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tailscale/net/dns/resolver/tsdns.go

1304 lines
38 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 (
"bufio"
"context"
"encoding/hex"
"errors"
"fmt"
"io"
"net"
"net/netip"
"os"
"runtime"
"sort"
"strconv"
"strings"
"sync"
"time"
dns "golang.org/x/net/dns/dnsmessage"
"tailscale.com/net/dns/resolvconffile"
"tailscale.com/net/netaddr"
"tailscale.com/net/tsaddr"
"tailscale.com/net/tsdial"
"tailscale.com/syncs"
"tailscale.com/types/dnstype"
"tailscale.com/types/logger"
"tailscale.com/util/clientmetric"
"tailscale.com/util/cloudenv"
"tailscale.com/util/dnsname"
"tailscale.com/util/strs"
"tailscale.com/wgengine/monitor"
)
const dnsSymbolicFQDN = "magicdns.localhost-tailscale-daemon."
// 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
// defaultTTL is the TTL of all responses from Resolver.
const defaultTTL = 600 * time.Second
var (
errNotQuery = errors.New("not a DNS query")
errNotOurName = errors.New("not a Tailscale DNS name")
)
type packet struct {
bs []byte
addr netip.AddrPort // 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][]*dnstype.Resolver
// LocalHosts is a map of FQDNs to corresponding IPs.
Hosts map[dnsname.FQDN][]netip.Addr
// LocalDomains is a list of DNS name suffixes that should not be
// routed to upstream resolvers.
LocalDomains []dnsname.FQDN
}
// WriteToBufioWriter write a debug version of c for logs to w, omitting
// spammy stuff like *.arpa entries and replacing it with a total count.
func (c *Config) WriteToBufioWriter(w *bufio.Writer) {
w.WriteString("{Routes:")
WriteRoutes(w, c.Routes)
fmt.Fprintf(w, " Hosts:%v LocalDomains:[", len(c.Hosts))
space := false
arpa := 0
for _, d := range c.LocalDomains {
if strings.HasSuffix(string(d), ".arpa.") {
arpa++
continue
}
if space {
w.WriteByte(' ')
}
w.WriteString(string(d))
space = true
}
w.WriteString("]")
if arpa > 0 {
fmt.Fprintf(w, "+%darpa", arpa)
}
if c := cloudenv.Get(); c != "" {
fmt.Fprintf(w, ", cloud=%q", string(c))
}
w.WriteString("}")
}
// WriteIPPorts writes vv to w.
func WriteIPPorts(w *bufio.Writer, vv []netip.AddrPort) {
w.WriteByte('[')
var b []byte
for i, v := range vv {
if i > 0 {
w.WriteByte(' ')
}
b = v.AppendTo(b[:0])
w.Write(b)
}
w.WriteByte(']')
}
// WriteDNSResolver writes r to w.
func WriteDNSResolver(w *bufio.Writer, r *dnstype.Resolver) {
io.WriteString(w, r.Addr)
if len(r.BootstrapResolution) > 0 {
w.WriteByte('(')
var b []byte
for _, ip := range r.BootstrapResolution {
ip.AppendTo(b[:0])
w.Write(b)
}
w.WriteByte(')')
}
}
// WriteDNSResolvers writes resolvers to w.
func WriteDNSResolvers(w *bufio.Writer, resolvers []*dnstype.Resolver) {
w.WriteByte('[')
for i, r := range resolvers {
if i > 0 {
w.WriteByte(' ')
}
WriteDNSResolver(w, r)
}
w.WriteByte(']')
}
// WriteRoutes writes routes to w, omitting *.arpa routes and instead
// summarizing how many of them there were.
