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tailscale/net/dns/resolver/forwarder.go

1196 lines
35 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package resolver
import (
"bytes"
"context"
"crypto/sha256"
"crypto/tls"
"encoding/base64"
"encoding/binary"
"errors"
"fmt"
"io"
"net"
"net/http"
"net/netip"
"net/url"
"sort"
"strings"
"sync"
"sync/atomic"
"time"
dns "golang.org/x/net/dns/dnsmessage"
"tailscale.com/control/controlknobs"
"tailscale.com/envknob"
"tailscale.com/health"
"tailscale.com/net/dns/publicdns"
"tailscale.com/net/dnscache"
"tailscale.com/net/neterror"
"tailscale.com/net/netmon"
"tailscale.com/net/sockstats"
"tailscale.com/net/tsdial"
"tailscale.com/types/dnstype"
"tailscale.com/types/logger"
"tailscale.com/types/nettype"
"tailscale.com/util/cloudenv"
"tailscale.com/util/dnsname"
"tailscale.com/util/race"
"tailscale.com/version"
)
// 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 (
// dohIdleConnTimeout is how long to keep idle HTTP connections
// open to DNS-over-HTTPS servers. 10 seconds is a sensible
// default, as it's long enough to handle a burst of queries
// coming in a row, but short enough to not keep idle connections
// open for too long. In theory, idle connections could be kept
// open for a long time without any battery impact as no traffic
// is supposed to be flowing on them.
// However, in practice, DoH servers will send TCP keepalives (e.g.
// NextDNS sends them every ~10s). Handling these keepalives
// wakes up the modem, and that uses battery. Therefore, we keep
// the idle timeout low enough to allow idle connections to be
// closed during an extended period with no DNS queries, killing
// keepalive network activity.
dohIdleConnTimeout = 10 * 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
// udpRaceTimeout is the timeout after which we will start a DNS query
// over TCP while waiting for the UDP query to complete.
udpRaceTimeout = 2 * time.Second
// tcpQueryTimeout is the timeout for a DNS query performed over TCP.
// It matches the default 5sec timeout of the 'dig' utility.
tcpQueryTimeout = 5 * time.Second
)
// 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)
}
// dnsForwarderFailing should be raised when the forwarder is unable to reach the
// upstream resolvers. This is a high severity warning as it results in "no internet".
// This warning must be cleared when the forwarder is working again.
//
// We allow for 5 second grace period to ensure this is not raised for spurious errors
// under the assumption that DNS queries are relatively frequent and a subsequent
// successful query will clear any one-off errors.
var dnsForwarderFailing = health.Register(&health.Warnable{
Code: "dns-forward-failing",
Title: "DNS unavailable",
Severity: health.SeverityMedium,
DependsOn: []*health.Warnable{health.NetworkStatusWarnable},
Text: health.StaticMessage("Tailscale can't reach the configured DNS servers. Internet connectivity may be affected."),
ImpactsConnectivity: true,
TimeToVisible: 15 * time.Second,
})
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
netMon *netmon.Monitor // always non-nil
linkSel ForwardLinkSelector // TODO(bradfitz): remove this when tsdial.Dialer absorbs it
dialer *tsdial.Dialer
health *health.Tracker // always non-nil
controlKnobs *controlknobs.Knobs // or nil
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
// cloudHostFallback are last resort resolvers to use if no per-suffix
// resolver matches. These are only populated on cloud hosts where the
// platform provides a well-known recursive resolver.
//
// That is, if we're running on GCP or AWS where there's always a well-known
// IP of a recursive resolver, return that rather than having callers return
// SERVFAIL. This fixes both normal 100.100.100.100 resolution when
// /etc/resolv.conf is missing/corrupt, and the peerapi ExitDNS stub
// resolver lookup.
cloudHostFallback []resolverAndDelay
// missingUpstreamRecovery, if non-nil, is set called when a SERVFAIL is
// returned due to missing upstream resolvers.
