Re: [Qemu-devel] virtio-net & dhcp & udp checksum

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 06, 2016 at 02:32:27PM +1000, Alexey Kardashevskiy wrote:
> Hi!
> 
> I am trying DHCP between 2 guests. So I am running first guest with:
> 
> -netdev tap,id=TAP0,helper=/home/aik/qemu-bridge-helper \
> -device "virtio-net-pci,id=vnet0,mac=C0:41:49:4b:ee:ee,netdev=TAP0"
> 
> and second one with:
> 
> -netdev tap,id=TAP0,vhost=on,helper=/home/aik/qemu-bridge-helper \
> -device "virtio-net-pci,id=vnet0,mac=C0:41:49:4b:00:01,netdev=TAP0" \
> 
> 
> Both tap are connected to br0 on the host:
> 
> aik@fstn1-p1:~$ brctl show
> bridge name     bridge id               STP enabled     interfaces
> br0             8000.fe397c73cecc       no              tap0
>                                                         tap1
> 
> Both guests are debian8 with v4.7 kernel, one is running Dnsmasq version
> 2.72, the other - isc-dhclient-4.3.1.
> 
> The very first response from dnsmasq has a bad UDP checksum:
> 
> 04:19:04.946754 c0:41:49:4b:ee:ee > c0:41:49:4b:00:01, ethertype IPv4 (0x0800)
> , length 346: (tos 0xc0, ttl 64, id 60635, offset 0, flags [none], proto UDP (
> 17), length 332)
>     192.168.1.250.67 > 192.168.1.1.68: [bad udp cksum 0x8595 -> 0x6e44!] BOOTP
> /DHCP, Reply, length 304, xid 0x38e6b51c, Flags [none] (0x0000)
> 
> 0x8595 looks like a UDP header checksum. Unlike dhclient (which uses
> PF_PACKET), dnsmasq seems to use AF_INET and DGRAM so I am wondering what
> exactly should do this checksum calculations in this case and why it does
> not do this?

Receiver should - the packet is clearly marked as such.
Of course old dhclient ignores this flag.  I think Debian used to
carry a patch to make it take it into account.

> 
> I read the old discussion at
> http://www.mail-archive.com/kvm@vger.kernel.org/msg41958.html
> 
> but it seems that in my case the broken thing is dnsmasq (which is hard to
> believe). Running the dnsmasq's guest with
> 
> -device virtio-net-pci,id=vnet0,csum=off,...
> 
> fixes the problem.
> 
> What is broken now? Thanks.

Maybe debian dropped the patch from dhcp?
http://marc.info/?l=kvm&m=129347023113779

Or you can use the host-side workaround using a checksum rule.

> 
> -- 
> Alexey