Source file src/net/ip.go

     1  // Copyright 2009 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  // IP address manipulations
     6  //
     7  // IPv4 addresses are 4 bytes; IPv6 addresses are 16 bytes.
     8  // An IPv4 address can be converted to an IPv6 address by
     9  // adding a canonical prefix (10 zeros, 2 0xFFs).
    10  // This library accepts either size of byte slice but always
    11  // returns 16-byte addresses.
    12  
    13  package net
    14  
    15  import (
    16  	"internal/bytealg"
    17  	"internal/itoa"
    18  )
    19  
    20  // IP address lengths (bytes).
    21  const (
    22  	IPv4len = 4
    23  	IPv6len = 16
    24  )
    25  
    26  // An IP is a single IP address, a slice of bytes.
    27  // Functions in this package accept either 4-byte (IPv4)
    28  // or 16-byte (IPv6) slices as input.
    29  //
    30  // Note that in this documentation, referring to an
    31  // IP address as an IPv4 address or an IPv6 address
    32  // is a semantic property of the address, not just the
    33  // length of the byte slice: a 16-byte slice can still
    34  // be an IPv4 address.
    35  type IP []byte
    36  
    37  // An IPMask is a bitmask that can be used to manipulate
    38  // IP addresses for IP addressing and routing.
    39  //
    40  // See type IPNet and func ParseCIDR for details.
    41  type IPMask []byte
    42  
    43  // An IPNet represents an IP network.
    44  type IPNet struct {
    45  	IP   IP     // network number
    46  	Mask IPMask // network mask
    47  }
    48  
    49  // IPv4 returns the IP address (in 16-byte form) of the
    50  // IPv4 address a.b.c.d.
    51  func IPv4(a, b, c, d byte) IP {
    52  	p := make(IP, IPv6len)
    53  	copy(p, v4InV6Prefix)
    54  	p[12] = a
    55  	p[13] = b
    56  	p[14] = c
    57  	p[15] = d
    58  	return p
    59  }
    60  
    61  var v4InV6Prefix = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff}
    62  
    63  // IPv4Mask returns the IP mask (in 4-byte form) of the
    64  // IPv4 mask a.b.c.d.
    65  func IPv4Mask(a, b, c, d byte) IPMask {
    66  	p := make(IPMask, IPv4len)
    67  	p[0] = a
    68  	p[1] = b
    69  	p[2] = c
    70  	p[3] = d
    71  	return p
    72  }
    73  
    74  // CIDRMask returns an IPMask consisting of 'ones' 1 bits
    75  // followed by 0s up to a total length of 'bits' bits.
    76  // For a mask of this form, CIDRMask is the inverse of IPMask.Size.
    77  func CIDRMask(ones, bits int) IPMask {
    78  	if bits != 8*IPv4len && bits != 8*IPv6len {
    79  		return nil
    80  	}
    81  	if ones < 0 || ones > bits {
    82  		return nil
    83  	}
    84  	l := bits / 8
    85  	m := make(IPMask, l)
    86  	n := uint(ones)
    87  	for i := 0; i < l; i++ {
    88  		if n >= 8 {
    89  			m[i] = 0xff
    90  			n -= 8
    91  			continue
    92  		}
    93  		m[i] = ^byte(0xff >> n)
    94  		n = 0
    95  	}
    96  	return m
    97  }
    98  
    99  // Well-known IPv4 addresses
   100  var (
   101  	IPv4bcast     = IPv4(255, 255, 255, 255) // limited broadcast
   102  	IPv4allsys    = IPv4(224, 0, 0, 1)       // all systems
   103  	IPv4allrouter = IPv4(224, 0, 0, 2)       // all routers
   104  	IPv4zero      = IPv4(0, 0, 0, 0)         // all zeros
   105  )
   106  
   107  // Well-known IPv6 addresses
   108  var (
   109  	IPv6zero                   = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   110  	IPv6unspecified            = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   111  	IPv6loopback               = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}
   112  	IPv6interfacelocalallnodes = IP{0xff, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   113  	IPv6linklocalallnodes      = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   114  	IPv6linklocalallrouters    = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x02}
   115  )
   116  
   117  // IsUnspecified reports whether ip is an unspecified address, either
   118  // the IPv4 address "0.0.0.0" or the IPv6 address "::".
