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net: RFC 6724 address selection

At least the most important parts, I think.

Fixes #10552

Change-Id: I1a03c5405bdbef337e0245d226e9247d3d067393
Reviewed-on: https://go-review.googlesource.com/12246
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
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bradfitz committed Jul 16, 2015
1 parent 29eb7d1 commit adb1e030136f14ef8434930f15a04a74568a5948
Showing with 602 additions and 2 deletions.
  1. +381 −0 src/net/addrselect.go
  2. +218 −0 src/net/addrselect_test.go
  3. +3 −2 src/net/dnsclient_unix.go
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@@ -0,0 +1,381 @@
// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build darwin dragonfly freebsd linux netbsd openbsd solaris
// Minimal RFC 6724 address selection.
package net
import "sort"
func sortByRFC6724(addrs []IPAddr) {
if len(addrs) < 2 {
return
}
sortByRFC6724withSrcs(addrs, srcAddrs(addrs))
}
func sortByRFC6724withSrcs(addrs []IPAddr, srcs []IP) {
if len(addrs) != len(srcs) {
panic("internal error")
}
addrAttr := make([]ipAttr, len(addrs))
srcAttr := make([]ipAttr, len(srcs))
for i, v := range addrs {
addrAttr[i] = ipAttrOf(v.IP)
srcAttr[i] = ipAttrOf(srcs[i])
}
sort.Stable(&byRFC6724{
addrs: addrs,
addrAttr: addrAttr,
srcs: srcs,
srcAttr: srcAttr,
})
}
// srcsAddrs tries to UDP-connect to each address to see if it has a
// route. (This doesn't send any packets). The destination port
// number is irrelevant.
func srcAddrs(addrs []IPAddr) []IP {
srcs := make([]IP, len(addrs))
for i := range addrs {
conn, err := Dial("udp", JoinHostPort(addrs[i].IP.String(), "1234"))
if err == nil {
if ua, ok := conn.LocalAddr().(*UDPAddr); ok {
srcs[i] = ua.IP
}
conn.Close()
}
}
return srcs
}
type ipAttr struct {
Scope scope
Precedence uint8
Label uint8
}
func ipAttrOf(ip IP) ipAttr {
if ip == nil {
return ipAttr{}
}
match := rfc6724policyTable.Classify(ip)
return ipAttr{
Scope: classifyScope(ip),
Precedence: match.Precedence,
Label: match.Label,
}
}
type byRFC6724 struct {
addrs []IPAddr // addrs to sort
addrAttr []ipAttr
srcs []IP // or nil if unreachable
srcAttr []ipAttr
}
func (s *byRFC6724) Len() int { return len(s.addrs) }
func (s *byRFC6724) Swap(i, j int) {
s.addrs[i], s.addrs[j] = s.addrs[j], s.addrs[i]
s.srcs[i], s.srcs[j] = s.srcs[j], s.srcs[i]
s.addrAttr[i], s.addrAttr[j] = s.addrAttr[j], s.addrAttr[i]
s.srcAttr[i], s.srcAttr[j] = s.srcAttr[j], s.srcAttr[i]
}
// Less reports whether i is a better destination address for this
// host than j.
//
// The algorithm and variable names comes from RFC 6724 section 6.
func (s *byRFC6724) Less(i, j int) bool {
DA := s.addrs[i].IP
DB := s.addrs[j].IP
SourceDA := s.srcs[i]
SourceDB := s.srcs[j]
attrDA := &s.addrAttr[i]
attrDB := &s.addrAttr[j]
attrSourceDA := &s.srcAttr[i]
attrSourceDB := &s.srcAttr[j]
const preferDA = true
const preferDB = false
// Rule 1: Avoid unusable destinations.
