Source file src/net/dial.go

Documentation: net

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package net
     6  
     7  import (
     8  	"context"
     9  	"internal/nettrace"
    10  	"internal/poll"
    11  	"syscall"
    12  	"time"
    13  )
    14  
    15  // defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
    16  // See golang.org/issue/31510
    17  const (
    18  	defaultTCPKeepAlive = 15 * time.Second
    19  )
    20  
    21  // A Dialer contains options for connecting to an address.
    22  //
    23  // The zero value for each field is equivalent to dialing
    24  // without that option. Dialing with the zero value of Dialer
    25  // is therefore equivalent to just calling the Dial function.
    26  type Dialer struct {
    27  	// Timeout is the maximum amount of time a dial will wait for
    28  	// a connect to complete. If Deadline is also set, it may fail
    29  	// earlier.
    30  	//
    31  	// The default is no timeout.
    32  	//
    33  	// When using TCP and dialing a host name with multiple IP
    34  	// addresses, the timeout may be divided between them.
    35  	//
    36  	// With or without a timeout, the operating system may impose
    37  	// its own earlier timeout. For instance, TCP timeouts are
    38  	// often around 3 minutes.
    39  	Timeout time.Duration
    40  
    41  	// Deadline is the absolute point in time after which dials
    42  	// will fail. If Timeout is set, it may fail earlier.
    43  	// Zero means no deadline, or dependent on the operating system
    44  	// as with the Timeout option.
    45  	Deadline time.Time
    46  
    47  	// LocalAddr is the local address to use when dialing an
    48  	// address. The address must be of a compatible type for the
    49  	// network being dialed.
    50  	// If nil, a local address is automatically chosen.
    51  	LocalAddr Addr
    52  
    53  	// DualStack previously enabled RFC 6555 Fast Fallback
    54  	// support, also known as "Happy Eyeballs", in which IPv4 is
    55  	// tried soon if IPv6 appears to be misconfigured and
    56  	// hanging.
    57  	//
    58  	// Deprecated: Fast Fallback is enabled by default. To
    59  	// disable, set FallbackDelay to a negative value.
    60  	DualStack bool
    61  
    62  	// FallbackDelay specifies the length of time to wait before
    63  	// spawning a RFC 6555 Fast Fallback connection. That is, this
    64  	// is the amount of time to wait for IPv6 to succeed before
    65  	// assuming that IPv6 is misconfigured and falling back to
    66  	// IPv4.
    67  	//
    68  	// If zero, a default delay of 300ms is used.
    69  	// A negative value disables Fast Fallback support.
    70  	FallbackDelay time.Duration
    71  
    72  	// KeepAlive specifies the interval between keep-alive
    73  	// probes for an active network connection.
    74  	// If zero, keep-alive probes are sent with a default value
    75  	// (currently 15 seconds), if supported by the protocol and operating
    76  	// system. Network protocols or operating systems that do
    77  	// not support keep-alives ignore this field.
    78  	// If negative, keep-alive probes are disabled.
    79  	KeepAlive time.Duration
    80  
    81  	// Resolver optionally specifies an alternate resolver to use.
    82  	Resolver *Resolver
    83  
    84  	// Cancel is an optional channel whose closure indicates that
    85  	// the dial should be canceled. Not all types of dials support
    86  	// cancellation.
    87  	//
    88  	// Deprecated: Use DialContext instead.
    89  	Cancel <-chan struct{}
    90  
    91  	// If Control is not nil, it is called after creating the network
    92  	// connection but before actually dialing.
    93  	//
    94  	// Network and address parameters passed to Control method are not
    95  	// necessarily the ones passed to Dial. For example, passing "tcp" to Dial
    96  	// will cause the Control function to be called with "tcp4" or "tcp6".