func WriteRoutes(w *bufio.Writer, routes map[dnsname.FQDN][]*dnstype.Resolver) {
var kk []dnsname.FQDN
arpa := 0
for k := range routes {
if strings.HasSuffix(string(k), ".arpa.") {
arpa++
continue
}
kk = append(kk, k)
}
sort.Slice(kk, func(i, j int) bool { return kk[i] < kk[j] })
w.WriteByte('{')
for i, k := range kk {
if i > 0 {
w.WriteByte(' ')
}
w.WriteString(string(k))
w.WriteByte(':')
WriteDNSResolvers(w, routes[k])
}
w.WriteByte('}')
if arpa > 0 {
fmt.Fprintf(w, "+%darpa", arpa)
}
}
// 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
dialer *tsdial.Dialer // non-nil
saveConfigForTests func(cfg Config) // used in tests to capture resolver config
// forwarder forwards requests to upstream nameservers.
forwarder *forwarder
// 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][]netip.Addr
ipToHost map[netip.Addr]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(netip.Addr) (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, dialer *tsdial.Dialer) *Resolver {
if dialer == nil {
panic("nil Dialer")
}
r := &Resolver{
logf: logger.WithPrefix(logf, "resolver: "),
linkMon: linkMon,
closed: make(chan struct{}),
hostToIP: map[dnsname.FQDN][]netip.Addr{},
ipToHost: map[netip.Addr]dnsname.FQDN{},
dialer: dialer,
}
r.forwarder = newForwarder(r.logf, linkMon, linkSel, dialer)
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)
}
reverse := make(map[netip.Addr]dnsname.FQDN, len(cfg.Hosts))
for host, ips := range cfg.Hosts {
for _, ip := range ips {
reverse[ip] = host
}
}
r.forwarder.setRoutes(cfg.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()
}
// dnsQueryTimeout is not intended to be user-visible (the users
// DNS resolver will retry well before that), just put an upper
// bound on per-query resource usage.
const dnsQueryTimeout = 10 * time.Second
func (r *Resolver) Query(ctx context.Context, bs []byte, from netip.AddrPort) ([]byte, error) {
metricDNSQueryLocal.Add(1)
select {
case <-r.closed:
metricDNSQueryErrorClosed.Add(1)
return nil, net.ErrClosed
default:
}
out, err := r.respond(bs)
if err == errNotOurName {
responses := make(chan packet, 1)
ctx, cancel := context.WithTimeout(ctx, dnsQueryTimeout)
defer close(responses)
defer cancel()
err = r.forwarder.forwardWithDestChan(ctx, packet{bs, from}, responses)
if err != nil {
select {
// Best effort: use any error response sent by forwardWithDestChan.
// This is present in some errors paths, such as when all upstream
// DNS servers replied with an error.
case resp := <-responses:
return resp.bs, err
default:
return nil, err
}
}
return (<-responses).bs, nil
}
return out, err
}
// parseExitNodeQuery parses a DNS request packet.
// It returns nil if it's malformed or lacking a question.
func parseExitNodeQuery(q []byte) *response {
p := dnsParserPool.Get().(*dnsParser)
defer dnsParserPool.Put(p)
p.zeroParser()
defer p.zeroParser()
if err := p.parseQuery(q); err != nil {
return nil
}
return p.response()
}
// HandleExitNodeDNSQuery handles a DNS query that arrived from a peer
// via the peerapi's DoH server. This is only used when the local
// node is being an exit node.
//
// The provided allowName callback is whether a DNS query for a name
// (as found by parsing q) is allowed.
//
// In most (all?) cases, err will be nil. A bogus DNS query q will
// still result in a response DNS packet (saying there's a failure)
// and a nil error.
// TODO: figure out if we even need an error result.
func (r *Resolver) HandleExitNodeDNSQuery(ctx context.Context, q []byte, from netip.AddrPort, allowName func(name string) bool) (res []byte, err error) {
metricDNSExitProxyQuery.Add(1)
ch := make(chan packet, 1)
resp := parseExitNodeQuery(q)
if resp == nil {
return nil, errors.New("bad query")
}
name := resp.Question.Name.String()
if !allowName(name) {
metricDNSExitProxyErrorName.Add(1)
resp.Header.RCode = dns.RCodeRefused
return marshalResponse(resp)
}
switch runtime.GOOS {
default:
return nil, errors.New("unsupported exit node OS")
case "windows", "android":
return handleExitNodeDNSQueryWithNetPkg(ctx, nil, resp)
case "darwin":
// /etc/resolv.conf is a lie and only says one upstream DNS
// but for now that's probably good enough. Later we'll
// want to blend in everything from scutil --dns.
