feat(listener): add SO_REUSEPORT support for TCP listeners (#654)

Add opt-in reuseport metadata option for tcp, mtcp, and redirect/tcp
listeners, allowing a GOST listener on a specific IP to coexist with
another process on the same port but different address (e.g., Nginx on
0.0.0.0:443 + GOST on 127.0.0.1:443). With SO_REUSEPORT, the kernel
routes to the most specific address match.

On Unix, sets SO_REUSEPORT via ListenConfig.Control before bind().
On Windows, the option is a no-op (SO_REUSEADDR is the default).

Closes go-gost/gost#654

listener/mtcp/listener.go

@@ -59,6 +59,9 @@ func (l *mtcpListener) Init(md md.Metadata) (err error) {
 		lc.SetMultipathTCP(true)
 		l.logger.Debugf("mptcp enabled: %v", lc.MultipathTCP())
 	}
+	if l.md.reuseport {
+		lc.Control = l.setReusePort
+	}
 	ln, err := lc.Listen(context.Background(), network, l.options.Addr)
 	if err != nil {
 		return

listener/mtcp/listener_unix.go

@@ -0,0 +1,17 @@
+//go:build !windows
+
+package mtcp
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func (l *mtcpListener) setReusePort(network, address string, c syscall.RawConn) error {
+	return c.Control(func(fd uintptr) {
+		if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+			l.logger.Errorf("failed to set SO_REUSEPORT: %v", err)
+		}
+	})
+}

listener/mtcp/listener_windows.go

@@ -0,0 +1,11 @@
+//go:build windows
+
+package mtcp
+
+import (
+	"syscall"
+)
+
+func (l *mtcpListener) setReusePort(network, address string, c syscall.RawConn) error {
+	return nil
+}

listener/mtcp/metadata.go

@@ -14,6 +14,7 @@ const (
 
 type metadata struct {
 	mptcp     bool
+	reuseport bool
 	muxCfg    *mux.Config
 	backlog           int
 	keepalive         bool
@@ -24,6 +25,7 @@ type metadata struct {
 
 func (l *mtcpListener) parseMetadata(md md.Metadata) (err error) {
 	l.md.mptcp = mdutil.GetBool(md, "mptcp")
+	l.md.reuseport = mdutil.GetBool(md, "reuseport")
 
 	l.md.muxCfg = &mux.Config{
 		Version:           mdutil.GetInt(md, "mux.version"),

listener/redirect/tcp/listener.go

@@ -53,7 +53,7 @@ func (l *redirectListener) Init(md md.Metadata) (err error) {
 		network = "tcp4"
 	}
 	lc := net.ListenConfig{}
-	if l.md.tproxy {
+	if l.md.tproxy || l.md.reuseport {
 		lc.Control = l.control
 	}
 	if l.md.mptcp {

listener/redirect/tcp/listener_darwin.go

@@ -2,8 +2,17 @@ package tcp
 
 import (
 	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
 func (l *redirectListener) control(network, address string, c syscall.RawConn) error {
+	if l.md.reuseport {
+		return c.Control(func(fd uintptr) {
+			if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+				l.logger.Errorf("failed to set SO_REUSEPORT: %v", err)
+			}
+		})
+	}
 	return nil
 }

listener/redirect/tcp/listener_linux.go

@@ -8,11 +8,18 @@ import (
 
 func (l *redirectListener) control(network, address string, c syscall.RawConn) error {
 	return c.Control(func(fd uintptr) {
+		if l.md.tproxy {
 			if err := unix.SetsockoptInt(int(fd), unix.SOL_IP, unix.IP_TRANSPARENT, 1); err != nil {
-			l.logger.Errorf("SetsockoptInt(SOL_IP, IP_TRANSPARENT, 1): %v", err)
+				l.logger.Errorf("failed to set IP_TRANSPARENT: %v", err)
 			}
 			if err := unix.SetsockoptInt(int(fd), unix.SOL_IPV6, unix.IPV6_TRANSPARENT, 1); err != nil {
-			l.logger.Errorf("SetsockoptInt(SOL_IPV6, IPV6_TRANSPARENT, 1): %v", err)
+				l.logger.Errorf("failed to set IPV6_TRANSPARENT: %v", err)
+			}
+		}
+		if l.md.reuseport {
+			if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+				l.logger.Errorf("failed to set SO_REUSEPORT: %v", err)
+			}
 		}
 	})
 }

