Basically I have a program which would send out IP packet periodically.

without modifying the original program, I want to write another program that could capture the IP packet, modify it and send it out.

Anyone have an idea how to do it?
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The exact detail is. I am doing a DNS server project. Using MaraDNS I want to do incoming traffic load balancing.

Right now I modified the original MaraDNS source code, and change the DNS reply packet manually.

For intuitive reason, I'm hoping that I don't need to modify the MaraDNS source code in anyway. Moreover I'm hoping that I can make it portable, that is running on other DNS server program also. Can someone give a hand?