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tailscale/net/dns/resolver/tsdns.go

668 lines
19 KiB
Go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package resolver implements a stub DNS resolver that can also serve
// records out of an internal local zone.
package resolver
import (
"encoding/hex"
"errors"
"runtime"
"sort"
"strings"
"sync"
"sync/atomic"
"time"
dns "golang.org/x/net/dns/dnsmessage"
"inet.af/netaddr"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
"tailscale.com/wgengine/monitor"
)
// maxResponseBytes is the maximum size of a response from a Resolver. The
// actual buffer size will be one larger than this so that we can detect
// truncation in a platform-agnostic way.
const maxResponseBytes = 4095
// maxActiveQueries returns the maximal number of DNS requests that be
// can running.
// If EnqueueRequest is called when this many requests are already pending,
// the request will be dropped to avoid blocking the caller.
func maxActiveQueries() int32 {
if runtime.GOOS == "ios" {
// For memory paranoia reasons on iOS, match the
// historical Tailscale 1.x..1.8 behavior for now
// (just before the 1.10 release).
return 64
}
// But for other platforms, allow more burstiness:
return 256
}
// defaultTTL is the TTL of all responses from Resolver.
const defaultTTL = 600 * time.Second
// ErrClosed indicates that the resolver has been closed and readers should exit.
var ErrClosed = errors.New("closed")
var (
errFullQueue = errors.New("request queue full")
errNotQuery = errors.New("not a DNS query")
errNotOurName = errors.New("not a Tailscale DNS name")
)
type packet struct {
bs []byte
addr netaddr.IPPort // src for a request, dst for a response
}
// Config is a resolver configuration.
// Given a Config, queries are resolved in the following order:
// If the query is an exact match for an entry in LocalHosts, return that.
// Else if the query suffix matches an entry in LocalDomains, return NXDOMAIN.
// Else forward the query to the most specific matching entry in Routes.
// Else return SERVFAIL.
type Config struct {
// Routes is a map of DNS name suffix to the resolvers to use for
// queries within that suffix.
// Queries only match the most specific suffix.
// To register a "default route", add an entry for ".".
Routes map[dnsname.FQDN][]netaddr.IPPort
// LocalHosts is a map of FQDNs to corresponding IPs.
Hosts map[dnsname.FQDN][]netaddr.IP
// LocalDomains is a list of DNS name suffixes that should not be
// routed to upstream resolvers.
LocalDomains []dnsname.FQDN
}
// Resolver is a DNS resolver for nodes on the Tailscale network,
// associating them with domain names of the form <mynode>.<mydomain>.<root>.
// If it is asked to resolve a domain that is not of that form,
// it delegates to upstream nameservers if any are set.
type Resolver struct {
logf logger.Logf
linkMon *monitor.Mon // or nil
saveConfigForTests func(cfg Config) // used in tests to capture resolver config
// forwarder forwards requests to upstream nameservers.
forwarder *forwarder
activeQueriesAtomic int32 // number of DNS queries in flight
// responses is an unbuffered channel to which responses are returned.
responses chan packet
// errors is an unbuffered channel to which errors are returned.
errors chan error
// closed signals all goroutines to stop.
closed chan struct{}
// wg signals when all goroutines have stopped.
wg sync.WaitGroup
// mu guards the following fields from being updated while used.
mu sync.Mutex
localDomains []dnsname.FQDN
hostToIP map[dnsname.FQDN][]netaddr.IP
ipToHost map[netaddr.IP]dnsname.FQDN
}
type ForwardLinkSelector interface {
// PickLink returns which network device should be used to query
// the DNS server at the given IP.
// The empty string means to use an unspecified default.
PickLink(netaddr.IP) (linkName string)
}
// New returns a new resolver.