    97  	Control func(network, address string, c syscall.RawConn) error
    98  }
    99  
   100  func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }
   101  
   102  func minNonzeroTime(a, b time.Time) time.Time {
   103  	if a.IsZero() {
   104  		return b
   105  	}
   106  	if b.IsZero() || a.Before(b) {
   107  		return a
   108  	}
   109  	return b
   110  }
   111  
   112  // deadline returns the earliest of:
   113  //   - now+Timeout
   114  //   - d.Deadline
   115  //   - the context's deadline
   116  // Or zero, if none of Timeout, Deadline, or context's deadline is set.
   117  func (d *Dialer) deadline(ctx context.Context, now time.Time) (earliest time.Time) {
   118  	if d.Timeout != 0 { // including negative, for historical reasons
   119  		earliest = now.Add(d.Timeout)
   120  	}
   121  	if d, ok := ctx.Deadline(); ok {
   122  		earliest = minNonzeroTime(earliest, d)
   123  	}
   124  	return minNonzeroTime(earliest, d.Deadline)
   125  }
   126  
   127  func (d *Dialer) resolver() *Resolver {
   128  	if d.Resolver != nil {
   129  		return d.Resolver
   130  	}
   131  	return DefaultResolver
   132  }
   133  
   134  // partialDeadline returns the deadline to use for a single address,
   135  // when multiple addresses are pending.
   136  func partialDeadline(now, deadline time.Time, addrsRemaining int) (time.Time, error) {
   137  	if deadline.IsZero() {
   138  		return deadline, nil
   139  	}
   140  	timeRemaining := deadline.Sub(now)
   141  	if timeRemaining <= 0 {
   142  		return time.Time{}, poll.ErrTimeout
   143  	}
   144  	// Tentatively allocate equal time to each remaining address.
   145  	timeout := timeRemaining / time.Duration(addrsRemaining)
   146  	// If the time per address is too short, steal from the end of the list.
   147  	const saneMinimum = 2 * time.Second
   148  	if timeout < saneMinimum {
   149  		if timeRemaining < saneMinimum {
   150  			timeout = timeRemaining
   151  		} else {
   152  			timeout = saneMinimum
   153  		}
   154  	}
   155  	return now.Add(timeout), nil
   156  }
   157  
   158  func (d *Dialer) fallbackDelay() time.Duration {
   159  	if d.FallbackDelay > 0 {
   160  		return d.FallbackDelay
   161  	} else {
   162  		return 300 * time.Millisecond
   163  	}
   164  }
   165  
   166  func parseNetwork(ctx context.Context, network string, needsProto bool) (afnet string, proto int, err error) {
   167  	i := last(network, ':')
   168  	if i < 0 { // no colon
   169  		switch network {
   170  		case "tcp", "tcp4", "tcp6":
   171  		case "udp", "udp4", "udp6":
   172  		case "ip", "ip4", "ip6":
   173  			if needsProto {
   174  				return "", 0, UnknownNetworkError(network)
   175  			}
   176  		case "unix", "unixgram", "unixpacket":
   177  		default:
   178  			return "", 0, UnknownNetworkError(network)
   179  		}
   180  		return network, 0, nil
   181  	}
   182  	afnet = network[:i]
   183  	switch afnet {
   184  	case "ip", "ip4", "ip6":
   185  		protostr := network[i+1:]
   186  		proto, i, ok := dtoi(protostr)
   187  		if !ok || i != len(protostr) {
   188  			proto, err = lookupProtocol(ctx, protostr)
   189  			if err != nil {
   190  				return "", 0, err
   191  			}
   192  		}
   193  		return afnet, proto, nil
   194  	}
   195  	return "", 0, UnknownNetworkError(network)
   196  }
   197  
   198  // resolveAddrList resolves addr using hint and returns a list of
   199  // addresses. The result contains at least one address when error is
   200  // nil.
