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

[Alexey Kardashevskiy]

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:

address@hidden:~$ 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?


I read the old discussion at
http://www.mail-archive.com/address@hidden/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.


-- 
Alexey