Add server example for DTLS CIDs

Adds a server example for DTLS Connection IDs. Random 8 byte CIDs are
used. Note that the built-in go-coap DTLS listener does not support this
functionality presently as it uses a UDP listener from pion/transport
that does not support routing datagrams based on attributes other than
remote address.

Signed-off-by: Daniel Mangum <[email protected]>

examples/dtls/cid/server/main.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"log"
+	"net"
+	"sync/atomic"
+	"time"
+
+	piondtls "github.com/pion/dtls/v2"
+	"github.com/plgd-dev/go-coap/v3/dtls/server"
+	"github.com/plgd-dev/go-coap/v3/message"
+	"github.com/plgd-dev/go-coap/v3/message/codes"
+	"github.com/plgd-dev/go-coap/v3/mux"
+	"github.com/plgd-dev/go-coap/v3/options"
+	udpClient "github.com/plgd-dev/go-coap/v3/udp/client"
+)
+
+func handleA(w mux.ResponseWriter, r *mux.Message) {
+	log.Printf("got message in handleA:  %+v from %v\n", r, w.Conn().RemoteAddr())
+	err := w.SetResponse(codes.GET, message.TextPlain, bytes.NewReader([]byte("A hello world")))
+	if err != nil {
+		log.Printf("cannot set response: %v", err)
+	}
+}
+
+func handleB(w mux.ResponseWriter, r *mux.Message) {
+	log.Printf("got message in handleB:  %+v from %v\n", r, w.Conn().RemoteAddr())
+	customResp := w.Conn().AcquireMessage(r.Context())
+	defer w.Conn().ReleaseMessage(customResp)
+	customResp.SetCode(codes.Content)
+	customResp.SetToken(r.Token())
+	customResp.SetContentFormat(message.TextPlain)
+	customResp.SetBody(bytes.NewReader([]byte("B hello world")))
+	err := w.Conn().WriteMessage(customResp)
+	if err != nil {
+		log.Printf("cannot set response: %v", err)
+	}
+}
+
+// wrappedListener wraps a net.Listener and implements a go-coap DTLS
+// server.Listener.
+// NOTE: this utility is for example purposes only. Context should be handled
+// properly in meaningful scenarios.
+type wrappedListener struct {
+	l      net.Listener
+	closed atomic.Bool
+}
+
+// AcceptWithContext disregards the passed context and calls the underlying
+// net.Listener Accept().
+func (w *wrappedListener) AcceptWithContext(_ context.Context) (net.Conn, error) {
+	return w.l.Accept()
+}
+
+// Close calls the underlying net.Listener Close().
+func (w *wrappedListener) Close() error {
+	return w.l.Close()
+}
+
+// wrapListener wraps a net.Listener and returns a DTLS server.Listener.
+func wrapListener(l net.Listener) server.Listener {
+	return &wrappedListener{
+		l: l,
+	}
+}
+
+func main() {
+	m := mux.NewRouter()
+	m.Handle("/a", mux.HandlerFunc(handleA))
+	m.Handle("/b", mux.HandlerFunc(handleB))
+	laddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:5688")
+	if err != nil {
+		log.Fatalf("Error dialing: %v", err)
+	}
+	l, err := piondtls.Listen("udp", laddr, &piondtls.Config{
+		PSK: func(hint []byte) ([]byte, error) {
+			fmt.Printf("Client's hint: %s \n", hint)
+			return []byte{0xAB, 0xC1, 0x23}, nil
+		},
+		PSKIdentityHint:       []byte("Pion DTLS Server"),
+		CipherSuites:          []piondtls.CipherSuiteID{piondtls.TLS_PSK_WITH_AES_128_CCM_8},
+		ConnectionIDGenerator: piondtls.RandomCIDGenerator(8),
+	})
+	if err != nil {
+		log.Fatalf("Error establishing DTLS listener: %v", err)
+	}
+	s := server.New(options.WithMux(m), options.WithInactivityMonitor(10*time.Second, func(cc *udpClient.Conn) {}))
+	s.Serve(wrapListener(l))
+}