6+ messages in thread
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2007-05-22 21:47 UDP checksum broken since 2.6.18? "Thomas B. Rücker"
2007-05-22 23:12 ` Stephen Hemminger
2007-05-22 23:33 ` "Thomas B. Rücker"
2007-05-23 10:01 ` Herbert Xu
2007-05-23 10:33 ` "Thomas B. Rücker"
2007-05-23 11:50 ` Herbert Xu
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