   119  func (ip IP) IsUnspecified() bool {
   120  	return ip.Equal(IPv4zero) || ip.Equal(IPv6unspecified)
   121  }
   122  
   123  // IsLoopback reports whether ip is a loopback address.
   124  func (ip IP) IsLoopback() bool {
   125  	if ip4 := ip.To4(); ip4 != nil {
   126  		return ip4[0] == 127
   127  	}
   128  	return ip.Equal(IPv6loopback)
   129  }
   130  
   131  // IsPrivate reports whether ip is a private address, according to
   132  // RFC 1918 (IPv4 addresses) and RFC 4193 (IPv6 addresses).
   133  func (ip IP) IsPrivate() bool {
   134  	if ip4 := ip.To4(); ip4 != nil {
   135  		// Following RFC 1918, Section 3. Private Address Space which says:
   136  		//   The Internet Assigned Numbers Authority (IANA) has reserved the
   137  		//   following three blocks of the IP address space for private internets:
   138  		//     10.0.0.0        -   10.255.255.255  (10/8 prefix)
   139  		//     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
   140  		//     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)
   141  		return ip4[0] == 10 ||
   142  			(ip4[0] == 172 && ip4[1]&0xf0 == 16) ||
   143  			(ip4[0] == 192 && ip4[1] == 168)
   144  	}
   145  	// Following RFC 4193, Section 8. IANA Considerations which says:
   146  	//   The IANA has assigned the FC00::/7 prefix to "Unique Local Unicast".
   147  	return len(ip) == IPv6len && ip[0]&0xfe == 0xfc
   148  }
   149  
   150  // IsMulticast reports whether ip is a multicast address.
   151  func (ip IP) IsMulticast() bool {
   152  	if ip4 := ip.To4(); ip4 != nil {
   153  		return ip4[0]&0xf0 == 0xe0
   154  	}
   155  	return len(ip) == IPv6len && ip[0] == 0xff
   156  }
   157  
   158  // IsInterfaceLocalMulticast reports whether ip is
   159  // an interface-local multicast address.
   160  func (ip IP) IsInterfaceLocalMulticast() bool {
   161  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x01
   162  }
   163  
   164  // IsLinkLocalMulticast reports whether ip is a link-local
   165  // multicast address.
   166  func (ip IP) IsLinkLocalMulticast() bool {
   167  	if ip4 := ip.To4(); ip4 != nil {
   168  		return ip4[0] == 224 && ip4[1] == 0 && ip4[2] == 0
   169  	}
   170  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x02
   171  }
   172  
   173  // IsLinkLocalUnicast reports whether ip is a link-local
   174  // unicast address.
   175  func (ip IP) IsLinkLocalUnicast() bool {
   176  	if ip4 := ip.To4(); ip4 != nil {
   177  		return ip4[0] == 169 && ip4[1] == 254
   178  	}
   179  	return len(ip) == IPv6len && ip[0] == 0xfe && ip[1]&0xc0 == 0x80
   180  }
   181  
   182  // IsGlobalUnicast reports whether ip is a global unicast
   183  // address.
   184  //
   185  // The identification of global unicast addresses uses address type
   186  // identification as defined in RFC 1122, RFC 4632 and RFC 4291 with
   187  // the exception of IPv4 directed broadcast addresses.
   188  // It returns true even if ip is in IPv4 private address space or
   189  // local IPv6 unicast address space.
   190  func (ip IP) IsGlobalUnicast() bool {
   191  	return (len(ip) == IPv4len || len(ip) == IPv6len) &&
   192  		!ip.Equal(IPv4bcast) &&
   193  		!ip.IsUnspecified() &&
   194  		!ip.IsLoopback() &&
   195  		!ip.IsMulticast() &&
   196  		!ip.IsLinkLocalUnicast()
   197  }
   198  
   199  // Is p all zeros?
