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tailscale/net/dns/resolver/forwarder.go

809 lines
22 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
import (
"bytes"
"context"
"encoding/binary"
"errors"
"fmt"
"io"
"io/ioutil"
"math/rand"
"net"
"net/http"
"net/url"
"runtime"
"sort"
"strconv"
"strings"
"sync"
"time"
dns "golang.org/x/net/dns/dnsmessage"
"inet.af/netaddr"
"tailscale.com/envknob"
"tailscale.com/hostinfo"
"tailscale.com/net/dns/publicdns"
"tailscale.com/net/dnscache"
"tailscale.com/net/neterror"
"tailscale.com/net/netns"
"tailscale.com/net/tsdial"
"tailscale.com/types/dnstype"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
"tailscale.com/wgengine/monitor"
)
// headerBytes is the number of bytes in a DNS message header.
const headerBytes = 12
// dnsFlagTruncated is set in the flags word when the packet is truncated.
const dnsFlagTruncated = 0x200
// truncatedFlagSet returns true if the DNS packet signals that it has
// been truncated. False is also returned if the packet was too small
// to be valid.
func truncatedFlagSet(pkt []byte) bool {
if len(pkt) < headerBytes {
return false
}
return (binary.BigEndian.Uint16(pkt[2:4]) & dnsFlagTruncated) != 0
}
const (
// responseTimeout is the maximal amount of time to wait for a DNS response.
responseTimeout = 5 * time.Second
// dohTransportTimeout is how long to keep idle HTTP
// connections open to DNS-over-HTTPs servers. This is pretty
// arbitrary.
dohTransportTimeout = 30 * time.Second
// dohTransportTimeout is how much of a head start to give a DoH query
// that was upgraded from a well-known public DNS provider's IP before
// normal UDP mode is attempted as a fallback.
dohHeadStart = 500 * time.Millisecond
// wellKnownHostBackupDelay is how long to artificially delay upstream
// DNS queries to the "fallback" DNS server IP for a known provider
// (e.g. how long to wait to query Google's 8.8.4.4 after 8.8.8.8).
wellKnownHostBackupDelay = 200 * time.Millisecond
)
var errNoUpstreams = errors.New("upstream nameservers not set")
// txid identifies a DNS transaction.
//
// As the standard DNS Request ID is only 16 bits, we extend it:
// the lower 32 bits are the zero-extended bits of the DNS Request ID;
// the upper 32 bits are the CRC32 checksum of the first question in the request.
// This makes probability of txid collision negligible.
type txid uint64
// getTxID computes the txid of the given DNS packet.
func getTxID(packet []byte) txid {
if len(packet) < headerBytes {
return 0
}
dnsid := binary.BigEndian.Uint16(packet[0:2])
// Previously, we hashed the question and combined it with the original txid
// which was useful when concurrent queries were multiplexed on a single
// local source port. We encountered some situations where the DNS server
// canonicalizes the question in the response (uppercase converted to
// lowercase in this case), which resulted in responses that we couldn't
// match to the original request due to hash mismatches.
return txid(dnsid)
}
func getRCode(packet []byte) dns.RCode {
if len(packet) < headerBytes {
// treat invalid packets as a refusal
return dns.RCode(5)
}
// get bottom 4 bits of 3rd byte
return dns.RCode(packet[3] & 0x0F)
}
// clampEDNSSize attempts to limit the maximum EDNS response size. This is not
// an exhaustive solution, instead only easy cases are currently handled in the
// interest of speed and reduced complexity. Only OPT records at the very end of
// the message with no option codes are addressed.
