ipv6: update example for the use of dual stack listener

Updates golang/go#13612.

Change-Id: Ic139b6f862a2a24d549bd1f502299a7eac2f0e38
Reviewed-on: https://go-review.googlesource.com/17799Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv6/doc.go

@@ -130,7 +130,7 @@
 //			// error handling
 //		}
 //		dst := &net.UDPAddr{IP: group, Port: 1024}
-//		wcm := ipv6.ControlMessage{TrafficClass: DiffServCS7, HopLimit: 1}
+//		wcm := ipv6.ControlMessage{TrafficClass: 0xe0, HopLimit: 1}
 //		for _, ifi := range []*net.Interface{en0, en1} {
 //			wcm.IfIndex = ifi.Index
 //			if _, err := p.WriteTo(data[:n], &wcm, dst); err != nil {

ipv6/example_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func ExampleConn_markingTCP() {
-	ln, err := net.Listen("tcp6", "[::]:1024")
+	ln, err := net.Listen("tcp", "[::]:1024")
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -29,12 +29,14 @@ func ExampleConn_markingTCP() {
 		}
 		go func(c net.Conn) {
 			defer c.Close()
-			p := ipv6.NewConn(c)
-			if err := p.SetTrafficClass(0x28); err != nil { // DSCP AF11
-				log.Fatal(err)
-			}
-			if err := p.SetHopLimit(128); err != nil {
-				log.Fatal(err)
+			if c.RemoteAddr().(*net.TCPAddr).IP.To16() != nil && c.RemoteAddr().(*net.TCPAddr).IP.To4() == nil {
+				p := ipv6.NewConn(c)
+				if err := p.SetTrafficClass(0x28); err != nil { // DSCP AF11
+					log.Fatal(err)
+				}
+				if err := p.SetHopLimit(128); err != nil {
+					log.Fatal(err)
+				}
 			}
 			if _, err := c.Write([]byte("HELLO-R-U-THERE-ACK")); err != nil {
 				log.Fatal(err)