bad udp cksum messages in tcpdump for wg0

bad udp cksum messages in tcpdump for wg0

[google gsuite]

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Hi

My DNS server is available via a wireguard interface. Because of many requests I did a tcpdump on the traffic and found messages like.

14:05:34.881307 IP (tos 0x0, ttl 63, id 33826, offset 0, flags [DF], proto UDP (17), length 54)
    10.99.97.15.53628 > 10.99.97.17.domain: [bad udp cksum 0xd719 -> 0x6360!] 8446+ A? postgres. (26)

I could turn that messages off with turning off check validation on the interface

ethtool -K wg0 tx off rx off
Actual changes:
rx-checksumming: off
tx-checksumming: off
tx-checksum-ip-generic: off

The tcpdump traffic looks now like this

14:08:36.494987 IP (tos 0x0, ttl 63, id 61627, offset 0, flags [DF], proto UDP (17), length 54)
    10.99.97.15.40185 > 10.99.97.17.domain: [udp sum ok] 1324+ A? postgres. (26)


I want to know if the behavior described above with the checksum errors is to be expected? Or is it necessary to turn those checks off on all the interfaces?

BR
ybaumy
 

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Re: bad udp cksum messages in tcpdump for wg0

[Matthias Urlichs]

Hi,
> ethtool -K wg0 tx off rx off
>
> I want to know if the behavior described above with the checksum
> errors is to be expected? Or is it necessary to turn those checks off
> on all the interfaces?

Yeah. I need that too, on one of my routers. Apparently some checksum
offloading hardware gets confused by wireguard.

Interestingly, on that machine fixing this problem requires turning off
checksumming on the wg interface _or_ on the hardware interface, either
way is sufficient.

-- 
-- Matthias Urlichs


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