// TODO: handle more situations if we discover that they happen often
func clampEDNSSize(packet []byte, maxSize uint16) {
// optFixedBytes is the size of an OPT record with no option codes.
const optFixedBytes = 11
const edns0Version = 0
if len(packet) < headerBytes+optFixedBytes {
return
}
arCount := binary.BigEndian.Uint16(packet[10:12])
if arCount == 0 {
// OPT shows up in an AR, so there must be no OPT
return
}
// https://datatracker.ietf.org/doc/html/rfc6891#section-6.1.2
opt := packet[len(packet)-optFixedBytes:]
if opt[0] != 0 {
// OPT NAME must be 0 (root domain)
return
}
if dns.Type(binary.BigEndian.Uint16(opt[1:3])) != dns.TypeOPT {
// Not an OPT record
return
}
requestedSize := binary.BigEndian.Uint16(opt[3:5])
// Ignore extended RCODE in opt[5]
if opt[6] != edns0Version {
// Be conservative and don't touch unknown versions.
return
}
// Ignore flags in opt[6:9]
if binary.BigEndian.Uint16(opt[9:11]) != 0 {
// RDLEN must be 0 (no variable length data). We're at the end of the
// packet so this should be 0 anyway)..
return
}
if requestedSize <= maxSize {
return
}
// Clamp the maximum size
binary.BigEndian.PutUint16(opt[3:5], maxSize)
}
type route struct {
Suffix dnsname.FQDN
Resolvers []resolverAndDelay
}
// resolverAndDelay is an upstream DNS resolver and a delay for how
// long to wait before querying it.
type resolverAndDelay struct {
// name is the upstream resolver.
name *dnstype.Resolver
// startDelay is an amount to delay this resolver at
// start. It's used when, say, there are four Google or
// Cloudflare DNS IPs (two IPv4 + two IPv6) and we don't want
// to race all four at once.
startDelay time.Duration
}
// forwarder forwards DNS packets to a number of upstream nameservers.
type forwarder struct {
logf logger.Logf
linkMon *monitor.Mon
linkSel ForwardLinkSelector // TODO(bradfitz): remove this when tsdial.Dialer absords it
dialer *tsdial.Dialer
dohSem chan struct{}
ctx context.Context // good until Close
ctxCancel context.CancelFunc // closes ctx
mu sync.Mutex // guards following
dohClient map[string]*http.Client // urlBase -> client
// routes are per-suffix resolvers to use, with
// the most specific routes first.
routes []route
}
func init() {
rand.Seed(time.Now().UnixNano())
}
func maxDoHInFlight(goos string) int {
if goos != "ios" {
return 1000 // effectively unlimited
}
// iOS < 15 limits the memory to 15MB for NetworkExtensions.
// iOS >= 15 gives us 50MB.
// See: https://tailscale.com/blog/go-linker/
ver := hostinfo.GetOSVersion()
if ver == "" {
// Unknown iOS version, be cautious.
return 10
}
major, _, ok := strings.Cut(ver, ".")
if !ok {
// Unknown iOS version, be cautious.
return 10
}
if m, err := strconv.Atoi(major); err != nil || m < 15 {
return 10
}
return 1000
}
func newForwarder(logf logger.Logf, linkMon *monitor.Mon, linkSel ForwardLinkSelector, dialer *tsdial.Dialer) *forwarder {
f := &forwarder{
logf: logger.WithPrefix(logf, "forward: "),
linkMon: linkMon,
linkSel: linkSel,
dialer: dialer,
dohSem: make(chan struct{}, maxDoHInFlight(runtime.GOOS)),
}
f.ctx, f.ctxCancel = context.WithCancel(context.Background())
return f
}
func (f *forwarder) Close() error {
f.ctxCancel()
return nil
}
// resolversWithDelays maps from a set of DNS server names to a slice of a type
// that included a startDelay, upgrading any well-known DoH (DNS-over-HTTP)
// servers in the process, insert a DoH lookup first before UDP fallbacks.
func resolversWithDelays(resolvers []*dnstype.Resolver) []resolverAndDelay {
rr := make([]resolverAndDelay, 0, len(resolvers)+2)
// Add the known DoH ones first, starting immediately.
didDoH := map[string]bool{}
for _, r := range resolvers {
ipp, ok := r.IPPort()
if !ok {
continue
}
dohBase, ok := publicdns.KnownDoH()[ipp.IP()]
if !ok || didDoH[dohBase] {
continue
}
didDoH[dohBase] = true
rr = append(rr, resolverAndDelay{name: &dnstype.Resolver{Addr: dohBase}})
}
type hostAndFam struct {
host string // some arbitrary string representing DNS host (currently the DoH base)
bits uint8 // either 32 or 128 for IPv4 vs IPv6s address family
}
done := map[hostAndFam]int{}
for _, r := range resolvers {
ipp, ok := r.IPPort()
if !ok {
// Pass non-IP ones through unchanged, without delay.
