derp: include src IPs in mesh watch messages

Updates tailscale/corp#13945

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_client.go

@@ -363,7 +363,12 @@ func (PeerGoneMessage) msg() {}
 
 // PeerPresentMessage is a ReceivedMessage that indicates that the client
 // is connected to the server. (Only used by trusted mesh clients)
-type PeerPresentMessage key.NodePublic
+type PeerPresentMessage struct {
+	// Key is the public key of the client.
+	Key key.NodePublic
+	// IPPort is the remote IP and port of the client.
+	IPPort netip.AddrPort
+}
 
 func (PeerPresentMessage) msg() {}
 
@@ -546,8 +551,15 @@ func (c *Client) recvTimeout(timeout time.Duration) (m ReceivedMessage, err erro
 				c.logf("[unexpected] dropping short peerPresent frame from DERP server")
 				continue
 			}
-			pg := PeerPresentMessage(key.NodePublicFromRaw32(mem.B(b[:keyLen])))
-			return pg, nil
+			var msg PeerPresentMessage
+			msg.Key = key.NodePublicFromRaw32(mem.B(b[:keyLen]))
+			if n >= keyLen+16+2 {
+				msg.IPPort = netip.AddrPortFrom(
+					netip.AddrFrom16([16]byte(b[keyLen:keyLen+16])).Unmap(),
+					binary.BigEndian.Uint16(b[keyLen+16:keyLen+16+2]),
+				)
+			}
+			return msg, nil
 
 		case frameRecvPacket:
 			var rp ReceivedPacket