Add SO_REUSEPORT support for EntryPoints

pkg/server/server_entrypoint_listenconfig_other.go

@@ -0,0 +1,15 @@
+//go:build !(linux || freebsd || openbsd || darwin)
+
+package server
+
+import (
+	"net"
+
+	"github.com/traefik/traefik/v3/pkg/config/static"
+)
+
+// newListenConfig creates a new net.ListenConfig for the given configuration of
+// the entry point.
+func newListenConfig(configuration *static.EntryPoint) (lc net.ListenConfig) {
+	return
+}

pkg/server/server_entrypoint_listenconfig_other_test.go

@@ -0,0 +1,44 @@
+//go:build !(linux || freebsd || openbsd || darwin)
+
+package server
+
+import (
+	"context"
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/traefik/traefik/v3/pkg/config/static"
+)
+
+func TestNewListenConfig(t *testing.T) {
+	ep := static.EntryPoint{Address: ":0"}
+	listenConfig := newListenConfig(&ep)
+	require.Nil(t, listenConfig.Control)
+	require.Zero(t, listenConfig.KeepAlive)
+
+	l1, err := listenConfig.Listen(context.Background(), "tcp", ep.Address)
+	require.NoError(t, err)
+	require.NotNil(t, l1)
+	defer l1.Close()
+
+	l2, err := listenConfig.Listen(context.Background(), "tcp", l1.Addr().String())
+	require.Error(t, err)
+	require.ErrorContains(t, err, "address already in use")
+	require.Nil(t, l2)
+
+	ep = static.EntryPoint{Address: ":0", ReusePort: true}
+	listenConfig = newListenConfig(&ep)
+	require.Nil(t, listenConfig.Control)
+	require.Zero(t, listenConfig.KeepAlive)
+
+	l3, err := listenConfig.Listen(context.Background(), "tcp", ep.Address)
+	require.NoError(t, err)
+	require.NotNil(t, l3)
+	defer l3.Close()
+
+	l4, err := listenConfig.Listen(context.Background(), "tcp", l3.Addr().String())
+	require.Error(t, err)
+	require.ErrorContains(t, err, "address already in use")
+	require.Nil(t, l4)
+}

pkg/server/server_entrypoint_listenconfig_unix.go

@@ -0,0 +1,44 @@
+//go:build linux || freebsd || openbsd || darwin
+
+package server
+
+import (
+	"fmt"
+	"net"
+	"syscall"
+
+	"github.com/traefik/traefik/v3/pkg/config/static"
+	"golang.org/x/sys/unix"
+)
+
+// newListenConfig creates a new net.ListenConfig for the given configuration of
+// the entry point.
+func newListenConfig(configuration *static.EntryPoint) (lc net.ListenConfig) {
+	if configuration != nil && configuration.ReusePort {
+		lc.Control = controlReusePort
+	}
+	return
+}
+
+// controlReusePort is a net.ListenConfig.Control function that enables SO_REUSEPORT
+// on the socket.
+func controlReusePort(network, address string, c syscall.RawConn) error {
+	var setSockOptErr error
+	err := c.Control(func(fd uintptr) {
+		// Note that net.ListenConfig enables unix.SO_REUSEADDR by default,
+		// as seen in https://go.dev/src/net/sockopt_linux.go. Therefore, no
+		// additional action is required to enable it here.
+
+		setSockOptErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unixSOREUSEPORT, 1)
+		if setSockOptErr != nil {
+			return
+		}
+	})
+	if err != nil {
+		return fmt.Errorf("control: %w", err)
+	}
+	if setSockOptErr != nil {
+		return fmt.Errorf("setsockopt: %w", setSockOptErr)
+	}
+	return nil
+}

pkg/server/server_entrypoint_listenconfig_unix_sockopt_freebsd.go

@@ -0,0 +1,7 @@
+//go:build freebsd
+
+package server
+
+import "golang.org/x/sys/unix"
+
+const unixSOREUSEPORT = unix.SO_REUSEPORT_LB

pkg/server/server_entrypoint_listenconfig_unix_sockopt_other.go

@@ -0,0 +1,7 @@
+//go:build linux || openbsd || darwin
+
+package server
+
+import "golang.org/x/sys/unix"
+
+const unixSOREUSEPORT = unix.SO_REUSEPORT

