In case your Xen domU's have networking trouble
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 http://wiki.xensource.com/xenwiki/XenFaq#head-4ce9767df34fe1c9cf4f85f7e07cb10110eae9b7 for the very small amount of extra information that is out there.