Tag Archives: chimney

Missing Packets and Chimnies

You’ve just fired up Wireshark on your Windows Server 2003 or 2008 system and you’re not seeing nearly the amount of traffic you should. What’s happening?

The Windows Server 2003 Scalable Networking Pack introduced a feature called TCP Chimney Offload. Chimney offloading lets the OS networking stack hand off established TCP connections to the NIC for processing. This frees up the CPU, bus, and memory for other things and lets you scale up the number of connections you can handle. Hooray! Once the OS hands a connection off to the NIC, that traffic completely bypasses WinPcap and therefore doesn’t show up in Wireshark. You see the TCP connection setup and non-TCP traffic but no TCP data. Oops.

How do you fix the problem? It depends on your environment.

You can disable chimney offloading as described in KB 91222 (Server 2003) or KB 951037 (Server 2008). If you have a gigabit NIC you can probably get away with leaving it disabled. If you have a 10 gig NIC this might affect your performance. You can also SPAN or tap in and capture on an external machine, assuming you’re sufficiently equipped.