How to fix UDP checksum errors in a Xen DomU running a DHCP Server.
In /etc/network/interfaces in the domU acting as dhcp-server, add the following iptables rule
auto eth0 iface eth0 inet static address 192.168.139.2 gateway 192.168.139.254 netmask 255.255.255.0 up iptables -t mangle -A POSTROUTING -o $IFACE -p udp --dport bootps -j CHECKSUM --checksum-fill