   201  func (r *Resolver) resolveAddrList(ctx context.Context, op, network, addr string, hint Addr) (addrList, error) {
   202  	afnet, _, err := parseNetwork(ctx, network, true)
   203  	if err != nil {
   204  		return nil, err
   205  	}
   206  	if op == "dial" && addr == "" {
   207  		return nil, errMissingAddress
   208  	}
   209  	switch afnet {
   210  	case "unix", "unixgram", "unixpacket":
   211  		addr, err := ResolveUnixAddr(afnet, addr)
   212  		if err != nil {
   213  			return nil, err
   214  		}
   215  		if op == "dial" && hint != nil && addr.Network() != hint.Network() {
   216  			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
   217  		}
   218  		return addrList{addr}, nil
   219  	}
   220  	addrs, err := r.internetAddrList(ctx, afnet, addr)
   221  	if err != nil || op != "dial" || hint == nil {
   222  		return addrs, err
   223  	}
   224  	var (
   225  		tcp      *TCPAddr
   226  		udp      *UDPAddr
   227  		ip       *IPAddr
   228  		wildcard bool
   229  	)
   230  	switch hint := hint.(type) {
   231  	case *TCPAddr:
   232  		tcp = hint
   233  		wildcard = tcp.isWildcard()
   234  	case *UDPAddr:
   235  		udp = hint
   236  		wildcard = udp.isWildcard()
   237  	case *IPAddr:
   238  		ip = hint
   239  		wildcard = ip.isWildcard()
   240  	}
   241  	naddrs := addrs[:0]
   242  	for _, addr := range addrs {
   243  		if addr.Network() != hint.Network() {
   244  			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
   245  		}
   246  		switch addr := addr.(type) {
   247  		case *TCPAddr:
   248  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(tcp.IP) {
   249  				continue
   250  			}
   251  			naddrs = append(naddrs, addr)
   252  		case *UDPAddr:
   253  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(udp.IP) {
   254  				continue
   255  			}
   256  			naddrs = append(naddrs, addr)
   257  		case *IPAddr:
   258  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(ip.IP) {
   259  				continue
   260  			}
   261  			naddrs = append(naddrs, addr)
   262  		}
   263  	}
   264  	if len(naddrs) == 0 {
   265  		return nil, &AddrError{Err: errNoSuitableAddress.Error(), Addr: hint.String()}
   266  	}
   267  	return naddrs, nil
   268  }
   269  
   270  // Dial connects to the address on the named network.
   271  //
   272  // Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only),
   273  // "udp", "udp4" (IPv4-only), "udp6" (IPv6-only), "ip", "ip4"
   274  // (IPv4-only), "ip6" (IPv6-only), "unix", "unixgram" and
   275  // "unixpacket".
   276  //
   277  // For TCP and UDP networks, the address has the form "host:port".
   278  // The host must be a literal IP address, or a host name that can be
   279  // resolved to IP addresses.
   280  // The port must be a literal port number or a service name.
   281  // If the host is a literal IPv6 address it must be enclosed in square
   282  // brackets, as in "[2001:db8::1]:80" or "[fe80::1%zone]:80".
   283  // The zone specifies the scope of the literal IPv6 address as defined
   284  // in RFC 4007.
   285  // The functions JoinHostPort and SplitHostPort manipulate a pair of
   286  // host and port in this form.
   287  // When using TCP, and the host resolves to multiple IP addresses,
   288  // Dial will try each IP address in order until one succeeds.
   289  //
   290  // Examples:
   291  //	Dial("tcp", "golang.org:http")
   292  //	Dial("tcp", "192.0.2.1:http")
   293  //	Dial("tcp", "198.51.100.1:80")
   294  //	Dial("udp", "[2001:db8::1]:domain")
   295  //	Dial("udp", "[fe80::1%lo0]:53")
   296  //	Dial("tcp", ":80")
   297  //
   298  // For IP networks, the network must be "ip", "ip4" or "ip6" followed
   299  // by a colon and a literal protocol number or a protocol name, and
   300  // the address has the form "host". The host must be a literal IP
   301  // address or a literal IPv6 address with zone.
   302  // It depends on each operating system how the operating system
   303  // behaves with a non-well known protocol number such as "0" or "255".