// linkMon optionally specifies a link monitor to use for socket rebinding.
func New(logf logger.Logf, linkMon *monitor.Mon, linkSel ForwardLinkSelector) *Resolver {
r := &Resolver{
logf: logger.WithPrefix(logf, "dns: "),
linkMon: linkMon,
responses: make(chan packet),
errors: make(chan error),
closed: make(chan struct{}),
hostToIP: map[dnsname.FQDN][]netaddr.IP{},
ipToHost: map[netaddr.IP]dnsname.FQDN{},
}
r.forwarder = newForwarder(r.logf, r.responses, linkMon, linkSel)
return r
}
func (r *Resolver) TestOnlySetHook(hook func(Config)) { r.saveConfigForTests = hook }
func (r *Resolver) SetConfig(cfg Config) error {
if r.saveConfigForTests != nil {
r.saveConfigForTests(cfg)
}
routes := make([]route, 0, len(cfg.Routes))
reverse := make(map[netaddr.IP]dnsname.FQDN, len(cfg.Hosts))
for host, ips := range cfg.Hosts {
for _, ip := range ips {
reverse[ip] = host
}
}
for suffix, ips := range cfg.Routes {
routes = append(routes, route{
Suffix: suffix,
Resolvers: ips,
})
}
// Sort from longest prefix to shortest.
sort.Slice(routes, func(i, j int) bool {
return routes[i].Suffix.NumLabels() > routes[j].Suffix.NumLabels()
})
r.forwarder.setRoutes(routes)
r.mu.Lock()
defer r.mu.Unlock()
r.localDomains = cfg.LocalDomains
r.hostToIP = cfg.Hosts
r.ipToHost = reverse
return nil
}
// Close shuts down the resolver and ensures poll goroutines have exited.
// The Resolver cannot be used again after Close is called.
func (r *Resolver) Close() {
select {
case <-r.closed:
return
default:
// continue
}
close(r.closed)
r.forwarder.Close()
}
// EnqueueRequest places the given DNS request in the resolver's queue.
// It takes ownership of the payload and does not block.
// If the queue is full, the request will be dropped and an error will be returned.
func (r *Resolver) EnqueueRequest(bs []byte, from netaddr.IPPort) error {
select {
case <-r.closed:
return ErrClosed
default:
}
if n := atomic.AddInt32(&r.activeQueriesAtomic, 1); n > maxActiveQueries() {
atomic.AddInt32(&r.activeQueriesAtomic, -1)
return errFullQueue
}
go r.handleQuery(packet{bs, from})
return nil
}
// NextResponse returns a DNS response to a previously enqueued request.
// It blocks until a response is available and gives up ownership of the response payload.
func (r *Resolver) NextResponse() (packet []byte, to netaddr.IPPort, err error) {
select {
case <-r.closed:
return nil, netaddr.IPPort{}, ErrClosed
case resp := <-r.responses:
return resp.bs, resp.addr, nil
case err := <-r.errors:
return nil, netaddr.IPPort{}, err
}
}
// resolveLocal returns an IP for the given domain, if domain is in
// the local hosts map and has an IP corresponding to the requested
// typ (A, AAAA, ALL).
// Returns dns.RCodeRefused to indicate that the local map is not
// authoritative for domain.
func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netaddr.IP, dns.RCode) {
// Reject .onion domains per RFC 7686.
if dnsname.HasSuffix(domain.WithoutTrailingDot(), ".onion") {
return netaddr.IP{}, dns.RCodeNameError
}
r.mu.Lock()
hosts := r.hostToIP
localDomains := r.localDomains
r.mu.Unlock()
addrs, found := hosts[domain]
if !found {
for _, suffix := range localDomains {
if suffix.Contains(domain) {
// We are authoritative for the queried domain.
return netaddr.IP{}, dns.RCodeNameError
}
}
// Not authoritative, signal that forwarding is advisable.
return netaddr.IP{}, dns.RCodeRefused
}
// Refactoring note: this must happen after we check suffixes,
// otherwise we will respond with NOTIMP to requests that should be forwarded.
//
// DNS semantics subtlety: when a DNS name exists, but no records
// are available for the requested record type, we must return
// RCodeSuccess with no data, not NXDOMAIN.
switch typ {
case dns.TypeA:
for _, ip := range addrs {
if ip.Is4() {
return ip, dns.RCodeSuccess
}
}
return netaddr.IP{}, dns.RCodeSuccess
case dns.TypeAAAA:
for _, ip := range addrs {
if ip.Is6() {
return ip, dns.RCodeSuccess
}
}
return netaddr.IP{}, dns.RCodeSuccess
case dns.TypeALL:
// Answer with whatever we've got.