pkg/server/server_entrypoint_listenconfig_unix_test.go

@@ -0,0 +1,56 @@
+//go:build linux || freebsd || openbsd || darwin
+
+package server
+
+import (
+	"context"
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/traefik/traefik/v3/pkg/config/static"
+)
+
+func TestNewListenConfig(t *testing.T) {
+	ep := static.EntryPoint{Address: ":0"}
+	listenConfig := newListenConfig(&ep)
+	require.Nil(t, listenConfig.Control)
+	require.Zero(t, listenConfig.KeepAlive)
+
+	l1, err := listenConfig.Listen(context.Background(), "tcp", ep.Address)
+	require.NoError(t, err)
+	require.NotNil(t, l1)
+	defer l1.Close()
+
+	l2, err := listenConfig.Listen(context.Background(), "tcp", l1.Addr().String())
+	require.Error(t, err)
+	require.ErrorContains(t, err, "address already in use")
+	require.Nil(t, l2)
+
+	ep = static.EntryPoint{Address: ":0", ReusePort: true}
+	listenConfig = newListenConfig(&ep)
+	require.NotNil(t, listenConfig.Control)
+	require.Zero(t, listenConfig.KeepAlive)
+
+	l3, err := listenConfig.Listen(context.Background(), "tcp", ep.Address)
+	require.NoError(t, err)
+	require.NotNil(t, l3)
+	defer l3.Close()
+
+	l4, err := listenConfig.Listen(context.Background(), "tcp", l3.Addr().String())
+	require.NoError(t, err)
+	require.NotNil(t, l4)
+	defer l4.Close()
+
+	_, l3Port, err := net.SplitHostPort(l3.Addr().String())
+	require.NoError(t, err)
+	l5, err := listenConfig.Listen(context.Background(), "tcp", "127.0.0.1:"+l3Port)
+	require.NoError(t, err)
+	require.NotNil(t, l5)
+	defer l5.Close()
+
+	l6, err := listenConfig.Listen(context.Background(), "tcp", l1.Addr().String())
+	require.Error(t, err)
+	require.ErrorContains(t, err, "address already in use")
+	require.Nil(t, l6)
+}

pkg/server/server_entrypoint_tcp.go

@@ -460,7 +460,8 @@ func buildProxyProtocolListener(ctx context.Context, entryPoint *static.EntryPoi
 }
 
 func buildListener(ctx context.Context, entryPoint *static.EntryPoint) (net.Listener, error) {
-	listener, err := net.Listen("tcp", entryPoint.GetAddress())
+	listenConfig := newListenConfig(entryPoint)
+	listener, err := listenConfig.Listen(ctx, "tcp", entryPoint.GetAddress())
 	if err != nil {
 		return nil, fmt.Errorf("error opening listener: %w", err)
 	}

pkg/server/server_entrypoint_tcp_http3.go

@@ -33,7 +33,8 @@ func newHTTP3Server(ctx context.Context, configuration *static.EntryPoint, https
 		return nil, errors.New("advertised port must be greater than or equal to zero")
 	}
 
-	conn, err := net.ListenPacket("udp", configuration.GetAddress())
+	listenConfig := newListenConfig(configuration)
+	conn, err := listenConfig.ListenPacket(ctx, "udp", configuration.GetAddress())
 	if err != nil {
 		return nil, fmt.Errorf("starting listener: %w", err)
 	}

pkg/server/server_entrypoint_udp.go

@@ -3,7 +3,6 @@ package server
 import (
 	"context"
 	"fmt"
-	"net"
 	"sync"
 	"time"
 
@@ -87,12 +86,8 @@ type UDPEntryPoint struct {
 
 // NewUDPEntryPoint returns a UDP entry point.
 func NewUDPEntryPoint(cfg *static.EntryPoint) (*UDPEntryPoint, error) {
-	addr, err := net.ResolveUDPAddr("udp", cfg.GetAddress())
-	if err != nil {
-		return nil, err
-	}
-
-	listener, err := udp.Listen("udp", addr, time.Duration(cfg.UDP.Timeout))
+	listenConfig := newListenConfig(cfg)
+	listener, err := udp.Listen(listenConfig, "udp", cfg.GetAddress(), time.Duration(cfg.UDP.Timeout))
 	if err != nil {
 		return nil, err
 	}