OpenStack instance not configuring network (DHCP) correctly

We ran into trouble when adding additional compute units to our railscomplete Hosting environment Show archive.org snapshot lately.

VM-instances on the new compute units where booting and requesting private IP addresses via DHCP correctly (DHCPDiscover), but after the answer of the dnsmasq dhcp server (DHCPOffer) we did not see any further traffic on the host machine. FYI: The instance should request the IP via DHCPRequest which in turn should be acknowledged by a DHCPAcknowledgment packet.

We assumed this DHCP UDP traffic did not arrive at the instance. And we were right after all...

The problem was discussed here Show archive.org snapshot and the background is given here Show archive.org snapshot .

The solution is to advise iptables to re-calculate checksums for outgoing DHCP traffic like this:

iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill