FIX: response source address on [::]-bound servers are wrong

.gitignore

@@ -11,7 +11,7 @@ client
 !client/
 vendor/
 v3/
-
+.idea/
 # Test binary, build with `go test -c`
 *.test
 

.vscode/launch.json

@@ -0,0 +1,12 @@
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Run simple server",
+            "type": "go",
+            "request": "launch",
+            "mode": "auto",
+            "program": "${workspaceFolder}/examples/observe/server"
+        }
+    ]
+}

udp/client/conn.go

@@ -206,6 +206,9 @@ type Conn struct {
 	msgID                 atomic.Uint32
 	blockwiseSZX          blockwise.SZX
 
+	localAddr      atomic.Pointer[net.IP]
+	interfaceIndex atomic.Int64
+
 	/*
 		An outstanding interaction is either a CON for which an ACK has not
 		yet been received but is still expected (message layer) or a request
@@ -550,6 +553,8 @@ func (cc *Conn) writeMessageAsync(req *pool.Message) error {
 func (cc *Conn) writeMessage(req *pool.Message) error {
 	req.UpsertType(message.Confirmable)
 	req.UpsertMessageID(cc.GetMessageID())
+	cc.upsertControlInformation(req)
+
 	if req.Type() != message.Confirmable {
 		return cc.writeMessageAsync(req)
 	}
@@ -755,6 +760,7 @@ func (cc *Conn) processResponse(reqType message.Type, reqMessageID int32, w *res
 		w.Message().SetType(message.Acknowledgement)
 		w.Message().SetMessageID(reqMessageID)
 		w.Message().SetToken(nil)
+
 		err := cc.addResponseToCache(w.Message())
 		if err != nil {
 			return fmt.Errorf("cannot cache response: %w", err)
@@ -824,7 +830,8 @@ func (cc *Conn) ProcessReceivedMessageWithHandler(req *pool.Message, handler con
 	}()
 	resp := cc.AcquireMessage(cc.Context())
 	resp.SetToken(req.Token())
-	ifIndex := req.ControlMessage().GetIfIndex()
+	cc.setControlInformation(req.ControlMessage())
+
 	w := responsewriter.New(resp, cc, req.Options()...)
 	defer func() {
 		cc.ReleaseMessage(w.Message())
@@ -839,7 +846,7 @@ func (cc *Conn) ProcessReceivedMessageWithHandler(req *pool.Message, handler con
 		// nothing to send
 		return
 	}
-	upsertInterfaceToMessage(w.Message(), ifIndex)
+	cc.upsertControlInformation(w.Message())
 	errW := cc.writeMessageAsync(w.Message())
 	if errW != nil {
 		cc.closeConnection()
@@ -851,13 +858,37 @@ func (cc *Conn) handlePong(w *responsewriter.ResponseWriter[*Conn], r *pool.Mess
 	cc.sendPong(w, r)
 }
 
-func upsertInterfaceToMessage(m *pool.Message, ifIndex int) {
-	if ifIndex >= 1 {
-		cm := coapNet.ControlMessage{
-			IfIndex: ifIndex,
-		}
-		m.UpsertControlMessage(&cm)
+func (cc *Conn) setControlInformation(cm *coapNet.ControlMessage) {
+	if cm == nil {
+		cc.interfaceIndex.Store(0)
+		cc.localAddr.Store(nil)
+		return
+	}
+
+	cc.interfaceIndex.Store(int64(cm.GetIfIndex()))
+	if len(cm.Dst) == 0 || cm.Dst.IsMulticast() {
+		cc.localAddr.Store(nil)
+		return
 	}
+
+	dst := make(net.IP, len(cm.Dst))
+	copy(dst, cm.Dst)
+	cc.localAddr.Store(&dst)
+}
+
+func (cc *Conn) upsertControlInformation(msg *pool.Message) {
+	ifIndex := int(cc.interfaceIndex.Load())
+	localAddrPtr := cc.localAddr.Load()
+	if ifIndex < 1 && localAddrPtr == nil {
+		return
+	}
+
+	var localAddr net.IP
+	if localAddrPtr != nil {
+		localAddr = *localAddrPtr
+	}
+
+	msg.UpsertControlMessage(&coapNet.ControlMessage{IfIndex: ifIndex, Src: localAddr})
 }
 
 func (cc *Conn) handleSpecialMessages(r *pool.Message) bool {
@@ -872,7 +903,6 @@ func (cc *Conn) handleSpecialMessages(r *pool.Message) bool {
 		elem.ReleaseMessage(cc)
 		resp := cc.AcquireMessage(cc.Context())
 		resp.SetToken(r.Token())
-		upsertInterfaceToMessage(resp, r.ControlMessage().GetIfIndex())
 		w := responsewriter.New(resp, cc, r.Options()...)
 		defer func() {
 			cc.ReleaseMessage(w.Message())