   200  func isZeros(p IP) bool {
   201  	for i := 0; i < len(p); i++ {
   202  		if p[i] != 0 {
   203  			return false
   204  		}
   205  	}
   206  	return true
   207  }
   208  
   209  // To4 converts the IPv4 address ip to a 4-byte representation.
   210  // If ip is not an IPv4 address, To4 returns nil.
   211  func (ip IP) To4() IP {
   212  	if len(ip) == IPv4len {
   213  		return ip
   214  	}
   215  	if len(ip) == IPv6len &&
   216  		isZeros(ip[0:10]) &&
   217  		ip[10] == 0xff &&
   218  		ip[11] == 0xff {
   219  		return ip[12:16]
   220  	}
   221  	return nil
   222  }
   223  
   224  // To16 converts the IP address ip to a 16-byte representation.
   225  // If ip is not an IP address (it is the wrong length), To16 returns nil.
   226  func (ip IP) To16() IP {
   227  	if len(ip) == IPv4len {
   228  		return IPv4(ip[0], ip[1], ip[2], ip[3])
   229  	}
   230  	if len(ip) == IPv6len {
   231  		return ip
   232  	}
   233  	return nil
   234  }
   235  
   236  // Default route masks for IPv4.
   237  var (
   238  	classAMask = IPv4Mask(0xff, 0, 0, 0)
   239  	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
   240  	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
   241  )
   242  
   243  // DefaultMask returns the default IP mask for the IP address ip.
   244  // Only IPv4 addresses have default masks; DefaultMask returns
   245  // nil if ip is not a valid IPv4 address.
   246  func (ip IP) DefaultMask() IPMask {
   247  	if ip = ip.To4(); ip == nil {
   248  		return nil
   249  	}
   250  	switch {
   251  	case ip[0] < 0x80:
   252  		return classAMask
   253  	case ip[0] < 0xC0:
   254  		return classBMask
   255  	default:
   256  		return classCMask
   257  	}
   258  }
   259  
   260  func allFF(b []byte) bool {
   261  	for _, c := range b {
   262  		if c != 0xff {
   263  			return false
   264  		}
   265  	}
   266  	return true
   267  }
   268  
   269  // Mask returns the result of masking the IP address ip with mask.
   270  func (ip IP) Mask(mask IPMask) IP {
   271  	if len(mask) == IPv6len && len(ip) == IPv4len && allFF(mask[:12]) {
   272  		mask = mask[12:]
   273  	}
   274  	if len(mask) == IPv4len && len(ip) == IPv6len && bytealg.Equal(ip[:12], v4InV6Prefix) {
   275  		ip = ip[12:]
   276  	}
   277  	n := len(ip)
   278  	if n != len(mask) {
   279  		return nil
   280  	}
   281  	out := make(IP, n)
   282  	for i := 0; i < n; i++ {
   283  		out[i] = ip[i] & mask[i]
   284  	}
   285  	return out
   286  }
   287  
   288  // ubtoa encodes the string form of the integer v to dst[start:] and
   289  // returns the number of bytes written to dst. The caller must ensure
   290  // that dst has sufficient length.
   291  func ubtoa(dst []byte, start int, v byte) int {
   292  	if v < 10 {
   293  		dst[start] = v + '0'
   294  		return 1
   295  	} else if v < 100 {
   296  		dst[start+1] = v%10 + '0'
   297  		dst[start] = v/10 + '0'
   298  		return 2
   299  	}
   300  
   301  	dst[start+2] = v%10 + '0'
   302  	dst[start+1] = (v/10)%10 + '0'
   303  	dst[start] = v/100 + '0'
   304  	return 3
   305  }
   306  
   307  // String returns the string form of the IP address ip.
