Source file src/net/dial.go

     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  	"syscall"
    11  	"time"
    12  )
    13  
    14  // defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
    15  // See golang.org/issue/31510
    16  const (
    17  	defaultTCPKeepAlive = 15 * time.Second
    18  )
    19  
    20  // A Dialer contains options for connecting to an address.
    21  //
    22  // The zero value for each field is equivalent to dialing
    23  // without that option. Dialing with the zero value of Dialer
    24  // is therefore equivalent to just calling the Dial function.
    25  //
    26  // It is safe to call Dialer's methods concurrently.
    27  type Dialer struct {
    28  	// Timeout is the maximum amount of time a dial will wait for
    29  	// a connect to complete. If Deadline is also set, it may fail
    30  	// earlier.
    31  	//
    32  	// The default is no timeout.
    33  	//
    34  	// When using TCP and dialing a host name with multiple IP
    35  	// addresses, the timeout may be divided between them.
    36  	//
    37  	// With or without a timeout, the operating system may impose
    38  	// its own earlier timeout. For instance, TCP timeouts are
    39  	// often around 3 minutes.
    40  	Timeout time.Duration
    41  
    42  	// Deadline is the absolute point in time after which dials
    43  	// will fail. If Timeout is set, it may fail earlier.
    44  	// Zero means no deadline, or dependent on the operating system
    45  	// as with the Timeout option.
    46  	Deadline time.Time
    47  
    48  	// LocalAddr is the local address to use when dialing an
    49  	// address. The address must be of a compatible type for the
    50  	// network being dialed.
    51  	// If nil, a local address is automatically chosen.
    52  	LocalAddr Addr
    53  
    54  	// DualStack previously enabled RFC 6555 Fast Fallback
    55  	// support, also known as "Happy Eyeballs", in which IPv4 is
    56  	// tried soon if IPv6 appears to be misconfigured and
    57  	// hanging.
    58  	//
    59  	// Deprecated: Fast Fallback is enabled by default. To
    60  	// disable, set FallbackDelay to a negative value.
    61  	DualStack bool
    62  
    63  	// FallbackDelay specifies the length of time to wait before
    64  	// spawning a RFC 6555 Fast Fallback connection. That is, this
    65  	// is the amount of time to wait for IPv6 to succeed before
    66  	// assuming that IPv6 is misconfigured and falling back to
    67  	// IPv4.
    68  	//
    69  	// If zero, a default delay of 300ms is used.
    70  	// A negative value disables Fast Fallback support.
    71  	FallbackDelay time.Duration
    72  
    73  	// KeepAlive specifies the interval between keep-alive
    74  	// probes for an active network connection.
    75  	// If zero, keep-alive probes are sent with a default value
    76  	// (currently 15 seconds), if supported by the protocol and operating
    77  	// system. Network protocols or operating systems that do
    78  	// not support keep-alives ignore this field.
    79  	// If negative, keep-alive probes are disabled.
    80  	KeepAlive time.Duration
    81  
    82  	// Resolver optionally specifies an alternate resolver to use.
    83  	Resolver *Resolver
    84  
    85  	// Cancel is an optional channel whose closure indicates that
    86  	// the dial should be canceled. Not all types of dials support
    87  	// cancellation.
    88  	//
    89  	// Deprecated: Use DialContext instead.
    90  	Cancel <-chan struct{}
    91  
    92  	// If Control is not nil, it is called after creating the network
    93  	// connection but before actually dialing.
    94  	//
    95  	// Network and address parameters passed to Control method are not
    96  	// necessarily the ones passed to Dial. For example, passing "tcp" to Dial
    97  	// will cause the Control function to be called with "tcp4" or "tcp6".
    98  	Control func(network, address string, c syscall.RawConn) error
    99  }
   100  
   101  func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }
   102  
   103  func minNonzeroTime(a, b time.Time) time.Time {
   104  	if a.IsZero() {
   105  		return b
   106  	}
   107  	if b.IsZero() || a.Before(b) {
   108  		return a
   109  	}
   110  	return b
   111  }
   112  
   113  // deadline returns the earliest of:
   114  //   - now+Timeout
   115  //   - d.Deadline
   116  //   - the context's deadline
   117  //
   118  // Or zero, if none of Timeout, Deadline, or context's deadline is set.