   304  //
   305  // Examples:
   306  //	Dial("ip4:1", "192.0.2.1")
   307  //	Dial("ip6:ipv6-icmp", "2001:db8::1")
   308  //	Dial("ip6:58", "fe80::1%lo0")
   309  //
   310  // For TCP, UDP and IP networks, if the host is empty or a literal
   311  // unspecified IP address, as in ":80", "0.0.0.0:80" or "[::]:80" for
   312  // TCP and UDP, "", "0.0.0.0" or "::" for IP, the local system is
   313  // assumed.
   314  //
   315  // For Unix networks, the address must be a file system path.
   316  func Dial(network, address string) (Conn, error) {
   317  	var d Dialer
   318  	return d.Dial(network, address)
   319  }
   320  
   321  // DialTimeout acts like Dial but takes a timeout.
   322  //
   323  // The timeout includes name resolution, if required.
   324  // When using TCP, and the host in the address parameter resolves to
   325  // multiple IP addresses, the timeout is spread over each consecutive
   326  // dial, such that each is given an appropriate fraction of the time
   327  // to connect.
   328  //
   329  // See func Dial for a description of the network and address
   330  // parameters.
   331  func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
   332  	d := Dialer{Timeout: timeout}
   333  	return d.Dial(network, address)
   334  }
   335  
   336  // sysDialer contains a Dial's parameters and configuration.
   337  type sysDialer struct {
   338  	Dialer
   339  	network, address string
   340  }
   341  
   342  // Dial connects to the address on the named network.
   343  //
   344  // See func Dial for a description of the network and address
   345  // parameters.
   346  func (d *Dialer) Dial(network, address string) (Conn, error) {
   347  	return d.DialContext(context.Background(), network, address)
   348  }
   349  
   350  // DialContext connects to the address on the named network using
   351  // the provided context.
   352  //
   353  // The provided Context must be non-nil. If the context expires before
   354  // the connection is complete, an error is returned. Once successfully
   355  // connected, any expiration of the context will not affect the
   356  // connection.
   357  //
   358  // When using TCP, and the host in the address parameter resolves to multiple
   359  // network addresses, any dial timeout (from d.Timeout or ctx) is spread
   360  // over each consecutive dial, such that each is given an appropriate
   361  // fraction of the time to connect.
   362  // For example, if a host has 4 IP addresses and the timeout is 1 minute,
   363  // the connect to each single address will be given 15 seconds to complete
   364  // before trying the next one.
   365  //
   366  // See func Dial for a description of the network and address
   367  // parameters.
   368  func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
   369  	if ctx == nil {
   370  		panic("nil context")
   371  	}
   372  	deadline := d.deadline(ctx, time.Now())
   373  	if !deadline.IsZero() {
   374  		if d, ok := ctx.Deadline(); !ok || deadline.Before(d) {
   375  			subCtx, cancel := context.WithDeadline(ctx, deadline)
   376  			defer cancel()
   377  			ctx = subCtx
   378  		}
   379  	}
   380  	if oldCancel := d.Cancel; oldCancel != nil {
   381  		subCtx, cancel := context.WithCancel(ctx)
   382  		defer cancel()
   383  		go func() {
   384  			select {
   385  			case <-oldCancel:
   386  				cancel()
   387  			case <-subCtx.Done():
   388  			}
   389  		}()
   390  		ctx = subCtx
   391  	}
   392  
   393  	// Shadow the nettrace (if any) during resolve so Connect events don't fire for DNS lookups.
