feat: add stateless UDP forwarding mode (#853)

Add stateless=true metadata option to UDP listener/handler for raw
datagram forwarding without per-client session tracking, similar to
NGINX Stream Module behavior.

- udp.NewListener: add Stateless field to ListenConfig; branch in
  NewListener/Accept to skip connPool/listenLoop when stateless
- datagramConn: lightweight net.Conn+PacketConn wrapping a single
  UDP datagram — no channels, mutexes, or pool logic
- forward handler: add handleRawDatagram for single request-response
  cycle; extract dialTarget to share hop selection/dial/proxyproto
  preamble with handleRawForwarding

Usage: gost -L "udp://:10000/127.0.0.1:2000?stateless=true"

handler/forward/local/forward.go

@@ -16,10 +16,20 @@ import (
 	xrecorder "github.com/go-gost/x/recorder"
 )
 
-// handleRawForwarding performs node selection, dials the target through the
-// router, and pipes the raw connection. It is used when sniffing is disabled
-// or the protocol was not HTTP/TLS.
-func (h *forwardHandler) handleRawForwarding(ctx context.Context, conn net.Conn, ro *xrecorder.HandlerRecorderObject, log logger.Logger, network, proto string) error {
+// dialResult is returned by dialTarget, carrying the dialed connection
+// and associated state that callers need for subsequent I/O and logging.
+type dialResult struct {
+	cc     net.Conn
+	log    logger.Logger
+	target *chain.Node
+}
+
+// dialTarget performs node selection, Router.Dial, proxy protocol wrapping,
+// and recorder population — the shared preamble for both stream forwarding
+// (handleRawForwarding) and datagram forwarding (handleRawDatagram).
+//
+// The caller must close cc when done.
+func (h *forwardHandler) dialTarget(ctx context.Context, conn net.Conn, ro *xrecorder.HandlerRecorderObject, log logger.Logger, network, proto string) (*dialResult, error) {
 	target := &chain.Node{}
 	if curHop := h.getHop(); curHop != nil {
 		target = curHop.Select(ctx,
@@ -28,7 +38,7 @@ func (h *forwardHandler) handleRawForwarding(ctx context.Context, conn net.Conn,
 	}
 	if target == nil {
 		log.Error(errNodeNotAvailable)
-		return errNodeNotAvailable
+		return nil, errNodeNotAvailable
 	}
 	addr := target.Addr
 	if opts := target.Options(); opts != nil {
@@ -63,12 +73,11 @@ func (h *forwardHandler) handleRawForwarding(ctx context.Context, conn net.Conn,
 		if marker := target.Marker(); marker != nil {
 			marker.Mark()
 		}
-		return err
+		return nil, err
 	}
 	if marker := target.Marker(); marker != nil {
 		marker.Reset()
 	}
-	defer cc.Close()
 
 	cc = proxyproto.WrapClientConn(
 		h.md.proxyProtocol,
@@ -80,14 +89,87 @@ func (h *forwardHandler) handleRawForwarding(ctx context.Context, conn net.Conn,
 	ro.SrcAddr = cc.LocalAddr().String()
 	ro.DstAddr = cc.RemoteAddr().String()
 
