It might be complicated, but depending on what you need to do, your changes might not be that hard to realize.

In the case that you're willing to invest some time to get into the code:
What do you want to do?

Besides: Do you understand everything on Main Page - Gnufu ?
If you want to do research on Gnutella, this should be a starting point to understand the concepts.
The step after that is to see how they were implemented: Gnutella Protocol Specification - Gnutella Specification

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