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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