+	return &dialResult{
+		cc:     cc,
+		log:    log,
+		target: target,
+	}, nil
+}
+
+// handleRawForwarding performs node selection, dials the target through the
+// router, and pipes the raw connection. It is used when sniffing is disabled
+// or the protocol was not HTTP/TLS.
+func (h *forwardHandler) handleRawForwarding(ctx context.Context, conn net.Conn, ro *xrecorder.HandlerRecorderObject, log logger.Logger, network, proto string) error {
+	dr, err := h.dialTarget(ctx, conn, ro, log, network, proto)
+	if err != nil {
+		return err
+	}
+	defer dr.cc.Close()
+
 	t := time.Now()
-	log.Infof("%s <-> %s", conn.RemoteAddr(), target.Addr)
-	if err := xnet.Pipe(ctx, conn, cc, xnet.WithReadTimeout(h.md.idleTimeout)); err != nil {
+	log.Infof("%s <-> %s", conn.RemoteAddr(), dr.target.Addr)
+	if err := xnet.Pipe(ctx, conn, dr.cc, xnet.WithReadTimeout(h.md.idleTimeout)); err != nil {
 		log.Debugf("pipe: %v", err)
 	}
 	log.WithFields(map[string]any{
 		"duration": time.Since(t),
-	}).Infof("%s >-< %s", conn.RemoteAddr(), target.Addr)
+	}).Infof("%s >-< %s", conn.RemoteAddr(), dr.target.Addr)
+
+	return nil
+}
+
+// handleRawDatagram forwards a single UDP datagram to the selected node and
+// writes the response back. Unlike handleRawForwarding (which uses xnet.Pipe
+// for bidirectional stream copy), this performs a single request-response
+// cycle — appropriate for stateless UDP forwarding where each packet is
+// independent and no session is maintained.
+//
+// The method reuses dialTarget for the shared hop-selection, Router.Dial,
+// proxyproto, and recorder preamble.
+func (h *forwardHandler) handleRawDatagram(ctx context.Context, conn net.Conn, ro *xrecorder.HandlerRecorderObject, log logger.Logger, network, proto string) error {
+	dr, err := h.dialTarget(ctx, conn, ro, log, network, proto)
+	if err != nil {
+		return err
+	}
+	defer dr.cc.Close()
+
+	t := time.Now()
+	log.Infof("%s <-> %s", conn.RemoteAddr(), dr.target.Addr)
+
+	bufp := make([]byte, h.md.bufferSize)
+	n, err := conn.Read(bufp)
+	if err != nil {
+		log.Debugf("conn read: %v", err)
+		return err
+	}
+
+	if _, err := dr.cc.Write(bufp[:n]); err != nil {
+		log.Debugf("outbound write: %v", err)
+		return err
+	}
+
+	// Read the response into the same buffer to avoid a second allocation.
+	if err := dr.cc.SetReadDeadline(time.Now().Add(h.md.readTimeout)); err != nil {
+		log.Debugf("set read deadline: %v", err)
+		return err
+	}
+	n, err = dr.cc.Read(bufp)
+	if err != nil {
+		log.Debugf("outbound read: %v", err)
+		return err
+	}
+	// Clear the deadline so it doesn't affect subsequent use of the
+	// underlying connection (e.g., keep-alive pools).
+	dr.cc.SetReadDeadline(time.Time{})
+
+	if _, err := conn.Write(bufp[:n]); err != nil {
+		log.Debugf("conn write: %v", err)
+		return err
+	}
+
+	log.WithFields(map[string]any{
+		"duration": time.Since(t),
+	}).Infof("%s >-< %s", conn.RemoteAddr(), dr.target.Addr)
 
 	return nil
 }

handler/forward/local/handler.go

@@ -206,6 +206,10 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 		return errRouterNotAvailable
 	}
 
+	if h.md.stateless {
+		return h.handleRawDatagram(ctx, conn, ro, log, network, "udp")
+	}
+
 	var proto string
 	if network == "tcp" && h.md.sniffing {
 		if h.md.sniffingTimeout > 0 {

handler/forward/local/metadata.go

@@ -36,6 +36,9 @@ type metadata struct {
 	privateKey  crypto.PrivateKey
 	alpn        string
 	mitmBypass  bypass.Bypass
+
+	stateless  bool
+	bufferSize int
 }
 
 func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
@@ -70,5 +73,11 @@ func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.alpn = mdutil.GetString(md, "mitm.alpn")
 	h.md.mitmBypass = registry.BypassRegistry().Get(mdutil.GetString(md, "mitm.bypass"))
 
+	h.md.stateless = mdutil.GetBool(md, "stateless")
+	h.md.bufferSize = mdutil.GetInt(md, "bufferSize", "readBufferSize", "udp.bufferSize")
+	if h.md.bufferSize <= 0 {
+		h.md.bufferSize = 4096
+	}
+
 	return
 }

internal/net/udp/listener.go

@@ -19,6 +19,7 @@ type ListenConfig struct {
 	ReadBufferSize int
 	TTL            time.Duration
 	Keepalive      bool
+	Stateless      bool
 	Logger         logger.Logger
 }
 type listener struct {
@@ -33,6 +34,11 @@ type listener struct {
 // NewListener creates a net.Listener from a net.PacketConn by demultiplexing
 // UDP datagrams into per-client net.Conn streams. Idle connections are cleaned
 // up according to cfg.TTL.
+//
+// When cfg.Stateless is true, the listener operates in raw datagram mode:
+// no connPool, no listenLoop goroutine, no per-client session tracking.
+// Each Accept call blocks on a ReadFrom and returns a lightweight
+// datagramConn wrapping a single packet.
 func NewListener(conn net.PacketConn, cfg *ListenConfig) net.Listener {
 	if cfg == nil {
 		cfg = &ListenConfig{}
@@ -40,11 +46,16 @@ func NewListener(conn net.PacketConn, cfg *ListenConfig) net.Listener {
 
 	ln := &listener{
 		conn:    conn,
-		cqueue:  make(chan net.Conn, cfg.Backlog),
 		closed:  make(chan struct{}),
 		errChan: make(chan error, 1),
 		config:  cfg,
 	}
+
+	if cfg.Stateless {
+		return ln
+	}
+
+	ln.cqueue = make(chan net.Conn, cfg.Backlog)
 	ln.connPool = newConnPool(cfg.TTL).WithLogger(cfg.Logger)
 	go ln.listenLoop()
 