// It could be IPv4, IPv6, or a zero addr.
// TODO: Return all available resolutions (A and AAAA, if we have them).
if len(addrs) == 0 {
return netaddr.IP{}, dns.RCodeSuccess
}
return addrs[0], dns.RCodeSuccess
// Leave some some record types explicitly unimplemented.
// These types relate to recursive resolution or special
// DNS semantics and might be implemented in the future.
case dns.TypeNS, dns.TypeSOA, dns.TypeAXFR, dns.TypeHINFO:
return netaddr.IP{}, dns.RCodeNotImplemented
// For everything except for the few types above that are explictly not implemented, return no records.
// This is what other DNS systems do: always return NOERROR
// without any records whenever the requested record type is unknown.
// You can try this with:
// dig -t TYPE9824 example.com
// and note that NOERROR is returned, despite that record type being made up.
default:
// The name exists, but no records exist of the requested type.
return netaddr.IP{}, dns.RCodeSuccess
}
}
// resolveReverse returns the unique domain name that maps to the given address.
func (r *Resolver) resolveLocalReverse(ip netaddr.IP) (dnsname.FQDN, dns.RCode) {
r.mu.Lock()
defer r.mu.Unlock()
name, ok := r.ipToHost[ip]
if !ok {
return "", dns.RCodeNameError
}
return name, dns.RCodeSuccess
}
func (r *Resolver) handleQuery(pkt packet) {
defer atomic.AddInt32(&r.activeQueriesAtomic, -1)
out, err := r.respond(pkt.bs)
if err == errNotOurName {
err = r.forwarder.forward(pkt)
if err == nil {
// forward will send response into r.responses, nothing to do.
return
}
}
if err != nil {
select {
case <-r.closed:
case r.errors <- err:
}
} else {
select {
case <-r.closed:
case r.responses <- packet{out, pkt.addr}:
}
}
}
type response struct {
Header dns.Header
Question dns.Question
// Name is the response to a PTR query.
Name dnsname.FQDN
// IP is the response to an A, AAAA, or ALL query.
IP netaddr.IP
}
var dnsParserPool = &sync.Pool{
New: func() interface{} {
return new(dnsParser)
},
}
// dnsParser parses DNS queries using x/net/dns/dnsmessage.
// These structs are pooled with dnsParserPool.
type dnsParser struct {
Header dns.Header
Question dns.Question
parser dns.Parser
}
func (p *dnsParser) response() *response {
return &response{Header: p.Header, Question: p.Question}
}
// zeroParser clears parser so it doesn't retain its most recently
// parsed DNS query's []byte while it's sitting in a sync.Pool.
// It's not useful to keep anyway: the next Start will do the same.
func (p *dnsParser) zeroParser() { p.parser = dns.Parser{} }
// parseQuery parses the query in given packet into p.Header and
// p.Question.
func (p *dnsParser) parseQuery(query []byte) error {
defer p.zeroParser()
var err error
p.Header, err = p.parser.Start(query)
if err != nil {
return err
}
if p.Header.Response {
return errNotQuery
}
p.Question, err = p.parser.Question()
return err
}
// marshalARecord serializes an A record into an active builder.
// The caller may continue using the builder following the call.
func marshalARecord(name dns.Name, ip netaddr.IP, builder *dns.Builder) error {
var answer dns.AResource
answerHeader := dns.ResourceHeader{
Name: name,
Type: dns.TypeA,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}
ipbytes := ip.As4()
copy(answer.A[:], ipbytes[:])
return builder.AResource(answerHeader, answer)
}
// marshalAAAARecord serializes an AAAA record into an active builder.
// The caller may continue using the builder following the call.
func marshalAAAARecord(name dns.Name, ip netaddr.IP, builder *dns.Builder) error {
var answer dns.AAAAResource
answerHeader := dns.ResourceHeader{
Name: name,
Type: dns.TypeAAAA,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}
ipbytes := ip.As16()
copy(answer.AAAA[:], ipbytes[:])
return builder.AAAAResource(answerHeader, answer)
}
// marshalPTRRecord serializes a PTR record into an active builder.