//
// This should attempt to properly (re)set the upstream resolvers.
missingUpstreamRecovery func()
}
func newForwarder(logf logger.Logf, netMon *netmon.Monitor, linkSel ForwardLinkSelector, dialer *tsdial.Dialer, health *health.Tracker, knobs *controlknobs.Knobs) *forwarder {
if netMon == nil {
panic("nil netMon")
}
f := &forwarder{
logf: logger.WithPrefix(logf, "forward: "),
netMon: netMon,
linkSel: linkSel,
dialer: dialer,
health: health,
controlKnobs: knobs,
missingUpstreamRecovery: func() {},
}
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)
type dohState uint8
const addedDoH = dohState(1)
const addedDoHAndDontAddUDP = dohState(2)
// Add the known DoH ones first, starting immediately.
didDoH := map[string]dohState{}
for _, r := range resolvers {
ipp, ok := r.IPPort()
if !ok {
continue
}
dohBase, dohOnly, ok := publicdns.DoHEndpointFromIP(ipp.Addr())
if !ok || didDoH[dohBase] != 0 {
continue
}
if dohOnly {
didDoH[dohBase] = addedDoHAndDontAddUDP
} else {
didDoH[dohBase] = addedDoH
}
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.Addr()
var startDelay time.Duration
if host, _, ok := publicdns.DoHEndpointFromIP(ip); ok {
if didDoH[host] == addedDoHAndDontAddUDP {
continue
}
// We already did the DoH query early. These
// are for normal dns53 UDP queries.
startDelay = dohHeadStart
key := hostAndFam{host, uint8(ip.BitLen())}
if done[key] > 0 {
startDelay += wellKnownHostBackupDelay
}
done[key]++
}
rr = append(rr, resolverAndDelay{
name: r,
startDelay: startDelay,
})
}
return rr
}
var (
cloudResolversOnce sync.Once
cloudResolversLazy []resolverAndDelay
)
func cloudResolvers() []resolverAndDelay {
cloudResolversOnce.Do(func() {
if ip := cloudenv.Get().ResolverIP(); ip != "" {
cloudResolver := []*dnstype.Resolver{{Addr: ip}}
cloudResolversLazy = resolversWithDelays(cloudResolver)
}
})
return cloudResolversLazy
}
// 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))
cloudHostFallback := cloudResolvers()
for suffix, rs := range routesBySuffix {
if suffix == "." && len(rs) == 0 && len(cloudHostFallback) > 0 {
routes = append(routes, route{
Suffix: suffix,
Resolvers: cloudHostFallback,
})
} else {
routes = append(routes, route{
Suffix: suffix,
Resolvers: resolversWithDelays(rs),
})
}
}
if cloudenv.Get().HasInternalTLD() && len(cloudHostFallback) > 0 {
if _, ok := routesBySuffix["internal."]; !ok {
routes = append(routes, route{
Suffix: "internal.",
Resolvers: cloudHostFallback,
})
}
}
// 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
f.cloudHostFallback = cloudHostFallback
}
var stdNetPacketListener nettype.PacketListenerWithNetIP = nettype.MakePacketListenerWithNetIP(new(net.ListenConfig))
func (f *forwarder) packetListener(ip netip.Addr) (nettype.PacketListenerWithNetIP, 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.netMon, linkName); err != nil {
return nil, err
}
return nettype.MakePacketListenerWithNetIP(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
}
dialer := dnscache.Dialer(f.getDialerType(), &dnscache.Resolver{
SingleHost: dohURL.Hostname(),
SingleHostStaticResult: allIPs,
Logf: f.logf,
})
tlsConfig := &tls.Config{
// Enforce TLS 1.3, as all of our supported DNS-over-HTTPS servers are compatible with it
// (see tailscale.com/net/dns/publicdns/publicdns.go).
MinVersion: tls.VersionTLS13,
}
c = &http.Client{
Transport: &http.Transport{
ForceAttemptHTTP2: true,
IdleConnTimeout: dohIdleConnTimeout,
// On mobile platforms TCP KeepAlive is disabled in the dialer,
// ensure that we timeout if the connection appears to be hung.
ResponseHeaderTimeout: 10 * time.Second,
MaxIdleConnsPerHost: 1,
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)
},
TLSClientConfig: tlsConfig,
},
}
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) sendDoH(ctx context.Context, urlBase string, c *http.Client, packet []byte) ([]byte, error) {
ctx = sockstats.WithSockStats(ctx, sockstats.LabelDNSForwarderDoH, f.logf)
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)
req.Header.Set("Accept", dohType)
req.Header.Set("User-Agent", "tailscaled/"+version.Long())
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)
if hres.StatusCode/100 == 5 {
// Translate 5xx HTTP server errors into SERVFAIL DNS responses.
