OSX Bridged Networking unreliable due to incorrect MTU setting!

[slebbon]

I setup a clean install of XP SP2 on VirtualBox on my new MacPro. I setup the network adapter (Intel Pro 1000 T Server) with Bridged mode. After getting into XP, I was getting an IP address from my router and a few websites would load, but others would not. For example google.com was OK but mozilla.com and microsoft.com would not load. Some experimenting pointed to an MTU issue.

I determined this WAS due to the MTU setting. My testing showed that when using Bridged Mode and performing "ping -f" to my home network router, I could see that setting the ping packet size (-l flag) larger than 1467 caused the packets to timeout, and a size over 1472 to get the expected "Packet needs to be fragmented but DF set." When using an adaptor in NAT mode, this problem does not occur, and pings up to 1472 in size are allowed as expected. Setting the MTU lower in Windows XP registry and rebooting provides a workaround, and allows all websites and pages to load OK again.

It's clear something about using Bridged mode is "padding" the packets with an additional 4 or 5 bits of data...

For reference Windows XP's registry key for MTU size:

System Key: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\[Adapter ID]]
Value Name: MTU
Data Type: REG_DWORD (DWORD Value)