ip.go

Documentation: net

     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 _ "unsafe" // for go:linkname
    16  
    17  // IP address lengths (bytes).
    18  const (
    19  	IPv4len = 4
    20  	IPv6len = 16
    21  )
    22  
    23  // An IP is a single IP address, a slice of bytes.
    24  // Functions in this package accept either 4-byte (IPv4)
    25  // or 16-byte (IPv6) slices as input.
    26  //
    27  // Note that in this documentation, referring to an
    28  // IP address as an IPv4 address or an IPv6 address
    29  // is a semantic property of the address, not just the
    30  // length of the byte slice: a 16-byte slice can still
    31  // be an IPv4 address.
    32  type IP []byte
    33  
    34  // An IP mask is an IP address.
    35  type IPMask []byte
    36  
    37  // An IPNet represents an IP network.
    38  type IPNet struct {
    39  	IP   IP     // network number
    40  	Mask IPMask // network mask
    41  }
    42  
    43  // IPv4 returns the IP address (in 16-byte form) of the
    44  // IPv4 address a.b.c.d.
    45  func IPv4(a, b, c, d byte) IP {
    46  	p := make(IP, IPv6len)
    47  	copy(p, v4InV6Prefix)
    48  	p[12] = a
    49  	p[13] = b
    50  	p[14] = c
    51  	p[15] = d
    52  	return p
    53  }
    54  
    55  var v4InV6Prefix = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff}
    56  
    57  // IPv4Mask returns the IP mask (in 4-byte form) of the
    58  // IPv4 mask a.b.c.d.
    59  func IPv4Mask(a, b, c, d byte) IPMask {
    60  	p := make(IPMask, IPv4len)
    61  	p[0] = a
    62  	p[1] = b
    63  	p[2] = c
    64  	p[3] = d
    65  	return p
    66  }
    67  
    68  // CIDRMask returns an IPMask consisting of `ones' 1 bits
    69  // followed by 0s up to a total length of `bits' bits.
    70  // For a mask of this form, CIDRMask is the inverse of IPMask.Size.
    71  func CIDRMask(ones, bits int) IPMask {
    72  	if bits != 8*IPv4len && bits != 8*IPv6len {
    73  		return nil
    74  	}
    75  	if ones < 0 || ones > bits {
    76  		return nil
    77  	}
    78  	l := bits / 8
    79  	m := make(IPMask, l)
    80  	n := uint(ones)
    81  	for i := 0; i < l; i++ {
    82  		if n >= 8 {
    83  			m[i] = 0xff
    84  			n -= 8
    85  			continue
    86  		}
    87  		m[i] = ^byte(0xff >> n)
    88  		n = 0
    89  	}
    90  	return m
    91  }
    92  
    93  // Well-known IPv4 addresses
    94  var (
    95  	IPv4bcast     = IPv4(255, 255, 255, 255) // limited broadcast
    96  	IPv4allsys    = IPv4(224, 0, 0, 1)       // all systems
    97  	IPv4allrouter = IPv4(224, 0, 0, 2)       // all routers
    98  	IPv4zero      = IPv4(0, 0, 0, 0)         // all zeros
    99  )
   100  
   101  // Well-known IPv6 addresses
   102  var (
   103  	IPv6zero                   = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   104  	IPv6unspecified            = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   105  	IPv6loopback               = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}
   106  	IPv6interfacelocalallnodes = IP{0xff, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   107  	IPv6linklocalallnodes      = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   108  	IPv6linklocalallrouters    = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x02}
   109  )
   110  
   111  // IsUnspecified reports whether ip is an unspecified address, either
   112  // the IPv4 address "0.0.0.0" or the IPv6 address "::".
   113  func (ip IP) IsUnspecified() bool {
   114  	return ip.Equal(IPv4zero) || ip.Equal(IPv6unspecified)
   115  }
   116  
   117  // IsLoopback reports whether ip is a loopback address.
   118  func (ip IP) IsLoopback() bool {
   119  	if ip4 := ip.To4(); ip4 != nil {
   120  		return ip4[0] == 127
   121  	}
   122  	return ip.Equal(IPv6loopback)
   123  }
   124  
   125  // IsMulticast reports whether ip is a multicast address.