fallthrough
case "linux", "freebsd", "openbsd", "illumos":
nameserver, err := stubResolverForOS()
if err != nil {
r.logf("stubResolverForOS: %v", err)
metricDNSExitProxyErrorResolvConf.Add(1)
return nil, err
}
// TODO: more than 1 resolver from /etc/resolv.conf?
var resolvers []resolverAndDelay
if nameserver == tsaddr.TailscaleServiceIP() || nameserver == tsaddr.TailscaleServiceIPv6() {
// If resolv.conf says 100.100.100.100, it's coming right back to us anyway
// so avoid the loop through the kernel and just do what we
// would've done anyway. By not passing any resolvers, the forwarder
// will use its default ones from our DNS config.
} else {
resolvers = []resolverAndDelay{{
name: &dnstype.Resolver{Addr: net.JoinHostPort(nameserver.String(), "53")},
}}
}
err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch, resolvers...)
if err != nil {
metricDNSExitProxyErrorForward.Add(1)
return nil, err
}
}
select {
case p, ok := <-ch:
if ok {
return p.bs, nil
}
panic("unexpected close chan")
default:
panic("unexpected unreadable chan")
}
}
// handleExitNodeDNSQueryWithNetPkg takes a DNS query message in q and
// return a reply (for the ExitDNS DoH service) using the net package's
// native APIs. This is only used on Windows for now.
//
// If resolver is nil, the net.Resolver zero value is used.
//
// response contains the pre-serialized response, which notably
// includes the original question and its header.
func handleExitNodeDNSQueryWithNetPkg(ctx context.Context, resolver *net.Resolver, resp *response) (res []byte, err error) {
if resp.Question.Class != dns.ClassINET {
return nil, errors.New("unsupported class")
}
r := resolver
if r == nil {
r = new(net.Resolver)
}
name := resp.Question.Name.String()
handleError := func(err error) (res []byte, _ error) {
if isGoNoSuchHostError(err) {
resp.Header.RCode = dns.RCodeNameError
return marshalResponse(resp)
}
// TODO: map other errors to RCodeServerFailure?
// Or I guess our caller should do that?
return nil, err
}
resp.Header.RCode = dns.RCodeSuccess // unless changed below
switch resp.Question.Type {
case dns.TypeA, dns.TypeAAAA:
network := "ip4"
if resp.Question.Type == dns.TypeAAAA {
network = "ip6"
}
ips, err := r.LookupIP(ctx, network, name)
if err != nil {
return handleError(err)
}
for _, stdIP := range ips {
if ip, ok := netip.AddrFromSlice(stdIP); ok {
resp.IPs = append(resp.IPs, ip.Unmap())
}
}
case dns.TypeTXT:
strs, err := r.LookupTXT(ctx, name)
if err != nil {
return handleError(err)
}
resp.TXT = strs
case dns.TypePTR:
ipStr, ok := unARPA(name)
if !ok {
// TODO: is this RCodeFormatError?
return nil, errors.New("bogus PTR name")
}
addrs, err := r.LookupAddr(ctx, ipStr)
if err != nil {
return handleError(err)
}
if len(addrs) > 0 {
resp.Name, _ = dnsname.ToFQDN(addrs[0])
}
case dns.TypeCNAME:
cname, err := r.LookupCNAME(ctx, name)
if err != nil {
return handleError(err)
}
resp.CNAME = cname
case dns.TypeSRV:
// Thanks, Go: "To accommodate services publishing SRV
// records under non-standard names, if both service
// and proto are empty strings, LookupSRV looks up
// name directly."
