Expose the ability to specify external port when adding port map

_examples/nat-tester.go

@@ -52,7 +52,7 @@ func main() {
 		}
 	}()
 
-	defer nat.DeletePortMapping("txp", 3080)
+	defer nat.DeletePortMapping("tcp", 3080)
 
 	http.ListenAndServe(":3080", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 		rw.Header().Set("Content-Type", "text/plain")

nat.go

@@ -30,6 +30,10 @@ type NAT interface {
 	// AddPortMapping maps a port on the local host to an external port.
 	AddPortMapping(protocol string, internalPort int, description string, timeout time.Duration) (mappedExternalPort int, err error)
 
+	// AddPortMappingSpecifyExternalPort maps a port on the local host to a specified external port.
+	AddPortMappingSpecifyExternalPort(protocol string, internalPort, externalPort int,
+		description string, timeout time.Duration) (mappedExternalPort int, err error)
+
 	// DeletePortMapping removes a port mapping.
 	DeletePortMapping(protocol string, internalPort int) (err error)
 }

natpmp.go

@@ -105,6 +105,24 @@ func (n *natpmpNAT) AddPortMapping(protocol string, internalPort int, descriptio
 	return 0, err
 }
 
+func (n *natpmpNAT) AddPortMappingSpecifyExternalPort(protocol string, internalPort, externalPort int,
+	description string, timeout time.Duration) (int, error) {
+
+	var (
+		err error
+	)
+
+	timeoutInSeconds := int(timeout / time.Second)
+
+	_, err = n.c.AddPortMapping(protocol, internalPort, externalPort, timeoutInSeconds)
+	if err == nil {
+		n.ports[internalPort] = externalPort
+		return externalPort, nil
+	}
+
+	return 0, err
+}
+
 func (n *natpmpNAT) DeletePortMapping(protocol string, internalPort int) (err error) {
 	delete(n.ports, internalPort)
 	return nil

upnp.go

@@ -188,6 +188,24 @@ func (u *upnp_NAT) AddPortMapping(protocol string, internalPort int, description
 	return 0, err
 }
 
+func (u *upnp_NAT) AddPortMappingSpecifyExternalPort(protocol string, internalPort, externalPort int,
+	description string, timeout time.Duration) (int, error) {
+
+	ip, err := u.GetInternalAddress()
+	if err != nil {
+		return 0, nil
+	}
+
+	timeoutInSeconds := uint32(timeout / time.Second)
+
+	err = u.c.AddPortMapping("", uint16(externalPort), mapProtocol(protocol), uint16(internalPort),
+		ip.String(), true, description, timeoutInSeconds)
+	if err != nil {
+		return 0, err
+	}
+	return externalPort, nil
+}
+
 func (u *upnp_NAT) DeletePortMapping(protocol string, internalPort int) error {
 	if externalPort := u.ports[internalPort]; externalPort > 0 {
 		delete(u.ports, internalPort)