   126  func (ip IP) IsMulticast() bool {
   127  	if ip4 := ip.To4(); ip4 != nil {
   128  		return ip4[0]&0xf0 == 0xe0
   129  	}
   130  	return len(ip) == IPv6len && ip[0] == 0xff
   131  }
   132  
   133  // IsInterfaceLocalMulticast reports whether ip is
   134  // an interface-local multicast address.
   135  func (ip IP) IsInterfaceLocalMulticast() bool {
   136  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x01
   137  }
   138  
   139  // IsLinkLocalMulticast reports whether ip is a link-local
   140  // multicast address.
   141  func (ip IP) IsLinkLocalMulticast() bool {
   142  	if ip4 := ip.To4(); ip4 != nil {
   143  		return ip4[0] == 224 && ip4[1] == 0 && ip4[2] == 0
   144  	}
   145  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x02
   146  }
   147  
   148  // IsLinkLocalUnicast reports whether ip is a link-local
   149  // unicast address.
   150  func (ip IP) IsLinkLocalUnicast() bool {
   151  	if ip4 := ip.To4(); ip4 != nil {
   152  		return ip4[0] == 169 && ip4[1] == 254
   153  	}
   154  	return len(ip) == IPv6len && ip[0] == 0xfe && ip[1]&0xc0 == 0x80
   155  }
   156  
   157  // IsGlobalUnicast reports whether ip is a global unicast
   158  // address.
   159  //
   160  // The identification of global unicast addresses uses address type
   161  // identification as defined in RFC 1122, RFC 4632 and RFC 4291 with
   162  // the exception of IPv4 directed broadcast addresses.
   163  // It returns true even if ip is in IPv4 private address space or
   164  // local IPv6 unicast address space.
   165  func (ip IP) IsGlobalUnicast() bool {
   166  	return (len(ip) == IPv4len || len(ip) == IPv6len) &&
   167  		!ip.Equal(IPv4bcast) &&
   168  		!ip.IsUnspecified() &&
   169  		!ip.IsLoopback() &&
   170  		!ip.IsMulticast() &&
   171  		!ip.IsLinkLocalUnicast()
   172  }
   173  
   174  // Is p all zeros?
   175  func isZeros(p IP) bool {
   176  	for i := 0; i < len(p); i++ {
   177  		if p[i] != 0 {
   178  			return false
   179  		}
   180  	}
   181  	return true
   182  }
   183  
   184  // To4 converts the IPv4 address ip to a 4-byte representation.
   185  // If ip is not an IPv4 address, To4 returns nil.
   186  func (ip IP) To4() IP {
   187  	if len(ip) == IPv4len {
   188  		return ip
   189  	}
   190  	if len(ip) == IPv6len &&
   191  		isZeros(ip[0:10]) &&
   192  		ip[10] == 0xff &&
   193  		ip[11] == 0xff {
   194  		return ip[12:16]
   195  	}
   196  	return nil
   197  }
   198  
   199  // To16 converts the IP address ip to a 16-byte representation.
   200  // If ip is not an IP address (it is the wrong length), To16 returns nil.
   201  func (ip IP) To16() IP {
   202  	if len(ip) == IPv4len {
   203  		return IPv4(ip[0], ip[1], ip[2], ip[3])
   204  	}
   205  	if len(ip) == IPv6len {
   206  		return ip
   207  	}
   208  	return nil
   209  }
   210  
   211  // Default route masks for IPv4.
   212  var (
   213  	classAMask = IPv4Mask(0xff, 0, 0, 0)
   214  	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
   215  	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
   216  )
   217  
   218  // DefaultMask returns the default IP mask for the IP address ip.
   219  // Only IPv4 addresses have default masks; DefaultMask returns
   220  // nil if ip is not a valid IPv4 address.
