Re: incorrect tcp checksum on Linux tun interfaces?

[Michael Richardson <mcr () sandelman ca>]

{sorry if I'm a bit tardy at removing the emergency moderation that I
turned on in order to keep junk off the list}

> > > > > "Gert" == Gert Doering <gert () greenie muc de> writes:
    Gert> I'm a bit irritated by a byproduct of a problem hunt today, incorrect
    Gert> TCP checksums on a *tun* interface...

    Gert> 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>

What's curious to me is that the chsum is not zero. If it was being
"offloaded" into a step after the PF_PACKET interface, it would be zero,
right?

    Gert> 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>

Both for IPv4 and IPv6?

    Gert> (The tun goes into openvpn, and out of the other side's tun
    Gert> comes a packet 

openvpn does IPv6 now?

I suppose the next step would be to hexdump packets from /dev/net/tun.

-- 
]       He who is tired of Weird Al is tired of life!           |  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] mcr () sandelman ottawa on ca http://www.sandelman.ottawa.on.ca/ |device driver[
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