   119  func (d *Dialer) deadline(ctx context.Context, now time.Time) (earliest time.Time) {
   120  	if d.Timeout != 0 { // including negative, for historical reasons
   121  		earliest = now.Add(d.Timeout)
   122  	}
   123  	if d, ok := ctx.Deadline(); ok {
   124  		earliest = minNonzeroTime(earliest, d)
   125  	}
   126  	return minNonzeroTime(earliest, d.Deadline)
   127  }
   128  
   129  func (d *Dialer) resolver() *Resolver {
   130  	if d.Resolver != nil {
   131  		return d.Resolver
   132  	}
   133  	return DefaultResolver
   134  }
   135  
   136  // partialDeadline returns the deadline to use for a single address,
   137  // when multiple addresses are pending.
   138  func partialDeadline(now, deadline time.Time, addrsRemaining int) (time.Time, error) {
   139  	if deadline.IsZero() {
   140  		return deadline, nil
   141  	}
   142  	timeRemaining := deadline.Sub(now)
   143  	if timeRemaining <= 0 {
   144  		return time.Time{}, errTimeout
   145  	}
   146  	// Tentatively allocate equal time to each remaining address.
   147  	timeout := timeRemaining / time.Duration(addrsRemaining)
   148  	// If the time per address is too short, steal from the end of the list.
   149  	const saneMinimum = 2 * time.Second
   150  	if timeout < saneMinimum {
   151  		if timeRemaining < saneMinimum {
   152  			timeout = timeRemaining
   153  		} else {
   154  			timeout = saneMinimum
   155  		}
   156  	}
   157  	return now.Add(timeout), nil
   158  }
   159  
   160  func (d *Dialer) fallbackDelay() time.Duration {
   161  	if d.FallbackDelay > 0 {
   162  		return d.FallbackDelay
   163  	} else {
   164  		return 300 * time.Millisecond
   165  	}
   166  }
   167  
   168  func parseNetwork(ctx context.Context, network string, needsProto bool) (afnet string, proto int, err error) {
   169  	i := last(network, ':')
   170  	if i < 0 { // no colon
   171  		switch network {
   172  		case "tcp", "tcp4", "tcp6":
   173  		case "udp", "udp4", "udp6":
   174  		case "ip", "ip4", "ip6":
   175  			if needsProto {
   176  				return "", 0, UnknownNetworkError(network)
   177  			}
   178  		case "unix", "unixgram", "unixpacket":
   179  		default:
   180  			return "", 0, UnknownNetworkError(network)
   181  		}
   182  		return network, 0, nil
   183  	}
   184  	afnet = network[:i]
   185  	switch afnet {
   186  	case "ip", "ip4", "ip6":
   187  		protostr := network[i+1:]
   188  		proto, i, ok := dtoi(protostr)
   189  		if !ok || i != len(protostr) {
   190  			proto, err = lookupProtocol(ctx, protostr)
   191  			if err != nil {
   192  				return "", 0, err
   193  			}
   194  		}
   195  		return afnet, proto, nil
   196  	}
   197  	return "", 0, UnknownNetworkError(network)
   198  }
   199  
   200  // resolveAddrList resolves addr using hint and returns a list of
   201  // addresses. The result contains at least one address when error is
   202  // nil.
