addrselect.go

Documentation: net

     1  // Copyright 2015 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  // Minimal RFC 6724 address selection.
     6  
     7  package net
     8  
     9  import "sort"
    10  
    11  func sortByRFC6724(addrs []IPAddr) {
    12  	if len(addrs) < 2 {
    13  		return
    14  	}
    15  	sortByRFC6724withSrcs(addrs, srcAddrs(addrs))
    16  }
    17  
    18  func sortByRFC6724withSrcs(addrs []IPAddr, srcs []IP) {
    19  	if len(addrs) != len(srcs) {
    20  		panic("internal error")
    21  	}
    22  	addrAttr := make([]ipAttr, len(addrs))
    23  	srcAttr := make([]ipAttr, len(srcs))
    24  	for i, v := range addrs {
    25  		addrAttr[i] = ipAttrOf(v.IP)
    26  		srcAttr[i] = ipAttrOf(srcs[i])
    27  	}
    28  	sort.Stable(&byRFC6724{
    29  		addrs:    addrs,
    30  		addrAttr: addrAttr,
    31  		srcs:     srcs,
    32  		srcAttr:  srcAttr,
    33  	})
    34  }
    35  
    36  // srcsAddrs tries to UDP-connect to each address to see if it has a
    37  // route. (This doesn't send any packets). The destination port
    38  // number is irrelevant.
    39  func srcAddrs(addrs []IPAddr) []IP {
    40  	srcs := make([]IP, len(addrs))
    41  	dst := UDPAddr{Port: 9}
    42  	for i := range addrs {
    43  		dst.IP = addrs[i].IP
    44  		dst.Zone = addrs[i].Zone
    45  		c, err := DialUDP("udp", nil, &dst)
    46  		if err == nil {
    47  			if src, ok := c.LocalAddr().(*UDPAddr); ok {
    48  				srcs[i] = src.IP
    49  			}
    50  			c.Close()
    51  		}
    52  	}
    53  	return srcs
    54  }
    55  
    56  type ipAttr struct {
    57  	Scope      scope
    58  	Precedence uint8
    59  	Label      uint8
    60  }
    61  
    62  func ipAttrOf(ip IP) ipAttr {
    63  	if ip == nil {
    64  		return ipAttr{}
    65  	}
    66  	match := rfc6724policyTable.Classify(ip)
    67  	return ipAttr{
    68  		Scope:      classifyScope(ip),
    69  		Precedence: match.Precedence,
    70  		Label:      match.Label,
    71  	}
    72  }
    73  
    74  type byRFC6724 struct {
    75  	addrs    []IPAddr // addrs to sort
    76  	addrAttr []ipAttr
    77  	srcs     []IP // or nil if unreachable
    78  	srcAttr  []ipAttr
    79  }
    80  
    81  func (s *byRFC6724) Len() int { return len(s.addrs) }
    82  
    83  func (s *byRFC6724) Swap(i, j int) {
    84  	s.addrs[i], s.addrs[j] = s.addrs[j], s.addrs[i]
    85  	s.srcs[i], s.srcs[j] = s.srcs[j], s.srcs[i]
    86  	s.addrAttr[i], s.addrAttr[j] = s.addrAttr[j], s.addrAttr[i]
    87  	s.srcAttr[i], s.srcAttr[j] = s.srcAttr[j], s.srcAttr[i]
    88  }
    89  
    90  // Less reports whether i is a better destination address for this
    91  // host than j.
    92  //
    93  // The algorithm and variable names comes from RFC 6724 section 6.
    94  func (s *byRFC6724) Less(i, j int) bool {
    95  	DA := s.addrs[i].IP
    96  	DB := s.addrs[j].IP
    97  	SourceDA := s.srcs[i]
    98  	SourceDB := s.srcs[j]
    99  	attrDA := &s.addrAttr[i]
   100  	attrDB := &s.addrAttr[j]
   101  	attrSourceDA := &s.srcAttr[i]
   102  	attrSourceDB := &s.srcAttr[j]
   103  
   104  	const preferDA = true
   105  	const preferDB = false
   106  
   107  	// Rule 1: Avoid unusable destinations.
   108  	// If DB is known to be unreachable or if Source(DB) is undefined, then
   109  	// prefer DA.  Similarly, if DA is known to be unreachable or if
   110  	// Source(DA) is undefined, then prefer DB.
   111  	if SourceDA == nil && SourceDB == nil {
   112  		return false // "equal"
   113  	}
   114  	if SourceDB == nil {
   115  		return preferDA
   116  	}
   117  	if SourceDA == nil {
   118  		return preferDB
   119  	}
   120  
   121  	// Rule 2: Prefer matching scope.
   122  	// If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
   123  	// then prefer DA.  Similarly, if Scope(DA) <> Scope(Source(DA)) and
   124  	// Scope(DB) = Scope(Source(DB)), then prefer DB.
