Add health check for DNS

vendor/golang.org/x/net/ipv4/payload.go

@@ -0,0 +1,23 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipv4
+
+import (
+	"net"
+
+	"golang.org/x/net/internal/socket"
+)
+
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
+// A payloadHandler represents the IPv4 datagram payload handler.
+type payloadHandler struct {
+	net.PacketConn
+	*socket.Conn
+	rawOpt
+}
+
+func (c *payloadHandler) ok() bool { return c != nil && c.PacketConn != nil && c.Conn != nil }