Add hole punch forwarding to master server.

If a server is behind a NAT gateway or firewall its UDP port may be
accessible to the master server but not to other clients. For some types
of gateway we can work around this by having the servers explicitly send
a packet to the client that is trying to connect to them ("hole punching").
The master server can serve as a trampoline to forward this request from
clients.

This is the initial master server-side part of implementing
chocolate-doom/chocolate-doom#469.

chocolate-master

@@ -40,6 +40,7 @@ MAX_RESPONSE_LEN = 1400
 
 NET_PACKET_TYPE_QUERY = 13
 NET_PACKET_TYPE_QUERY_RESPONSE = 14
+NET_PACKET_TYPE_NAT_HOLE_PUNCH = 16
 
 # Packet types, matches the constants in net_defs.h.
 
@@ -53,6 +54,7 @@ NET_MASTER_PACKET_TYPE_SIGN_START = 6
 NET_MASTER_PACKET_TYPE_SIGN_START_RESPONSE = 7
 NET_MASTER_PACKET_TYPE_SIGN_END = 8
 NET_MASTER_PACKET_TYPE_SIGN_END_RESPONSE = 9
+NET_MASTER_PACKET_TYPE_NAT_HOLE_PUNCH = 10
 
 def bind_socket_to(sock, config):
     """ Bind the specified socket to the address/port configuration from
@@ -356,6 +358,31 @@ class MasterServer:
             self.send_message(addr, NET_MASTER_PACKET_TYPE_SIGN_END_RESPONSE,
                               signature)
 
+    def send_hole_punch(self, server, client_addr):
+        """Send a hole punch request to a server on behalf of a client."""
+        client_addr_str = "%s:%d" % client_addr
+        packet = client_addr_str.encode("utf8") + b'\0'
+        self.send_message(server.addr, NET_MASTER_PACKET_TYPE_NAT_HOLE_PUNCH,
+                          packet)
+        self.log_output(client_addr, "Sent hole punch to %s" % server)
+
+    def process_hole_punch(self, data, addr):
+        """Process a NAT hole punch request from a client."""
+
+        # Packet just contains the address of the the server. Check it's really
+        # a server that we have registered.
+        _, server_addr_str = read_string(data)
+        self.log_output(addr, "Hole punch request for %r" % server_addr_str)
+        a, p = server_addr_str.split(":", 1)
+        server_addr = (a, int(p))
+
+        if server_addr not in self.servers:
+            self.log_output(addr, "Unknown server to hole punch")
+            return
+
+        # Forward hole punch request to the server:
+        self.send_hole_punch(self.servers[server_addr], addr)
+
     def process_packet(self, data, addr):
         """ Process a packet received from a server. """
 
@@ -371,6 +398,8 @@ class MasterServer:
             self.sign_start_message(addr)
         elif packet_type == NET_MASTER_PACKET_TYPE_SIGN_END:
             self.sign_end_message(data[2:], addr)
+        elif packet_type == NET_MASTER_PACKET_TYPE_NAT_HOLE_PUNCH:
+            self.process_hole_punch(data[2:], addr)
 
     def is_blocked(self, addr):
         addr_str = "%s:%i" % addr

chocolate-master-test.py

@@ -28,6 +28,8 @@ import sys
 import struct
 import json
 
+NET_PACKET_TYPE_NAT_HOLE_PUNCH = 16
+
 NET_MASTER_PACKET_TYPE_ADD = 0
 NET_MASTER_PACKET_TYPE_ADD_RESPONSE = 1
 NET_MASTER_PACKET_TYPE_QUERY = 2
@@ -38,6 +40,7 @@ NET_MASTER_PACKET_TYPE_SIGN_START = 6
 NET_MASTER_PACKET_TYPE_SIGN_START_RESPONSE = 7
 NET_MASTER_PACKET_TYPE_SIGN_END = 8
 NET_MASTER_PACKET_TYPE_SIGN_END_RESPONSE = 9
+NET_MASTER_PACKET_TYPE_NAT_HOLE_PUNCH = 10
 
 UDP_PORT = 2342
 
@@ -222,17 +225,39 @@ def sign_end(addr_str):
                             NET_MASTER_PACKET_TYPE_SIGN_END_RESPONSE)
     print(response)
 
+def hole_punch(master_addr_str, server_addr_str):
+    """Send a NAT hole punch request to the master server."""
+
+    master_addr = parse_address(master_addr_str)
+    server_addr = parse_address(server_addr_str)
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+
+    # Send request
+    print("Sending hole punch request to master at %s" % str(master_addr))
+    packet = server_addr_str.encode("utf8") + b"\x00"
+    send_message(sock, master_addr, NET_MASTER_PACKET_TYPE_NAT_HOLE_PUNCH,
+                 packet)
+
+    # Wait for response.
+    print("Waiting for response...")
+    response = get_response(sock, server_addr,
+                            NET_PACKET_TYPE_NAT_HOLE_PUNCH)
+
+    print("Got hole punch request from server via master server.")
+
+
 commands = [
     ("query",          query_master),
     ("add",            add_to_master),
     ("get-metadata",   get_metadata),
     ("sign-start",     sign_start),
     ("sign-end",       sign_end),
+    ("hole-punch",     hole_punch),
 ]
 
 for name, callback in commands:
     if len(sys.argv) > 2 and name == sys.argv[1]:
-        callback(sys.argv[2])
+        callback(*sys.argv[2:])
         break
 else:
     print("Usage:")
@@ -241,4 +266,5 @@ else:
     print("chocolate-master-test.py get-metadata <address>")
     print("chocolate-master-test.py sign-start <address>")
     print("chocolate-master-test.py sign-end <address>")
+    print("chocolate-master-test.py hole-punch <master> <server>")