   394  	resolveCtx := ctx
   395  	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
   396  		shadow := *trace
   397  		shadow.ConnectStart = nil
   398  		shadow.ConnectDone = nil
   399  		resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
   400  	}
   401  
   402  	addrs, err := d.resolver().resolveAddrList(resolveCtx, "dial", network, address, d.LocalAddr)
   403  	if err != nil {
   404  		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
   405  	}
   406  
   407  	sd := &sysDialer{
   408  		Dialer:  *d,
   409  		network: network,
   410  		address: address,
   411  	}
   412  
   413  	var primaries, fallbacks addrList
   414  	if d.dualStack() && network == "tcp" {
   415  		primaries, fallbacks = addrs.partition(isIPv4)
   416  	} else {
   417  		primaries = addrs
   418  	}
   419  
   420  	var c Conn
   421  	if len(fallbacks) > 0 {
   422  		c, err = sd.dialParallel(ctx, primaries, fallbacks)
   423  	} else {
   424  		c, err = sd.dialSerial(ctx, primaries)
   425  	}
   426  	if err != nil {
   427  		return nil, err
   428  	}
   429  
   430  	if tc, ok := c.(*TCPConn); ok && d.KeepAlive >= 0 {
   431  		setKeepAlive(tc.fd, true)
   432  		ka := d.KeepAlive
   433  		if d.KeepAlive == 0 {
   434  			ka = defaultTCPKeepAlive
   435  		}
   436  		setKeepAlivePeriod(tc.fd, ka)
   437  		testHookSetKeepAlive(ka)
   438  	}
   439  	return c, nil
   440  }
   441  
   442  // dialParallel races two copies of dialSerial, giving the first a
   443  // head start. It returns the first established connection and
   444  // closes the others. Otherwise it returns an error from the first
   445  // primary address.
   446  func (sd *sysDialer) dialParallel(ctx context.Context, primaries, fallbacks addrList) (Conn, error) {
   447  	if len(fallbacks) == 0 {
   448  		return sd.dialSerial(ctx, primaries)
   449  	}
   450  
   451  	returned := make(chan struct{})
   452  	defer close(returned)
   453  
   454  	type dialResult struct {
   455  		Conn
   456  		error
   457  		primary bool
   458  		done    bool
   459  	}
   460  	results := make(chan dialResult) // unbuffered
   461  
   462  	startRacer := func(ctx context.Context, primary bool) {
   463  		ras := primaries
   464  		if !primary {
   465  			ras = fallbacks
   466  		}
   467  		c, err := sd.dialSerial(ctx, ras)
   468  		select {
   469  		case results <- dialResult{Conn: c, error: err, primary: primary, done: true}:
   470  		case <-returned:
   471  			if c != nil {
   472  				c.Close()
   473  			}
   474  		}
   475  	}
   476  
   477  	var primary, fallback dialResult
   478  
   479  	// Start the main racer.
   480  	primaryCtx, primaryCancel := context.WithCancel(ctx)
   481  	defer primaryCancel()
   482  	go startRacer(primaryCtx, true)
   483  
   484  	// Start the timer for the fallback racer.
   485  	fallbackTimer := time.NewTimer(sd.fallbackDelay())
   486  	defer fallbackTimer.Stop()
   487  
   488  	for {
   489  		select {
   490  		case <-fallbackTimer.C:
   491  			fallbackCtx, fallbackCancel := context.WithCancel(ctx)
   492  			defer fallbackCancel()
   493  			go startRacer(fallbackCtx, false)
   494  
   495  		case res := <-results:
   496  			if res.error == nil {
   497  				return res.Conn, nil
   498  			}
   499  			if res.primary {
   500  				primary = res
   501  			} else {
   502  				fallback = res
   503  			}
   504  			if primary.done && fallback.done {
   505  				return nil, primary.error
   506  			}
   507  			if res.primary && fallbackTimer.Stop() {
   508  				// If we were able to stop the timer, that means it
   509  				// was running (hadn't yet started the fallback), but
   510  				// we just got an error on the primary path, so start
   511  				// the fallback immediately (in 0 nanoseconds).
   512  				fallbackTimer.Reset(0)
   513  			}
   514  		}
   515  	}
   516  }
   517  
   518  // dialSerial connects to a list of addresses in sequence, returning
   519  // either the first successful connection, or the first error.