   308  // It returns one of 4 forms:
   309  //   - "<nil>", if ip has length 0
   310  //   - dotted decimal ("192.0.2.1"), if ip is an IPv4 or IP4-mapped IPv6 address
   311  //   - IPv6 ("2001:db8::1"), if ip is a valid IPv6 address
   312  //   - the hexadecimal form of ip, without punctuation, if no other cases apply
   313  func (ip IP) String() string {
   314  	p := ip
   315  
   316  	if len(ip) == 0 {
   317  		return "<nil>"
   318  	}
   319  
   320  	// If IPv4, use dotted notation.
   321  	if p4 := p.To4(); len(p4) == IPv4len {
   322  		const maxIPv4StringLen = len("255.255.255.255")
   323  		b := make([]byte, maxIPv4StringLen)
   324  
   325  		n := ubtoa(b, 0, p4[0])
   326  		b[n] = '.'
   327  		n++
   328  
   329  		n += ubtoa(b, n, p4[1])
   330  		b[n] = '.'
   331  		n++
   332  
   333  		n += ubtoa(b, n, p4[2])
   334  		b[n] = '.'
   335  		n++
   336  
   337  		n += ubtoa(b, n, p4[3])
   338  		return string(b[:n])
   339  	}
   340  	if len(p) != IPv6len {
   341  		return "?" + hexString(ip)
   342  	}
   343  
   344  	// Find longest run of zeros.
   345  	e0 := -1
   346  	e1 := -1
   347  	for i := 0; i < IPv6len; i += 2 {
   348  		j := i
   349  		for j < IPv6len && p[j] == 0 && p[j+1] == 0 {
   350  			j += 2
   351  		}
   352  		if j > i && j-i > e1-e0 {
   353  			e0 = i
   354  			e1 = j
   355  			i = j
   356  		}
   357  	}
   358  	// The symbol "::" MUST NOT be used to shorten just one 16 bit 0 field.
   359  	if e1-e0 <= 2 {
   360  		e0 = -1
   361  		e1 = -1
   362  	}
   363  
   364  	const maxLen = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")
   365  	b := make([]byte, 0, maxLen)
   366  
   367  	// Print with possible :: in place of run of zeros
   368  	for i := 0; i < IPv6len; i += 2 {
   369  		if i == e0 {
   370  			b = append(b, ':', ':')
   371  			i = e1
   372  			if i >= IPv6len {
   373  				break
   374  			}
   375  		} else if i > 0 {
   376  			b = append(b, ':')
   377  		}
   378  		b = appendHex(b, (uint32(p[i])<<8)|uint32(p[i+1]))
   379  	}
   380  	return string(b)
   381  }
   382  
   383  func hexString(b []byte) string {
   384  	s := make([]byte, len(b)*2)
   385  	for i, tn := range b {
   386  		s[i*2], s[i*2+1] = hexDigit[tn>>4], hexDigit[tn&0xf]
   387  	}
   388  	return string(s)
   389  }
   390  
   391  // ipEmptyString is like ip.String except that it returns
   392  // an empty string when ip is unset.
   393  func ipEmptyString(ip IP) string {
   394  	if len(ip) == 0 {
   395  		return ""
   396  	}
   397  	return ip.String()
   398  }
   399  
   400  // MarshalText implements the encoding.TextMarshaler interface.
   401  // The encoding is the same as returned by String, with one exception:
   402  // When len(ip) is zero, it returns an empty slice.
   403  func (ip IP) MarshalText() ([]byte, error) {
   404  	if len(ip) == 0 {
   405  		return []byte(""), nil
   406  	}
   407  	if len(ip) != IPv4len && len(ip) != IPv6len {
   408  		return nil, &AddrError{Err: "invalid IP address", Addr: hexString(ip)}
   409  	}
   410  	return []byte(ip.String()), nil
   411  }
   412  
   413  // UnmarshalText implements the encoding.TextUnmarshaler interface.
   414  // The IP address is expected in a form accepted by ParseIP.