@@ -52,6 +63,10 @@ func NewListener(conn net.PacketConn, cfg *ListenConfig) net.Listener {
 }
 
 func (ln *listener) Accept() (conn net.Conn, err error) {
+	if ln.config.Stateless {
+		return ln.acceptStateless()
+	}
+
 	select {
 	case conn = <-ln.cqueue:
 		return
@@ -65,6 +80,30 @@ func (ln *listener) Accept() (conn net.Conn, err error) {
 	}
 }
 
+func (ln *listener) acceptStateless() (net.Conn, error) {
+	b := bufpool.Get(ln.config.ReadBufferSize)
+
+	n, raddr, err := ln.conn.ReadFrom(b)
+	if err != nil {
+		bufpool.Put(b)
+		// Surface the error so the service loop can decide whether to
+		// continue accepting.
+		select {
+		case <-ln.closed:
+			return nil, net.ErrClosed
+		default:
+		}
+		return nil, err
+	}
+
+	return &datagramConn{
+		pc:         ln.conn,
+		data:       b[:n],
+		localAddr:  ln.Addr(),
+		remoteAddr: raddr,
+	}, nil
+}
+
 func (ln *listener) listenLoop() {
 	for {
 		select {
@@ -243,3 +282,67 @@ func (c *conn) WriteQueue(b []byte) error {
 		return errors.New("recv queue is full")
 	}
 }
+
+// datagramConn is a lightweight net.Conn that wraps a single UDP datagram.
+// It is used in stateless mode where each Accept returns a new datagramConn
+// for a single packet — no channels, no mutexes, no pool tracking.
+type datagramConn struct {
+	pc         net.PacketConn
+	data       []byte
+	offset     int
+	localAddr  net.Addr
+	remoteAddr net.Addr
+}
+
+func (c *datagramConn) Read(b []byte) (n int, err error) {
+	if c.data == nil || c.offset >= len(c.data) {
+		return 0, net.ErrClosed
+	}
+	n = copy(b, c.data[c.offset:])
+	c.offset += n
+	return
+}
+
+func (c *datagramConn) Write(b []byte) (n int, err error) {
+	return c.pc.WriteTo(b, c.remoteAddr)
+}
+
+// ReadFrom implements net.PacketConn. It reads the buffered datagram and
+// returns the sender's address.
+func (c *datagramConn) ReadFrom(b []byte) (n int, addr net.Addr, err error) {
+	n, err = c.Read(b)
+	addr = c.remoteAddr
+	return
+}
+
+// WriteTo implements net.PacketConn. It sends b to addr via the underlying
+// PacketConn, ignoring the stored remoteAddr.
+func (c *datagramConn) WriteTo(b []byte, addr net.Addr) (n int, err error) {
+	return c.pc.WriteTo(b, addr)
+}
+
+func (c *datagramConn) Close() error {
+	bufpool.Put(c.data)
+	c.data = nil
+	return nil
+}
+
+func (c *datagramConn) LocalAddr() net.Addr {
+	return c.localAddr
+}
+
+func (c *datagramConn) RemoteAddr() net.Addr {
+	return c.remoteAddr
+}
+
+func (c *datagramConn) SetDeadline(t time.Time) error {
+	return c.pc.SetReadDeadline(t)
+}
+
+func (c *datagramConn) SetReadDeadline(t time.Time) error {
+	return c.pc.SetReadDeadline(t)
+}
+
+func (c *datagramConn) SetWriteDeadline(t time.Time) error {
+	return c.pc.SetWriteDeadline(t)
+}

listener/udp/listener.go

@@ -76,6 +76,7 @@ func (l *udpListener) Init(md md.Metadata) (err error) {
 		ReadBufferSize: l.md.readBufferSize,
 		Keepalive:      l.md.keepalive,
 		TTL:            l.md.ttl,
+		Stateless:      l.md.stateless,
 		Logger:         l.logger,
 	})
 	return

listener/udp/metadata.go

@@ -20,6 +20,7 @@ type metadata struct {
 	backlog        int
 	keepalive      bool
 	ttl            time.Duration
+	stateless      bool
 }
 
 func (l *udpListener) parseMetadata(md mdata.Metadata) (err error) {
@@ -43,5 +44,7 @@ func (l *udpListener) parseMetadata(md mdata.Metadata) (err error) {
 		l.md.readQueueSize = defaultReadQueueSize
 	}
 
+	l.md.stateless = mdutil.GetBool(md, "stateless")
+
 	return
 }