   203  func (r *Resolver) resolveAddrList(ctx context.Context, op, network, addr string, hint Addr) (addrList, error) {
   204  	afnet, _, err := parseNetwork(ctx, network, true)
   205  	if err != nil {
   206  		return nil, err
   207  	}
   208  	if op == "dial" && addr == "" {
   209  		return nil, errMissingAddress
   210  	}
   211  	switch afnet {
   212  	case "unix", "unixgram", "unixpacket":
   213  		addr, err := ResolveUnixAddr(afnet, addr)
   214  		if err != nil {
   215  			return nil, err
   216  		}
   217  		if op == "dial" && hint != nil && addr.Network() != hint.Network() {
   218  			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
   219  		}
   220  		return addrList{addr}, nil
   221  	}
   222  	addrs, err := r.internetAddrList(ctx, afnet, addr)
   223  	if err != nil || op != "dial" || hint == nil {
   224  		return addrs, err
   225  	}
   226  	var (
   227  		tcp      *TCPAddr
   228  		udp      *UDPAddr
   229  		ip       *IPAddr
   230  		wildcard bool
   231  	)
   232  	switch hint := hint.(type) {
   233  	case *TCPAddr:
   234  		tcp = hint
   235  		wildcard = tcp.isWildcard()
   236  	case *UDPAddr:
   237  		udp = hint
   238  		wildcard = udp.isWildcard()
   239  	case *IPAddr:
   240  		ip = hint
   241  		wildcard = ip.isWildcard()
   242  	}
   243  	naddrs := addrs[:0]
   244  	for _, addr := range addrs {
   245  		if addr.Network() != hint.Network() {
   246  			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
   247  		}
   248  		switch addr := addr.(type) {
   249  		case *TCPAddr:
   250  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(tcp.IP) {
   251  				continue
   252  			}
   253  			naddrs = append(naddrs, addr)
   254  		case *UDPAddr:
   255  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(udp.IP) {
   256  				continue
   257  			}
   258  			naddrs = append(naddrs, addr)
   259  		case *IPAddr:
   260  			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(ip.IP) {
   261  				continue
   262  			}
   263  			naddrs = append(naddrs, addr)
   264  		}
   265  	}
   266  	if len(naddrs) == 0 {
   267  		return nil, &AddrError{Err: errNoSuitableAddress.Error(), Addr: hint.String()}
   268  	}
   269  	return naddrs, nil
   270  }
   271  
   272  // Dial connects to the address on the named network.
   273  //
   274  // Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only),
   275  // "udp", "udp4" (IPv4-only), "udp6" (IPv6-only), "ip", "ip4"
   276  // (IPv4-only), "ip6" (IPv6-only), "unix", "unixgram" and
   277  // "unixpacket".
   278  //
   279  // For TCP and UDP networks, the address has the form "host:port".
   280  // The host must be a literal IP address, or a host name that can be
   281  // resolved to IP addresses.
   282  // The port must be a literal port number or a service name.
   283  // If the host is a literal IPv6 address it must be enclosed in square
   284  // brackets, as in "[2001:db8::1]:80" or "[fe80::1%zone]:80".
   285  // The zone specifies the scope of the literal IPv6 address as defined
   286  // in RFC 4007.
   287  // The functions JoinHostPort and SplitHostPort manipulate a pair of
   288  // host and port in this form.
   289  // When using TCP, and the host resolves to multiple IP addresses,
   290  // Dial will try each IP address in order until one succeeds.
   291  //
   292  // Examples:
   293  //
   294  //	Dial("tcp", "golang.org:http")
   295  //	Dial("tcp", "192.0.2.1:http")
   296  //	Dial("tcp", "198.51.100.1:80")
   297  //	Dial("udp", "[2001:db8::1]:domain")
   298  //	Dial("udp", "[fe80::1%lo0]:53")
   299  //	Dial("tcp", ":80")
   300  //
   301  // For IP networks, the network must be "ip", "ip4" or "ip6" followed
   302  // by a colon and a literal protocol number or a protocol name, and
   303  // the address has the form "host". The host must be a literal IP
   304  // address or a literal IPv6 address with zone.
   305  // It depends on each operating system how the operating system
   306  // behaves with a non-well known protocol number such as "0" or "255".