_, srvs, err := r.LookupSRV(ctx, "", "", name)
if err != nil {
return handleError(err)
}
resp.SRVs = srvs
case dns.TypeNS:
nss, err := r.LookupNS(ctx, name)
if err != nil {
return handleError(err)
}
resp.NSs = nss
default:
return nil, fmt.Errorf("unsupported record type %v", resp.Question.Type)
}
return marshalResponse(resp)
}
func isGoNoSuchHostError(err error) bool {
if de, ok := err.(*net.DNSError); ok {
return de.IsNotFound
}
return false
}
type resolvConfCache struct {
mod time.Time
size int64
ip netip.Addr
// TODO: inode/dev?
}
// resolvConfCacheValue contains the most recent stat metadata and parsed
// version of /etc/resolv.conf.
var resolvConfCacheValue syncs.AtomicValue[resolvConfCache]
var errEmptyResolvConf = errors.New("resolv.conf has no nameservers")
// stubResolverForOS returns the IP address of the first nameserver in
// /etc/resolv.conf.
func stubResolverForOS() (ip netip.Addr, err error) {
fi, err := os.Stat(resolvconffile.Path)
if err != nil {
return netip.Addr{}, err
}
cur := resolvConfCache{
mod: fi.ModTime(),
size: fi.Size(),
}
if c, ok := resolvConfCacheValue.LoadOk(); ok && c.mod == cur.mod && c.size == cur.size {
return c.ip, nil
}
conf, err := resolvconffile.ParseFile(resolvconffile.Path)
if err != nil {
return netip.Addr{}, err
}
if len(conf.Nameservers) == 0 {
return netip.Addr{}, errEmptyResolvConf
}
ip = conf.Nameservers[0]
cur.ip = ip
resolvConfCacheValue.Store(cur)
return ip, nil
}
// 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) (netip.Addr, dns.RCode) {
metricDNSResolveLocal.Add(1)
// Reject .onion domains per RFC 7686.
if dnsname.HasSuffix(domain.WithoutTrailingDot(), ".onion") {
metricDNSResolveLocalErrorOnion.Add(1)
return netip.Addr{}, dns.RCodeNameError
}
// We return a symbolic domain if someone does a reverse lookup on the
// DNS endpoint. To round out this special case, we also do the inverse
// (returning the endpoint IP if someone looks up the symbolic domain).
if domain == dnsSymbolicFQDN {
switch typ {
case dns.TypeA:
return tsaddr.TailscaleServiceIP(), dns.RCodeSuccess
case dns.TypeAAAA:
return tsaddr.TailscaleServiceIPv6(), dns.RCodeSuccess
}
}
// Special-case: 4via6 DNS names.
if ip, ok := r.parseViaDomain(domain, typ); ok {
return ip, dns.RCodeSuccess
}
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.
metricDNSResolveLocalErrorMissing.Add(1)
return netip.Addr{}, dns.RCodeNameError
}
}
// Not authoritative, signal that forwarding is advisable.
return netip.Addr{}, 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() {
metricDNSResolveLocalOKA.Add(1)
return ip, dns.RCodeSuccess
}
}
metricDNSResolveLocalNoA.Add(1)
return netip.Addr{}, dns.RCodeSuccess
case dns.TypeAAAA:
for _, ip := range addrs {
if ip.Is6() {
metricDNSResolveLocalOKAAAA.Add(1)
return ip, dns.RCodeSuccess
}
}
metricDNSResolveLocalNoAAAA.Add(1)
return netip.Addr{}, 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 {
metricDNSResolveLocalNoAll.Add(1)
return netip.Addr{}, dns.RCodeSuccess
}
metricDNSResolveLocalOKAll.Add(1)
return addrs[0], dns.RCodeSuccess
// Leave 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:
metricDNSResolveNotImplType.Add(1)
return netip.Addr{}, dns.RCodeNotImplemented
// For everything except for the few types above that are explicitly 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:
metricDNSResolveNoRecordType.Add(1)
// The name exists, but no records exist of the requested type.
return netip.Addr{}, dns.RCodeSuccess
}
}
// parseViaDomain synthesizes an IP address for quad-A DNS requests of the form
// `<IPv4-address-with-hypens-instead-of-dots>-via-<siteid>[.*]`. Two prior formats that
// didn't pan out (due to a Chrome issue and DNS search ndots issues) were
// `<IPv4-address>.via-<X>` and the older `via-<X>.<IPv4-address>`,
// where X is a decimal, or hex-encoded number with a '0x' prefix.
