Try to map external port the same as internal port

natpmp.go

@@ -93,6 +93,13 @@ func (n *natpmpNAT) AddPortMapping(protocol string, internalPort int, descriptio
 		}
 	}
 
+	// try to map external port the same as internal port
+	_, err = n.c.AddPortMapping(protocol, internalPort, internalPort, timeoutInSeconds)
+	if err == nil {
+		n.ports[internalPort] = internalPort
+		return internalPort, nil
+	}
+
 	for i := 0; i < 3; i++ {
 		externalPort := randomPort()
 		_, err = n.c.AddPortMapping(protocol, internalPort, externalPort, timeoutInSeconds)

upnp.go

@@ -176,6 +176,12 @@ func (u *upnp_NAT) AddPortMapping(protocol string, internalPort int, description
 		}
 	}
 
+	// try to map external port the same as internal port
+	err = u.c.AddPortMapping("", uint16(internalPort), mapProtocol(protocol), uint16(internalPort), ip.String(), true, description, timeoutInSeconds)
+	if err == nil {
+		return internalPort, nil
+	}
+
 	for i := 0; i < 3; i++ {
 		externalPort := randomPort()
 		err = u.c.AddPortMapping("", uint16(externalPort), mapProtocol(protocol), uint16(internalPort), ip.String(), true, description, timeoutInSeconds)