   415  func (ip *IP) UnmarshalText(text []byte) error {
   416  	if len(text) == 0 {
   417  		*ip = nil
   418  		return nil
   419  	}
   420  	s := string(text)
   421  	x := ParseIP(s)
   422  	if x == nil {
   423  		return &ParseError{Type: "IP address", Text: s}
   424  	}
   425  	*ip = x
   426  	return nil
   427  }
   428  
   429  // Equal reports whether ip and x are the same IP address.
   430  // An IPv4 address and that same address in IPv6 form are
   431  // considered to be equal.
   432  func (ip IP) Equal(x IP) bool {
   433  	if len(ip) == len(x) {
   434  		return bytealg.Equal(ip, x)
   435  	}
   436  	if len(ip) == IPv4len && len(x) == IPv6len {
   437  		return bytealg.Equal(x[0:12], v4InV6Prefix) && bytealg.Equal(ip, x[12:])
   438  	}
   439  	if len(ip) == IPv6len && len(x) == IPv4len {
   440  		return bytealg.Equal(ip[0:12], v4InV6Prefix) && bytealg.Equal(ip[12:], x)
   441  	}
   442  	return false
   443  }
   444  
   445  func (ip IP) matchAddrFamily(x IP) bool {
   446  	return ip.To4() != nil && x.To4() != nil || ip.To16() != nil && ip.To4() == nil && x.To16() != nil && x.To4() == nil
   447  }
   448  
   449  // If mask is a sequence of 1 bits followed by 0 bits,
   450  // return the number of 1 bits.
   451  func simpleMaskLength(mask IPMask) int {
   452  	var n int
   453  	for i, v := range mask {
   454  		if v == 0xff {
   455  			n += 8
   456  			continue
   457  		}
   458  		// found non-ff byte
   459  		// count 1 bits
   460  		for v&0x80 != 0 {
   461  			n++
   462  			v <<= 1
   463  		}
   464  		// rest must be 0 bits
   465  		if v != 0 {
   466  			return -1
   467  		}
   468  		for i++; i < len(mask); i++ {
   469  			if mask[i] != 0 {
   470  				return -1
   471  			}
   472  		}
   473  		break
   474  	}
   475  	return n
   476  }
   477  
   478  // Size returns the number of leading ones and total bits in the mask.
   479  // If the mask is not in the canonical form--ones followed by zeros--then
   480  // Size returns 0, 0.
   481  func (m IPMask) Size() (ones, bits int) {
   482  	ones, bits = simpleMaskLength(m), len(m)*8
   483  	if ones == -1 {
   484  		return 0, 0
   485  	}
   486  	return
   487  }
   488  
   489  // String returns the hexadecimal form of m, with no punctuation.
   490  func (m IPMask) String() string {
   491  	if len(m) == 0 {
   492  		return "<nil>"
   493  	}
   494  	return hexString(m)
   495  }
   496  
   497  func networkNumberAndMask(n *IPNet) (ip IP, m IPMask) {
   498  	if ip = n.IP.To4(); ip == nil {
   499  		ip = n.IP
   500  		if len(ip) != IPv6len {
   501  			return nil, nil
   502  		}
   503  	}
   504  	m = n.Mask
   505  	switch len(m) {
   506  	case IPv4len:
   507  		if len(ip) != IPv4len {
   508  			return nil, nil
   509  		}
   510  	case IPv6len:
   511  		if len(ip) == IPv4len {
   512  			m = m[12:]
   513  		}
   514  	default:
   515  		return nil, nil
   516  	}
   517  	return
   518  }
   519  
   520  // Contains reports whether the network includes ip.
   521  func (n *IPNet) Contains(ip IP) bool {
   522  	nn, m := networkNumberAndMask(n)
   523  	if x := ip.To4(); x != nil {
   524  		ip = x
   525  	}
   526  	l := len(ip)
   527  	if l != len(nn) {
   528  		return false
   529  	}
   530  	for i := 0; i < l; i++ {
   531  		if nn[i]&m[i] != ip[i]&m[i] {
   532  			return false
   533  		}
   534  	}
   535  	return true
   536  }
   537  
   538  // Network returns the address's network name, "ip+net".