return nil, fmt.Errorf("%w: %s", errServerFailure, hres.Status)
}
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 := io.ReadAll(hres.Body)
if err != nil {
metricDNSFwdDoHErrorBody.Add(1)
}
if truncatedFlagSet(res) {
metricDNSFwdTruncated.Add(1)
}
return res, err
}
var (
verboseDNSForward = envknob.RegisterBool("TS_DEBUG_DNS_FORWARD_SEND")
skipTCPRetry = envknob.RegisterBool("TS_DNS_FORWARD_SKIP_TCP_RETRY")
// For correlating log messages in the send() function; only used when
// verboseDNSForward() is true.
forwarderCount atomic.Uint64
)
// 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() {
id := forwarderCount.Add(1)
domain, typ, _ := nameFromQuery(fq.packet)
f.logf("forwarder.send(%q, %d, %v, %d) [%d] ...", rr.name.Addr, fq.txid, typ, len(domain), id)
defer func() {
f.logf("forwarder.send(%q, %d, %v, %d) [%d] = %v, %v", rr.name.Addr, fq.txid, typ, len(domain), id, 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 custom 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")
}
ctx, cancel := context.WithCancel(ctx)
defer cancel()
isUDPQuery := fq.family == "udp"
skipTCP := skipTCPRetry() || (f.controlKnobs != nil && f.controlKnobs.DisableDNSForwarderTCPRetries.Load())
// Print logs about retries if this was because of a truncated response.
var explicitRetry atomic.Bool // true if truncated UDP response retried
defer func() {
if !explicitRetry.Load() {
return
}
if err == nil {
f.logf("forwarder.send(%q): successfully retried via TCP", rr.name.Addr)
} else {
f.logf("forwarder.send(%q): could not retry via TCP: %v", rr.name.Addr, err)
}
}()
firstUDP := func(ctx context.Context) ([]byte, error) {
resp, err := f.sendUDP(ctx, fq, rr)
if err != nil {
return nil, err
}
if !truncatedFlagSet(resp) {
// Successful, non-truncated response; no retry.
return resp, nil
}
// If this is a UDP query, return it regardless of whether the
// response is truncated or not; the client can retry
// communicating with tailscaled over TCP. There's no point
// falling back to TCP for a truncated query if we can't return
// the results to the client.
if isUDPQuery {
return resp, nil
}
if skipTCP {
// Envknob or control knob disabled the TCP retry behaviour;
// just return what we have.
return resp, nil
}
// This is a TCP query from the client, and the UDP response
// from the upstream DNS server is truncated; map this to an
// error to cause our retry helper to immediately kick off the
// TCP retry.
explicitRetry.Store(true)
return nil, truncatedResponseError{resp}
}
thenTCP := func(ctx context.Context) ([]byte, error) {
// If we're skipping the TCP fallback, then wait until the
// context is canceled and return that error (i.e. not
// returning anything).
if skipTCP {
<-ctx.Done()
return nil, ctx.Err()
}
return f.sendTCP(ctx, fq, rr)
}
// If the input query is TCP, then don't have a timeout between
// starting UDP and TCP.
timeout := udpRaceTimeout
if !isUDPQuery {
timeout = 0
}
// Kick off the race between the UDP and TCP queries.
rh := race.New(timeout, firstUDP, thenTCP)
resp, err := rh.Start(ctx)
if err == nil {
return resp, nil
}
// If we got a truncated UDP response, return that instead of an error.
var trErr truncatedResponseError
if errors.As(err, &trErr) {
return trErr.res, nil
}
return nil, err
}
type truncatedResponseError struct {
res []byte
}
func (tr truncatedResponseError) Error() string { return "response truncated" }
var errServerFailure = errors.New("response code indicates server issue")
var errTxIDMismatch = errors.New("txid doesn't match")
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)
ctx = sockstats.WithSockStats(ctx, sockstats.LabelDNSForwarderUDP, f.logf)
ln, err := f.packetListener(ipp.Addr())
if err != nil {
return nil, err
}
// Specify the exact UDP family to work around https://github.com/golang/go/issues/52264
udpFam := "udp4"
if ipp.Addr().Is6() {
udpFam = "udp6"
}
conn, err := ln.ListenPacket(ctx, udpFam, ":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.WriteToUDPAddrPort(fq.packet, ipp); 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.ReadFromUDPAddrPort(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, errTxIDMismatch
}
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, errServerFailure
}
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
}
func (f *forwarder) getDialerType() dnscache.DialContextFunc {
if f.controlKnobs != nil && f.controlKnobs.UserDialUseRoutes.Load() {
// It is safe to use UserDial as it dials external servers without going through Tailscale
// and closes connections on interface change in the same way as SystemDial does,
// thus preventing DNS resolution issues when switching between WiFi and cellular,
// but can also dial an internal DNS server on the Tailnet or via a subnet router.