udp/client/controlmessage_test.go

@@ -0,0 +1,49 @@
+package client
+
+import (
+	"net"
+	"testing"
+
+	coapNet "github.com/plgd-dev/go-coap/v3/net"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSetControlInformationNil(t *testing.T) {
+	var cc Conn
+	addr := net.ParseIP("192.0.2.1").To4()
+	cc.localAddr.Store(&addr)
+	cc.interfaceIndex.Store(7)
+
+	cc.setControlInformation(nil)
+
+	require.Equal(t, int64(0), cc.interfaceIndex.Load())
+	require.Nil(t, cc.localAddr.Load())
+}
+
+func TestSetControlInformationUnicastCopiesAddress(t *testing.T) {
+	var cc Conn
+	dst := net.ParseIP("2001:db8::1")
+	cm := &coapNet.ControlMessage{Dst: append(net.IP(nil), dst...), IfIndex: 9}
+
+	cc.setControlInformation(cm)
+
+	require.Equal(t, int64(9), cc.interfaceIndex.Load())
+	stored := cc.localAddr.Load()
+	require.NotNil(t, stored)
+	expected := append(net.IP(nil), dst...)
+	require.True(t, stored.Equal(expected))
+
+	cm.Dst[0] ^= 0xff
+	require.True(t, stored.Equal(expected))
+}
+
+func TestSetControlInformationMulticastClearsAddress(t *testing.T) {
+	var cc Conn
+	addr := net.ParseIP("192.0.2.1").To4()
+	cc.localAddr.Store(&addr)
+
+	cc.setControlInformation(&coapNet.ControlMessage{Dst: net.ParseIP("ff02::1"), IfIndex: 11})
+
+	require.Equal(t, int64(11), cc.interfaceIndex.Load())
+	require.Nil(t, cc.localAddr.Load())
+}

udp/server/server.go

@@ -159,7 +159,19 @@ func (s *Server) Serve(l *coapNet.UDPConn) error {
 			}
 		}
 		buf = buf[:n]
-		cc, err := s.getConn(l, raddr, true)
+
+		// UDPConn.LocalAddr() only takes into account the address it is bound to.
+		// In the case of a wildcard address, the actual destination address is in the control message.
+		// On server-initiated exchanges, listener's LocalAddr can be used as the client has no assumptions of the source.
+		laddr, err := s.getListenerLocalAddr(l)
+		if err != nil {
+			return err
+		}
+		if cm != nil && len(cm.Dst) > 0 && !cm.Dst.IsMulticast() {
+			laddr.IP = cm.Dst
+		}
+
+		cc, err := s.getConn(l, raddr, laddr, true)
 		if err != nil {
 			s.cfg.Errors(fmt.Errorf("%v: cannot get client connection: %w", raddr, err))
 			continue
@@ -178,6 +190,15 @@ func (s *Server) getListener() *coapNet.UDPConn {
 	return s.listen
 }
 
+func (s *Server) getListenerLocalAddr(l *coapNet.UDPConn) (*net.UDPAddr, error) {
+	localAddr, ok := l.LocalAddr().(*net.UDPAddr)
+	if !ok || localAddr == nil {
+		return nil, fmt.Errorf("unexpected listener local addr type: %T", l.LocalAddr())
+	}
+	laddrVal := *localAddr
+	return &laddrVal, nil
+}
+
 // Stop stops server without wait of ends Serve function.
 func (s *Server) Stop() {
 	s.cancel()
@@ -254,10 +275,21 @@ func getClose(cc *client.Conn) func() {
 	return closeFn
 }
 
-func (s *Server) getOrCreateConn(udpConn *coapNet.UDPConn, raddr *net.UDPAddr) (cc *client.Conn, created bool) {
+func getConnKey(raddr *net.UDPAddr, laddr *net.UDPAddr) string {
+	normalizedLocalAddr := *laddr
+	if len(normalizedLocalAddr.IP) > 0 && normalizedLocalAddr.IP.IsMulticast() {
+		// Multicast destination address does not identify a unique server-side source address.
+		// Normalize it to avoid creating one conn key per multicast group.
+		normalizedLocalAddr.IP = nil
+		normalizedLocalAddr.Zone = ""
+	}
+	return raddr.String() + "-" + normalizedLocalAddr.String()
+}
+
+func (s *Server) getOrCreateConn(udpConn *coapNet.UDPConn, raddr *net.UDPAddr, laddr *net.UDPAddr) (cc *client.Conn, created bool) {
 	s.connsMutex.Lock()
 	defer s.connsMutex.Unlock()
-	key := raddr.String()
+	key := getConnKey(raddr, laddr)
 	cc = s.conns[key]
 
 	if cc != nil {
@@ -345,8 +377,19 @@ func (s *Server) getOrCreateConn(udpConn *coapNet.UDPConn, raddr *net.UDPAddr) (
 	return cc, true
 }
 
