fix building on plan9

udp.go

@@ -1,4 +1,4 @@
-// +build !windows
+// +build !windows,!plan9
 
 package dns
 

udp_other.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux,!plan9
 
 package dns
 

udp_plan9.go

@@ -0,0 +1,34 @@
+package dns
+
+import (
+	"net"
+)
+
+func setUDPSocketOptions(conn *net.UDPConn) error { return nil }
+
+// SessionUDP holds the remote address and the associated
+// out-of-band data.
+type SessionUDP struct {
+	raddr   *net.UDPAddr
+	context []byte
+}
+
+// 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.
+func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
+	oob := make([]byte, 40)
+	n, oobn, _, raddr, err := conn.ReadMsgUDP(b, oob)
+	if err != nil {
+		return n, nil, err
+	}
+	return n, &SessionUDP{raddr, oob[:oobn]}, err
+}
+
+// WriteToSessionUDP acts just like net.UDPConn.WritetTo(), but uses a *SessionUDP instead of a net.Addr.
+func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
+	n, _, err := conn.WriteMsgUDP(b, session.context, session.raddr)
+	return n, err
+}