//
// This exists as a convenient mapping into Tailscales 'Via Range'.
//
// TODO(maisem/bradfitz/tom): `<IPv4-address>.via-<X>` was introduced
// (2022-06-02) to work around an issue in Chrome where it would treat
// "http://via-1.1.2.3.4" as a search string instead of a URL. We should rip out
// the old format in early 2023.
func (r *Resolver) parseViaDomain(domain dnsname.FQDN, typ dns.Type) (netip.Addr, bool) {
fqdn := string(domain.WithoutTrailingDot())
if typ != dns.TypeAAAA {
return netip.Addr{}, false
}
if len(fqdn) < len("via-X.0.0.0.0") {
return netip.Addr{}, false // too short to be valid
}
var siteID string
var ip4Str string
switch {
case strings.Contains(fqdn, "-via-"):
// Format number 3: "192-168-1-2-via-7" or "192-168-1-2-via-7.foo.ts.net."
// Third time's a charm. The earlier two formats follow after this block.
firstLabel, domain, _ := strings.Cut(fqdn, ".") // "192-168-1-2-via-7"
if !(domain == "" || dnsname.HasSuffix(domain, "ts.net") || dnsname.HasSuffix(domain, "tailscale.net")) {
return netip.Addr{}, false
}
v4hyphens, suffix, ok := strings.Cut(firstLabel, "-via-")
if !ok {
return netip.Addr{}, false
}
siteID = suffix
ip4Str = strings.ReplaceAll(v4hyphens, "-", ".")
case strings.HasPrefix(fqdn, "via-"):
firstDot := strings.Index(fqdn, ".")
if firstDot < 0 {
return netip.Addr{}, false // missing dot delimiters
}
siteID = fqdn[len("via-"):firstDot]
ip4Str = fqdn[firstDot+1:]
default:
lastDot := strings.LastIndex(fqdn, ".")
if lastDot < 0 {
return netip.Addr{}, false // missing dot delimiters
}
suffix := fqdn[lastDot+1:]
if !strings.HasPrefix(suffix, "via-") {
return netip.Addr{}, false
}
siteID = suffix[len("via-"):]
ip4Str = fqdn[:lastDot]
}
ip4, err := netip.ParseAddr(ip4Str)
if err != nil {
return netip.Addr{}, false // badly formed, don't respond
}
prefix, err := strconv.ParseUint(siteID, 0, 32)
if err != nil {
return netip.Addr{}, false // badly formed, don't respond
}
// MapVia will never error when given an ipv4 netip.Prefix.
out, _ := tsaddr.MapVia(uint32(prefix), netip.PrefixFrom(ip4, ip4.BitLen()))
return out.Addr(), true
}
// resolveReverse returns the unique domain name that maps to the given address.
func (r *Resolver) resolveLocalReverse(name dnsname.FQDN) (dnsname.FQDN, dns.RCode) {
var ip netip.Addr
var ok bool
switch {
case strings.HasSuffix(name.WithTrailingDot(), rdnsv4Suffix):
ip, ok = rdnsNameToIPv4(name)
case strings.HasSuffix(name.WithTrailingDot(), rdnsv6Suffix):
ip, ok = rdnsNameToIPv6(name)
}
if !ok {
// This isn't a well-formed in-addr.arpa or ip6.arpa name, but
// who knows what upstreams might do, try kicking it up to
// them. We definitely won't handle it.
return "", dns.RCodeRefused
}
r.mu.Lock()
defer r.mu.Unlock()
// If the requested IP is part of the IPv6 4-to-6 range, it might
// correspond to an IPv4 address (assuming IPv4 is enabled).
if ip4, ok := tsaddr.Tailscale6to4(ip); ok {
fqdn, code := r.fqdnForIPLocked(ip4, name)
if code == dns.RCodeSuccess {
return fqdn, code
}
}
return r.fqdnForIPLocked(ip, name)
}
// r.mu must be held.
