diff --git a/chocolate-master b/chocolate-master
index 9d6646e..50e01a3 100755
--- a/chocolate-master
+++ b/chocolate-master
@@ -39,6 +39,11 @@ SERVER_TIMEOUT = 2 * 60 * 60  # 2 hours
 
 MAX_RESPONSE_LEN = 1000
 
+# Normal packet types.
+
+NET_PACKET_TYPE_QUERY = 13
+NET_PACKET_TYPE_QUERY_RESPONSE = 14
+
 # Packet types, matches the constants in net_defs.h.
 
 NET_MASTER_PACKET_TYPE_ADD = 0
@@ -56,6 +61,7 @@ class Server:
 
     def __init__(self, addr):
         self.addr = addr
+        self.verified = False
         self.refresh()
 
     def refresh(self):
@@ -90,6 +96,42 @@ class MasterServer:
         self.server_addr = (address, port)
         self.open_socket()
 
+    def send_query(self, server):
+        """ Send a query to the specified server. """
+
+        packet = struct.pack(">h", NET_PACKET_TYPE_QUERY)
+        
+        self.query_sock.sendto(packet, server.addr)
+
+    def parse_query_response(self, data, addr):
+        """ Parse a packet received (presumably) in response to a
+            query that we sent to a server. """
+
+        # Unknown?
+
+        if addr not in self.servers:
+            return
+
+        server = self.servers[addr]
+
+        # Check packet type
+
+        packet_type, = struct.unpack(">h", data[0:2])
+
+        if packet_type != NET_PACKET_TYPE_QUERY_RESPONSE:
+            return
+
+        # TODO: Process rest of details so that we can maintain
+        # some information about list of servers?
+
+        # Server responded to our query, so it is verified.
+        # We can send a positive response to its add request.
+
+        if not server.verified:
+            self.log_output(server.addr, "Server responded to query, added")
+            server.verified = True
+            self.send_add_response(server, 1)
+
     def send_message(self, addr, message_type, payload):
         """ Send a message of the specified type to the specified
             remote address. """
@@ -108,6 +150,11 @@ class MasterServer:
         for server in self.servers.values():
             encoded_addr = server.encode_addr()
 
+            # Only include verified servers.
+
+            if not server.verified:
+                continue
+
             # Start a new packet?
 
             if len(packets[-1]) + len(encoded_addr) > MAX_RESPONSE_LEN:
@@ -117,6 +164,13 @@ class MasterServer:
 
         return packets
 
+    def send_add_response(self, server, success):
+        """ Send a response to a server's add request. """
+
+        self.send_message(server.addr,
+                          NET_MASTER_PACKET_TYPE_ADD_RESPONSE,
+                          struct.pack(">h", success))
+
     def process_add_to_master(self, addr):
         """ Process an "add to master" request received from a server. """
 
@@ -125,15 +179,21 @@ class MasterServer:
             server = self.servers[addr]
             server.refresh()
         else:
-            self.log_output(addr, "Add to master")
             server = Server(addr)
             self.servers[addr] = server
 
-        # Send a reply indicating successful
+        # If the server has already been verified, we can send a
+        # reply immediately.  Otherwise, query the server via a
+        # different socket first to verify it.
+        # Why is this needed?  The server might be behind a NAT
+        # gateway.  In this case, the master might be able to
+        # communicate with it, but other machines might not.
 
-        self.send_message(addr,
-                          NET_MASTER_PACKET_TYPE_ADD_RESPONSE,
-                          struct.pack(">h", 1))
+        if server.verified:
+            self.send_add_response(1)
+        else:
+            self.log_output(addr, "Add request, sending query to confirm")
+            self.send_query(server)
 
     def process_query(self, addr):
         """ Process a query message received from a client. """
@@ -166,6 +226,16 @@ class MasterServer:
         except Exception, e:
             print e
 
+    def rx_packet_query_sock(self):
+        """ Invoked when a packet is received on the query socket. """
+
+        data, addr = self.query_sock.recvfrom(1024)
+
+        try:
+            self.parse_query_response(data, addr)
+        except Exception, e:
+            print e
+
     def age_servers(self):
         """ Check server timestamps and flush out stale servers. """
 
@@ -176,25 +246,42 @@ class MasterServer:
                                 (time() - server.add_time))
                 del self.servers[server.addr]
 
+            # Expect a response to queries quickly, otherwise add
+            # requests are rejected.
+
+            if not server.verified and time() - server.add_time > 5:
+                self.log_output(server.addr,
+                                "No response to query, add rejected")
+                self.send_add_response(server, 0)
+                del self.servers[server.addr]
+
     def open_socket(self):
         """ Open the server socket and bind to the listening address. """
 
         self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
         self.sock.bind(self.server_addr)
 
+        # Query socket, used to send queries to servers to check that
+        # they are actually accessible.
+
+        self.query_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+
     def run(self):
         """ Run the server main loop, listening for packets. """
 
         self.log_output(self.server_addr, "Server started.")
 
         while True:
-            r, w, x = select([self.sock], [], [], 5)
+            r, w, x = select([self.sock, self.query_sock], [], [], 5)
 
             self.age_servers()
 
             if self.sock in r:
                 self.rx_packet()
 
+            if self.query_sock in r:
+                self.rx_packet_query_sock()
+
 if __name__ == "__main__":
     server = MasterServer(UDP_ADDRESS, UDP_PORT)
     server.run()