   221  func (ip IP) DefaultMask() IPMask {
   222  	if ip = ip.To4(); ip == nil {
   223  		return nil
   224  	}
   225  	switch true {
   226  	case ip[0] < 0x80:
   227  		return classAMask
   228  	case ip[0] < 0xC0:
   229  		return classBMask
   230  	default:
   231  		return classCMask
   232  	}
   233  }
   234  
   235  func allFF(b []byte) bool {
   236  	for _, c := range b {
   237  		if c != 0xff {
   238  			return false
   239  		}
   240  	}
   241  	return true
   242  }
   243  
   244  // Mask returns the result of masking the IP address ip with mask.
   245  func (ip IP) Mask(mask IPMask) IP {
   246  	if len(mask) == IPv6len && len(ip) == IPv4len && allFF(mask[:12]) {
   247  		mask = mask[12:]
   248  	}
   249  	if len(mask) == IPv4len && len(ip) == IPv6len && bytesEqual(ip[:12], v4InV6Prefix) {
   250  		ip = ip[12:]
   251  	}
   252  	n := len(ip)
   253  	if n != len(mask) {
   254  		return nil
   255  	}
   256  	out := make(IP, n)
   257  	for i := 0; i < n; i++ {
   258  		out[i] = ip[i] & mask[i]
   259  	}
   260  	return out
   261  }
   262  
   263  // String returns the string form of the IP address ip.
   264  // It returns one of 4 forms:
   265  //   - "<nil>", if ip has length 0
   266  //   - dotted decimal ("192.0.2.1"), if ip is an IPv4 or IP4-mapped IPv6 address
   267  //   - IPv6 ("2001:db8::1"), if ip is a valid IPv6 address
   268  //   - the hexadecimal form of ip, without punctuation, if no other cases apply
   269  func (ip IP) String() string {
   270  	p := ip
   271  
   272  	if len(ip) == 0 {
   273  		return "<nil>"
   274  	}
   275  
   276  	// If IPv4, use dotted notation.
   277  	if p4 := p.To4(); len(p4) == IPv4len {
   278  		return uitoa(uint(p4[0])) + "." +
   279  			uitoa(uint(p4[1])) + "." +
   280  			uitoa(uint(p4[2])) + "." +
   281  			uitoa(uint(p4[3]))
   282  	}
   283  	if len(p) != IPv6len {
   284  		return "?" + hexString(ip)
   285  	}
   286  
   287  	// Find longest run of zeros.
   288  	e0 := -1
   289  	e1 := -1
   290  	for i := 0; i < IPv6len; i += 2 {
   291  		j := i
   292  		for j < IPv6len && p[j] == 0 && p[j+1] == 0 {
   293  			j += 2
   294  		}
   295  		if j > i && j-i > e1-e0 {
   296  			e0 = i
   297  			e1 = j
   298  			i = j
   299  		}
   300  	}
   301  	// The symbol "::" MUST NOT be used to shorten just one 16 bit 0 field.
   302  	if e1-e0 <= 2 {
   303  		e0 = -1
   304  		e1 = -1
   305  	}
   306  
   307  	const maxLen = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")
   308  	b := make([]byte, 0, maxLen)
   309  
   310  	// Print with possible :: in place of run of zeros
   311  	for i := 0; i < IPv6len; i += 2 {
   312  		if i == e0 {
   313  			b = append(b, ':', ':')
   314  			i = e1
   315  			if i >= IPv6len {
   316  				break
   317  			}
   318  		} else if i > 0 {
   319  			b = append(b, ':')
   320  		}
   321  		b = appendHex(b, (uint32(p[i])<<8)|uint32(p[i+1]))
   322  	}
   323  	return string(b)
   324  }
   325  
   326  func hexString(b []byte) string {
   327  	s := make([]byte, len(b)*2)
   328  	for i, tn := range b {
   329  		s[i*2], s[i*2+1] = hexDigit[tn>>4], hexDigit[tn&0xf]
   330  	}
   331  	return string(s)
   332  }
   333  
   334  // ipEmptyString is like ip.String except that it returns
   335  // an empty string when ip is unset.
   336  func ipEmptyString(ip IP) string {
   337  	if len(ip) == 0 {
   338  		return ""
   339  	}
   340  	return ip.String()
   341  }
   342  
   343  // MarshalText implements the encoding.TextMarshaler interface.