   539  func (n *IPNet) Network() string { return "ip+net" }
   540  
   541  // String returns the CIDR notation of n like "192.0.2.0/24"
   542  // or "2001:db8::/48" as defined in RFC 4632 and RFC 4291.
   543  // If the mask is not in the canonical form, it returns the
   544  // string which consists of an IP address, followed by a slash
   545  // character and a mask expressed as hexadecimal form with no
   546  // punctuation like "198.51.100.0/c000ff00".
   547  func (n *IPNet) String() string {
   548  	nn, m := networkNumberAndMask(n)
   549  	if nn == nil || m == nil {
   550  		return "<nil>"
   551  	}
   552  	l := simpleMaskLength(m)
   553  	if l == -1 {
   554  		return nn.String() + "/" + m.String()
   555  	}
   556  	return nn.String() + "/" + itoa.Uitoa(uint(l))
   557  }
   558  
   559  // Parse IPv4 address (d.d.d.d).
   560  func parseIPv4(s string) IP {
   561  	var p [IPv4len]byte
   562  	for i := 0; i < IPv4len; i++ {
   563  		if len(s) == 0 {
   564  			// Missing octets.
   565  			return nil
   566  		}
   567  		if i > 0 {
   568  			if s[0] != '.' {
   569  				return nil
   570  			}
   571  			s = s[1:]
   572  		}
   573  		n, c, ok := dtoi(s)
   574  		if !ok || n > 0xFF {
   575  			return nil
   576  		}
   577  		if c > 1 && s[0] == '0' {
   578  			// Reject non-zero components with leading zeroes.
   579  			return nil
   580  		}
   581  		s = s[c:]
   582  		p[i] = byte(n)
   583  	}
   584  	if len(s) != 0 {
   585  		return nil
   586  	}
   587  	return IPv4(p[0], p[1], p[2], p[3])
   588  }
   589  
   590  // parseIPv6Zone parses s as a literal IPv6 address and its associated zone
   591  // identifier which is described in RFC 4007.
   592  func parseIPv6Zone(s string) (IP, string) {
   593  	s, zone := splitHostZone(s)
   594  	return parseIPv6(s), zone
   595  }
   596  
   597  // parseIPv6 parses s as a literal IPv6 address described in RFC 4291
   598  // and RFC 5952.
   599  func parseIPv6(s string) (ip IP) {
   600  	ip = make(IP, IPv6len)
   601  	ellipsis := -1 // position of ellipsis in ip
   602  
   603  	// Might have leading ellipsis
   604  	if len(s) >= 2 && s[0] == ':' && s[1] == ':' {
   605  		ellipsis = 0
   606  		s = s[2:]
   607  		// Might be only ellipsis
   608  		if len(s) == 0 {
   609  			return ip
   610  		}
   611  	}
   612  
   613  	// Loop, parsing hex numbers followed by colon.
   614  	i := 0
   615  	for i < IPv6len {
   616  		// Hex number.
   617  		n, c, ok := xtoi(s)
   618  		if !ok || n > 0xFFFF {
   619  			return nil
   620  		}
   621  
   622  		// If followed by dot, might be in trailing IPv4.
   623  		if c < len(s) && s[c] == '.' {
   624  			if ellipsis < 0 && i != IPv6len-IPv4len {
   625  				// Not the right place.
   626  				return nil
   627  			}
   628  			if i+IPv4len > IPv6len {
   629  				// Not enough room.
   630  				return nil
   631  			}
   632  			ip4 := parseIPv4(s)
   633  			if ip4 == nil {
   634  				return nil
   635  			}
   636  			ip[i] = ip4[12]
   637  			ip[i+1] = ip4[13]
   638  			ip[i+2] = ip4[14]
   639  			ip[i+3] = ip4[15]
   640  			s = ""
   641  			i += IPv4len
   642  			break
   643  		}
   644  
   645  		// Save this 16-bit chunk.