   307  //
   308  // Examples:
   309  //
   310  //	Dial("ip4:1", "192.0.2.1")
   311  //	Dial("ip6:ipv6-icmp", "2001:db8::1")
   312  //	Dial("ip6:58", "fe80::1%lo0")
   313  //
   314  // For TCP, UDP and IP networks, if the host is empty or a literal
   315  // unspecified IP address, as in ":80", "0.0.0.0:80" or "[::]:80" for
   316  // TCP and UDP, "", "0.0.0.0" or "::" for IP, the local system is
   317  // assumed.
   318  //
   319  // For Unix networks, the address must be a file system path.
   320  func Dial(network, address string) (Conn, error) {
   321  	var d Dialer
   322  	return d.Dial(network, address)
   323  }
   324  
   325  // DialTimeout acts like Dial but takes a timeout.
   326  //
   327  // The timeout includes name resolution, if required.
   328  // When using TCP, and the host in the address parameter resolves to
   329  // multiple IP addresses, the timeout is spread over each consecutive
   330  // dial, such that each is given an appropriate fraction of the time
   331  // to connect.
   332  //
   333  // See func Dial for a description of the network and address
   334  // parameters.
   335  func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
   336  	d := Dialer{Timeout: timeout}
   337  	return d.Dial(network, address)
   338  }
   339  
   340  // sysDialer contains a Dial's parameters and configuration.
   341  type sysDialer struct {
   342  	Dialer
   343  	network, address string
   344  	testHookDialTCP  func(ctx context.Context, net string, laddr, raddr *TCPAddr) (*TCPConn, error)
   345  }
   346  
   347  // Dial connects to the address on the named network.
   348  //
   349  // See func Dial for a description of the network and address
   350  // parameters.
   351  //
   352  // Dial uses context.Background internally; to specify the context, use
   353  // DialContext.
   354  func (d *Dialer) Dial(network, address string) (Conn, error) {
   355  	return d.DialContext(context.Background(), network, address)
   356  }
   357  
   358  // DialContext connects to the address on the named network using
   359  // the provided context.
   360  //
   361  // The provided Context must be non-nil. If the context expires before
   362  // the connection is complete, an error is returned. Once successfully
   363  // connected, any expiration of the context will not affect the
   364  // connection.
   365  //
   366  // When using TCP, and the host in the address parameter resolves to multiple
   367  // network addresses, any dial timeout (from d.Timeout or ctx) is spread
   368  // over each consecutive dial, such that each is given an appropriate
   369  // fraction of the time to connect.
   370  // For example, if a host has 4 IP addresses and the timeout is 1 minute,
   371  // the connect to each single address will be given 15 seconds to complete
   372  // before trying the next one.
   373  //
   374  // See func Dial for a description of the network and address
   375  // parameters.
   376  func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
   377  	if ctx == nil {
   378  		panic("nil context")
   379  	}
   380  	deadline := d.deadline(ctx, time.Now())
   381  	if !deadline.IsZero() {
   382  		if d, ok := ctx.Deadline(); !ok || deadline.Before(d) {
   383  			subCtx, cancel := context.WithDeadline(ctx, deadline)
   384  			defer cancel()
   385  			ctx = subCtx
   386  		}
   387  	}
   388  	if oldCancel := d.Cancel; oldCancel != nil {
   389  		subCtx, cancel := context.WithCancel(ctx)
   390  		defer cancel()
   391  		go func() {
   392  			select {
   393  			case <-oldCancel:
   394  				cancel()
   395  			case <-subCtx.Done():
   396  			}
   397  		}()
   398  		ctx = subCtx
   399  	}
   400  
   401  	// Shadow the nettrace (if any) during resolve so Connect events don't fire for DNS lookups.
