1: Understanding the interface

Before writing the agent and its handler, we need to understand how Havoc handles third party agents.

Callback specifications

Since we are handling our own custom agent's callbacks ourselves, there isn't a fixed callback data structure we have to follow. However, there is a fixed set of bytes that must be prepended to each callback that gives Havoc some basic information on how to handle the callback. These bytes are as such:
[4 bytes] size - this is the total size (in bytes) of the request (including the 12 byte header)
[4 bytes] magic value - this is the value that is used to identify the custom agent type. It has to be common between all agents of the same type.
[4 bytes] agent ID - this is a 4 byte agent ID. This can be any value, but it should be unique per agent, so randomly generating it during runtime is a good idea.
So the structure of a typical callback should be something like this:
Callback structure

The third party agent system

The way Havoc third party agents work, is that any data sent to a valid C2 listening interface (http/s, external c2 etc.) is checked for its magic value, and forwarded to the appropriate agent handler. For example:
Havoc third party agent system
That's pretty much all you need to understand about Havoc's third party agent interface.