func (r *Resolver) fqdnForIPLocked(ip netip.Addr, name dnsname.FQDN) (dnsname.FQDN, dns.RCode) {
// If someone curiously does a reverse lookup on the DNS IP, we
// return a domain that helps indicate that Tailscale is using
// this IP for a special purpose and it is not a node on their
// tailnet.
if ip == tsaddr.TailscaleServiceIP() || ip == tsaddr.TailscaleServiceIPv6() {
return dnsSymbolicFQDN, dns.RCodeSuccess
}
ret, ok := r.ipToHost[ip]
if !ok {
for _, suffix := range r.localDomains {
if suffix.Contains(name) {
// We are authoritative for this chunk of IP space.
return "", dns.RCodeNameError
}
}
// Not authoritative, signal that forwarding is advisable.
return "", dns.RCodeRefused
}
return ret, dns.RCodeSuccess
}
type response struct {
Header dns.Header
Question dns.Question
// Name is the response to a PTR query.
Name dnsname.FQDN
// IP and IPs are the responses to an A, AAAA, or ALL query.
// Either/both/neither can be populated.
IP netip.Addr
IPs []netip.Addr
// TXT is the response to a TXT query.
// Each one is its own RR with one string.
TXT []string
// CNAME is the response to a CNAME query.
CNAME string
// SRVs are the responses to a SRV query.
SRVs []*net.SRV
// NSs are the responses to an NS query.
NSs []*net.NS
}
var dnsParserPool = &sync.Pool{
New: func() any {
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()
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 netip.Addr, 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 netip.Addr, 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)
}
func marshalIP(name dns.Name, ip netip.Addr, builder *dns.Builder) error {
if ip.Is4() {
return marshalARecord(name, ip, builder)
}
if ip.Is6() {
return marshalAAAARecord(name, ip, builder)
}
return nil
}
// 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)
}
func marshalTXT(queryName dns.Name, txts []string, builder *dns.Builder) error {
for _, txt := range txts {
if err := builder.TXTResource(dns.ResourceHeader{
Name: queryName,
Type: dns.TypeTXT,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}, dns.TXTResource{
TXT: []string{txt},
}); err != nil {
return err
}
}
return nil
}
func marshalCNAME(queryName dns.Name, cname string, builder *dns.Builder) error {
if cname == "" {
return nil
}
name, err := dns.NewName(cname)
if err != nil {
return err
}
return builder.CNAMEResource(dns.ResourceHeader{
Name: queryName,
Type: dns.TypeCNAME,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}, dns.CNAMEResource{
CNAME: name,
})
}
func marshalNS(queryName dns.Name, nss []*net.NS, builder *dns.Builder) error {
for _, ns := range nss {
name, err := dns.NewName(ns.Host)
if err != nil {
return err
}
err = builder.NSResource(dns.ResourceHeader{
Name: queryName,
Type: dns.TypeNS,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}, dns.NSResource{NS: name})
if err != nil {
return err
}
}
return nil
}
func marshalSRV(queryName dns.Name, srvs []*net.SRV, builder *dns.Builder) error {
for _, s := range srvs {
srvName, err := dns.NewName(s.Target)
if err != nil {
return err
}
err = builder.SRVResource(dns.ResourceHeader{
Name: queryName,
Type: dns.TypeSRV,
Class: dns.ClassINET,
TTL: uint32(defaultTTL / time.Second),
}, dns.SRVResource{
Target: srvName,
Priority: s.Priority,
Port: s.Port,
Weight: s.Weight,
})
if err != nil {
return err
}
}
return nil
}
// 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)
// TODO(bradfitz): I'm not sure why this wasn't enabled
// before, but for now (2021-12-09) enable it at least when
// there's more than 1 record (which was never the case
// before), where it really helps.
