I'm using a Linux (Debian based) system on which IP forwarding is enabled (/proc/sys/net/ipv4/ip_forward is set to 1). I can read the incoming packets using a C++ raw sockets program, but I want to modify their content before they are forwarded (using BSD sockets only). This link mentions that I can use an "iptables REDIRECT rule" to send the incoming packets to a port my program is listening on, but I didn't find any information on how exactly this should be done. Mainly, I want to know:
How I should forward the packets after modifying them. Do I have to send them manually from the program to their destination address after they are received and modified?
What I should do in case the program isn't listening on a particular port (e.g. a program concerned with only the IP headers of the packets).
In case it matters receiving and forwarding are done on the same wireless interface. I have to forward packets to, and receive packets from devices on the same WLAN.