net/portmapper: relax source port check for UPnP responses

Per a packet capture provided, some gateways will reply to a UPnP
discovery packet with a UDP packet with a source port that does not come
from the UPnP port. Accept these packets with a log message.

Updates #7377

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I5d4d5b2a0275009ed60f15c20b484fe2025d094b

net/portmapper/portmapper.go

@@ -805,11 +805,10 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 			continue
 		}
 		ip = ip.Unmap()
-		port := uint16(addr.(*net.UDPAddr).Port)
+
-		switch port {
+		handleUPnPResponse := func() {
-		case c.upnpPort():
 			metricUPnPResponse.Add(1)
-			if mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
+
 			if ip != gw {
 				// https://github.com/tailscale/tailscale/issues/5502
 				c.logf("UPnP discovery response from %v, but gateway IP is %v", ip, gw)
@@ -818,7 +817,7 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 			if err != nil {
 				metricUPnPParseErr.Add(1)
 				c.logf("unrecognized UPnP discovery response; ignoring: %v", err)
-					continue
+				return
 			}
 			metricUPnPOK.Add(1)
 			c.logf("[v1] UPnP reply %+v, %q", meta, buf[:n])
@@ -832,6 +831,22 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 			}
 			c.mu.Unlock()
 		}
+
+		port := uint16(addr.(*net.UDPAddr).Port)
+		switch port {
+		case c.upnpPort():
+			if mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
+				handleUPnPResponse()
+			}
+
+		default:
+			// https://github.com/tailscale/tailscale/issues/7377
+			if mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
+				c.logf("UPnP discovery response from non-UPnP port %d", port)
+				metricUPnPResponseAlternatePort.Add(1)
+				handleUPnPResponse()
+			}
+
 		case c.pxpPort(): // same value for PMP and PCP
 			metricPXPResponse.Add(1)
 			if pres, ok := parsePCPResponse(buf[:n]); ok {
@@ -983,6 +998,10 @@ var (
 	// metricUPnPResponse counts the number of times we received a UPnP response.
 	metricUPnPResponse = clientmetric.NewCounter("portmap_upnp_response")
 
+	// metricUPnPResponseAlternatePort counts the number of times we
+	// received a UPnP response from a port other than the UPnP port.
+	metricUPnPResponseAlternatePort = clientmetric.NewCounter("portmap_upnp_response_alternate_port")
+
 	// metricUPnPParseErr counts the number of times we failed to parse a UPnP response.
 	metricUPnPParseErr = clientmetric.NewCounter("portmap_upnp_parse_err")