if len(resp.IPs) > 1 {
builder.EnableCompression()
}
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 err := marshalIP(resp.Question.Name, resp.IP, &builder); err != nil {
return nil, err
}
for _, ip := range resp.IPs {
if err := marshalIP(resp.Question.Name, ip, &builder); err != nil {
return nil, err
}
}
case dns.TypePTR:
err = marshalPTRRecord(resp.Question.Name, resp.Name, &builder)
case dns.TypeTXT:
err = marshalTXT(resp.Question.Name, resp.TXT, &builder)
case dns.TypeCNAME:
err = marshalCNAME(resp.Question.Name, resp.CNAME, &builder)
case dns.TypeSRV:
err = marshalSRV(resp.Question.Name, resp.SRVs, &builder)
case dns.TypeNS:
err = marshalNS(resp.Question.Name, resp.NSs, &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 {
s := name.WithTrailingDot()
base, rest, ok := strings.Cut(s, ".")
if !ok {
return false // shouldn't happen
}
switch base {
case "b", "db", "r", "dr", "lb":
default:
return false
}
return strings.HasPrefix(rest, "_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 netip.Addr, ok bool) {
s := strings.TrimSuffix(name.WithTrailingDot(), rdnsv4Suffix)
ip, err := netip.ParseAddr(s)
if err != nil {
return netip.Addr{}, false
}
if !ip.Is4() {
return netip.Addr{}, 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 netip.Addr, 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 netip.Addr{}, 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 netip.Addr{}, 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 netip.Addr{}, false
}
return netip.AddrFrom16(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) {
metricDNSReverseMissBonjour.Add(1)
return nil, errNotOurName
}
resp.Name, resp.Header.RCode = r.resolveLocalReverse(name)
if resp.Header.RCode == dns.RCodeRefused {
metricDNSReverseMissOther.Add(1)
return nil, errNotOurName
}
metricDNSMagicDNSSuccessReverse.Add(1)
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) {
metricDNSErrorParseNoQ.Add(1)
r.logf("parseQuery(%02x): no DNS questions", query)
} else {
metricDNSErrorParseQuery.Add(1)
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 {
metricDNSErrorNotFQDN.Add(1)
// 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)
}
// unARPA maps from "4.4.8.8.in-addr.arpa." to "8.8.4.4", etc.
func unARPA(a string) (ipStr string, ok bool) {
const suf4 = ".in-addr.arpa."
if s, ok := strs.CutSuffix(a, suf4); ok {
// Parse and reverse octets.
ip, err := netip.ParseAddr(s)
if err != nil || !ip.Is4() {
return "", false
}
a4 := ip.As4()
return netaddr.IPv4(a4[3], a4[2], a4[1], a4[0]).String(), true
}
const suf6 = ".ip6.arpa."
if len(a) == len("e.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.b.0.8.0.a.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa.") &&
strings.HasSuffix(a, suf6) {
var hx [32]byte
var a16 [16]byte
for i := range hx {
hx[31-i] = a[i*2]
if a[i*2+1] != '.' {
return "", false
}
}
hex.Decode(a16[:], hx[:])
return netip.AddrFrom16(a16).Unmap().String(), true
}
return "", false
}
var (
metricDNSQueryLocal = clientmetric.NewCounter("dns_query_local")
metricDNSQueryErrorClosed = clientmetric.NewCounter("dns_query_local_error_closed")
metricDNSErrorParseNoQ = clientmetric.NewCounter("dns_query_respond_error_no_question")
metricDNSErrorParseQuery = clientmetric.NewCounter("dns_query_respond_error_parse")
metricDNSErrorNotFQDN = clientmetric.NewCounter("dns_query_respond_error_not_fqdn")
metricDNSMagicDNSSuccessName = clientmetric.NewCounter("dns_query_magic_success_name")
