fix: initialize source address in readLoop from remote addr

Previously, the `readLoop` would lazily initialize the source address (`src`) based on the first incoming packet. This behavior meant that for a short window during initialization, the session might accept a packet from an arbitrary address, potentially allowing connection hijacking or address spoofing.

This change ensures that if `s.remote` is already known (e.g., in `Dial` mode), `src` is initialized immediately before entering the loop. This locks the session to the expected remote address from thefix: initialize source address in readLoop from remote addr

Previously, the `readLoop` would lazily initialize the source address (`src`) based on the first incoming packet. This behavior meant that for a short window during initialization, the session might accept a packet from an arbitrary address, potentially allowing connection hijacking or address spoofing.

This change ensures that if `s.remote` is already known (e.g., in `Dial` mode), `src` is initialized immediately before entering the loop. This locks the session to the expected remote address from the start.

readloop.go

@@ -33,6 +33,9 @@ func (s *UDPSession) defaultReadLoop() {
 	buf := make([]byte, mtuLimit)
 
 	var src string
+	if s.remote != nil {
+		src = s.remote.String()
+	}
 	for {
 		n, addr, err := s.conn.ReadFrom(buf)
 		if err != nil {
@@ -45,14 +48,9 @@ func (s *UDPSession) defaultReadLoop() {
 		}
 
 		// make sure the packet is from the same source
-		switch src {
-		case "":
-			// set source address if not set
+		if src == "" { // set source address if nil
 			src = addr.String()
-		case addr.String():
-			// source valid
-		default:
-			// source invalid
+		} else if addr.String() != src {
 			atomic.AddUint64(&DefaultSnmp.InErrs, 1)
 			continue
 		}

readloop_linux.go

@@ -46,6 +46,9 @@ func (s *UDPSession) readLoop() {
 
 	// x/net version
 	var src string
+	if s.remote != nil {
+		src = s.remote.String()
+	}
 	msgs := make([]ipv4.Message, batchSize)
 	for k := range msgs {
 		msgs[k].Buffers = [][]byte{make([]byte, mtuLimit)}
@@ -66,14 +69,9 @@ func (s *UDPSession) readLoop() {
 			msg := &msgs[i]
 
 			// make sure the packet is from the same source
-			switch src {
-			case "":
-				// set source address if not set
+			if src == "" { // set source address if nil
 				src = msg.Addr.String()
-			case msg.Addr.String():
-				// source valid
-			default:
-				// source invalid
+			} else if msg.Addr.String() != src {
 				atomic.AddUint64(&DefaultSnmp.InErrs, 1)
 				continue
 			}