How to Write a Server

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This tutorial is being created to document what it takes to write a stand-alone server to interact with Notchian clients. The tutorial is incomplete but will be updated whenever more information is discovered.

Note that a lot of the information here is out of date (last updated in 2016), especially as a result of recent changes in 1.20.2. You should probably look elsewhere instead, e.g. the Protocol FAQ.

Before You Get Started

  • Make sure you don't want to fork or join an existing project.
  • Wonder why you want to do this in the first place.
  • Choose a language that has good networking, like Java, C#, or Python. (If you want to be different, choose a faster language like C or Rust!)

If you're not using a wrapper/library

If you're not using a wrapper/library, then you'll need to do all the dirty work by hand.

Accepting TCP Packets

The Minecraft server accepts connections from TCP clients and communicates with them using packets. A packet is a sequence of bytes sent over the TCP connection. The meaning of a packet depends both on its packet ID and the current state of the connection. The initial state of each connection is Handshaking, and state is switched using the packets Handshake (Handshaking, 0x00, serverbound) and Login Success (Login, 0x02, clientbound). You can read more about Minecraft Packets on the Protocol page.

Server List Ping

Your server needs to accept a Handshake(0x00 with state=1) and a Request packet, which you'll respond with a Response packet including JSON detailing the server. You can read more about this on the Server List Ping page.

FSM example of handling new TCP connections

As an example of how to handle new TCP connections we can have a look at this informal FSM to get an example of how one could write the server's states.

FSM showing an example of how to handle new connections

To distinguish between FSM-states and the states of the connection, the different connection states has been colour-coded to make it easier to understand the transitions. The given connection states are:

  • Handshake
  • Status
  • Login
  • Play
  • Closed (added for clarity, as Pong closed the TCP socket)

It should be noted that outgoing packets are not directed towards the TCP node in the middle. This is just because it made a simpler FSM. There is also a possibility to have the Set compression to be sent after Login success, but it will have a different packet id because the connection state will be Play instead of Login.