On my Ubuntu 12.04, when I sent TCP packets, wireshark will show all the TCP checksums are wrong, this is understandable because of checksum offloading by the NIC (Intel WiFi NIC). However, when it sent thousands of UDP packets, I don't see even one UDP packets with wrong checksum in Wireshark. What could be going on here? I need some packets with wrong UDP checksum to test my application. asked 28 Oct '15, 12:24  pktUser1001 |
One Answer:
Maybe checksum offloading is not on for UDP. Check your network adapter properties. Checksum offloading can be enabled and disabled separately for IP, TCP, and UDP. Or download and install Colasoft Packet Builder. This is a free tool that will let you craft packets. It can automatically re-calculate the checksum when you change the packets so that the checksum is always correct, but this feature can be turned off. answered 28 Oct '15, 12:54  Jim Aragon |
Thanks @jim-aragon.In the output for "ethtool -k", the setting for tcp and udp are same. Thanks for the link to Colasoft Packet Builder, I was able to use a binary editor to change the content of udp packet to make the checksum wrong (on purpose).