   646  		ip[i] = byte(n >> 8)
   647  		ip[i+1] = byte(n)
   648  		i += 2
   649  
   650  		// Stop at end of string.
   651  		s = s[c:]
   652  		if len(s) == 0 {
   653  			break
   654  		}
   655  
   656  		// Otherwise must be followed by colon and more.
   657  		if s[0] != ':' || len(s) == 1 {
   658  			return nil
   659  		}
   660  		s = s[1:]
   661  
   662  		// Look for ellipsis.
   663  		if s[0] == ':' {
   664  			if ellipsis >= 0 { // already have one
   665  				return nil
   666  			}
   667  			ellipsis = i
   668  			s = s[1:]
   669  			if len(s) == 0 { // can be at end
   670  				break
   671  			}
   672  		}
   673  	}
   674  
   675  	// Must have used entire string.
   676  	if len(s) != 0 {
   677  		return nil
   678  	}
   679  
   680  	// If didn't parse enough, expand ellipsis.
   681  	if i < IPv6len {
   682  		if ellipsis < 0 {
   683  			return nil
   684  		}
   685  		n := IPv6len - i
   686  		for j := i - 1; j >= ellipsis; j-- {
   687  			ip[j+n] = ip[j]
   688  		}
   689  		for j := ellipsis + n - 1; j >= ellipsis; j-- {
   690  			ip[j] = 0
   691  		}
   692  	} else if ellipsis >= 0 {
   693  		// Ellipsis must represent at least one 0 group.
   694  		return nil
   695  	}
   696  	return ip
   697  }
   698  
   699  // ParseIP parses s as an IP address, returning the result.
   700  // The string s can be in IPv4 dotted decimal ("192.0.2.1"), IPv6
   701  // ("2001:db8::68"), or IPv4-mapped IPv6 ("::ffff:192.0.2.1") form.
   702  // If s is not a valid textual representation of an IP address,
   703  // ParseIP returns nil.
   704  func ParseIP(s string) IP {
   705  	for i := 0; i < len(s); i++ {
   706  		switch s[i] {
   707  		case '.':
   708  			return parseIPv4(s)
   709  		case ':':
   710  			return parseIPv6(s)
   711  		}
   712  	}
   713  	return nil
   714  }
   715  
   716  // parseIPZone parses s as an IP address, return it and its associated zone
   717  // identifier (IPv6 only).
   718  func parseIPZone(s string) (IP, string) {
   719  	for i := 0; i < len(s); i++ {
   720  		switch s[i] {
   721  		case '.':
   722  			return parseIPv4(s), ""
   723  		case ':':
   724  			return parseIPv6Zone(s)
   725  		}
   726  	}
   727  	return nil, ""
   728  }
   729  
   730  // ParseCIDR parses s as a CIDR notation IP address and prefix length,
   731  // like "192.0.2.0/24" or "2001:db8::/32", as defined in
   732  // RFC 4632 and RFC 4291.
   733  //
   734  // It returns the IP address and the network implied by the IP and
   735  // prefix length.
   736  // For example, ParseCIDR("192.0.2.1/24") returns the IP address
   737  // 192.0.2.1 and the network 192.0.2.0/24.
   738  func ParseCIDR(s string) (IP, *IPNet, error) {
   739  	i := bytealg.IndexByteString(s, '/')
   740  	if i < 0 {
   741  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   742  	}
   743  	addr, mask := s[:i], s[i+1:]
   744  	iplen := IPv4len
   745  	ip := parseIPv4(addr)
   746  	if ip == nil {
   747  		iplen = IPv6len
   748  		ip = parseIPv6(addr)
   749  	}
   750  	n, i, ok := dtoi(mask)
   751  	if ip == nil || !ok || i != len(mask) || n < 0 || n > 8*iplen {
   752  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   753  	}
   754  	m := CIDRMask(n, 8*iplen)
   755  	return ip, &IPNet{IP: ip.Mask(m), Mask: m}, nil
   756  }
   757  

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