So I fired up a copy on a spare whitebox only to find HTTP and SMB outbound traffic timing out. Bizarrely ping and traceroute were working, so ICMP and UDP were working, as was inbound SMB connections – just not outbound. Did the usual tricks – upgrade network drivers, disabled NIC-based offloading and modified the usual suspects via netsh (Task Offload, Chimney Offload, RWIN tuning) to no avail.
It this point I compared the ‘netsh int tcp show global’ and ‘netsh int ip show global’ outputs with the defaults from a Windows Server 2008 R2 box and noticed that ‘ECN Capability’ in the TCP Global Parameters for Windows Server 8 Developer Preview was Enabled. I set this to disabled using:
netsh int tcp set global ecn=disabled
and outbound connectivity was established.