   402  	resolveCtx := ctx
   403  	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
   404  		shadow := *trace
   405  		shadow.ConnectStart = nil
   406  		shadow.ConnectDone = nil
   407  		resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
   408  	}
   409  
   410  	addrs, err := d.resolver().resolveAddrList(resolveCtx, "dial", network, address, d.LocalAddr)
   411  	if err != nil {
   412  		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
   413  	}
   414  
   415  	sd := &sysDialer{
   416  		Dialer:  *d,
   417  		network: network,
   418  		address: address,
   419  	}
   420  
   421  	var primaries, fallbacks addrList
   422  	if d.dualStack() && network == "tcp" {
   423  		primaries, fallbacks = addrs.partition(isIPv4)
   424  	} else {
   425  		primaries = addrs
   426  	}
   427  
   428  	c, err := sd.dialParallel(ctx, primaries, fallbacks)
   429  	if err != nil {
   430  		return nil, err
   431  	}
   432  
   433  	if tc, ok := c.(*TCPConn); ok && d.KeepAlive >= 0 {
   434  		setKeepAlive(tc.fd, true)
   435  		ka := d.KeepAlive
   436  		if d.KeepAlive == 0 {
   437  			ka = defaultTCPKeepAlive
   438  		}
   439  		setKeepAlivePeriod(tc.fd, ka)
   440  		testHookSetKeepAlive(ka)
   441  	}
   442  	return c, nil
   443  }
   444  
   445  // dialParallel races two copies of dialSerial, giving the first a
   446  // head start. It returns the first established connection and
   447  // closes the others. Otherwise it returns an error from the first
   448  // primary address.
   449  func (sd *sysDialer) dialParallel(ctx context.Context, primaries, fallbacks addrList) (Conn, error) {
   450  	if len(fallbacks) == 0 {
   451  		return sd.dialSerial(ctx, primaries)
   452  	}
   453  
   454  	returned := make(chan struct{})
   455  	defer close(returned)
   456  
   457  	type dialResult struct {
   458  		Conn
   459  		error
   460  		primary bool
   461  		done    bool
   462  	}
   463  	results := make(chan dialResult) // unbuffered
   464  
   465  	startRacer := func(ctx context.Context, primary bool) {
   466  		ras := primaries
   467  		if !primary {
   468  			ras = fallbacks
   469  		}
   470  		c, err := sd.dialSerial(ctx, ras)
   471  		select {
   472  		case results <- dialResult{Conn: c, error: err, primary: primary, done: true}:
   473  		case <-returned:
   474  			if c != nil {
   475  				c.Close()
   476  			}
   477  		}
   478  	}
   479  
   480  	var primary, fallback dialResult
   481  
   482  	// Start the main racer.
   483  	primaryCtx, primaryCancel := context.WithCancel(ctx)
   484  	defer primaryCancel()
   485  	go startRacer(primaryCtx, true)
   486  
   487  	// Start the timer for the fallback racer.
   488  	fallbackTimer := time.NewTimer(sd.fallbackDelay())
   489  	defer fallbackTimer.Stop()
   490  
   491  	for {
   492  		select {
   493  		case <-fallbackTimer.C:
   494  			fallbackCtx, fallbackCancel := context.WithCancel(ctx)
   495  			defer fallbackCancel()
   496  			go startRacer(fallbackCtx, false)
   497  
   498  		case res := <-results:
   499  			if res.error == nil {
   500  				return res.Conn, nil
   501  			}
   502  			if res.primary {
   503  				primary = res
   504  			} else {
   505  				fallback = res
   506  			}
   507  			if primary.done && fallback.done {
   508  				return nil, primary.error
   509  			}
   510  			if res.primary && fallbackTimer.Stop() {
   511  				// If we were able to stop the timer, that means it
   512  				// was running (hadn't yet started the fallback), but
   513  				// we just got an error on the primary path, so start
   514  				// the fallback immediately (in 0 nanoseconds).
   515  				fallbackTimer.Reset(0)
   516  			}
   517  		}
   518  	}
   519  }
   520  
   521  // dialSerial connects to a list of addresses in sequence, returning
   522  // either the first successful connection, or the first error.
   523  func (sd *sysDialer) dialSerial(ctx context.Context, ras addrList) (Conn, error) {
   524  	var firstErr error // The error from the first address is most relevant.
