Rollback PR #738 because it breaks compatibility with Windows (#765)

udp.go

@@ -1,3 +1,5 @@
+// +build !windows
+
 package dns
 
 import (

udp_test.go

@@ -1,3 +1,5 @@
+// +build linux,!appengine
+
 package dns
 
 import (

udp_windows.go

@@ -0,0 +1,37 @@
+// +build windows
+
+package dns
+
+import "net"
+
+// SessionUDP holds the remote address
+type SessionUDP struct {
+	raddr *net.UDPAddr
+}
+
+// RemoteAddr returns the remote network address.
+func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
+
+// ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
+// net.UDPAddr.
+// TODO(fastest963): Once go1.10 is released, use ReadMsgUDP.
+func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
+	n, raddr, err := conn.ReadFrom(b)
+	if err != nil {
+		return n, nil, err
+	}
+	session := &SessionUDP{raddr.(*net.UDPAddr)}
+	return n, session, err
+}
+
+// WriteToSessionUDP acts just like net.UDPConn.WriteTo(), but uses a *SessionUDP instead of a net.Addr.
+// TODO(fastest963): Once go1.10 is released, use WriteMsgUDP.
+func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
+	n, err := conn.WriteTo(b, session.raddr)
+	return n, err
+}
+
+// TODO(fastest963): Once go1.10 is released and we can use *MsgUDP methods
+// use the standard method in udp.go for these.
+func setUDPSocketOptions(*net.UDPConn) error { return nil }
+func parseDstFromOOB([]byte, net.IP) net.IP  { return nil }