Wednesday, October 19, 2011

NIC Offloading and Packet Capturing

Doug Burk, the guy who wrote and maintains the Security Onion Live DVD, posted a nice article on NIC offloading and the issues it can cause capturing network traffic here. Rather than try to regurgitate Doug's article, I'll summarize by saying it deals with the ability of modern network cards to offload the processing of traffic from the CPU, which is a great thing on fast (Gigabit) networks, but not what you want when capturing traffic. Why not? Because the controller does it's own reassembly at sizes greater than the MTU of your link, causing the sensor or sniffing program to miss part of the packet, in some cases, substantial parts of it. Doug is looking for feedback on this, so if you test and find you're getting "super-sized" packets in your packet captures (Wireshark will flag this for you as "Packet size limited during capture", let him know if you can.

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