We have root caused the problem. This issue happens on the Hyper-Vs on Windows 
Server 2012 (Win 8.0) and earlier releases. On these releases, the UPD checksum 
offloading on host side does not work properly. The workaround is to disable 
UPD checksum offloading in the FreeBSD guest through 'ifconfig'. We are also 
working on a patch to turn off UPD checksum offloading in the netvsc driver 
when detecting the Hyper-V releases. 

The UDP checksum offloading works fine on Windows Server 2012R2 and Win 8.1 
hosts. 

Thanks Pavel and Slawa for the support.

Wei


> -----Original Message-----
> From: owner-freebsd-virtualizat...@freebsd.org [mailto:owner-freebsd-
> virtualizat...@freebsd.org] On Behalf Of Pavel Timofeev
> Sent: Wednesday, July 8, 2015 4:06 PM
> To: Slawa Olhovchenkov
> Cc: freebsd-curr...@freebsd.org; freebsd-virtualization@freebsd.org
> Subject: Re: MS DNS doesn't answer to CURRENT under Hyper-V
> 
> 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?
> 
> 
> 2015-07-07 18:49 GMT+03:00 Slawa Olhovchenkov <s...@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).
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