Source file src/pkg/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		"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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