From owner-freebsd-current@freebsd.org  Wed Jul  8 08:22:30 2015
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Date: Wed, 8 Jul 2015 11:22:24 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Pavel Timofeev <timp87@gmail.com>
Cc: freebsd-current@freebsd.org,
 "freebsd-virtualization@freebsd.org" <freebsd-virtualization@freebsd.org>
Subject: Re: MS DNS doesn't answer to CURRENT under Hyper-V
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On Wed, Jul 08, 2015 at 11:05:39AM +0300, Pavel Timofeev wrote:

> Ok, r284746 is the root of the problem. MS DNS works under r284745 and
> doesn't work under r284746.
> Slawa, what should I look at in wireshark output?

I think developers can want look at same packet before entering in NIC
and after receiving MS DNS server. I.e. hexdump of all packet.

FreeBSD's tcpdump do this by `tcpdump -X` for IPv4 payload and
`tcpdump -XX` for IPv4 and Ethernet payload (wireshark by default
include ethernet, for easy comparasion do same).

wireshar do same by 'Export Packet Dissections' 'as plain text' with
checked 'Packet Deatils - All expanded' and 'Packet Bytes'.

This is will be very verbose output.

> 2015-07-07 18:49 GMT+03:00 Slawa Olhovchenkov <slw@zxy.spb.ru>:
> > On Tue, Jul 07, 2015 at 06:04:46PM +0300, Pavel Timofeev wrote:
> >
> >> Well, turning off checksum offloading by `ifconfig hn0 -txcsum
> >> -rxcsum` definitely helps.
> >>
> >> As for tcpdump I'm not completely sure if I did it right, but I see
> >> "bad udp cksum" phrase:
> >>
> >> # tcpdump -i hn0 -vvv -nn udp dst port 53
> >> tcpdump: listening on hn0, link-type EN10MB (Ethernet), capture size
> >> 262144 bytes
> >> 18:01:19.139994 IP (tos 0x0, ttl 64, id 61218, offset 0, flags [none],
> >> proto UDP (17), length 51)
> >>     192.168.25.26.45683 > 192.168.25.3.53: [bad udp cksum 0xb39e ->
> >> 0xf210!] 52886+ A? ya.ru. (23)
> >> 18:01:24.140544 IP (tos 0x0, ttl 64, id 17293, offset 0, flags [none],
> >> proto UDP (17), length 51)
> >>     192.168.25.26.12575 > 192.168.25.3.53: [bad udp cksum 0xb39e ->
> >> 0x7365!] 52886+ A? ya.ru. (23)
> >
> > tcpdump "bad udp cksum" is normal on FreeBSD host in case checksum
> > offload (and may be need only for help finding issuse in code). Need
> > wireshark capturing from MS DNS host (or from mirroring port).