Fix for #1626

Port used for PortMapping was not properly randomized causing multiple clients on the same lan to request the same UPnP port, and not all routers handle this gracefully.

Also fixes issue where the portmapper wasn't started at all if a secondary port wasn't specified, or if the tertiary port was manually specified.

service/OneService.cpp

@@ -1002,27 +1002,16 @@ public:
 				// If we're running uPnP/NAT-PMP, bind a *third* port for that. We can't
 				// use the other two ports for that because some NATs do really funky
 				// stuff with ports that are explicitly mapped that breaks things.
-				if (_ports[1]) {
-					if (_tertiaryPort) {
-						_ports[2] = _tertiaryPort;
-					} else {
-						_ports[2] = 20000 + (_ports[0] % 40000);
-						for(int i=0;;++i) {
-							if (i > 1000) {
-								_ports[2] = 0;
-								break;
-							} else if (++_ports[2] >= 65536) {
-								_ports[2] = 20000;
-							}
-							if (_trialBind(_ports[2]))
-								break;
-						}
-						if (_ports[2]) {
-							char uniqueName[64];
-							OSUtils::ztsnprintf(uniqueName,sizeof(uniqueName),"ZeroTier/%.10llx@%u",_node->address(),_ports[2]);
-							_portMapper = new PortMapper(_ports[2],uniqueName);
-						}
-					}
+				if (_tertiaryPort) {
+					_ports[2] = _tertiaryPort;
+				} else {
+					_ports[2] = _getRandomPort();
+				}
+
+				if (_ports[2]) {
+					char uniqueName[64];
+					OSUtils::ztsnprintf(uniqueName,sizeof(uniqueName),"ZeroTier/%.10llx@%u",_node->address(),_ports[2]);
+					_portMapper = new PortMapper(_ports[2],uniqueName);
 				}
 			}
 #endif