// (e.g. DNS-over-ExitDNS when using an exit node)
rr = append(rr, resolverAndDelay{name: r})
continue
}
ip := ipp.IP()
var startDelay time.Duration
if host, ok := publicdns.KnownDoH()[ip]; ok {
// We already did the DoH query early. These
startDelay = dohHeadStart
key := hostAndFam{host, ip.BitLen()}
if done[key] > 0 {
startDelay += wellKnownHostBackupDelay
}
done[key]++
}
rr = append(rr, resolverAndDelay{
name: r,
startDelay: startDelay,
})
}
return rr
}
// setRoutes sets the routes to use for DNS forwarding. It's called by
// Resolver.SetConfig on reconfig.
//
// The memory referenced by routesBySuffix should not be modified.
func (f *forwarder) setRoutes(routesBySuffix map[dnsname.FQDN][]*dnstype.Resolver) {
routes := make([]route, 0, len(routesBySuffix))
for suffix, rs := range routesBySuffix {
routes = append(routes, route{
Suffix: suffix,
Resolvers: resolversWithDelays(rs),
})
}
// Sort from longest prefix to shortest.
sort.Slice(routes, func(i, j int) bool {
return routes[i].Suffix.NumLabels() > routes[j].Suffix.NumLabels()
})
f.mu.Lock()
defer f.mu.Unlock()
f.routes = routes
}
var stdNetPacketListener packetListener = new(net.ListenConfig)
type packetListener interface {
ListenPacket(ctx context.Context, network, address string) (net.PacketConn, error)
}
func (f *forwarder) packetListener(ip netaddr.IP) (packetListener, error) {
if f.linkSel == nil || initListenConfig == nil {
return stdNetPacketListener, nil
}
linkName := f.linkSel.PickLink(ip)
if linkName == "" {
return stdNetPacketListener, nil
}
lc := new(net.ListenConfig)
if err := initListenConfig(lc, f.linkMon, linkName); err != nil {
return nil, err
}
return lc, nil
}
// getKnownDoHClientForProvider returns an HTTP client for a specific DoH
// provider named by its DoH base URL (like "https://dns.google/dns-query").
//
// The returned client race/Happy Eyeballs dials all IPs for urlBase (usually
// 4), as statically known by the publicdns package.
func (f *forwarder) getKnownDoHClientForProvider(urlBase string) (c *http.Client, ok bool) {
f.mu.Lock()
defer f.mu.Unlock()
if c, ok := f.dohClient[urlBase]; ok {
return c, true
}
allIPs := publicdns.DoHIPsOfBase()[urlBase]
if len(allIPs) == 0 {
return nil, false
}
dohURL, err := url.Parse(urlBase)
if err != nil {
return nil, false
}
nsDialer := netns.NewDialer(f.logf)
dialer := dnscache.Dialer(nsDialer.DialContext, &dnscache.Resolver{
SingleHost: dohURL.Hostname(),
SingleHostStaticResult: allIPs,
})
c = &http.Client{
Transport: &http.Transport{
IdleConnTimeout: dohTransportTimeout,
DialContext: func(ctx context.Context, netw, addr string) (net.Conn, error) {
if !strings.HasPrefix(netw, "tcp") {
return nil, fmt.Errorf("unexpected network %q", netw)
}
return dialer(ctx, netw, addr)
},
},
}
if f.dohClient == nil {
f.dohClient = map[string]*http.Client{}
}
f.dohClient[urlBase] = c
return c, true
}
const dohType = "application/dns-message"
func (f *forwarder) releaseDoHSem() { <-f.dohSem }
func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client, packet []byte) ([]byte, error) {
// Bound the number of HTTP requests in flight. This primarily
// matters for iOS where we're very memory constrained and
// HTTP requests are heavier on iOS where we don't include
// HTTP/2 for binary size reasons (as binaries on iOS linked
// with Go code cost memory proportional to the binary size,
// for reasons not fully understood).