   525  
   526  	for i, ra := range ras {
   527  		select {
   528  		case <-ctx.Done():
   529  			return nil, &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: mapErr(ctx.Err())}
   530  		default:
   531  		}
   532  
   533  		dialCtx := ctx
   534  		if deadline, hasDeadline := ctx.Deadline(); hasDeadline {
   535  			partialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)
   536  			if err != nil {
   537  				// Ran out of time.
   538  				if firstErr == nil {
   539  					firstErr = &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: err}
   540  				}
   541  				break
   542  			}
   543  			if partialDeadline.Before(deadline) {
   544  				var cancel context.CancelFunc
   545  				dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
   546  				defer cancel()
   547  			}
   548  		}
   549  
   550  		c, err := sd.dialSingle(dialCtx, ra)
   551  		if err == nil {
   552  			return c, nil
   553  		}
   554  		if firstErr == nil {
   555  			firstErr = err
   556  		}
   557  	}
   558  
   559  	if firstErr == nil {
   560  		firstErr = &OpError{Op: "dial", Net: sd.network, Source: nil, Addr: nil, Err: errMissingAddress}
   561  	}
   562  	return nil, firstErr
   563  }
   564  
   565  // dialSingle attempts to establish and returns a single connection to
   566  // the destination address.
   567  func (sd *sysDialer) dialSingle(ctx context.Context, ra Addr) (c Conn, err error) {
   568  	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
   569  	if trace != nil {
   570  		raStr := ra.String()
   571  		if trace.ConnectStart != nil {
   572  			trace.ConnectStart(sd.network, raStr)
   573  		}
   574  		if trace.ConnectDone != nil {
   575  			defer func() { trace.ConnectDone(sd.network, raStr, err) }()
   576  		}
   577  	}
   578  	la := sd.LocalAddr
   579  	switch ra := ra.(type) {
   580  	case *TCPAddr:
   581  		la, _ := la.(*TCPAddr)
   582  		c, err = sd.dialTCP(ctx, la, ra)
   583  	case *UDPAddr:
   584  		la, _ := la.(*UDPAddr)
   585  		c, err = sd.dialUDP(ctx, la, ra)
   586  	case *IPAddr:
   587  		la, _ := la.(*IPAddr)
   588  		c, err = sd.dialIP(ctx, la, ra)
   589  	case *UnixAddr:
   590  		la, _ := la.(*UnixAddr)
   591  		c, err = sd.dialUnix(ctx, la, ra)
   592  	default:
   593  		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: sd.address}}
   594  	}
   595  	if err != nil {
   596  		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
   597  	}
   598  	return c, nil
   599  }
   600  
   601  // ListenConfig contains options for listening to an address.
   602  type ListenConfig struct {
   603  	// If Control is not nil, it is called after creating the network
   604  	// connection but before binding it to the operating system.
   605  	//
   606  	// Network and address parameters passed to Control method are not
   607  	// necessarily the ones passed to Listen. For example, passing "tcp" to
   608  	// Listen will cause the Control function to be called with "tcp4" or "tcp6".
   609  	Control func(network, address string, c syscall.RawConn) error
   610  
   611  	// KeepAlive specifies the keep-alive period for network
   612  	// connections accepted by this listener.
   613  	// If zero, keep-alives are enabled if supported by the protocol
   614  	// and operating system. Network protocols or operating systems
   615  	// that do not support keep-alives ignore this field.
   616  	// If negative, keep-alives are disabled.
   617  	KeepAlive time.Duration
   618  }
   619  
   620  // Listen announces on the local network address.
   621  //
   622  // See func Listen for a description of the network and address
   623  // parameters.
