maintenance.py

import sys
import json
import SocketServer

SocketServer.TCPServer.allow_reuse_address = True

class MCHandler(SocketServer.BaseRequestHandler):

    @classmethod
    def pack_varint(cls, d):
        o = ""
        while True:
            b = d & 0x7F
            d >>= 7
            o += chr(b | (0x80 if d > 0 else 0))
            if d == 0:
                break
        return o

    def unpack_varint(self):
        o = 0
        for i in range(5):
            b = ord(self.unpack(1))
            o |= (b & 0x7F) << 7*i
            if not b & 0x80:
                return o

    def unpack(self, n):
        o = ""
        while len(o) < n:
            o += self.request.recv(n-len(o))
        return o

    def handle(self):
        # Discard packet header
        self.unpack_varint() # length
        self.unpack_varint() # ident

        # Store protocol version
        protocol_version = self.unpack_varint()

        # Discard server addr/port
        self.unpack(self.unpack_varint())
        self.unpack(2)

        # Store next mode
        next_mode = self.unpack_varint()

        # Discard a packet (its content isn't useful)
        self.unpack(self.unpack_varint())

        # Server list ping
        if next_mode == 1:
            response = {
                "description": {"text": self.server.motd},
                "players": {"online": 0, "max": 0},
                "version": {"name": "???", "protocol": protocol_version}}

        # Login attempt
        elif next_mode == 2:
            response = self.server.motd

        # Send response
        d = json.dumps(response)
        d = d.encode("utf-8")
        d = MCHandler.pack_varint(len(d)) + d
        d = MCHandler.pack_varint(0) + d
        d = MCHandler.pack_varint(len(d)) + d
        self.request.sendall(d)

        # Close the connection
        self.request.close()


if __name__ == '__main__':
    args = sys.argv[1:]
    if len(args) != 3:
        print "usage: python maintenance.py <listen-ip> <listen-port> <motd>"
    else:
        host = args[0]
        port = int(args[1])
        motd = args[2]

        server = SocketServer.TCPServer((host, port), MCHandler)
        server.motd = motd

        server.serve_forever()