select {
case f.dohSem <- struct{}{}:
case <-ctx.Done():
return nil, ctx.Err()
}
defer f.releaseDoHSem()
metricDNSFwdDoH.Add(1)
req, err := http.NewRequestWithContext(ctx, "POST", urlBase, bytes.NewReader(packet))
if err != nil {
return nil, err
}
req.Header.Set("Content-Type", dohType)
// Note: we don't currently set the Accept header (which is
// only a SHOULD in the spec) as iOS doesn't use HTTP/2 and
// we'd rather save a few bytes on outgoing requests when
// empirically no provider cares about the Accept header's
// absence.
hres, err := c.Do(req)
if err != nil {
metricDNSFwdDoHErrorTransport.Add(1)
return nil, err
}
defer hres.Body.Close()
if hres.StatusCode != 200 {
metricDNSFwdDoHErrorStatus.Add(1)
return nil, errors.New(hres.Status)
}
if ct := hres.Header.Get("Content-Type"); ct != dohType {
metricDNSFwdDoHErrorCT.Add(1)
return nil, fmt.Errorf("unexpected response Content-Type %q", ct)
}
res, err := ioutil.ReadAll(hres.Body)
if err != nil {
metricDNSFwdDoHErrorBody.Add(1)
}
if truncatedFlagSet(res) {
metricDNSFwdTruncated.Add(1)
}
return res, err
}
var verboseDNSForward = envknob.Bool("TS_DEBUG_DNS_FORWARD_SEND")
// send sends packet to dst. It is best effort.
//
// send expects the reply to have the same txid as txidOut.
func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDelay) (ret []byte, err error) {
if verboseDNSForward {
f.logf("forwarder.send(%q) ...", rr.name.Addr)
defer func() {
f.logf("forwarder.send(%q) = %v, %v", rr.name.Addr, len(ret), err)
}()
}
if strings.HasPrefix(rr.name.Addr, "http://") {
return f.sendDoH(ctx, rr.name.Addr, f.dialer.PeerAPIHTTPClient(), fq.packet)
}
if strings.HasPrefix(rr.name.Addr, "https://") {
// Only known DoH providers are supported currently. Specifically, we
// only support DoH providers where we can TCP connect to them on port
// 443 at the same IP address they serve normal UDP DNS from (1.1.1.1,
// 8.8.8.8, 9.9.9.9, etc.) That's why OpenDNS and custon DoH providers
// aren't currently supported. There's no backup DNS resolution path for
// them.
urlBase := rr.name.Addr
if hc, ok := f.getKnownDoHClientForProvider(urlBase); ok {
return f.sendDoH(ctx, urlBase, hc, fq.packet)
}
metricDNSFwdErrorType.Add(1)
return nil, fmt.Errorf("arbitrary https:// resolvers not supported yet")
}
if strings.HasPrefix(rr.name.Addr, "tls://") {
metricDNSFwdErrorType.Add(1)
return nil, fmt.Errorf("tls:// resolvers not supported yet")
}
return f.sendUDP(ctx, fq, rr)
}
func (f *forwarder) sendUDP(ctx context.Context, fq *forwardQuery, rr resolverAndDelay) (ret []byte, err error) {
ipp, ok := rr.name.IPPort()
if !ok {
metricDNSFwdErrorType.Add(1)
return nil, fmt.Errorf("unrecognized resolver type %q", rr.name.Addr)
}
metricDNSFwdUDP.Add(1)
ln, err := f.packetListener(ipp.IP())
if err != nil {
return nil, err
}
conn, err := ln.ListenPacket(ctx, "udp", ":0")
if err != nil {
f.logf("ListenPacket failed: %v", err)
return nil, err
}
defer conn.Close()
fq.closeOnCtxDone.Add(conn)
defer fq.closeOnCtxDone.Remove(conn)
if _, err := conn.WriteTo(fq.packet, ipp.UDPAddr()); err != nil {
metricDNSFwdUDPErrorWrite.Add(1)
if err := ctx.Err(); err != nil {
return nil, err
}
return nil, err
}
metricDNSFwdUDPWrote.Add(1)
// The 1 extra byte is to detect packet truncation.