   125  	if attrDA.Scope == attrSourceDA.Scope && attrDB.Scope != attrSourceDB.Scope {
   126  		return preferDA
   127  	}
   128  	if attrDA.Scope != attrSourceDA.Scope && attrDB.Scope == attrSourceDB.Scope {
   129  		return preferDB
   130  	}
   131  
   132  	// Rule 3: Avoid deprecated addresses.
   133  	// If Source(DA) is deprecated and Source(DB) is not, then prefer DB.
   134  	// Similarly, if Source(DA) is not deprecated and Source(DB) is
   135  	// deprecated, then prefer DA.
   136  
   137  	// TODO(bradfitz): implement? low priority for now.
   138  
   139  	// Rule 4: Prefer home addresses.
   140  	// If Source(DA) is simultaneously a home address and care-of address
   141  	// and Source(DB) is not, then prefer DA.  Similarly, if Source(DB) is
   142  	// simultaneously a home address and care-of address and Source(DA) is
   143  	// not, then prefer DB.
   144  
   145  	// TODO(bradfitz): implement? low priority for now.
   146  
   147  	// Rule 5: Prefer matching label.
   148  	// If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB),
   149  	// then prefer DA.  Similarly, if Label(Source(DA)) <> Label(DA) and
   150  	// Label(Source(DB)) = Label(DB), then prefer DB.
   151  	if attrSourceDA.Label == attrDA.Label &&
   152  		attrSourceDB.Label != attrDB.Label {
   153  		return preferDA
   154  	}
   155  	if attrSourceDA.Label != attrDA.Label &&
   156  		attrSourceDB.Label == attrDB.Label {
   157  		return preferDB
   158  	}
   159  
   160  	// Rule 6: Prefer higher precedence.
   161  	// If Precedence(DA) > Precedence(DB), then prefer DA.  Similarly, if
   162  	// Precedence(DA) < Precedence(DB), then prefer DB.
   163  	if attrDA.Precedence > attrDB.Precedence {
   164  		return preferDA
   165  	}
   166  	if attrDA.Precedence < attrDB.Precedence {
   167  		return preferDB
   168  	}
   169  
   170  	// Rule 7: Prefer native transport.
   171  	// If DA is reached via an encapsulating transition mechanism (e.g.,
   172  	// IPv6 in IPv4) and DB is not, then prefer DB.  Similarly, if DB is
   173  	// reached via encapsulation and DA is not, then prefer DA.
   174  
   175  	// TODO(bradfitz): implement? low priority for now.
   176  
   177  	// Rule 8: Prefer smaller scope.
   178  	// If Scope(DA) < Scope(DB), then prefer DA.  Similarly, if Scope(DA) >
   179  	// Scope(DB), then prefer DB.
   180  	if attrDA.Scope < attrDB.Scope {
   181  		return preferDA
   182  	}
   183  	if attrDA.Scope > attrDB.Scope {
   184  		return preferDB
   185  	}
   186  
   187  	// Rule 9: Use longest matching prefix.
   188  	// When DA and DB belong to the same address family (both are IPv6 or
   189  	// both are IPv4 [but see below]): If CommonPrefixLen(Source(DA), DA) >
   190  	// CommonPrefixLen(Source(DB), DB), then prefer DA.  Similarly, if
   191  	// CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB),
   192  	// then prefer DB.
   193  	//
   194  	// However, applying this rule to IPv4 addresses causes
   195  	// problems (see issues 13283 and 18518), so limit to IPv6.
   196  	if DA.To4() == nil && DB.To4() == nil {
   197  		commonA := commonPrefixLen(SourceDA, DA)
   198  		commonB := commonPrefixLen(SourceDB, DB)
   199  
   200  		if commonA > commonB {
   201  			return preferDA
   202  		}
   203  		if commonA < commonB {
   204  			return preferDB
   205  		}
   206  	}
   207  
   208  	// Rule 10: Otherwise, leave the order unchanged.
   209  	// If DA preceded DB in the original list, prefer DA.
   210  	// Otherwise, prefer DB.
   211  	return false // "equal"
   212  }
   213  
   214  type policyTableEntry struct {
   215  	Prefix     *IPNet
   216  	Precedence uint8
   217  	Label      uint8
   218  }
   219  
   220  type policyTable []policyTableEntry
   221  
   222  // RFC 6724 section 2.1.
   223  var rfc6724policyTable = policyTable{
   224  	{
   225  		Prefix:     mustCIDR("::1/128"),
   226  		Precedence: 50,
   227  		Label:      0,
   228  	},
   229  	{
   230  		Prefix:     mustCIDR("::/0"),
   231  		Precedence: 40,
   232  		Label:      1,
   233  	},
   234  	{
   235  		// IPv4-compatible, etc.
