I found this problem on a new server I built. This system acts as a local
router and DNS server, and also runs vmware-server. VMware is configured with
vmnet0 as a bridge to eth1, which physically links to a 10/100Mbps switch for
the local LAN.
VMware instances could communicate with the host system's IP over TCP but not
UDP, so DNS queries would fail. Other systems on the physical LAN can query
DNS just fine.
A tcpdump capture and subsequent wireshark analysis revealed that all
UDP replies coming from the host server system included a bad UDP
checksum. Even UDP replies going to other systems on the physical LAN
show bad checksums, but I assume those systems ignore it. VMware
Server's vmnet-bridge driver might silently drop these packets.
After turning off hardware checksum offloading (ethtool -K eth1 rx off
tx off), everything worked perfectly. Wireshark revealed the reply
packets from the host server include a correct UDP checksum.
Kernel/hardware details:
Ubuntu Server 7.04 x86_64 (2.6.20-16-generic)
CPU: AMD Opteron 1218 (2.6GHz dual core)
Motherboard: TYAN S2925A2NRF AM2 NVIDIA nForce Professional 3400 ATX Server
Motherboard
NICs: 2x onboard 1000Mbps ports, using forcedeth network driver ("forcedeth.c:
Reverse Engineered nForce ethernet driver. Version 0.59.")
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ethereal and tcpdump show a lot of packets with incorrect checksum
https://bugs.launchpad.net/bugs/31273
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