   520  func (sd *sysDialer) dialSerial(ctx context.Context, ras addrList) (Conn, error) {
   521  	var firstErr error // The error from the first address is most relevant.
   522  
   523  	for i, ra := range ras {
   524  		select {
   525  		case <-ctx.Done():
   526  			return nil, &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: mapErr(ctx.Err())}
   527  		default:
   528  		}
   529  
   530  		deadline, _ := ctx.Deadline()
   531  		partialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)
   532  		if err != nil {
   533  			// Ran out of time.
   534  			if firstErr == nil {
   535  				firstErr = &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: err}
   536  			}
   537  			break
   538  		}
   539  		dialCtx := ctx
   540  		if partialDeadline.Before(deadline) {
   541  			var cancel context.CancelFunc
   542  			dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
   543  			defer cancel()
   544  		}
   545  
   546  		c, err := sd.dialSingle(dialCtx, ra)
   547  		if err == nil {
   548  			return c, nil
   549  		}
   550  		if firstErr == nil {
   551  			firstErr = err
   552  		}
   553  	}
   554  
   555  	if firstErr == nil {
   556  		firstErr = &OpError{Op: "dial", Net: sd.network, Source: nil, Addr: nil, Err: errMissingAddress}
   557  	}
   558  	return nil, firstErr
   559  }
   560  
   561  // dialSingle attempts to establish and returns a single connection to
   562  // the destination address.
   563  func (sd *sysDialer) dialSingle(ctx context.Context, ra Addr) (c Conn, err error) {
   564  	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
   565  	if trace != nil {
   566  		raStr := ra.String()
   567  		if trace.ConnectStart != nil {
   568  			trace.ConnectStart(sd.network, raStr)
   569  		}
   570  		if trace.ConnectDone != nil {
   571  			defer func() { trace.ConnectDone(sd.network, raStr, err) }()
   572  		}
   573  	}
   574  	la := sd.LocalAddr
   575  	switch ra := ra.(type) {
   576  	case *TCPAddr:
   577  		la, _ := la.(*TCPAddr)
   578  		c, err = sd.dialTCP(ctx, la, ra)
   579  	case *UDPAddr:
   580  		la, _ := la.(*UDPAddr)
   581  		c, err = sd.dialUDP(ctx, la, ra)
   582  	case *IPAddr:
   583  		la, _ := la.(*IPAddr)
   584  		c, err = sd.dialIP(ctx, la, ra)
   585  	case *UnixAddr:
   586  		la, _ := la.(*UnixAddr)
   587  		c, err = sd.dialUnix(ctx, la, ra)
   588  	default:
   589  		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: sd.address}}
   590  	}
   591  	if err != nil {
   592  		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
   593  	}
   594  	return c, nil
   595  }
   596  
   597  // ListenConfig contains options for listening to an address.
   598  type ListenConfig struct {
   599  	// If Control is not nil, it is called after creating the network
   600  	// connection but before binding it to the operating system.
   601  	//
   602  	// Network and address parameters passed to Control method are not
   603  	// necessarily the ones passed to Listen. For example, passing "tcp" to
   604  	// Listen will cause the Control function to be called with "tcp4" or "tcp6".
   605  	Control func(network, address string, c syscall.RawConn) error
   606  
   607  	// KeepAlive specifies the keep-alive period for network
   608  	// connections accepted by this listener.
   609  	// If zero, keep-alives are enabled if supported by the protocol
   610  	// and operating system. Network protocols or operating systems
   611  	// that do not support keep-alives ignore this field.
   612  	// If negative, keep-alives are disabled.
   613  	KeepAlive time.Duration
   614  }
   615  
   616  // Listen announces on the local network address.
   617  //
   618  // See func Listen for a description of the network and address
   619  // parameters.
