Luanti protocol (incomplete, early draft):
Updated 2011-06-18

A custom protocol over UDP.
Integers are big endian.
Refer to connection.{h,cpp} for further reference.

Initialization:
- A dummy reliable packet with peer_id=PEER_ID_INEXISTENT=0 is sent to the server:
	- Actually this can be sent without the reliable packet header, too, i guess,
	  but the sequence number in the header allows the sender to re-send the
	  packet without accidentally getting a double initialization.
	- Packet content:
		# Basic header
		u32 protocol_id = PROTOCOL_ID = 0x4f457403
		u16 sender_peer_id = PEER_ID_INEXISTENT = 0
		u8 channel = 0
		# Reliable packet header
		u8 type = TYPE_RELIABLE = 3
		u16 seqnum = SEQNUM_INITIAL = 65500
		# Original packet header
		u8 type = TYPE_ORIGINAL = 1
		# And no actual payload.
- Server responds with something like this:
	- Packet content:
		# Basic header
		u32 protocol_id = PROTOCOL_ID = 0x4f457403
		u16 sender_peer_id = PEER_ID_INEXISTENT = 0
		u8 channel = 0
		# Reliable packet header
		u8 type = TYPE_RELIABLE = 3
		u16 seqnum = SEQNUM_INITIAL = 65500
		# Control packet header
		u8 type = TYPE_CONTROL = 0
		u8 controltype = CONTROLTYPE_SET_PEER_ID = 1
		u16 peer_id_new = assigned peer id to client (other than 0 or 1)
- Then the connection can be disconnected by sending:
	- Packet content:
		# Basic header
		u32 protocol_id = PROTOCOL_ID = 0x4f457403
		u16 sender_peer_id = whatever was gotten in CONTROLTYPE_SET_PEER_ID
		u8 channel = 0
		# Control packet header
		u8 type = TYPE_CONTROL = 0
		u8 controltype = CONTROLTYPE_DISCO = 3

- Here's a quick untested connect-disconnect done in PHP:
# host: ip of server (use gethostbyname(hostname) to get from a dns name)
# port: port of server
function check_if_minetestserver_up($host, $port)
{
	$socket = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
	$timeout = array("sec" => 1, "usec" => 0);
	socket_set_option($socket, SOL_SOCKET, SO_RCVTIMEO, $timeout);
	$buf = "\x4f\x45\x74\x03\x00\x00\x00\x03\xff\xdc\x01";
	socket_sendto($socket, $buf, strlen($buf), 0, $host, $port);
	$buf = socket_read($socket, 1000);
	if($buf != "")
	{
		# We got a reply! read the peer id from it.
		$peer_id = substr($buf, 9, 2);

		# Disconnect
		$buf = "\x4f\x45\x74\x03".$peer_id."\x00\x00\x03";
		socket_sendto($socket, $buf, strlen($buf), 0, $host, $port);
		socket_close($socket);

		return true;
	}
	return false;
}

- Here's a Python script for checking if a Luanti server is up, confirmed working

#!/usr/bin/env python3
import sys, time, socket

address = ""
port = 30000
if len(sys.argv) <= 1:
    print("Usage: %s <address>" % sys.argv[0])
    exit()
if ":" in sys.argv[1]:
    address = sys.argv[1].split(":")[0]
    try:
        port = int(sys.argv[1].split(":")[1])
    except ValueError:
        print("Please specify a valid port")
        exit()
else:
    address = sys.argv[1]

try:
    start = time.time()
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.settimeout(2.0)
    buf = b"\x4f\x45\x74\x03\x00\x00\x00\x01"
    sock.sendto(buf, (address, port))
    data, addr = sock.recvfrom(1000)
    if data:
        peer_id = data[12:14]
        buf = b"\x4f\x45\x74\x03" + peer_id + b"\x00\x00\x03"
        sock.sendto(buf, (address, port))
        sock.close()
        end = time.time()
        print("%s is up (%0.5fms)" % (sys.argv[1], end - start))
    else:
        print("%s seems to be down " % sys.argv[1])
except Exception as err:
    print("%s seems to be down (%s) " % (sys.argv[1], str(err)))