   624  func (lc *ListenConfig) Listen(ctx context.Context, network, address string) (Listener, error) {
   625  	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
   626  	if err != nil {
   627  		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
   628  	}
   629  	sl := &sysListener{
   630  		ListenConfig: *lc,
   631  		network:      network,
   632  		address:      address,
   633  	}
   634  	var l Listener
   635  	la := addrs.first(isIPv4)
   636  	switch la := la.(type) {
   637  	case *TCPAddr:
   638  		l, err = sl.listenTCP(ctx, la)
   639  	case *UnixAddr:
   640  		l, err = sl.listenUnix(ctx, la)
   641  	default:
   642  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
   643  	}
   644  	if err != nil {
   645  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // l is non-nil interface containing nil pointer
   646  	}
   647  	return l, nil
   648  }
   649  
   650  // ListenPacket announces on the local network address.
   651  //
   652  // See func ListenPacket for a description of the network and address
   653  // parameters.
   654  func (lc *ListenConfig) ListenPacket(ctx context.Context, network, address string) (PacketConn, error) {
   655  	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
   656  	if err != nil {
   657  		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
   658  	}
   659  	sl := &sysListener{
   660  		ListenConfig: *lc,
   661  		network:      network,
   662  		address:      address,
   663  	}
   664  	var c PacketConn
   665  	la := addrs.first(isIPv4)
   666  	switch la := la.(type) {
   667  	case *UDPAddr:
   668  		c, err = sl.listenUDP(ctx, la)
   669  	case *IPAddr:
   670  		c, err = sl.listenIP(ctx, la)
   671  	case *UnixAddr:
   672  		c, err = sl.listenUnixgram(ctx, la)
   673  	default:
   674  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
   675  	}
   676  	if err != nil {
   677  		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // c is non-nil interface containing nil pointer
   678  	}
   679  	return c, nil
   680  }
   681  
   682  // sysListener contains a Listen's parameters and configuration.
   683  type sysListener struct {
   684  	ListenConfig
   685  	network, address string
   686  }
   687  
   688  // Listen announces on the local network address.
   689  //
   690  // The network must be "tcp", "tcp4", "tcp6", "unix" or "unixpacket".
   691  //
   692  // For TCP networks, if the host in the address parameter is empty or
   693  // a literal unspecified IP address, Listen listens on all available
   694  // unicast and anycast IP addresses of the local system.
   695  // To only use IPv4, use network "tcp4".
   696  // The address can use a host name, but this is not recommended,
   697  // because it will create a listener for at most one of the host's IP
   698  // addresses.
   699  // If the port in the address parameter is empty or "0", as in
   700  // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
   701  // The Addr method of Listener can be used to discover the chosen
   702  // port.
   703  //
   704  // See func Dial for a description of the network and address
   705  // parameters.
   706  //
   707  // Listen uses context.Background internally; to specify the context, use
   708  // ListenConfig.Listen.
   709  func Listen(network, address string) (Listener, error) {
   710  	var lc ListenConfig
   711  	return lc.Listen(context.Background(), network, address)
   712  }
   713  
   714  // ListenPacket announces on the local network address.
   715  //
   716  // The network must be "udp", "udp4", "udp6", "unixgram", or an IP
   717  // transport. The IP transports are "ip", "ip4", or "ip6" followed by
   718  // a colon and a literal protocol number or a protocol name, as in
   719  // "ip:1" or "ip:icmp".
   720  //
   721  // For UDP and IP networks, if the host in the address parameter is
   722  // empty or a literal unspecified IP address, ListenPacket listens on
   723  // all available IP addresses of the local system except multicast IP
   724  // addresses.
   725  // To only use IPv4, use network "udp4" or "ip4:proto".
   726  // The address can use a host name, but this is not recommended,
   727  // because it will create a listener for at most one of the host's IP
   728  // addresses.
   729  // If the port in the address parameter is empty or "0", as in
   730  // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
   731  // The LocalAddr method of PacketConn can be used to discover the
   732  // chosen port.
   733  //
   734  // See func Dial for a description of the network and address
   735  // parameters.
   736  //
   737  // ListenPacket uses context.Background internally; to specify the context, use
   738  // ListenConfig.ListenPacket.
   739  func ListenPacket(network, address string) (PacketConn, error) {
   740  	var lc ListenConfig
   741  	return lc.ListenPacket(context.Background(), network, address)
   742  }
   743  

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