   344  // The encoding is the same as returned by String, with one exception:
   345  // When len(ip) is zero, it returns an empty slice.
   346  func (ip IP) MarshalText() ([]byte, error) {
   347  	if len(ip) == 0 {
   348  		return []byte(""), nil
   349  	}
   350  	if len(ip) != IPv4len && len(ip) != IPv6len {
   351  		return nil, &AddrError{Err: "invalid IP address", Addr: hexString(ip)}
   352  	}
   353  	return []byte(ip.String()), nil
   354  }
   355  
   356  // UnmarshalText implements the encoding.TextUnmarshaler interface.
   357  // The IP address is expected in a form accepted by ParseIP.
   358  func (ip *IP) UnmarshalText(text []byte) error {
   359  	if len(text) == 0 {
   360  		*ip = nil
   361  		return nil
   362  	}
   363  	s := string(text)
   364  	x := ParseIP(s)
   365  	if x == nil {
   366  		return &ParseError{Type: "IP address", Text: s}
   367  	}
   368  	*ip = x
   369  	return nil
   370  }
   371  
   372  // Equal reports whether ip and x are the same IP address.
   373  // An IPv4 address and that same address in IPv6 form are
   374  // considered to be equal.
   375  func (ip IP) Equal(x IP) bool {
   376  	if len(ip) == len(x) {
   377  		return bytesEqual(ip, x)
   378  	}
   379  	if len(ip) == IPv4len && len(x) == IPv6len {
   380  		return bytesEqual(x[0:12], v4InV6Prefix) && bytesEqual(ip, x[12:])
   381  	}
   382  	if len(ip) == IPv6len && len(x) == IPv4len {
   383  		return bytesEqual(ip[0:12], v4InV6Prefix) && bytesEqual(ip[12:], x)
   384  	}
   385  	return false
   386  }
   387  
   388  // bytes.Equal is implemented in runtime/asm_$goarch.s
   389  //go:linkname bytesEqual bytes.Equal
   390  func bytesEqual(x, y []byte) bool
   391  
   392  func (ip IP) matchAddrFamily(x IP) bool {
   393  	return ip.To4() != nil && x.To4() != nil || ip.To16() != nil && ip.To4() == nil && x.To16() != nil && x.To4() == nil
   394  }
   395  
   396  // If mask is a sequence of 1 bits followed by 0 bits,
   397  // return the number of 1 bits.
   398  func simpleMaskLength(mask IPMask) int {
   399  	var n int
   400  	for i, v := range mask {
   401  		if v == 0xff {
   402  			n += 8
   403  			continue
   404  		}
   405  		// found non-ff byte
   406  		// count 1 bits
   407  		for v&0x80 != 0 {
   408  			n++
   409  			v <<= 1
   410  		}
   411  		// rest must be 0 bits
   412  		if v != 0 {
   413  			return -1
   414  		}
   415  		for i++; i < len(mask); i++ {
   416  			if mask[i] != 0 {
   417  				return -1
   418  			}
   419  		}
   420  		break
   421  	}
   422  	return n
   423  }
   424  
   425  // Size returns the number of leading ones and total bits in the mask.
   426  // If the mask is not in the canonical form--ones followed by zeros--then
   427  // Size returns 0, 0.
   428  func (m IPMask) Size() (ones, bits int) {
   429  	ones, bits = simpleMaskLength(m), len(m)*8
   430  	if ones == -1 {
   431  		return 0, 0
   432  	}
   433  	return
   434  }
   435  
   436  // String returns the hexadecimal form of m, with no punctuation.
   437  func (m IPMask) String() string {
   438  	if len(m) == 0 {
   439  		return "<nil>"
   440  	}
   441  	return hexString(m)
   442  }
   443  
   444  func networkNumberAndMask(n *IPNet) (ip IP, m IPMask) {
   445  	if ip = n.IP.To4(); ip == nil {
   446  		ip = n.IP
   447  		if len(ip) != IPv6len {
   448  			return nil, nil
   449  		}
   450  	}
   451  	m = n.Mask
   452  	switch len(m) {
   453  	case IPv4len:
   454  		if len(ip) != IPv4len {
   455  			return nil, nil
   456  		}
   457  	case IPv6len:
   458  		if len(ip) == IPv4len {
   459  			m = m[12:]
   460  		}
   461  	default:
   462  		return nil, nil
   463  	}
   464  	return
   465  }
   466  
   467  // Contains reports whether the network includes ip.
