Howto update tcp checksum after modifying IP address

qnguyendang · 01-31-2009, 03:34 PM

Hi everybody,

I am writing a program which modifies source IP address of packet before writing this packet to tap interface. I face only one problem with tcp checksum. I have current checksum, to get new checksum, I add new source IP address and add one complement of old source IP address. That is, I update checksum with difference of old and new source IP address. It sounds not bad but does not work.

Could anybody help me?
Thank in advance.

Nguyen.

curos · 02-02-2009, 01:46 AM

I don't know too much about this, but shouldn't you be computing the checksum on the whole TCP header?

From what I can remember, you have to:
1) zero out the checksum field
2) compute the TCP checksum over the whole tcp header
3) put the computed value back into the checksum field.