create links by both ends registering with the same token

Using the actual connection is the only reliable way to know source IP and port because of NATing that sometimes mangles
ports

server/udp_dyn_proxy

@@ -26,6 +26,7 @@ import socket, sys
 # init values
 UDP_IP = "0.0.0.0"
 linklookup = dict()
+tokenlookup = dict()
 
 try:
     UDP_PORT = int(sys.argv[1])
@@ -39,7 +40,6 @@ if not isinstance( UDP_PORT, int ) or not  1024 <= UDP_PORT <= 65535:
     print 'Valid port range: 1024 - 65535'
     sys.exit(1)
 
-
 # create a socket
 try:
     sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
@@ -63,9 +63,19 @@ def addlink():
     except:
         sock.sendto("Error while executing: " + " ".join(command) + '\n', srcaddr)
 
+def addtoken(token, addr):
+    if token in tokenlookup:
+        tokenlookup[token].append(addr)
+        linklookup[tokenlookup[token][0]] = tokenlookup[token][1]
+        linklookup[tokenlookup[token][1]] = tokenlookup[token][0]
+        del tokenlookup[token]
+    else:
+        tokenlookup[token] = [addr]
+
 def clear():
     try:
         linklookup.clear()
+        tokenlookup.clear()
     except:
         sock.sendto("Clearing linklookup table failed\n", srcaddr)
 
@@ -106,10 +116,19 @@ while True:
             except:
                 pass
 
+    # check token
+    elif data[0:7] == '_TOKEN ':
+        token = data.split()
+        try:
+            addtoken(token[1], srcaddr)
+        except:
+            print "error with parsing token"
+
     # forward data according to lookup table
-    try:
-        destaddr = linklookup[srcaddr]
-        sock.sendto(data, destaddr)
-    except:
-        pass
+    else:
+        try:
+            destaddr = linklookup[srcaddr]
+            sock.sendto(data, destaddr)
+        except:
+            pass