   468  func (n *IPNet) Contains(ip IP) bool {
   469  	nn, m := networkNumberAndMask(n)
   470  	if x := ip.To4(); x != nil {
   471  		ip = x
   472  	}
   473  	l := len(ip)
   474  	if l != len(nn) {
   475  		return false
   476  	}
   477  	for i := 0; i < l; i++ {
   478  		if nn[i]&m[i] != ip[i]&m[i] {
   479  			return false
   480  		}
   481  	}
   482  	return true
   483  }
   484  
   485  // Network returns the address's network name, "ip+net".
   486  func (n *IPNet) Network() string { return "ip+net" }
   487  
   488  // String returns the CIDR notation of n like "192.0.2.1/24"
   489  // or "2001:db8::/48" as defined in RFC 4632 and RFC 4291.
   490  // If the mask is not in the canonical form, it returns the
   491  // string which consists of an IP address, followed by a slash
   492  // character and a mask expressed as hexadecimal form with no
   493  // punctuation like "198.51.100.1/c000ff00".
   494  func (n *IPNet) String() string {
   495  	nn, m := networkNumberAndMask(n)
   496  	if nn == nil || m == nil {
   497  		return "<nil>"
   498  	}
   499  	l := simpleMaskLength(m)
   500  	if l == -1 {
   501  		return nn.String() + "/" + m.String()
   502  	}
   503  	return nn.String() + "/" + uitoa(uint(l))
   504  }
   505  
   506  // Parse IPv4 address (d.d.d.d).
   507  func parseIPv4(s string) IP {
   508  	var p [IPv4len]byte
   509  	for i := 0; i < IPv4len; i++ {
   510  		if len(s) == 0 {
   511  			// Missing octets.
   512  			return nil
   513  		}
   514  		if i > 0 {
   515  			if s[0] != '.' {
   516  				return nil
   517  			}
   518  			s = s[1:]
   519  		}
   520  		n, c, ok := dtoi(s)
   521  		if !ok || n > 0xFF {
   522  			return nil
   523  		}
   524  		s = s[c:]
   525  		p[i] = byte(n)
   526  	}
   527  	if len(s) != 0 {
   528  		return nil
   529  	}
   530  	return IPv4(p[0], p[1], p[2], p[3])
   531  }
   532  
   533  // parseIPv6 parses s as a literal IPv6 address described in RFC 4291
   534  // and RFC 5952.  It can also parse a literal scoped IPv6 address with
   535  // zone identifier which is described in RFC 4007 when zoneAllowed is
   536  // true.
   537  func parseIPv6(s string, zoneAllowed bool) (ip IP, zone string) {
   538  	ip = make(IP, IPv6len)
   539  	ellipsis := -1 // position of ellipsis in ip
   540  
   541  	if zoneAllowed {
   542  		s, zone = splitHostZone(s)
   543  	}
   544  
   545  	// Might have leading ellipsis
   546  	if len(s) >= 2 && s[0] == ':' && s[1] == ':' {
   547  		ellipsis = 0
   548  		s = s[2:]
   549  		// Might be only ellipsis
   550  		if len(s) == 0 {
   551  			return ip, zone
   552  		}
   553  	}
   554  
   555  	// Loop, parsing hex numbers followed by colon.
   556  	i := 0
   557  	for i < IPv6len {
   558  		// Hex number.
   559  		n, c, ok := xtoi(s)
   560  		if !ok || n > 0xFFFF {
   561  			return nil, zone
   562  		}
   563  
   564  		// If followed by dot, might be in trailing IPv4.
   565  		if c < len(s) && s[c] == '.' {
   566  			if ellipsis < 0 && i != IPv6len-IPv4len {
   567  				// Not the right place.
