github.com/noisysockets/noisysockets@v0.21.2-0.20240515114641-7f467e651c90/internal/dns/addrselect/addrselect.go

// SPDX-License-Identifier: MPL-2.0
/*
 * Copyright (C) 2024 The Noisy Sockets Authors.
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Portions of this file are based on code originally from the Go project,
 *
 * Copyright (c) 2015 The Go Authors. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above
 *     copyright notice, this list of conditions and the following disclaimer
 *     in the documentation and/or other materials provided with the
 *     distribution.
 *   * Neither the name of Google Inc. nor the names of its
 *     contributors may be used to endorse or promote products derived from
 *     this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

// Package addrselect implements RFC 6724, which describes the
// preferred source and destination address selection algorithm for
// Internet Protocol version 6 (IPv6) and Internet Protocol version 4
// (IPv4).
package addrselect

import (
	stdnet "net"
	"net/netip"
	"sort"

	"github.com/noisysockets/noisysockets/network"
)

func SortByRFC6724(net network.Network, addrs []netip.Addr) {
	if len(addrs) < 2 {
		return
	}
	SortByRFC6724withSrcs(net, addrs, srcAddrs(net, addrs))
}

func SortByRFC6724withSrcs(net network.Network, addrs []netip.Addr, srcs []netip.Addr) {
	if len(addrs) != len(srcs) {
		panic("internal error")
	}
	addrAttr := make([]ipAttr, len(addrs))
	srcAttr := make([]ipAttr, len(srcs))
	for i, v := range addrs {
		addrAttrIP, _ := netip.AddrFromSlice(v.AsSlice())
		addrAttr[i] = ipAttrOf(addrAttrIP)
		srcAttr[i] = ipAttrOf(srcs[i])
	}
	sort.Stable(&byRFC6724{
		addrs:    addrs,
		addrAttr: addrAttr,
		srcs:     srcs,
		srcAttr:  srcAttr,
	})
}

// srcAddrs 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(net network.Network, addrs []netip.Addr) []netip.Addr {
	srcs := make([]netip.Addr, len(addrs))
	for i := range addrs {
		c, err := net.Dial("udp", stdnet.JoinHostPort(addrs[i].String(), "9"))
		if err == nil {
			if src, ok := c.LocalAddr().(*stdnet.UDPAddr); ok {
				srcs[i], _ = netip.AddrFromSlice(src.IP)
			}
			_ = c.Close()
		}
	}
	return srcs
}

type ipAttr struct {
	Scope      scope
	Precedence uint8
	Label      uint8
}

func ipAttrOf(ip netip.Addr) ipAttr {
	if !ip.IsValid() {
		return ipAttr{}
	}
	match := rfc6724policyTable.Classify(ip)
	return ipAttr{
		Scope:      classifyScope(ip),
		Precedence: match.Precedence,
		Label:      match.Label,
	}
}

type byRFC6724 struct {
	addrs    []netip.Addr // addrs to sort
	addrAttr []ipAttr
	srcs     []netip.Addr // or not valid addr 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]
	DB := s.addrs[j]
	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.IsValid() && !SourceDB.IsValid() {
		return false // "equal"
	}
	if !SourceDB.IsValid() {
		return preferDA
	}
	if !SourceDA.IsValid() {
		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 the longest matching prefix.
	// When DA and DB belong to the same address family (both are IPv6 or
	// both are IPv4 [but see below]): 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.
	//
	// However, applying this rule to IPv4 addresses causes
	// problems (see issues 13283 and 18518), so limit to IPv6.
	if DA.Is6() && DB.Is6() {
		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     netip.Prefix
	Precedence uint8
	Label      uint8
}

type policyTable []policyTableEntry

// RFC 6724 section 2.1.
// Items are sorted by the size of their Prefix.Mask.Size,
var rfc6724policyTable = policyTable{
	{
		// "::1/128"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}), 128),
		Precedence: 50,
		Label:      0,
	},
	{
		// "::ffff:0:0/96"
		// IPv4-compatible, etc.
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff}), 96),
		Precedence: 35,
		Label:      4,
	},
	{
		// "::/96"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{}), 96),
		Precedence: 1,
		Label:      3,
	},
	{
		// "2001::/32"
		// Teredo
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0x20, 0x01}), 32),
		Precedence: 5,
		Label:      5,
	},
	{
		// "2002::/16"
		// 6to4
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0x20, 0x02}), 16),
		Precedence: 30,
		Label:      2,
	},
	{
		// "3ffe::/16"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0x3f, 0xfe}), 16),
		Precedence: 1,
		Label:      12,
	},
	{
		// "fec0::/10"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0xfe, 0xc0}), 10),
		Precedence: 1,
		Label:      11,
	},
	{
		// "fc00::/7"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{0xfc}), 7),
		Precedence: 3,
		Label:      13,
	},
	{
		// "::/0"
		Prefix:     netip.PrefixFrom(netip.AddrFrom16([16]byte{}), 0),
		Precedence: 40,
		Label:      1,
	},
}

// 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 netip.Addr) policyTableEntry {
	// Prefix.Contains() will not match an IPv6 prefix for an IPv4 address.
	if ip.Is4() {
		ip = netip.AddrFrom16(ip.As16())
	}
	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 netip.Addr) scope {
	if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
		return scopeLinkLocal
	}
	ipv6 := ip.Is6() && !ip.Is4In6()
	ipv6AsBytes := ip.As16()
	if ipv6 && ip.IsMulticast() {
		return scope(ipv6AsBytes[1] & 0xf)
	}
	// Site-local addresses are defined in RFC 3513 section 2.5.6
	// (and deprecated in RFC 3879).
	if ipv6 && ipv6AsBytes[0] == 0xfe && ipv6AsBytes[1]&0xc0 == 0xc0 {
		return scopeSiteLocal
	}
	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 netip.Addr) (cpl int) {
	// Normalize IPv4-mapped IPv6 addresses to their IPv4 representations
	if a.Is4In6() {
		a = a.Unmap()
	}
	if b.Is4In6() {
		b = b.Unmap()
	}

	// If a and b are of different IP versions, return 0
	if a.Is4() != b.Is4() {
		return 0
	}

	aAsSlice := a.AsSlice()
	bAsSlice := b.AsSlice()

	// If IPv6, only up to the prefix (first 64 bits)
	if len(aAsSlice) > 8 {
		aAsSlice = aAsSlice[:8]
		bAsSlice = bAsSlice[:8]
	}

	for len(aAsSlice) > 0 {
		if aAsSlice[0] == bAsSlice[0] {
			cpl += 8
			aAsSlice = aAsSlice[1:]
			bAsSlice = bAsSlice[1:]
			continue
		}
		bits := 8
		ab, bb := aAsSlice[0], bAsSlice[0]
		for {
			ab >>= 1
			bb >>= 1
			bits--
			if ab == bb {
				cpl += bits
				return
			}
		}
	}
	return
}