net/portmapper: handle pcp ADDRESS_MISMATCH response

These show up a fair amount in our logs.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

net/portmapper/pcp.go

@@ -28,6 +28,12 @@ const (
 
 	pcpCodeOK            = 0
 	pcpCodeNotAuthorized = 2
+	// From RFC 6887:
+	// ADDRESS_MISMATCH: The source IP address of the request packet does
+	// not match the contents of the PCP Client's IP Address field, due
+	// to an unexpected NAT on the path between the PCP client and the
+	// PCP-controlled NAT or firewall.
+	pcpCodeAddressMismatch = 12
 
 	pcpOpReply    = 0x80 // OR'd into request's op code on response
 	pcpOpAnnounce = 0

net/portmapper/portmapper.go

@@ -791,6 +791,10 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 						// provide port mapping services.
 						res.PCP = false
 						continue
+					case pcpCodeAddressMismatch:
+						// A PCP service is running, but it is behind a NAT, so it can't help us.
+						res.PCP = false
+						continue
 					default:
 						// Fall through to unexpected log line.
 					}