incorrect tcp checksum on Linux tun interfaces?

[Gert Doering <gert () greenie muc de>]

Hi,

I'm a bit irritated by a byproduct of a problem hunt today, incorrect
TCP checksums on a *tun* interface...

23:17:39.862001 IP6 (hlim 64, next-header TCP (6) payload length: 40) fd00:abcd:194:7::1.33509 > 
fd00:abcd:194:7::1000.2: S, cksum 0x6502 (incorrect (-> 0x3962), 1541095226:1541095226(0) win 14400 <mss 
1440,sackOK,timestamp 178211075 0,nop,wscale 6>
23:17:39.926409 IP6 (hlim 64, next-header TCP (6) payload length: 20) fd00:abcd:194:7::1000.2 > 
fd00:abcd:194:7::1.33509: R, cksum 0x2cff (correct), 0:0(0) ack 1541095227 win 0

23:18:14.295000 IP (tos 0x10, ttl 64, id 4904, offset 0, flags [DF], proto TCP (6), length 60) 10.194.7.1.52647 > 
10.194.7.6.2: S, cksum 0x23b9 (incorrect (-> 0xd832), 2395069612:2395069612(0) win 14600 <mss 1460,sackOK,timestamp 
178245508 0,nop,wscale 6>
23:18:14.322183 IP (tos 0x10, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40) 10.194.7.6.2 > 
10.194.7.1.52647: R, cksum 0x532d (correct), 0:0(0) ack 2395069613 win 0

this is "tcpdump -v -s0 -i tun6" on a Linux 3.3.8 kernel.

I'm quite used to see "incorrect" TCP checksums on interfaces that have
hardware TCP checksum offloading (because pcap sees the packet before it's
handed to the hardware for checksumming) - but on a *tun* interface, with
no hardware to actually offload it to?

Is this something new in the way Linux tun operates?

(The tun goes into openvpn, and out of the other side's tun comes a packet
with a perfectly valid TCP checksum, so "what openvpn sees" has the correct
checksum, only "what tcpdum sees" does not)


For the fun of it, I asked "ethtool", and it tells me "no checksum 
offloading"...

gert@gentoo $ ethtool  -k tun6
Offload parameters for tun6:
rx-checksumming: off
tx-checksumming: off
scatter-gather: off
tcp-segmentation-offload: off
udp-fragmentation-offload: off
generic-segmentation-offload: off
generic-receive-offload: on
large-receive-offload: off
rx-vlan-offload: off
tx-vlan-offload: off
ntuple-filters: off
receive-hashing: off

... weird.

gert

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Gert Doering - Munich, Germany                             gert () greenie muc de
fax: +49-89-35655025                        gert () net informatik tu-muenchen de
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