// The caller may continue using the builder following the call.
func marshalPTRRecord(queryName dns.Name, name dnsname.FQDN, builder *dns.Builder) error {
var answer dns.PTRResource
var err error
answerHeader := dns.ResourceHeader{
Name: queryName,
Type: dns.TypePTR,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}
answer.PTR, err = dns.NewName(name.WithTrailingDot())
if err != nil {
return err
}
return builder.PTRResource(answerHeader, answer)
}
// marshalResponse serializes the DNS response into a new buffer.
func marshalResponse(resp *response) ([]byte, error) {
resp.Header.Response = true
resp.Header.Authoritative = true
if resp.Header.RecursionDesired {
resp.Header.RecursionAvailable = true
}
builder := dns.NewBuilder(nil, resp.Header)
isSuccess := resp.Header.RCode == dns.RCodeSuccess
if resp.Question.Type != 0 || isSuccess {
err := builder.StartQuestions()
if err != nil {
return nil, err
}
err = builder.Question(resp.Question)
if err != nil {
return nil, err
}
}
// Only successful responses contain answers.
if !isSuccess {
return builder.Finish()
}
err := builder.StartAnswers()
if err != nil {
return nil, err
}
switch resp.Question.Type {
case dns.TypeA, dns.TypeAAAA, dns.TypeALL:
if resp.IP.Is4() {
err = marshalARecord(resp.Question.Name, resp.IP, &builder)
} else if resp.IP.Is6() {
err = marshalAAAARecord(resp.Question.Name, resp.IP, &builder)
}
case dns.TypePTR:
err = marshalPTRRecord(resp.Question.Name, resp.Name, &builder)
}
if err != nil {
return nil, err
}
return builder.Finish()
}
const (
rdnsv4Suffix = ".in-addr.arpa."
rdnsv6Suffix = ".ip6.arpa."
)
// hasRDNSBonjourPrefix reports whether name has a Bonjour Service Prefix..
//
// https://tools.ietf.org/html/rfc6763 lists
// "five special RR names" for Bonjour service discovery:
//
// b._dns-sd._udp.<domain>.
// db._dns-sd._udp.<domain>.
// r._dns-sd._udp.<domain>.
// dr._dns-sd._udp.<domain>.
// lb._dns-sd._udp.<domain>.
func hasRDNSBonjourPrefix(name dnsname.FQDN) bool {
// Even the shortest name containing a Bonjour prefix is long,
// so check length (cheap) and bail early if possible.
if len(name) < len("*._dns-sd._udp.0.0.0.0.in-addr.arpa.") {
return false
}
s := name.WithTrailingDot()
dot := strings.IndexByte(s, '.')
if dot == -1 {
return false // shouldn't happen
}
switch s[:dot] {
case "b", "db", "r", "dr", "lb":
default:
return false
}
return strings.HasPrefix(s[dot:], "._dns-sd._udp.")
}
// rawNameToLower converts a raw DNS name to a string, lowercasing it.
func rawNameToLower(name []byte) string {
var sb strings.Builder
sb.Grow(len(name))
for _, b := range name {
if 'A' <= b && b <= 'Z' {
b = b - 'A' + 'a'
}
sb.WriteByte(b)
}
return sb.String()
}
// ptrNameToIPv4 transforms a PTR name representing an IPv4 address to said address.
// Such names are IPv4 labels in reverse order followed by .in-addr.arpa.
// For example,
// 4.3.2.1.in-addr.arpa
// is transformed to
// 1.2.3.4
func rdnsNameToIPv4(name dnsname.FQDN) (ip netaddr.IP, ok bool) {
s := strings.TrimSuffix(name.WithTrailingDot(), rdnsv4Suffix)
ip, err := netaddr.ParseIP(s)
if err != nil {
return netaddr.IP{}, false
}
if !ip.Is4() {
return netaddr.IP{}, false
}
b := ip.As4()
return netaddr.IPv4(b[3], b[2], b[1], b[0]), true
}
// ptrNameToIPv6 transforms a PTR name representing an IPv6 address to said address.
