On Tue, Jul 07, 2015 at 02:50:49PM +0300, Pavel Timofeev wrote:

> Hi!
> I have a test virtual machine which runs CURRENT under Hyper-V. It's
> amd64 r285198 now.
> It can't get any response from MS DNS server. Well, it could two or
> three weeks ago, but after upgrade it's not able to do it anymore.
> Google DNS answers without problems meanwhile (sic!).
> What I do:
> # host google.ru
> I see that MS DNS ( server receives these packets, but
> ignores them.
> And no matter how my system asks MS DNS. Every daemon can't get response too.
> I know that nothing was changed in MS DNS server. No doubt.
> Then I tried different available CURRENT snapshot ISOs.
> FreeBSD-11.0-CURRENT-amd64-20150630-r284969-disc1.iso - MS DNS does not 
> answer.
> FreeBSD-11.0-CURRENT-amd64-20150625-r284814-disc1.iso - MS DNS does not 
> answer.
> FreeBSD-11.0-CURRENT-amd64-20150618-r284544-disc1.iso - MS DNS answers!
> So something was committed to CURRENT between 20150618 and 20150625.
> This something ruins communication with MS DNS.
> Then I tried latest
> FreeBSD-11.0-CURRENT-amd64-20150630-r284969-disc1.iso on bare metal -
> MS DNS answered!
> Looks like that something is related to Hyper-V code.
> Maybe it changes packets somehow? I can gather and provide more info
> (tcpdump?) if you ask, it's not a problem!

Author: whu
Date: Wed Jun 24 06:01:29 2015
New Revision: 284746
URL: https://svnweb.freebsd.org/changeset/base/284746

  TSO and checksum offloading support for Netvsc driver on Hyper-V.


Try tcpdump/wireshark on FreeBSD and MS DNS host.
Check validating IP/UDP checksums.
Try off checksum offloading on network interface
(ifconfig ifname -txcsum -rxcsum)
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