client: add UDPSourcePortRange parameter (#988)

Co-authored-by: Bjørn Steen Saethre <[email protected]>

Author: aler9

client.go

@@ -497,6 +497,9 @@ type Client struct {
 	DisableRTCPSenderReports bool
 	// explicitly request back channels to the server.
 	RequestBackChannels bool
+	// Range of ports used as source port in outgoing UDP packets.
+	// It defaults to [10000, 65535].
+	UDPSourcePortRange [2]uint16
 
 	//
 	// system functions (all optional)
@@ -614,6 +617,9 @@ func (c *Client) Start() error {
 	if c.UserAgent == "" {
 		c.UserAgent = clientUserAgent
 	}
+	if c.UDPSourcePortRange == [2]uint16{0, 0} {
+		c.UDPSourcePortRange = [2]uint16{10000, 65535}
+	}
 
 	// system functions
 	if c.DialContext == nil {

client_media.go

@@ -77,15 +77,15 @@ func createUDPListenerPair(
 		return l1, l2, nil
 	}
 
-	// pick two consecutive ports in range 65535-10000
-	// RTP port must be even and RTCP port odd
+	// pick two consecutive ports.
+	// RTP port must be even and RTCP port odd.
 	for {
-		v, err := randInRange((65535 - 10000) / 2)
+		v, err := randInRange(int((c.UDPSourcePortRange[1] - c.UDPSourcePortRange[0]) / 2))
 		if err != nil {
 			return nil, nil, err
 		}
 
-		rtpPort := v*2 + 10000
+		rtpPort := v*2 + int(c.UDPSourcePortRange[0])
 		rtcpPort := rtpPort + 1
 
 		l1 := &clientUDPListener{