// Such names are dot-separated nibbles in reverse order followed by .ip6.arpa.
// For example,
// b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.
// is transformed to
// 2001:db8::567:89ab
func rdnsNameToIPv6(name dnsname.FQDN) (ip netaddr.IP, ok bool) {
var b [32]byte
var ipb [16]byte
s := strings.TrimSuffix(name.WithTrailingDot(), rdnsv6Suffix)
// 32 nibbles and 31 dots between them.
if len(s) != 63 {
return netaddr.IP{}, false
}
// Dots and hex digits alternate.
prevDot := true
// i ranges over name backward; j ranges over b forward.
for i, j := len(s)-1, 0; i >= 0; i-- {
thisDot := (s[i] == '.')
if prevDot == thisDot {
return netaddr.IP{}, false
}
prevDot = thisDot
if !thisDot {
// This is safe assuming alternation.
// We do not check that non-dots are hex digits: hex.Decode below will do that.
b[j] = s[i]
j++
}
}
_, err := hex.Decode(ipb[:], b[:])
if err != nil {
return netaddr.IP{}, false
}
return netaddr.IPFrom16(ipb), true
}
// respondReverse returns a DNS response to a PTR query.
// It is assumed that resp.Question is populated by respond before this is called.
func (r *Resolver) respondReverse(query []byte, name dnsname.FQDN, resp *response) ([]byte, error) {
if hasRDNSBonjourPrefix(name) {
return nil, errNotOurName
}
var ip netaddr.IP
var ok bool
switch {
case strings.HasSuffix(name.WithTrailingDot(), rdnsv4Suffix):
ip, ok = rdnsNameToIPv4(name)
case strings.HasSuffix(name.WithTrailingDot(), rdnsv6Suffix):
ip, ok = rdnsNameToIPv6(name)
default:
return nil, errNotOurName
}
// It is more likely that we failed in parsing the name than that it is actually malformed.
// To avoid frustrating users, just log and delegate.
if !ok {
r.logf("parsing rdns: malformed name: %s", name)
return nil, errNotOurName
}
resp.Name, resp.Header.RCode = r.resolveLocalReverse(ip)
if resp.Header.RCode == dns.RCodeNameError {
return nil, errNotOurName
}
return marshalResponse(resp)
}
// respond returns a DNS response to query if it can be resolved locally.
// Otherwise, it returns errNotOurName.
func (r *Resolver) respond(query []byte) ([]byte, error) {
parser := dnsParserPool.Get().(*dnsParser)
defer dnsParserPool.Put(parser)
// ParseQuery is sufficiently fast to run on every DNS packet.
// This is considerably simpler than extracting the name by hand
// to shave off microseconds in case of delegation.
err := parser.parseQuery(query)
// We will not return this error: it is the sender's fault.
if err != nil {
if errors.Is(err, dns.ErrSectionDone) {
r.logf("parseQuery(%02x): no DNS questions", query)
} else {
r.logf("parseQuery(%02x): %v", query, err)
}
resp := parser.response()
resp.Header.RCode = dns.RCodeFormatError
return marshalResponse(resp)
}
rawName := parser.Question.Name.Data[:parser.Question.Name.Length]
name, err := dnsname.ToFQDN(rawNameToLower(rawName))
if err != nil {
// DNS packet unexpectedly contains an invalid FQDN.
resp := parser.response()
resp.Header.RCode = dns.RCodeFormatError
return marshalResponse(resp)
}
// Always try to handle reverse lookups; delegate inside when not found.
// This way, queries for existent nodes do not leak,
// but we behave gracefully if non-Tailscale nodes exist in CGNATRange.
if parser.Question.Type == dns.TypePTR {
return r.respondReverse(query, name, parser.response())
}
ip, rcode := r.resolveLocal(name, parser.Question.Type)
if rcode == dns.RCodeRefused {
return nil, errNotOurName // sentinel error return value: it requests forwarding
}
resp := parser.response()
resp.Header.RCode = rcode
resp.IP = ip
return marshalResponse(resp)
}