c - how to modify packet header(IP header, TCP Header) before the host send them into the network

Question

I want to modify packet header(IP header, TCP Header) before the host send them into the network.

For example, if I'm using firefox for browsing, then I want to intercept all the packets from firefox and modify the IP/TCP header and then send them into the network.

so basically, there are two requirements: 1 intercept all the packets from firefox(but not other programs). if this is not possible, is it possible to intercept the packets from a specific port or ip/port pair

2 modify the IP/TCP header and then inject them into the network

are there ways, APIs to achieve this? how about libpcap? are there similar source code snippets? I'm working with linux c

thanks!

Answer 1

To modify the packets, it is better as simple as injecting the packet which is modified into your network. C has a library libnet which does every thing related to packet injection. Python has scapy to inject the packets .

Now you can do quite interesting things , with libent and scapy , instead of modifying your packet ip/tcp headers , perform arp spoofing and then inject falsify packets and redirect them to the victim.

Libpcap is used for capturing packets but not for modifying packets . So you can build your own sniffer using libpcap , but to inject/modify packets you need to use libnet .

To make life simpler , there are tools like burp suite etc..where you can modify the http headers only before forwarding it .