How to set initial sequence number of TCP socket

Question

As a part of testing, I am needing to be able to initialize the ISN (initial sequence number) of a TCP connection to a specific value. Normally this is a randomized value picked by the OS/Network Stack, but I need to control the initial value.

Is there any way to do this in C or Python?

Answer 1

• Download lwip and lwip-contrib .
• Create a new tap interface and bridge it with your normal network card
• Build the lwip-contrib/ports/unix/proj/minimal app and run it
• Now you should be able to ping the lwip stack
• TCP sequence number are in struct tcp_pcb , values rcv_nxt and snd_nxt
• Hack the TCP code to not randomize these values and add a TCP connection to the minimal app