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forwarder.go
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forwarder.go
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// 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"
"hash/crc32"
"io"
"math/rand"
"net"
"sync"
"time"
dns "golang.org/x/net/dns/dnsmessage"
"inet.af/netaddr"
"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
const (
// responseTimeout is the maximal amount of time to wait for a DNS response.
responseTimeout = 5 * time.Second
)
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])
qcount := binary.BigEndian.Uint16(packet[4:6])
if qcount == 0 {
return txid(dnsid)
}
offset := headerBytes
for i := uint16(0); i < qcount; i++ {
// Note: this relies on the fact that names are not compressed in questions,
// so they are guaranteed to end with a NUL byte.
//
// Justification:
// RFC 1035 doesn't seem to explicitly prohibit compressing names in questions,
// but this is exceedingly unlikely to be done in practice. A DNS request
// with multiple questions is ill-defined (which questions do the header flags apply to?)
// and a single question would have to contain a pointer to an *answer*,
// which would be excessively smart, pointless (an answer can just as well refer to the question)
// and perhaps even prohibited: a draft RFC (draft-ietf-dnsind-local-compression-05) states:
//
// > It is important that these pointers always point backwards.
//
// This is said in summarizing RFC 1035, although that phrase does not appear in the original RFC.
// Additionally, (https://cr.yp.to/djbdns/notes.html) states:
//
// > The precise rule is that a name can be compressed if it is a response owner name,
// > the name in NS data, the name in CNAME data, the name in PTR data, the name in MX data,
// > or one of the names in SOA data.
namebytes := bytes.IndexByte(packet[offset:], 0)
// ... | name | NUL | type | class
// ?? 1 2 2
offset = offset + namebytes + 5
if len(packet) < offset {
// Corrupt packet; don't crash.
return txid(dnsid)
}
}
hash := crc32.ChecksumIEEE(packet[headerBytes:offset])
return (txid(hash) << 32) | txid(dnsid)
}
type route struct {
Suffix dnsname.FQDN
Resolvers []netaddr.IPPort
}
// forwarder forwards DNS packets to a number of upstream nameservers.
type forwarder struct {
logf logger.Logf
linkMon *monitor.Mon
linkSel ForwardLinkSelector
ctx context.Context // good until Close
ctxCancel context.CancelFunc // closes ctx
// responses is a channel by which responses are returned.
responses chan packet
mu sync.Mutex // guards following
// routes are per-suffix resolvers to use, with
// the most specific routes first.
routes []route
}
func init() {
rand.Seed(time.Now().UnixNano())
}
func newForwarder(logf logger.Logf, responses chan packet, linkMon *monitor.Mon, linkSel ForwardLinkSelector) *forwarder {
f := &forwarder{
logf: logger.WithPrefix(logf, "forward: "),
linkMon: linkMon,
linkSel: linkSel,
responses: responses,
}
f.ctx, f.ctxCancel = context.WithCancel(context.Background())
return f
}
func (f *forwarder) Close() error {
f.ctxCancel()
return nil
}
func (f *forwarder) setRoutes(routes []route) {
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
}
// send sends packet to dst. It is best effort.
//
// send expects the reply to have the same txid as txidOut.
//
// The provided 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.
func (f *forwarder) send(ctx context.Context, txidOut txid, closeOnCtxDone *closePool, packet []byte, dst netaddr.IPPort) ([]byte, error) {
// TODO(bradfitz): if dst.IP is 8.8.8.8 or 8.8.4.4 or 1.1.1.1, etc, or
// something dynamically probed earlier to support DoH or DoT,
// do that here instead.
ln, err := f.packetListener(dst.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()
closeOnCtxDone.Add(conn)
defer closeOnCtxDone.Remove(conn)
if _, err := conn.WriteTo(packet, dst.UDPAddr()); err != nil {
if err := ctx.Err(); err != nil {
return nil, err
}
return nil, err
}
// 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 packetWasTruncated(err) {
err = nil
} else {
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 != txidOut {
return nil, errors.New("txid doesn't match")
}
if truncated {
const dnsFlagTruncated = 0x200
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.
}
return out, nil
}
// resolvers returns the resolvers to use for domain.
func (f *forwarder) resolvers(domain dnsname.FQDN) []netaddr.IPPort {
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
}
// forward forwards the query to all upstream nameservers and returns the first response.
func (f *forwarder) forward(query packet) error {
domain, err := nameFromQuery(query.bs)
if err != nil {
return err
}
txid := getTxID(query.bs)
resolvers := f.resolvers(domain)
if len(resolvers) == 0 {
return errNoUpstreams
}
closeOnCtxDone := new(closePool)
defer closeOnCtxDone.Close()
ctx, cancel := context.WithTimeout(f.ctx, responseTimeout)
defer cancel()
resc := make(chan []byte, 1)
var (
mu sync.Mutex
firstErr error
)
for _, ipp := range resolvers {
go func(ipp netaddr.IPPort) {
resb, err := f.send(ctx, txid, closeOnCtxDone, query.bs, ipp)
if err != nil {
mu.Lock()
defer mu.Unlock()
if firstErr == nil {
firstErr = err
}
return
}
select {
case resc <- resb:
default:
}
}(ipp)
}
select {
case v := <-resc:
select {
case <-ctx.Done():
return ctx.Err()
case f.responses <- packet{v, query.addr}:
return nil
}
case <-ctx.Done():
mu.Lock()
defer mu.Unlock()
if firstErr != nil {
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))
}
// 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
}