Julia Evans

Day 23: Started writing a TCP stack in Python

For the last couple of days I’ve been trying to understand how TCP works, because networking is really fun. Yesterday I realized that I really hadn’t been writing as much code as I want to be, though.

So I’ve started working on a mini TCP stack! I’m not sure yet how far this will go, but so far I’m running into tons of problems, so I think this might be a good choice.

The code so far is at https://github.com/jvns/teeceepee.

Here are the parameters:

  • Built on top of scapy, so I don’t have to actually know how the bits of a TCP packet are put together
  • I do, however, need to use raw sockets in order to be able to send TCP packets, so everything has to run as root

So far I have

  • A TCPSocket class, which manages connections to servers and keeps some state. Right now my states are CLOSED, SYN-SENT, and ESTABLISHED. I’m using this diagram as a reference.
  • A TCPListener class, which is responsible for managing all the sockets that might be open. Basically it has a dictionary that maps ports to TCPSocket instances. I have no idea if this is actually how socket libraries work.

There’s a thread that listens for packets that are coming in and sends them off to the appropriate TCPSocket instance for handling, using the dispatch method. This is my first time using threads pretty much ever. Expecting all the problems. The listener thread starts when I import the tcp module and ends when the program importing it ends.

Right now I can

  • establish a connection (send a TCP handshake)
  • send some data (but not parse the response)

Tomorrow I’m hoping to be able to tear down a connection properly and assemble the packets I’m receiving into some actual data.

One of the things that is the most challenging so far is that I keep starting and stopping the library, so there are all kinds of connections that just die. Also I think the servers I’m interacting with think I’m unreliable, so sometimes they send me packets slowly.