   568  				return nil, zone
   569  			}
   570  			if i+IPv4len > IPv6len {
   571  				// Not enough room.
   572  				return nil, zone
   573  			}
   574  			ip4 := parseIPv4(s)
   575  			if ip4 == nil {
   576  				return nil, zone
   577  			}
   578  			ip[i] = ip4[12]
   579  			ip[i+1] = ip4[13]
   580  			ip[i+2] = ip4[14]
   581  			ip[i+3] = ip4[15]
   582  			s = ""
   583  			i += IPv4len
   584  			break
   585  		}
   586  
   587  		// Save this 16-bit chunk.
   588  		ip[i] = byte(n >> 8)
   589  		ip[i+1] = byte(n)
   590  		i += 2
   591  
   592  		// Stop at end of string.
   593  		s = s[c:]
   594  		if len(s) == 0 {
   595  			break
   596  		}
   597  
   598  		// Otherwise must be followed by colon and more.
   599  		if s[0] != ':' || len(s) == 1 {
   600  			return nil, zone
   601  		}
   602  		s = s[1:]
   603  
   604  		// Look for ellipsis.
   605  		if s[0] == ':' {
   606  			if ellipsis >= 0 { // already have one
   607  				return nil, zone
   608  			}
   609  			ellipsis = i
   610  			s = s[1:]
   611  			if len(s) == 0 { // can be at end
   612  				break
   613  			}
   614  		}
   615  	}
   616  
   617  	// Must have used entire string.
   618  	if len(s) != 0 {
   619  		return nil, zone
   620  	}
   621  
   622  	// If didn't parse enough, expand ellipsis.
   623  	if i < IPv6len {
   624  		if ellipsis < 0 {
   625  			return nil, zone
   626  		}
   627  		n := IPv6len - i
   628  		for j := i - 1; j >= ellipsis; j-- {
   629  			ip[j+n] = ip[j]
   630  		}
   631  		for j := ellipsis + n - 1; j >= ellipsis; j-- {
   632  			ip[j] = 0
   633  		}
   634  	} else if ellipsis >= 0 {
   635  		// Ellipsis must represent at least one 0 group.
   636  		return nil, zone
   637  	}
   638  	return ip, zone
   639  }
   640  
   641  // ParseIP parses s as an IP address, returning the result.
   642  // The string s can be in dotted decimal ("192.0.2.1")
   643  // or IPv6 ("2001:db8::68") form.
   644  // If s is not a valid textual representation of an IP address,
   645  // ParseIP returns nil.
   646  func ParseIP(s string) IP {
   647  	for i := 0; i < len(s); i++ {
   648  		switch s[i] {
   649  		case '.':
   650  			return parseIPv4(s)
   651  		case ':':
   652  			ip, _ := parseIPv6(s, false)
   653  			return ip
   654  		}
   655  	}
   656  	return nil
   657  }
   658  
   659  // ParseCIDR parses s as a CIDR notation IP address and prefix length,
   660  // like "192.0.2.0/24" or "2001:db8::/32", as defined in
   661  // RFC 4632 and RFC 4291.
   662  //
   663  // It returns the IP address and the network implied by the IP and
   664  // prefix length.
   665  // For example, ParseCIDR("192.0.2.1/24") returns the IP address
   666  // 192.0.2.1 and the network 192.0.2.0/24.
   667  func ParseCIDR(s string) (IP, *IPNet, error) {
   668  	i := byteIndex(s, '/')
   669  	if i < 0 {
   670  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   671  	}
   672  	addr, mask := s[:i], s[i+1:]
   673  	iplen := IPv4len
   674  	ip := parseIPv4(addr)
   675  	if ip == nil {
   676  		iplen = IPv6len
   677  		ip, _ = parseIPv6(addr, false)
   678  	}
   679  	n, i, ok := dtoi(mask)
   680  	if ip == nil || !ok || i != len(mask) || n < 0 || n > 8*iplen {
   681  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   682  	}
   683  	m := CIDRMask(n, 8*iplen)
   684  	return ip, &IPNet{IP: ip.Mask(m), Mask: m}, nil
   685  }
   686
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