   236  		Prefix:     mustCIDR("::ffff:0:0/96"),
   237  		Precedence: 35,
   238  		Label:      4,
   239  	},
   240  	{
   241  		// 6to4
   242  		Prefix:     mustCIDR("2002::/16"),
   243  		Precedence: 30,
   244  		Label:      2,
   245  	},
   246  	{
   247  		// Teredo
   248  		Prefix:     mustCIDR("2001::/32"),
   249  		Precedence: 5,
   250  		Label:      5,
   251  	},
   252  	{
   253  		Prefix:     mustCIDR("fc00::/7"),
   254  		Precedence: 3,
   255  		Label:      13,
   256  	},
   257  	{
   258  		Prefix:     mustCIDR("::/96"),
   259  		Precedence: 1,
   260  		Label:      3,
   261  	},
   262  	{
   263  		Prefix:     mustCIDR("fec0::/10"),
   264  		Precedence: 1,
   265  		Label:      11,
   266  	},
   267  	{
   268  		Prefix:     mustCIDR("3ffe::/16"),
   269  		Precedence: 1,
   270  		Label:      12,
   271  	},
   272  }
   273  
   274  func init() {
   275  	sort.Sort(sort.Reverse(byMaskLength(rfc6724policyTable)))
   276  }
   277  
   278  // byMaskLength sorts policyTableEntry by the size of their Prefix.Mask.Size,
   279  // from smallest mask, to largest.
   280  type byMaskLength []policyTableEntry
   281  
   282  func (s byMaskLength) Len() int      { return len(s) }
   283  func (s byMaskLength) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   284  func (s byMaskLength) Less(i, j int) bool {
   285  	isize, _ := s[i].Prefix.Mask.Size()
   286  	jsize, _ := s[j].Prefix.Mask.Size()
   287  	return isize < jsize
   288  }
   289  
   290  // mustCIDR calls ParseCIDR and panics on any error, or if the network
   291  // is not IPv6.
   292  func mustCIDR(s string) *IPNet {
   293  	ip, ipNet, err := ParseCIDR(s)
   294  	if err != nil {
   295  		panic(err.Error())
   296  	}
   297  	if len(ip) != IPv6len {
   298  		panic("unexpected IP length")
   299  	}
   300  	return ipNet
   301  }
   302  
   303  // Classify returns the policyTableEntry of the entry with the longest
   304  // matching prefix that contains ip.
   305  // The table t must be sorted from largest mask size to smallest.
   306  func (t policyTable) Classify(ip IP) policyTableEntry {
   307  	for _, ent := range t {
   308  		if ent.Prefix.Contains(ip) {
   309  			return ent
   310  		}
   311  	}
   312  	return policyTableEntry{}
   313  }
   314  
   315  // RFC 6724 section 3.1.
   316  type scope uint8
   317  
   318  const (
   319  	scopeInterfaceLocal scope = 0x1
   320  	scopeLinkLocal      scope = 0x2
   321  	scopeAdminLocal     scope = 0x4
   322  	scopeSiteLocal      scope = 0x5
   323  	scopeOrgLocal       scope = 0x8
   324  	scopeGlobal         scope = 0xe
   325  )
   326  
   327  func classifyScope(ip IP) scope {
   328  	if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
   329  		return scopeLinkLocal
   330  	}
   331  	ipv6 := len(ip) == IPv6len && ip.To4() == nil
   332  	if ipv6 && ip.IsMulticast() {
   333  		return scope(ip[1] & 0xf)
   334  	}
   335  	// Site-local addresses are defined in RFC 3513 section 2.5.6
   336  	// (and deprecated in RFC 3879).
   337  	if ipv6 && ip[0] == 0xfe && ip[1]&0xc0 == 0xc0 {
   338  		return scopeSiteLocal
   339  	}
   340  	return scopeGlobal
   341  }
   342  
   343  // commonPrefixLen reports the length of the longest prefix (looking
   344  // at the most significant, or leftmost, bits) that the
   345  // two addresses have in common, up to the length of a's prefix (i.e.,
   346  // the portion of the address not including the interface ID).
   347  //
   348  // If a or b is an IPv4 address as an IPv6 address, the IPv4 addresses
   349  // are compared (with max common prefix length of 32).
   350  // If a and b are different IP versions, 0 is returned.
   351  //
   352  // See https://tools.ietf.org/html/rfc6724#section-2.2
   353  func commonPrefixLen(a, b IP) (cpl int) {
   354  	if a4 := a.To4(); a4 != nil {
   355  		a = a4
   356  	}
   357  	if b4 := b.To4(); b4 != nil {
   358  		b = b4
   359  	}
   360  	if len(a) != len(b) {
   361  		return 0
   362  	}
   363  	// If IPv6, only up to the prefix (first 64 bits)
   364  	if len(a) > 8 {
   365  		a = a[:8]
   366  		b = b[:8]
   367  	}
   368  	for len(a) > 0 {
   369  		if a[0] == b[0] {
   370  			cpl += 8
   371  			a = a[1:]
   372  			b = b[1:]
   373  			continue
   374  		}
   375  		bits := 8
   376  		ab, bb := a[0], b[0]
   377  		for {
   378  			ab >>= 1
   379  			bb >>= 1
   380  			bits--
   381  			if ab == bb {
   382  				cpl += bits
   383  				return
   384  			}
   385  		}
   386  	}
   387  	return
   388  }
   389
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