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tailcfg, ipn/ipnlocal, net/dns: forward exit node DNS on Unix to system DNS

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Updates #1713

Change-Id: I4c073fec0992d9e01a9a4ce97087d5af0efdc68d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick
2021-11-29 14:18:09 -08:00
committed by Brad Fitzpatrick
parent d9c21936c3
commit 135580a5a8
7 changed files with 251 additions and 57 deletions
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17
net/dns/resolver/forwarder.go
View File

@@ -580,9 +580,9 @@ func (f *forwarder) forward(query packet) error {
// It either sends to responseChan and returns nil, or returns a
// non-nil error (without sending to the channel).
//
// If backupResolvers are specified, they're used in the case that no
// upstreams are available.
func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, responseChan chan<- packet, backupResolvers ...resolverAndDelay) error {
// 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 {
@@ -601,13 +601,12 @@ func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, respo
clampEDNSSize(query.bs, maxResponseBytes)
resolvers := f.resolvers(domain)
if len(resolvers) == 0 {
resolvers = backupResolvers
}
if len(resolvers) == 0 {
metricDNSFwdErrorNoUpstream.Add(1)
return errNoUpstreams
resolvers = f.resolvers(domain)
if len(resolvers) == 0 {
metricDNSFwdErrorNoUpstream.Add(1)
return errNoUpstreams
}
}
fq := &forwardQuery{

171
net/dns/resolver/tsdns.go
View File

@@ -8,11 +8,14 @@ package resolver
import (
"bufio"
"bytes"
"context"
"encoding/hex"
"errors"
"fmt"
"io"
"net"
"os"
"runtime"
"sort"
"strings"
@@ -20,12 +23,15 @@ import (
"sync/atomic"
"time"
"go4.org/mem"
dns "golang.org/x/net/dns/dnsmessage"
"inet.af/netaddr"
"tailscale.com/net/tsaddr"
"tailscale.com/types/dnstype"
"tailscale.com/types/logger"
"tailscale.com/util/clientmetric"
"tailscale.com/util/dnsname"
"tailscale.com/util/lineread"
"tailscale.com/wgengine/monitor"
)
@@ -303,48 +309,83 @@ func (r *Resolver) NextResponse() (packet []byte, to netaddr.IPPort, err error)
}
}
// 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.
func (r *Resolver) HandleExitNodeDNSQuery(ctx context.Context, q []byte, from netaddr.IPPort) (res []byte, err error) {
metricDNSQueryForPeer.Add(1)
//
// 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 netaddr.IPPort, allowName func(name string) bool) (res []byte, err error) {
metricDNSExitProxyQuery.Add(1)
ch := make(chan packet, 1)
err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch)
if err == errNoUpstreams {
// Handle to the system resolver.
switch runtime.GOOS {
case "linux":
// Assume for now that we don't have an upstream because
// they're using systemd-resolved and we're in Split DNS mode
// where we don't know the base config.
//
// TODO(bradfitz): this is a lazy assumption. Do better, and
// maybe move the HandleExitNodeDNSQuery method to the dns.Manager
// instead? But this works for now.
err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch, resolverAndDelay{
name: dnstype.Resolver{
Addr: "127.0.0.1:53",
},
})
default:
// TODO(bradfitz): if we're on an exit node
// on, say, Windows, we need to parse the DNS
// packet in q and call OS-native APIs for
// each question. But we'll want to strip out
// questions for MagicDNS names probably, so
// they don't loop back into
// 100.100.100.100. We don't want to resolve
// MagicDNS names across Tailnets once we
// permit sharing exit nodes.
//
// For now, just return an error.
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":
// TODO: use DnsQueryEx and write to ch.
// See https://docs.microsoft.com/en-us/windows/win32/api/windns/nf-windns-dnsqueryex.
return nil, errors.New("TODO: windows exit node suport")
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() {
// 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
}
}
if err != nil {
metricDNSQueryForPeerError.Add(1)
return nil, err
}
select {
case p, ok := <-ch:
@@ -357,6 +398,59 @@ func (r *Resolver) HandleExitNodeDNSQuery(ctx context.Context, q []byte, from ne
}
}
type resolvConfCache struct {
mod time.Time
size int64
ip netaddr.IP
// TODO: inode/dev?
}
// resolvConfCacheValue contains the most recent stat metadata and parsed
// version of /etc/resolv.conf.
var resolvConfCacheValue atomic.Value // of 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 netaddr.IP, err error) {
fi, err := os.Stat("/etc/resolv.conf")
if err != nil {
return netaddr.IP{}, err
}
cur := resolvConfCache{
mod: fi.ModTime(),
size: fi.Size(),
}
if c, ok := resolvConfCacheValue.Load().(resolvConfCache); ok && c.mod == cur.mod && c.size == cur.size {
return c.ip, nil
}
err = lineread.File("/etc/resolv.conf", func(line []byte) error {
if !ip.IsZero() {
return nil
}
line = bytes.TrimSpace(line)
if len(line) == 0 || line[0] == '#' {
return nil
}
if mem.HasPrefix(mem.B(line), mem.S("nameserver ")) {
s := strings.TrimSpace(strings.TrimPrefix(string(line), "nameserver "))
ip, err = netaddr.ParseIP(s)
return err
}
return nil
})
if err != nil {
return netaddr.IP{}, err
}
if !ip.IsValid() {
return netaddr.IP{}, errEmptyResolvConf
}
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).
@@ -538,6 +632,7 @@ func (p *dnsParser) zeroParser() { p.parser = dns.Parser{} }
// 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 {
@@ -837,8 +932,10 @@ var (
metricDNSMagicDNSSuccessName = clientmetric.NewCounter("dns_query_magic_success_name")
metricDNSMagicDNSSuccessReverse = clientmetric.NewCounter("dns_query_magic_success_reverse")
metricDNSQueryForPeer = clientmetric.NewCounter("dns_query_peerapi")
metricDNSQueryForPeerError = clientmetric.NewCounter("dns_query_peerapi_error")
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")