//
// TODO(nickkhyl): Update tsdial.Dialer to reuse the bart.Table we create in net/tstun.Wrapper
// to avoid having two bart tables in memory, especially on iOS. Once that's done,
// we can get rid of the nodeAttr/control knob and always use UserDial for DNS.
//
// See https://github.com/tailscale/tailscale/issues/12027.
return f.dialer.UserDial
}
return f.dialer.SystemDial
}
func (f *forwarder) sendTCP(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)
}
metricDNSFwdTCP.Add(1)
ctx = sockstats.WithSockStats(ctx, sockstats.LabelDNSForwarderTCP, f.logf)
// Specify the exact family to work around https://github.com/golang/go/issues/52264
tcpFam := "tcp4"
if ipp.Addr().Is6() {
tcpFam = "tcp6"
}
ctx, cancel := context.WithTimeout(ctx, tcpQueryTimeout)
defer cancel()
conn, err := f.getDialerType()(ctx, tcpFam, ipp.String())
if err != nil {
return nil, err
}
defer conn.Close()
fq.closeOnCtxDone.Add(conn)
defer fq.closeOnCtxDone.Remove(conn)
ctxOrErr := func(err2 error) ([]byte, error) {
if err := ctx.Err(); err != nil {
return nil, err
}
return nil, err2
}
// Write the query to the server.
query := make([]byte, len(fq.packet)+2)
binary.BigEndian.PutUint16(query, uint16(len(fq.packet)))
copy(query[2:], fq.packet)
if _, err := conn.Write(query); err != nil {
metricDNSFwdTCPErrorWrite.Add(1)
return ctxOrErr(err)
}
metricDNSFwdTCPWrote.Add(1)
// Read the header length back from the server
var length uint16
if err := binary.Read(conn, binary.BigEndian, &length); err != nil {
metricDNSFwdTCPErrorRead.Add(1)
return ctxOrErr(err)
}
// Now read the response
out := make([]byte, length)
n, err := io.ReadFull(conn, out)
if err != nil {
metricDNSFwdTCPErrorRead.Add(1)
return ctxOrErr(err)
}
if n < int(length) {
f.logf("sendTCP: packet too small (%d bytes)", n)
return nil, io.ErrUnexpectedEOF
}
out = out[:n]
txid := getTxID(out)
if txid != fq.txid {
metricDNSFwdTCPErrorTxID.Add(1)
return nil, errTxIDMismatch
}
rcode := getRCode(out)
// don't forward transient errors back to the client when the server fails
if rcode == dns.RCodeServerFailure {
f.logf("sendTCP: response code indicating server failure: %d", rcode)
metricDNSFwdTCPErrorServer.Add(1)
return nil, errServerFailure
}
// TODO(andrew): do we need to do this?
//clampEDNSSize(out, maxResponseBytes)
metricDNSFwdTCPSuccess.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
cloudHostFallback := f.cloudHostFallback
f.mu.Unlock()
for _, route := range routes {
if route.Suffix == "." || route.Suffix.Contains(domain) {
return route.Resolvers
}
}
return cloudHostFallback // or nil if no fallback
}
// GetUpstreamResolvers returns the resolvers that would be used to resolve
// the given FQDN.
func (f *forwarder) GetUpstreamResolvers(name dnsname.FQDN) []*dnstype.Resolver {
resolvers := f.resolvers(name)
upstreamResolvers := make([]*dnstype.Resolver, 0, len(resolvers))
for _, r := range resolvers {
upstreamResolvers = append(upstreamResolvers, r.name)
}
return upstreamResolvers
}
// 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
family string // "tcp" or "udp"
// 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, typ, 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 {
return err
}
select {
case <-ctx.Done():
return fmt.Errorf("waiting to send NXDOMAIN: %w", ctx.Err())
case responseChan <- res:
return nil
}
}
if fl := fwdLogAtomic.Load(); fl != nil {
fl.addName(string(domain))
}
clampEDNSSize(query.bs, maxResponseBytes)
if len(resolvers) == 0 {
resolvers = f.resolvers(domain)
if len(resolvers) == 0 {
metricDNSFwdErrorNoUpstream.Add(1)
f.health.SetUnhealthy(dnsForwarderFailing, health.Args{health.ArgDNSServers: ""})
f.logf("no upstream resolvers set, returning SERVFAIL")
// Attempt to recompile the DNS configuration
// If we are being asked to forward queries and we have no
// nameservers, the network is in a bad state.
