portmap plugin should flush previous udp connections

conntrack does not have any way to track UDP connections, so
it relies on timers to delete a connection.
The problem is that UDP is connectionless, so a client will keep
sending traffic despite the server has gone, thus renewing the
conntrack entries.
Pods that use portmaps to expose UDP services need to flush the existing
conntrack entries on the port exposed when they are created,
otherwise conntrack will keep sending the traffic to the previous IP
until the connection age (the client stops sending traffic)

Signed-off-by: Antonio Ojea <aojea@redhat.com>

pkg/testutils/echo/server/main.go

@@ -10,6 +10,7 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"log"
 	"net"
 	"os"
 	"strings"
@@ -17,21 +18,50 @@ import (
 )
 
 func main() {
+	// Start TCP server
 	listener, err := net.Listen("tcp", ":")
 	if err != nil {
 		panic(err)
 	}
+	defer listener.Close()
+	// use the same port for UDP
 	_, port, err := net.SplitHostPort(listener.Addr().String())
 	if err != nil {
 		panic(err)
 	}
 	fmt.Printf("127.0.0.1:%s\n", port)
-	for {
-		conn, err := listener.Accept()
-		if err != nil {
-			panic(err)
+	go func() {
+		for {
+			conn, err := listener.Accept()
+			if err != nil {
+				panic(err)
+			}
+			go handleConnection(conn)
+		}
+	}()
+
+	// Start UDP server
+	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%s", port))
+	if err != nil {
+		log.Fatalf("Error from net.ResolveUDPAddr(): %s", err)
+	}
+	sock, err := net.ListenUDP("udp", addr)
+	if err != nil {
+		log.Fatalf("Error from ListenUDP(): %s", err)
+	}
+	defer sock.Close()
+
+	buffer := make([]byte, 1024)
+	for {
+		n, addr, err := sock.ReadFrom(buffer)
+		if err != nil {
+			log.Fatalf("Error from ReadFrom(): %s", err)
+		}
+		sock.SetWriteDeadline(time.Now().Add(1 * time.Minute))
+		n, err = sock.WriteTo(buffer[0:n], addr)
+		if err != nil {
+			return
 		}
-		go handleConnection(conn)
 	}
 }
 
@@ -53,5 +83,4 @@ func handleConnection(conn net.Conn) {
 		fmt.Fprint(os.Stderr, err.Error())
 		return
 	}
-
 }