listener/redirect/tcp/listener_other.go

@@ -1,12 +1,24 @@
-//go:build !linux && !darwin
+//go:build !linux && !darwin && !windows
 
 package tcp
 
 import (
 	"errors"
 	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
 func (l *redirectListener) control(network, address string, c syscall.RawConn) error {
+	if l.md.tproxy {
 		return errors.New("TProxy is not available on non-linux platform")
 	}
+	if l.md.reuseport {
+		return c.Control(func(fd uintptr) {
+			if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+				l.logger.Errorf("failed to set SO_REUSEPORT: %v", err)
+			}
+		})
+	}
+	return nil
+}

listener/redirect/tcp/listener_windows.go

@@ -0,0 +1,14 @@
+package tcp
+
+import (
+	"errors"
+	"syscall"
+)
+
+func (l *redirectListener) control(network, address string, c syscall.RawConn) error {
+	if l.md.tproxy {
+		return errors.New("TProxy is not available on non-linux platform")
+	}
+	// SO_REUSEADDR is the default on Windows, so reuseport is a no-op.
+	return nil
+}

listener/redirect/tcp/metadata.go

@@ -8,11 +8,13 @@ import (
 type metadata struct {
 	tproxy    bool
 	mptcp     bool
+	reuseport bool
 }
 
 func (l *redirectListener) parseMetadata(md mdata.Metadata) (err error) {
 	l.md.tproxy = mdutil.GetBool(md, "tproxy")
 	l.md.mptcp = mdutil.GetBool(md, "mptcp")
+	l.md.reuseport = mdutil.GetBool(md, "reuseport")
 
 	return
 }

listener/tcp/listener.go

@@ -56,6 +56,9 @@ func (l *tcpListener) Init(md md.Metadata) (err error) {
 		lc.SetMultipathTCP(true)
 		l.logger.Debugf("mptcp enabled: %v", lc.MultipathTCP())
 	}
+	if l.md.reuseport {
+		lc.Control = l.setReusePort
+	}
 	ln, err := lc.Listen(context.Background(), network, l.options.Addr)
 	if err != nil {
 		return

listener/tcp/listener_unix.go

@@ -0,0 +1,17 @@
+//go:build !windows
+
+package tcp
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func (l *tcpListener) setReusePort(network, address string, c syscall.RawConn) error {
+	return c.Control(func(fd uintptr) {
+		if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+			l.logger.Errorf("failed to set SO_REUSEPORT: %v", err)
+		}
+	})
+}

listener/tcp/listener_windows.go

@@ -0,0 +1,11 @@
+//go:build windows
+
+package tcp
+
+import (
+	"syscall"
+)
+
+func (l *tcpListener) setReusePort(network, address string, c syscall.RawConn) error {
+	return nil
+}

listener/tcp/metadata.go

@@ -9,6 +9,7 @@ import (
 
 type metadata struct {
 	mptcp     bool
+	reuseport bool
 
 	keepalive         bool
 	keepaliveIdle     time.Duration
@@ -18,6 +19,7 @@ type metadata struct {
 
 func (l *tcpListener) parseMetadata(md md.Metadata) (err error) {
 	l.md.mptcp = mdutil.GetBool(md, "mptcp")
+	l.md.reuseport = mdutil.GetBool(md, "reuseport")
 
 	l.md.keepalive = mdutil.GetBool(md, "keepalive")
 	l.md.keepaliveIdle = mdutil.GetDuration(md, "keepalive.idle")