TCP checksum calculation for divert socket

Question

I have a divert socket. I am trying to change the port on packets as I see them. When I do this - do I need to recalculate the TCP checksum, and the IP checksum?

I am trying to do this, but I am seeing reset packets in tcpdump. I dont know if this is because I am calculating the tcp checksum wrong, or if its something else going on.

I used the cksum methods from this site:

http://www.enderunix.org/docs/en/rawipspoof/

I thought with using a divert socket, if I change one thing, it should be fairly straightforward, but doesnt seem to be. is there a TCP checksum calculator out there I can use to verify I am getting the right value?

Answer 1

You do need to recompute the checksum. Here is a description of the calculation: TCP/IP Checksum calculation

Answer 2

See Section 3.3 of RFC 1631 for the checksum adjustments that a NAT/PAT must make. You don't have to go through the entire packet, you just need to adjust the checksum based on the bytes that you modify.