Add sharedsock example (#1116)

sharedsock/example/main.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"context"
+	"github.com/netbirdio/netbird/sharedsock"
+	log "github.com/sirupsen/logrus"
+	"os"
+	"os/signal"
+)
+
+func main() {
+
+	port := 51820
+	rawSock, err := sharedsock.Listen(port, sharedsock.NewIncomingSTUNFilter())
+	if err != nil {
+		panic(err)
+	}
+
+	log.Infof("attached to to the raw socket on port %d", port)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	// read packets
+	go func() {
+		buf := make([]byte, 1500)
+		for {
+			select {
+			case <-ctx.Done():
+				log.Debugf("stopped reading from the shared socket")
+				return
+			default:
+				size, addr, err := rawSock.ReadFrom(buf)
+				if err != nil {
+					log.Errorf("error while reading packet from the shared socket: %s", err)
+					continue
+				}
+				log.Infof("read a STUN packet of size %d from %s", size, addr.String())
+			}
+		}
+	}()
+
+	// terminate the program on ^C
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, os.Interrupt)
+	go func() {
+		for range c {
+			log.Infof("received ^C signal, stopping the program")
+			cancel()
+			err = rawSock.Close()
+			if err != nil {
+				log.Errorf("failed closing raw socket")
+			}
+		}
+	}()
+
+	<-ctx.Done()
+}