out := make([]byte, maxResponseBytes+1)
n, _, err := conn.ReadFrom(out)
if err != nil {
if err := ctx.Err(); err != nil {
return nil, err
}
if neterror.PacketWasTruncated(err) {
err = nil
} else {
metricDNSFwdUDPErrorRead.Add(1)
return nil, err
}
}
truncated := n > maxResponseBytes
if truncated {
n = maxResponseBytes
}
if n < headerBytes {
f.logf("recv: packet too small (%d bytes)", n)
}
out = out[:n]
txid := getTxID(out)
if txid != fq.txid {
metricDNSFwdUDPErrorTxID.Add(1)
return nil, errors.New("txid doesn't match")
}
rcode := getRCode(out)
// don't forward transient errors back to the client when the server fails
if rcode == dns.RCodeServerFailure {
f.logf("recv: response code indicating server failure: %d", rcode)
metricDNSFwdUDPErrorServer.Add(1)
return nil, errors.New("response code indicates server issue")
}
if truncated {
// Set the truncated bit if it wasn't already.
flags := binary.BigEndian.Uint16(out[2:4])
flags |= dnsFlagTruncated
binary.BigEndian.PutUint16(out[2:4], flags)
// TODO(#2067): Remove any incomplete records? RFC 1035 section 6.2
// states that truncation should head drop so that the authority
// section can be preserved if possible. However, the UDP read with
// a too-small buffer has already dropped the end, so that's the
// best we can do.
}
if truncatedFlagSet(out) {
metricDNSFwdTruncated.Add(1)
}
clampEDNSSize(out, maxResponseBytes)
metricDNSFwdUDPSuccess.Add(1)
return out, nil
}
// resolvers returns the resolvers to use for domain.
func (f *forwarder) resolvers(domain dnsname.FQDN) []resolverAndDelay {
f.mu.Lock()
routes := f.routes
f.mu.Unlock()
for _, route := range routes {
if route.Suffix == "." || route.Suffix.Contains(domain) {
return route.Resolvers
}
}
return nil
}
// forwardQuery is information and state about a forwarded DNS query that's
// being sent to 1 or more upstreams.
//
// In the case of racing against multiple equivalent upstreams
// (e.g. Google or CloudFlare's 4 DNS IPs: 2 IPv4 + 2 IPv6), this type
// handles racing them more intelligently than just blasting away 4
// queries at once.
type forwardQuery struct {
txid txid
packet []byte
// closeOnCtxDone lets send register values to Close if the
// caller's ctx expires. This avoids send from allocating its
// own waiting goroutine to interrupt the ReadFrom, as memory
// is tight on iOS and we want the number of pending DNS
// lookups to be bursty without too much associated
// goroutine/memory cost.
closeOnCtxDone *closePool
// TODO(bradfitz): add race delay state:
// mu sync.Mutex
// ...
}
// forwardWithDestChan forwards the query to all upstream nameservers
// and waits for the first response.
//
// It either sends to responseChan and returns nil, or returns a
// non-nil error (without sending to the channel).
//
// If resolvers is non-empty, it's used explicitly (notably, for exit
// node DNS proxy queries), otherwise f.resolvers is used.
func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, responseChan chan<- packet, resolvers ...resolverAndDelay) error {
metricDNSFwd.Add(1)
domain, err := nameFromQuery(query.bs)
if err != nil {
metricDNSFwdErrorName.Add(1)
return err
}
// Guarantee that the ctx we use below is done when this function returns.
ctx, cancel := context.WithCancel(ctx)
defer cancel()
// Drop DNS service discovery spam, primarily for battery life
// on mobile. Things like Spotify on iOS generate this traffic,
// when browsing for LAN devices. But even when filtering this
// out, playing on Sonos still works.