// If DB is known to be unreachable or if Source(DB) is undefined, then
// prefer DA. Similarly, if DA is known to be unreachable or if
// Source(DA) is undefined, then prefer DB.
if SourceDA == nil && SourceDB == nil {
return false // "equal"
}
if SourceDB == nil {
return preferDA
}
if SourceDA == nil {
return preferDB
}
// Rule 2: Prefer matching scope.
// If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
// then prefer DA. Similarly, if Scope(DA) <> Scope(Source(DA)) and
// Scope(DB) = Scope(Source(DB)), then prefer DB.
if attrDA.Scope == attrSourceDA.Scope && attrDB.Scope != attrSourceDB.Scope {
return preferDA
}
if attrDA.Scope != attrSourceDA.Scope && attrDB.Scope == attrSourceDB.Scope {
return preferDB
}
// Rule 3: Avoid deprecated addresses.
// If Source(DA) is deprecated and Source(DB) is not, then prefer DB.
// Similarly, if Source(DA) is not deprecated and Source(DB) is
// deprecated, then prefer DA.
// TODO(bradfitz): implement? low priority for now.
// Rule 4: Prefer home addresses.
// If Source(DA) is simultaneously a home address and care-of address
// and Source(DB) is not, then prefer DA. Similarly, if Source(DB) is
// simultaneously a home address and care-of address and Source(DA) is
// not, then prefer DB.
// TODO(bradfitz): implement? low priority for now.
// Rule 5: Prefer matching label.
// If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB),
// then prefer DA. Similarly, if Label(Source(DA)) <> Label(DA) and
// Label(Source(DB)) = Label(DB), then prefer DB.
if attrSourceDA.Label == attrDA.Label &&
attrSourceDB.Label != attrDB.Label {
return preferDA
}
if attrSourceDA.Label != attrDA.Label &&
attrSourceDB.Label == attrDB.Label {
return preferDB
}
// Rule 6: Prefer higher precedence.
// If Precedence(DA) > Precedence(DB), then prefer DA. Similarly, if
// Precedence(DA) < Precedence(DB), then prefer DB.
if attrDA.Precedence > attrDB.Precedence {
return preferDA
}
if attrDA.Precedence < attrDB.Precedence {
return preferDB
}
// Rule 7: Prefer native transport.
// If DA is reached via an encapsulating transition mechanism (e.g.,
// IPv6 in IPv4) and DB is not, then prefer DB. Similarly, if DB is
// reached via encapsulation and DA is not, then prefer DA.
// TODO(bradfitz): implement? low priority for now.
// Rule 8: Prefer smaller scope.
// If Scope(DA) < Scope(DB), then prefer DA. Similarly, if Scope(DA) >
// Scope(DB), then prefer DB.
if attrDA.Scope < attrDB.Scope {
return preferDA
}
if attrDA.Scope > attrDB.Scope {
return preferDB
}
// Rule 9: Use longest matching prefix.
// When DA and DB belong to the same address family (both are IPv6 or
// both are IPv4): If CommonPrefixLen(Source(DA), DA) >
// CommonPrefixLen(Source(DB), DB), then prefer DA. Similarly, if
// CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB),
// then prefer DB.
da4 := DA.To4() != nil
db4 := DB.To4() != nil
if da4 == db4 {
commonA := commonPrefixLen(SourceDA, DA)
commonB := commonPrefixLen(SourceDB, DB)
if commonA > commonB {
return preferDA
}
if commonA < commonB {
return preferDB
}
}
// Rule 10: Otherwise, leave the order unchanged.
// If DA preceded DB in the original list, prefer DA.
// Otherwise, prefer DB.
return false // "equal"
}
type policyTableEntry struct {
Prefix *IPNet
Precedence uint8
Label uint8
}
type policyTable []policyTableEntry
// RFC 6724 section 2.1.
var rfc6724policyTable = policyTable{
{
Prefix: mustCIDR("::1/128"),
Precedence: 50,
Label: 0,
},
{
Prefix: mustCIDR("::/0"),
Precedence: 40,
Label: 1,
},
{
// IPv4-compatible, etc.
