minefake.py

Minecraft server placeholder returning a sensible disconnect message

#!/usr/bin/python3

import socket, json

server_address = ('', 25565)
disconnect_message = "Server currently unavailable"

# encode an integer into a protobuf-compatible VarInt
def varint(i):
    if i < 0:
        # dynamic length two's complement
        i = (1 << (i.bit_length() + 1)) + i
    repres = bytearray()
    while True:
        rem = i % 128
        i //= 128
        if i:
            repres.append(rem + 128)
        else:
            repres.append(rem)
            break
    return repres

# socket.AF_INET6 for an IPv6 or dual stack socket
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as ss:
    ss.bind(server_address)
    ss.listen(1)
    while True:
        (cs, caddr) = ss.accept()
        handshake_data = cs.recv(1024)
        # crude 7/14 bit VarInt decoder
        if handshake_data[0] > 127:
            handshake_type_offset = 128 * handshake_data[1] + handshake_data[0] - 128
        else:
            handshake_type_offset = handshake_data[0]
        handshake_type = handshake_data[handshake_type_offset]
        if handshake_type == 2:
            # login connection
            disconnect_reason = json.dumps({"text": disconnect_message}).encode("utf-8")
            response = b'\x00' + varint(len(disconnect_reason)) + disconnect_reason
        else:
            # status connection (0x01)
            # protocol version 1 is always older than the client
            # (Minecraft 1.8 uses 47, 15w39a/15w39b currently uses 74)
            status_message = json.dumps({"version": {"name": "offline", "protocol": 1},
                                         "description": {"text": disconnect_message}}).encode("utf-8")
            response = b'\x00' + varint(len(status_message)) + status_message
        cs.sendall(varint(len(response)) + response)
        cs.close()