wgengine/magicsock: use netaddr.IP in listenPacket

It must be an IP address; enforce that at the type level.

Suggested-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

wgengine/magicsock/magicsock.go

@@ -2596,10 +2596,17 @@ func (c *Conn) initialBind() error {
 	return nil
 }
 
-// listenPacket opens a listener. Host must be "" or an IP address.
-func (c *Conn) listenPacket(network, host string, port uint16) (net.PacketConn, error) {
+// listenPacket opens a packet listener.
+// The network must be "udp4" or "udp6".
+// Host is the (local) IP address to listen on; use the zero IP to leave unspecified.
+func (c *Conn) listenPacket(network string, host netaddr.IP, port uint16) (net.PacketConn, error) {
 	ctx := context.Background() // unused without DNS name to resolve
-	addr := net.JoinHostPort(host, fmt.Sprint(port))
+	// Translate host to package net: "" for the zero value, the IP address string otherwise.
+	var s string
+	if !host.IsZero() {
+		s = host.String()
+	}
+	addr := net.JoinHostPort(s, fmt.Sprint(port))
 	if c.packetListener != nil {
 		return c.packetListener.ListenPacket(ctx, network, addr)
 	}
@@ -2611,11 +2618,15 @@ func (c *Conn) listenPacket(network, host string, port uint16) (net.PacketConn,
 // If rucPtr had an existing UDP socket bound, it closes that socket.
 // The caller is responsible for informing the portMapper of any changes.
 func (c *Conn) bindSocket(rucPtr **RebindingUDPConn, network string) error {
-	host := ""
+	var host netaddr.IP
 	if inTest() && !c.simulatedNetwork {
-		host = "127.0.0.1"
-		if network == "udp6" {
-			host = "::1"
+		switch network {
+		case "udp4":
+			host = netaddr.MustParseIP("127.0.0.1")
+		case "udp6":
+			host = netaddr.MustParseIP("::1")
+		default:
+			panic("unrecognized network in bindSocket: " + network)
 		}
 	}