if hasRDNSBonjourPrefix(domain) {
metricDNSFwdDropBonjour.Add(1)
res, err := nxDomainResponse(query)
if err != nil {
f.logf("error parsing bonjour query: %v", err)
return nil
}
select {
case <-ctx.Done():
return ctx.Err()
case responseChan <- res:
return nil
}
}
if fl, ok := fwdLogAtomic.Load().(*fwdLog); ok {
fl.addName(string(domain))
}
clampEDNSSize(query.bs, maxResponseBytes)
if len(resolvers) == 0 {
resolvers = f.resolvers(domain)
if len(resolvers) == 0 {
metricDNSFwdErrorNoUpstream.Add(1)
return errNoUpstreams
}
}
fq := &forwardQuery{
txid: getTxID(query.bs),
packet: query.bs,
closeOnCtxDone: new(closePool),
}
defer fq.closeOnCtxDone.Close()
resc := make(chan []byte, 1) // it's fine buffered or not
errc := make(chan error, 1) // it's fine buffered or not too
for i := range resolvers {
go func(rr *resolverAndDelay) {
if rr.startDelay > 0 {
timer := time.NewTimer(rr.startDelay)
select {
case <-timer.C:
case <-ctx.Done():
timer.Stop()
return
}
}
resb, err := f.send(ctx, fq, *rr)
if err != nil {
select {
case errc <- err:
case <-ctx.Done():
}
return
}
select {
case resc <- resb:
case <-ctx.Done():
}
}(&resolvers[i])
}
var firstErr error
var numErr int
for {
select {
case v := <-resc:
select {
case <-ctx.Done():
metricDNSFwdErrorContext.Add(1)
return ctx.Err()
case responseChan <- packet{v, query.addr}:
metricDNSFwdSuccess.Add(1)
return nil
}
case err := <-errc:
if firstErr == nil {
firstErr = err
}
numErr++
if numErr == len(resolvers) {
return firstErr
}
case <-ctx.Done():
metricDNSFwdErrorContext.Add(1)
if firstErr != nil {
metricDNSFwdErrorContextGotError.Add(1)
return firstErr
}
return ctx.Err()
}
}
}
var initListenConfig func(_ *net.ListenConfig, _ *monitor.Mon, tunName string) error
// nameFromQuery extracts the normalized query name from bs.
func nameFromQuery(bs []byte) (dnsname.FQDN, error) {
var parser dns.Parser
hdr, err := parser.Start(bs)
if err != nil {
return "", err
}
if hdr.Response {
return "", errNotQuery
}
q, err := parser.Question()
if err != nil {
return "", err
}
n := q.Name.Data[:q.Name.Length]
return dnsname.ToFQDN(rawNameToLower(n))
}
// nxDomainResponse returns an NXDomain DNS reply for the provided request.
func nxDomainResponse(req packet) (res packet, err error) {
p := dnsParserPool.Get().(*dnsParser)
defer dnsParserPool.Put(p)
if err := p.parseQuery(req.bs); err != nil {
return packet{}, err
}
h := p.Header
h.Response = true
h.RecursionAvailable = h.RecursionDesired
h.RCode = dns.RCodeNameError
b := dns.NewBuilder(nil, h)
// TODO(bradfitz): should we add an SOA record in the Authority
// section too? (for the nxdomain negative caching TTL)
// For which zone? Does iOS care?
res.bs, err = b.Finish()
res.addr = req.addr
return res, err
}
// closePool is a dynamic set of io.Closers to close as a group.
// It's intended to be Closed at most once.
//
// The zero value is ready for use.
type closePool struct {
mu sync.Mutex
m map[io.Closer]bool
closed bool
}
func (p *closePool) Add(c io.Closer) {
p.mu.Lock()
defer p.mu.Unlock()
if p.closed {
c.Close()
return
}
if p.m == nil {
p.m = map[io.Closer]bool{}
}
p.m[c] = true
}
func (p *closePool) Remove(c io.Closer) {
p.mu.Lock()
defer p.mu.Unlock()
if p.closed {
return
}
delete(p.m, c)
}
func (p *closePool) Close() error {
p.mu.Lock()
defer p.mu.Unlock()
if p.closed {
return nil
}
p.closed = true
for c := range p.m {
c.Close()
}
return nil
}