Prefix: mustCIDR("::ffff:0:0/96"),
Precedence: 35,
Label: 4,
},
{
// 6to4
Prefix: mustCIDR("2002::/16"),
Precedence: 30,
Label: 2,
},
{
// Teredo
Prefix: mustCIDR("2001::/32"),
Precedence: 5,
Label: 5,
},
{
Prefix: mustCIDR("fc00::/7"),
Precedence: 3,
Label: 13,
},
{
Prefix: mustCIDR("::/96"),
Precedence: 1,
Label: 3,
},
{
Prefix: mustCIDR("fec0::/10"),
Precedence: 1,
Label: 11,
},
{
Prefix: mustCIDR("3ffe::/16"),
Precedence: 1,
Label: 12,
},
}
func init() {
sort.Sort(sort.Reverse(byMaskLength(rfc6724policyTable)))
}
// byMaskLength sorts policyTableEntry by the size of their Prefix.Mask.Size,
// from smallest mask, to largest.
type byMaskLength []policyTableEntry
func (s byMaskLength) Len() int { return len(s) }
func (s byMaskLength) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
func (s byMaskLength) Less(i, j int) bool {
isize, _ := s[i].Prefix.Mask.Size()
jsize, _ := s[j].Prefix.Mask.Size()
return isize < jsize
}
// mustCIDR calls ParseCIDR and panics on any error, or if the network
// is not IPv6.
func mustCIDR(s string) *IPNet {
ip, ipNet, err := ParseCIDR(s)
if err != nil {
panic(err.Error())
}
if len(ip) != IPv6len {
panic("unexpected IP length")
}
return ipNet
}
// Classify returns the policyTableEntry of the entry with the longest
// matching prefix that contains ip.
// The table t must be sorted from largest mask size to smallest.
func (t policyTable) Classify(ip IP) policyTableEntry {
for _, ent := range t {
if ent.Prefix.Contains(ip) {
return ent
}
}
return policyTableEntry{}
}
// RFC 6724 section 3.1.
type scope uint8
const (
scopeInterfaceLocal scope = 0x1
scopeLinkLocal scope = 0x2
scopeAdminLocal scope = 0x4
scopeSiteLocal scope = 0x5
scopeOrgLocal scope = 0x8
scopeGlobal scope = 0xe
)
func classifyScope(ip IP) scope {
if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
return scopeLinkLocal
}
if len(ip) == IPv6len && ip.To4() == nil && ip.IsMulticast() {
return scope(ip[1] & 0xf)
}
// TODO: are there unicast scopeAdminLocal, scopeSiteLocal,
// scopeOrgLocal? Better question: are those even used?
return scopeGlobal
}
// commonPrefixLen reports the length of the longest prefix (looking
// at the most significant, or leftmost, bits) that the
// two addresses have in common, up to the length of a's prefix (i.e.,
// the portion of the address not including the interface ID).
//
// If a or b is an IPv4 address as an IPv6 address, the IPv4 addresses
// are compared (with max common prefix length of 32).
// If a and b are different IP versions, 0 is returned.
//
// See https://tools.ietf.org/html/rfc6724#section-2.2
func commonPrefixLen(a, b IP) (cpl int) {
if a4 := a.To4(); a4 != nil {
a = a4
}
if b4 := b.To4(); b4 != nil {
b = b4
}
if len(a) != len(b) {
return 0
}
// If IPv6, only up to the prefix (first 64 bits)
if len(a) > 8 {
a = a[:8]
b = b[:8]
}
for len(a) > 0 {
if a[0] == b[0] {
cpl += 8
a = a[1:]
b = b[1:]
continue
}
bits := 8
ab, bb := a[0], b[0]
for {
ab >>= 1
bb >>= 1
bits--
if ab == bb {
cpl += bits
return
}
}
}
return
}
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