metricDNSMagicDNSSuccessReverse = clientmetric.NewCounter("dns_query_magic_success_reverse")
metricDNSExitProxyQuery = clientmetric.NewCounter("dns_exit_node_query")
metricDNSExitProxyErrorName = clientmetric.NewCounter("dns_exit_node_error_name")
metricDNSExitProxyErrorForward = clientmetric.NewCounter("dns_exit_node_error_forward")
metricDNSExitProxyErrorResolvConf = clientmetric.NewCounter("dns_exit_node_error_resolvconf")
metricDNSFwd = clientmetric.NewCounter("dns_query_fwd")
metricDNSFwdDropBonjour = clientmetric.NewCounter("dns_query_fwd_drop_bonjour")
metricDNSFwdErrorName = clientmetric.NewCounter("dns_query_fwd_error_name")
metricDNSFwdErrorNoUpstream = clientmetric.NewCounter("dns_query_fwd_error_no_upstream")
metricDNSFwdSuccess = clientmetric.NewCounter("dns_query_fwd_success")
metricDNSFwdErrorContext = clientmetric.NewCounter("dns_query_fwd_error_context")
metricDNSFwdErrorContextGotError = clientmetric.NewCounter("dns_query_fwd_error_context_got_error")
metricDNSFwdErrorType = clientmetric.NewCounter("dns_query_fwd_error_type")
metricDNSFwdErrorParseAddr = clientmetric.NewCounter("dns_query_fwd_error_parse_addr")
metricDNSFwdTruncated = clientmetric.NewCounter("dns_query_fwd_truncated")
metricDNSFwdUDP = clientmetric.NewCounter("dns_query_fwd_udp") // on entry
metricDNSFwdUDPWrote = clientmetric.NewCounter("dns_query_fwd_udp_wrote") // sent UDP packet
metricDNSFwdUDPErrorWrite = clientmetric.NewCounter("dns_query_fwd_udp_error_write")
metricDNSFwdUDPErrorServer = clientmetric.NewCounter("dns_query_fwd_udp_error_server")
metricDNSFwdUDPErrorTxID = clientmetric.NewCounter("dns_query_fwd_udp_error_txid")
metricDNSFwdUDPErrorRead = clientmetric.NewCounter("dns_query_fwd_udp_error_read")
metricDNSFwdUDPSuccess = clientmetric.NewCounter("dns_query_fwd_udp_success")
metricDNSFwdDoH = clientmetric.NewCounter("dns_query_fwd_doh")
metricDNSFwdDoHErrorStatus = clientmetric.NewCounter("dns_query_fwd_doh_error_status")
metricDNSFwdDoHErrorCT = clientmetric.NewCounter("dns_query_fwd_doh_error_content_type")
metricDNSFwdDoHErrorTransport = clientmetric.NewCounter("dns_query_fwd_doh_error_transport")
metricDNSFwdDoHErrorBody = clientmetric.NewCounter("dns_query_fwd_doh_error_body")
metricDNSResolveLocal = clientmetric.NewCounter("dns_resolve_local")
metricDNSResolveLocalErrorOnion = clientmetric.NewCounter("dns_resolve_local_error_onion")
metricDNSResolveLocalErrorMissing = clientmetric.NewCounter("dns_resolve_local_error_missing")
metricDNSResolveLocalErrorRefused = clientmetric.NewCounter("dns_resolve_local_error_refused")
metricDNSResolveLocalOKA = clientmetric.NewCounter("dns_resolve_local_ok_a")
metricDNSResolveLocalOKAAAA = clientmetric.NewCounter("dns_resolve_local_ok_aaaa")
metricDNSResolveLocalOKAll = clientmetric.NewCounter("dns_resolve_local_ok_all")
metricDNSResolveLocalNoA = clientmetric.NewCounter("dns_resolve_local_no_a")
metricDNSResolveLocalNoAAAA = clientmetric.NewCounter("dns_resolve_local_no_aaaa")
metricDNSResolveLocalNoAll = clientmetric.NewCounter("dns_resolve_local_no_all")
metricDNSResolveNotImplType = clientmetric.NewCounter("dns_resolve_local_not_impl_type")
metricDNSResolveNoRecordType = clientmetric.NewCounter("dns_resolve_local_no_record_type")
metricDNSReverseMissBonjour = clientmetric.NewCounter("dns_reverse_miss_bonjour")
metricDNSReverseMissOther = clientmetric.NewCounter("dns_reverse_miss_other")
)