In case your Xen domU's have networking trouble

If your domUs have networking trouble with TCP, or some other protocol that ends up needing fragmentation such as large ICMP pings, you need to read this.

If it seems TCP handshakes complete, but no data is transferred -- especially, no actual data gets sent out from the domU -- you're likely hitting a bug in how Xen interacts with TCP segmentation offload.

The bug seems to depend on the actual network interface card the traffic is going out from. I hear tg3 is one of the cards that triggers it, and I'm seeing it on my home box with 8139too's.

The fix is pretty simple, but hard to figure unless you know what to look for: inside the domU, run

ethtool -K eth0 tx off

for each interface affected.

See for the very small amount of extra information that is out there.

  • xen
  • networking
  • computer