   620  func (lc *ListenConfig) Listen(ctx context.Context, network, address string) (Listener, error) {
   621  	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
   622  	if err != nil {
   623  		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
   624  	}
   625  	sl := &sysListener{
   626  		ListenConfig: *lc,
   627  		network:      network,
   628  		address:      address,
   629  	}
   630  	var l Listener
   631  	la := addrs.first(isIPv4)
   632  	switch la := la.(type) {
   633  	case *TCPAddr:
   634  		l, err = sl.listenTCP(ctx, la)
   635  	case *UnixAddr:
   636  		l, err = sl.listenUnix(ctx, la)
   637  	default:
   638  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
   639  	}
   640  	if err != nil {
   641  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // l is non-nil interface containing nil pointer
   642  	}
   643  	return l, nil
   644  }
   645  
   646  // ListenPacket announces on the local network address.
   647  //
   648  // See func ListenPacket for a description of the network and address
   649  // parameters.
   650  func (lc *ListenConfig) ListenPacket(ctx context.Context, network, address string) (PacketConn, error) {
   651  	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
   652  	if err != nil {
   653  		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
   654  	}
   655  	sl := &sysListener{
   656  		ListenConfig: *lc,
   657  		network:      network,
   658  		address:      address,
   659  	}
   660  	var c PacketConn
   661  	la := addrs.first(isIPv4)
   662  	switch la := la.(type) {
   663  	case *UDPAddr:
   664  		c, err = sl.listenUDP(ctx, la)
   665  	case *IPAddr:
   666  		c, err = sl.listenIP(ctx, la)
   667  	case *UnixAddr:
   668  		c, err = sl.listenUnixgram(ctx, la)
   669  	default:
   670  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
   671  	}
   672  	if err != nil {
   673  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // c is non-nil interface containing nil pointer
   674  	}
   675  	return c, nil
   676  }
   677  
   678  // sysListener contains a Listen's parameters and configuration.
   679  type sysListener struct {
   680  	ListenConfig
   681  	network, address string
   682  }
   683  
   684  // Listen announces on the local network address.
   685  //
   686  // The network must be "tcp", "tcp4", "tcp6", "unix" or "unixpacket".
   687  //
   688  // For TCP networks, if the host in the address parameter is empty or
   689  // a literal unspecified IP address, Listen listens on all available
   690  // unicast and anycast IP addresses of the local system.
   691  // To only use IPv4, use network "tcp4".
   692  // The address can use a host name, but this is not recommended,
   693  // because it will create a listener for at most one of the host's IP
   694  // addresses.
   695  // If the port in the address parameter is empty or "0", as in
   696  // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
   697  // The Addr method of Listener can be used to discover the chosen
   698  // port.
   699  //
   700  // See func Dial for a description of the network and address
   701  // parameters.
   702  func Listen(network, address string) (Listener, error) {
   703  	var lc ListenConfig
   704  	return lc.Listen(context.Background(), network, address)
   705  }
   706  
   707  // ListenPacket announces on the local network address.
   708  //
   709  // The network must be "udp", "udp4", "udp6", "unixgram", or an IP
   710  // transport. The IP transports are "ip", "ip4", or "ip6" followed by
   711  // a colon and a literal protocol number or a protocol name, as in
   712  // "ip:1" or "ip:icmp".
   713  //
   714  // For UDP and IP networks, if the host in the address parameter is
   715  // empty or a literal unspecified IP address, ListenPacket listens on
   716  // all available IP addresses of the local system except multicast IP
   717  // addresses.
   718  // To only use IPv4, use network "udp4" or "ip4:proto".
   719  // The address can use a host name, but this is not recommended,
   720  // because it will create a listener for at most one of the host's IP
   721  // addresses.
   722  // If the port in the address parameter is empty or "0", as in
   723  // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
   724  // The LocalAddr method of PacketConn can be used to discover the
   725  // chosen port.
   726  //
   727  // See func Dial for a description of the network and address
   728  // parameters.
   729  func ListenPacket(network, address string) (PacketConn, error) {
   730  	var lc ListenConfig
   731  	return lc.ListenPacket(context.Background(), network, address)
   732  }
   733  

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