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

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