Ok, so here is the breakdown of what i was able to find.

DHCP server using dnsmasq (2.65.5.el6) on CentoOS 6.4 a kvm guest with virtio 
+ Also running cobbler with some preseed building scripts based on 12.04 LTS

node-1 a bare (guest) to PXE boot from DHCP server, on its own linux bridge 
(node_1_net) in routed mode, guest nic is virtio
node-2  a bare (guest) to PXE boot from DHCP server, on its own linux bridge 
(node_2_net) in routed mode, guest nic is e1000
node-3 a bare (guest) to PXE from DHCP server on the same linux bridge as DHCP 
server with virtio nic

In the kvm host I have stopped dnsmasq (which starts automatically) on default, 
node_1_net, node_2_net 
In the kvm host I have started dhcp-helper -i node_1_net -i node_2_net -s <DHCP 
server IP> (dhcp-helper 1.1)

when booting node-1 it will PXE just fine, but when running the
installer, the node aborts attempting to request and address from DHCP,
running dhclient by hand yields the magical "bad udp checksum" message.

when booting node-2 it will boot and install just fine, the only change being 
the nic driver
when booting node-3 it will boot and install just fine but in this case, the 
DHCP requests aren't relayed over dhcp-helper.

So it appears that some drivers won't perform the udp checksum when some
dhcp servers / relays are involved in the conversation.

I will try to test the same using a 13.10 loader and report back

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https://bugs.launchpad.net/bugs/610391

Title:
  UDP checksum offload breaks DHCP on virtual machines

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