cmd/natc: use ListenPacket

Now that tsnet supports it, use it.

Updates tailscale/corp#20503

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/natc/natc.go

@@ -35,7 +35,6 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsnet"
 	"tailscale.com/tsweb"
-	"tailscale.com/types/nettype"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/mak"
 )
@@ -169,24 +168,27 @@ func (c *connector) run(ctx context.Context) {
 		log.Fatalf("failed to advertise routes: %v", err)
 	}
 	c.ts.RegisterFallbackTCPHandler(c.handleTCPFlow)
+	c.serveDNS()
+}
 
-	ln, err := c.ts.Listen("udp", net.JoinHostPort(c.dnsAddr.String(), "53"))
+func (c *connector) serveDNS() {
+	pc, err := c.ts.ListenPacket("udp", net.JoinHostPort(c.dnsAddr.String(), "53"))
 	if err != nil {
 		log.Fatalf("failed listening on port 53: %v", err)
 	}
-	defer ln.Close()
-	log.Printf("Listening for DNS on %s", ln.Addr())
-	c.serveDNS(ln)
-}
-
-func (c *connector) serveDNS(ln net.Listener) {
+	defer pc.Close()
+	log.Printf("Listening for DNS on %s", pc.LocalAddr().String())
 	for {
-		conn, err := ln.Accept()
+		buf := make([]byte, 1500)
+		n, addr, err := pc.ReadFrom(buf)
 		if err != nil {
-			log.Printf("serveDNS accept: %v", err)
-			return
+			if errors.Is(err, net.ErrClosed) {
+				return
+			}
+			log.Printf("serveDNS.ReadFrom failed: %v", err)
+			continue
 		}
-		go c.handleDNS(conn.(nettype.ConnPacketConn))
+		go c.handleDNS(pc, buf[:n], addr.(*net.UDPAddr))
 	}
 }
 
@@ -201,27 +203,17 @@ func (c *connector) serveDNS(ln net.Listener) {
 //
 // This assignment later allows the connector to determine where to forward
 // traffic based on the destination IP address.
-func (c *connector) handleDNS(conn nettype.ConnPacketConn) {
-	defer conn.Close()
+func (c *connector) handleDNS(pc net.PacketConn, buf []byte, remoteAddr *net.UDPAddr) {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	remoteAddr := conn.RemoteAddr().(*net.UDPAddr).AddrPort()
 	who, err := c.lc.WhoIs(ctx, remoteAddr.String())
 	if err != nil {
 		log.Printf("HandleDNS: WhoIs failed: %v\n", err)
 		return
 	}
-	conn.SetReadDeadline(time.Now().Add(5 * time.Second))
-
-	buf := make([]byte, 1500)
-	n, err := conn.Read(buf)
-	if err != nil {
-		log.Printf("HandleDNS: read failed: %v\n ", err)
-		return
-	}
 
 	var msg dnsmessage.Message
-	err = msg.Unpack(buf[:n])
+	err = msg.Unpack(buf)
 	if err != nil {
 		log.Printf("HandleDNS: dnsmessage unpack failed: %v\n ", err)
 		return
@@ -236,7 +228,7 @@ func (c *connector) handleDNS(conn nettype.ConnPacketConn) {
 		return
 	}
 	// This connector handled the DNS request
-	_, err = conn.Write(resp)
+	_, err = pc.WriteTo(resp, remoteAddr)
 	if err != nil {
 		log.Printf("HandleDNS: write failed: %v\n", err)
 	}