// It is possible to have a disconnected client in this tiny window of time.
func CheckPortAvailability(host, port string, opts TCPOptions) (err error) {
	lc := &net.ListenConfig{
		Control: func(network, address string, c syscall.RawConn) error {
			c.Control(func(fdPtr uintptr) {
				if opts.Interface != "" {
					// When interface is specified look for specifically port availability on

func (sl *sysListener) listenTCP(ctx context.Context, laddr *TCPAddr) (*TCPListener, error) {
	fd, err := listenPlan9(ctx, sl.network, laddr)
	if err != nil {
		return nil, err
	}
	return &TCPListener{fd: fd, lc: sl.ListenConfig}, nil

package net

import (
	"bytes"
	"context"
	"errors"
	"syscall"
	"testing"
)

func newLocalListenerMPTCP(t *testing.T, envVar bool) Listener {
	lc := &ListenConfig{}

	if envVar {
		if !lc.MultipathTCP() {
			t.Fatal("MultipathTCP Listen is not on despite GODEBUG=multipathtcp=1")
		}
	} else {
		if lc.MultipathTCP() {
			t.Error("MultipathTCP should be off by default")

	listeners := make([]net.Listener, 0, len(serverAddrs))
	listenErrs = make([]error, len(serverAddrs))

	// Unix listener with special TCP options.
	listenCfg := net.ListenConfig{
		Control: setTCPParametersFn(opts),
	}

	for i, serverAddr := range serverAddrs {
		l, e := listenCfg.Listen(ctx, "tcp", serverAddr)
		if e != nil {
			if opts.Trace != nil {

	// at most 2 message: ipv4 and ipv6
	c := make(chan ReturnCode, 2)
	hasAtLeastOneListener := false
	for _, addr := range s.Config.ServerListenAddress {
		log.Infof("Listening on %v", addr)
		config := &net.ListenConfig{Control: reuseAddr}

		l, err := config.Listen(context.Background(), "tcp", addr) // bind to the address:port
		if err != nil {
			log.Errorf("Error on listening: %v", err)
			continue
		}

pkg go/types, method (*Var) Embedded() bool
pkg net, method (*ListenConfig) Listen(context.Context, string, string) (Listener, error)
pkg net, method (*ListenConfig) ListenPacket(context.Context, string, string) (PacketConn, error)
pkg net, type Dialer struct, Control func(string, string, syscall.RawConn) error
pkg net, type ListenConfig struct
pkg net, type ListenConfig struct, Control func(string, string, syscall.RawConn) error

				continue
			}
			ln := newLocalListener(t, network, &ListenConfig{Control: controlOnConnSetup})
			ln.Close()
		}
	})
	t.Run("PacketListen", func(t *testing.T) {
		for _, network := range []string{"udp", "udp4", "udp6", "unixgram"} {
			if !testableNetwork(network) {
				continue
			}
			c := newLocalPacketListener(t, network, &ListenConfig{Control: controlOnConnSetup})
			c.Close()
		}
	})

	}
	if s.BindPort <= 0 && s.Listener == nil {
		return nil
	}

	if s.Listener == nil {
		var err error
		addr := net.JoinHostPort(s.BindAddress.String(), strconv.Itoa(s.BindPort))

		c := net.ListenConfig{}

		ctls := multipleControls{}
		if s.PermitPortSharing {
			ctls = append(ctls, permitPortReuse)
		}
		if s.PermitAddressSharing {
			ctls = append(ctls, permitAddressReuse)
		}

pkg net, type KeepAliveConfig struct, Enable bool #62254
pkg net, type KeepAliveConfig struct, Idle time.Duration #62254
pkg net, type KeepAliveConfig struct, Interval time.Duration #62254
pkg net, type ListenConfig struct, KeepAliveConfig KeepAliveConfig #62254
pkg net/http, func ParseCookie(string) ([]*Cookie, error) #66008
pkg net/http, func ParseSetCookie(string) (*Cookie, error) #66008

pkg net/http, var ErrSchemeMismatch error #44855
pkg net, method (*Dialer) MultipathTCP() bool #56539
pkg net, method (*Dialer) SetMultipathTCP(bool) #56539
pkg net, method (*ListenConfig) MultipathTCP() bool #56539
pkg net, method (*ListenConfig) SetMultipathTCP(bool) #56539
pkg net, method (*TCPConn) MultipathTCP() (bool, error) #59166
pkg reflect, method (Value) Clear() #55002
pkg reflect, type SliceHeader //deprecated #56906