net/portmapper: send discovery packet for IGD specifically.

There appear to be devices out there which send only their
first descriptor in response to a discovery packet for
`ssdp:all`, for example the Sagemcom FAST3890V3 only sends
urn:schemas-wifialliance-org:device:WFADevice:1

Send both ssdp:all and a discovery frame for
InternetGatewayDevice specifically.

Updates https://github.com/tailscale/tailscale/issues/3557

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/portmapper/portmapper.go

@@ -749,9 +749,16 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 		// See https://github.com/tailscale/tailscale/issues/3197 for
 		// an example of a device that strictly implements UPnP, and
 		// only responds to multicast queries.
+		//
+		// Then we send a discovery packet looking for
+		// urn:schemas-upnp-org:device:InternetGatewayDevice:1 specifically, not
+		// just ssdp:all, because there appear to be devices which only send
+		// their first descriptor (like urn:schemas-wifialliance-org:device:WFADevice:1)
+		// in response to ssdp:all. https://github.com/tailscale/tailscale/issues/3557
 		metricUPnPSent.Add(1)
 		uc.WriteTo(uPnPPacket, upnpAddr)
 		uc.WriteTo(uPnPPacket, upnpMulticastAddr)
+		uc.WriteTo(uPnPIGDPacket, upnpMulticastAddr)
 	}
 
 	buf := make([]byte, 1500)
@@ -877,6 +884,15 @@ var uPnPPacket = []byte("M-SEARCH * HTTP/1.1\r\n" +
 	"MAN: \"ssdp:discover\"\r\n" +
 	"MX: 2\r\n\r\n")
 
+// Send a discovery frame for InternetGatewayDevice, since some devices respond
+// to ssdp:all with only their first descriptor (which is often not IGD).
+// https://github.com/tailscale/tailscale/issues/3557
+var uPnPIGDPacket = []byte("M-SEARCH * HTTP/1.1\r\n" +
+	"HOST: 239.255.255.250:1900\r\n" +
+	"ST: urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\n" +
+	"MAN: \"ssdp:discover\"\r\n" +
+	"MX: 2\r\n\r\n")
+
 // PCP/PMP metrics
 var (
 	// metricPXPResponse counts the number of times we received a PMP/PCP response.