-func (s *Server) getConn(l *coapNet.UDPConn, raddr *net.UDPAddr, firstTime bool) (*client.Conn, error) {
-	cc, created := s.getOrCreateConn(l, raddr)
+func (s *Server) getConn(l *coapNet.UDPConn, raddr *net.UDPAddr, laddr *net.UDPAddr, firstTime bool) (*client.Conn, error) {
+	if raddr == nil {
+		return nil, errors.New("invalid remote address")
+	}
+	if laddr == nil {
+		var err error
+		laddr, err = s.getListenerLocalAddr(l)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	cc, created := s.getOrCreateConn(l, raddr, laddr)
 	if created {
 		if s.cfg.OnNewConn != nil {
 			s.cfg.OnNewConn(cc)
@@ -367,18 +410,30 @@ func (s *Server) getConn(l *coapNet.UDPConn, raddr *net.UDPAddr, firstTime bool)
 			closeFn()
 		}
 		if firstTime {
-			return s.getConn(l, raddr, false)
+			return s.getConn(l, raddr, laddr, false)
 		}
 		return nil, errors.New("connection is closed")
 	}
 	return cc, nil
 }
 
-func (s *Server) NewConn(addr *net.UDPAddr) (*client.Conn, error) {
+// NewConn creates or gets a connection for the provided remote address.
+//
+// Optional laddr may be used to pin a concrete local address when the listener is bound to a wildcard address.
+// If laddr is omitted or nil, listener's local address is used.
+func (s *Server) NewConn(addr *net.UDPAddr, laddr ...*net.UDPAddr) (*client.Conn, error) {
+	if len(laddr) > 1 {
+		return nil, fmt.Errorf("invalid number of local addresses: %d", len(laddr))
+	}
+	var localAddr *net.UDPAddr
+	if len(laddr) == 1 {
+		localAddr = laddr[0]
+	}
+
 	l := s.getListener()
 	if l == nil {
 		// server is not started/stopped
 		return nil, errors.New("server is not running")
 	}
-	return s.getConn(l, addr, true)
+	return s.getConn(l, addr, localAddr, true)
 }

udp/server/server_key_test.go

@@ -0,0 +1,26 @@
+package server
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetConnKeyIgnoresMulticastLocalAddress(t *testing.T) {
+	raddr := &net.UDPAddr{IP: net.ParseIP("2001:db8::1"), Port: 56830}
+	mcastV6 := &net.UDPAddr{IP: net.ParseIP("ff02::fd"), Port: 5683}
+	mcastV4 := &net.UDPAddr{IP: net.ParseIP("224.0.1.187"), Port: 5683}
+	normalized := &net.UDPAddr{Port: 5683}
+
+	require.Equal(t, getConnKey(raddr, mcastV6), getConnKey(raddr, normalized))
+	require.Equal(t, getConnKey(raddr, mcastV4), getConnKey(raddr, normalized))
+}
+
+func TestGetConnKeyKeepsUnicastLocalAddressDistinct(t *testing.T) {
+	raddr := &net.UDPAddr{IP: net.ParseIP("2001:db8::1"), Port: 56830}
+	laddrA := &net.UDPAddr{IP: net.ParseIP("2001:db8::10"), Port: 5683}
+	laddrB := &net.UDPAddr{IP: net.ParseIP("2001:db8::11"), Port: 5683}
+
+	require.NotEqual(t, getConnKey(raddr, laddrA), getConnKey(raddr, laddrB))
+}

udp/server_test.go

@@ -467,6 +467,9 @@ func TestServerNewClient(t *testing.T) {
 
 	time.Sleep(time.Second)
 
+	_, err = s1.NewConn(nil)
+	require.ErrorContains(t, err, "invalid remote address")
+
 	cc, err := s1.NewConn(peer)
 	require.NoError(t, err)
 
@@ -478,7 +481,7 @@ func TestServerNewClient(t *testing.T) {
 	require.NoError(t, err)
 
 	// repeat ping - new client should be created
-	cc, err = s1.NewConn(peer)
+	cc, err = s1.NewConn(peer, nil)
 	require.NoError(t, err)
 	err = cc.Ping(ctx)
 	require.NoError(t, err)
@@ -626,7 +629,7 @@ func TestServerReconnectNewClient(t *testing.T) {
 	}
 
 	// new client
-	cc, err = s1.NewConn(peer)
+	cc, err = s1.NewConn(peer, nil)
 	require.NoError(t, err)
 	ctx, cancel = context.WithTimeout(context.Background(), time.Second*1)
 	defer cancel()