TMG 2010 and hyper-v guest issue with windows update 80072EE2

I was trying to set up a temporary hyper-v server for demo/training purposes this week and ran into a windows update issue with a 2008 r2 guest on it. I turned on logging at my TMG server just to make sure the packets were getting out okay and then I noticed an odd error that popped up - 0xc0040018 FWX_E_BAD_LENGTH_PACKET_DROPPED - the packet was dropped because its IP length field does not fall within the allowed range or is inconsistent with the actual length.


So I tried looking that one up on the web and came up pretty dry other than the list of error codes from Technet. I decided to change gears and approach it from the other direction and went off in search of the windows update error code 80072EE2 which led me to this blog post:

http://msmvps.com/blogs/thenakedmvp/archive/2009/11/08/windows-update-error-80072ee2-hyper-v-guest-issues.aspx

And then it all clicked together. The offload settings weren't working right on the sub-standard hyper-v server I was using and the effect I was seeing on the firewall was incorrect packet lengths. As soon as I turned off the offload settings on the host server for that network card the windows update problems on the guest OS all went away.