if f.missingUpstreamRecovery != nil {
f.missingUpstreamRecovery()
}
res, err := servfailResponse(query)
if err != nil {
return err
}
select {
case <-ctx.Done():
return fmt.Errorf("waiting to send SERVFAIL: %w", ctx.Err())
case responseChan <- res:
return nil
}
} else {
f.health.SetHealthy(dnsForwarderFailing)
}
}
fq := &forwardQuery{
txid: getTxID(query.bs),
packet: query.bs,
family: query.family,
closeOnCtxDone: new(closePool),
}
defer fq.closeOnCtxDone.Close()
if verboseDNSForward() {
domainSha256 := sha256.Sum256([]byte(domain))
domainSig := base64.RawStdEncoding.EncodeToString(domainSha256[:3])
f.logf("request(%d, %v, %d, %s) %d...", fq.txid, typ, len(domain), domainSig, len(fq.packet))
}
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 {
err = fmt.Errorf("resolving using %q: %w", rr.name.Addr, err)
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 fmt.Errorf("waiting to send response: %w", ctx.Err())
case responseChan <- packet{v, query.family, query.addr}:
if verboseDNSForward() {
f.logf("response(%d, %v, %d) = %d, nil", fq.txid, typ, len(domain), len(v))
}
metricDNSFwdSuccess.Add(1)
f.health.SetHealthy(dnsForwarderFailing)
return nil
}
case err := <-errc:
if firstErr == nil {
firstErr = err
}
numErr++
if numErr == len(resolvers) {
if errors.Is(firstErr, errServerFailure) {
res, err := servfailResponse(query)
if err != nil {
f.logf("building servfail response: %v", err)
return firstErr
}
select {
case <-ctx.Done():
metricDNSFwdErrorContext.Add(1)
metricDNSFwdErrorContextGotError.Add(1)
var resolverAddrs []string
for _, rr := range resolvers {
resolverAddrs = append(resolverAddrs, rr.name.Addr)
}
f.health.SetUnhealthy(dnsForwarderFailing, health.Args{health.ArgDNSServers: strings.Join(resolverAddrs, ",")})
case responseChan <- res:
if verboseDNSForward() {
f.logf("forwarder response(%d, %v, %d) = %d, %v", fq.txid, typ, len(domain), len(res.bs), firstErr)
}
return nil
}
}
return firstErr
}
case <-ctx.Done():
metricDNSFwdErrorContext.Add(1)
if firstErr != nil {
metricDNSFwdErrorContextGotError.Add(1)
return firstErr
}
// If we haven't got an error or a successful response,
// include all resolvers in the error message so we can
// at least see what what servers we're trying to
// query.
var resolverAddrs []string
for _, rr := range resolvers {
resolverAddrs = append(resolverAddrs, rr.name.Addr)
}
f.health.SetUnhealthy(dnsForwarderFailing, health.Args{health.ArgDNSServers: strings.Join(resolverAddrs, ",")})
return fmt.Errorf("waiting for response or error from %v: %w", resolverAddrs, ctx.Err())
}
}
}
var initListenConfig func(_ *net.ListenConfig, _ *netmon.Monitor, tunName string) error
// nameFromQuery extracts the normalized query name from bs.
func nameFromQuery(bs []byte) (dnsname.FQDN, dns.Type, error) {
var parser dns.Parser
hdr, err := parser.Start(bs)
if err != nil {
return "", 0, err
}
if hdr.Response {
return "", 0, errNotQuery
}
q, err := parser.Question()
if err != nil {
return "", 0, err
}
n := q.Name.Data[:q.Name.Length]
fqdn, err := dnsname.ToFQDN(rawNameToLower(n))
if err != nil {
return "", 0, err
}
return fqdn, q.Type, nil
}
// 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?
b.StartQuestions()
b.Question(p.Question)
res.bs, err = b.Finish()
res.addr = req.addr
return res, err
}
// servfailResponse returns a SERVFAIL error reply for the provided request.
func servfailResponse(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.Authoritative = true
h.RCode = dns.RCodeServerFailure
b := dns.NewBuilder(nil, h)
b.StartQuestions()
b.Question(p.Question)
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
}