mirror of https://github.com/tailscale/tailscale/
tsdns: delegate requests asynchronously (#687)
Signed-Off-By: Dmytro Shynkevych <dmytro@tailscale.com>reviewable/pr696/r1
Dmytro Shynkevych
4 years ago
committed by
GitHub
parent
a583e498b0
commit
1af70e2468
@ -0,0 +1,325 @@
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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package tsdns
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import (
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"bytes"
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"encoding/binary"
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"errors"
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"hash/crc32"
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"math/rand"
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"net"
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"os"
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"sync"
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"time"
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"inet.af/netaddr"
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"tailscale.com/types/logger"
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)
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// headerBytes is the number of bytes in a DNS message header.
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const headerBytes = 12
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// forwardQueueSize is the maximal number of requests that can be pending delegation.
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// Note that this is distinct from the number of requests that are pending a response,
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// which is not limited (except by txid collisions).
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const forwardQueueSize = 64
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// connCount is the number of UDP connections to use for forwarding.
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const connCount = 32
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const (
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// cleanupInterval is the interval between purged of timed-out entries from txMap.
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cleanupInterval = 30 * time.Second
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// responseTimeout is the maximal amount of time to wait for a DNS response.
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responseTimeout = 5 * time.Second
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)
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var errNoUpstreams = errors.New("upstream nameservers not set")
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var aLongTimeAgo = time.Unix(0, 1)
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type forwardedPacket struct {
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payload []byte
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dst net.Addr
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}
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type forwardingRecord struct {
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src netaddr.IPPort
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createdAt time.Time
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}
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// txid identifies a DNS transaction.
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//
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// As the standard DNS Request ID is only 16 bits, we extend it:
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// the lower 32 bits are the zero-extended bits of the DNS Request ID;
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// the upper 32 bits are the CRC32 checksum of the first question in the request.
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// This makes probability of txid collision negligible.
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type txid uint64
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// getTxID computes the txid of the given DNS packet.
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func getTxID(packet []byte) txid {
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if len(packet) < headerBytes {
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return 0
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}
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dnsid := binary.BigEndian.Uint16(packet[0:2])
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qcount := binary.BigEndian.Uint16(packet[4:6])
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if qcount == 0 {
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return txid(dnsid)
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}
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offset := headerBytes
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for i := uint16(0); i < qcount; i++ {
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// Note: this relies on the fact that names are not compressed in questions,
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// so they are guaranteed to end with a NUL byte.
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//
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// Justification:
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// RFC 1035 doesn't seem to explicitly prohibit compressing names in questions,
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// but this is exceedingly unlikely to be done in practice. A DNS request
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// with multiple questions is ill-defined (which questions do the header flags apply to?)
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// and a single question would have to contain a pointer to an *answer*,
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// which would be excessively smart, pointless (an answer can just as well refer to the question)
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// and perhaps even prohibited: a draft RFC (draft-ietf-dnsind-local-compression-05) states:
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//
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// > It is important that these pointers always point backwards.
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//
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// This is said in summarizing RFC 1035, although that phrase does not appear in the original RFC.
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// Additionally, (https://cr.yp.to/djbdns/notes.html) states:
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//
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// > The precise rule is that a name can be compressed if it is a response owner name,
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// > the name in NS data, the name in CNAME data, the name in PTR data, the name in MX data,
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// > or one of the names in SOA data.
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namebytes := bytes.IndexByte(packet[offset:], 0)
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// ... | name | NUL | type | class
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// ?? 1 2 2
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offset = offset + namebytes + 5
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if len(packet) < offset {
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// Corrupt packet; don't crash.
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return txid(dnsid)
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}
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}
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hash := crc32.ChecksumIEEE(packet[headerBytes:offset])
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return (txid(hash) << 32) | txid(dnsid)
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}
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// forwarder forwards DNS packets to a number of upstream nameservers.
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type forwarder struct {
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logf logger.Logf
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// queue is the queue for delegated packets.
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queue chan forwardedPacket
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// responses is a channel by which responses are returned.
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responses chan Packet
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// closed signals all goroutines to stop.
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closed chan struct{}
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// wg signals when all goroutines have stopped.
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wg sync.WaitGroup
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// conns are the UDP connections used for forwarding.
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// A random one is selected for each request, regardless of the target upstream.
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conns []*net.UDPConn
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mu sync.Mutex
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// upstreams are the nameserver addresses that should be used for forwarding.
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upstreams []net.Addr
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// txMap maps DNS txids to active forwarding records.
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txMap map[txid]forwardingRecord
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}
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func init() {
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rand.Seed(time.Now().UnixNano())
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}
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func newForwarder(logf logger.Logf, responses chan Packet) *forwarder {
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return &forwarder{
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logf: logger.WithPrefix(logf, "forward: "),
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responses: responses,
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queue: make(chan forwardedPacket, forwardQueueSize),
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closed: make(chan struct{}),
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conns: make([]*net.UDPConn, connCount),
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txMap: make(map[txid]forwardingRecord),
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}
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}
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func (f *forwarder) Start() error {
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var err error
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for i := range f.conns {
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f.conns[i], err = net.ListenUDP("udp", nil)
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if err != nil {
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return err
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}
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}
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f.wg.Add(connCount + 2)
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for idx, conn := range f.conns {
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go f.recv(uint16(idx), conn)
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}
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go f.send()
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go f.cleanMap()
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return nil
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}
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func (f *forwarder) Close() {
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select {
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case <-f.closed:
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return
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default:
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// continue
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}
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close(f.closed)
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for _, conn := range f.conns {
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conn.SetDeadline(aLongTimeAgo)
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}
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f.wg.Wait()
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for _, conn := range f.conns {
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conn.Close()
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}
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}
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func (f *forwarder) setUpstreams(upstreams []net.Addr) {
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f.mu.Lock()
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f.upstreams = upstreams
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f.mu.Unlock()
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}
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func (f *forwarder) send() {
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defer f.wg.Done()
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var packet forwardedPacket
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for {
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select {
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case <-f.closed:
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return
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case packet = <-f.queue:
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// continue
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}
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connIdx := rand.Intn(connCount)
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conn := f.conns[connIdx]
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_, err := conn.WriteTo(packet.payload, packet.dst)
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if err != nil {
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// Do not log errors due to expired deadline.
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if !errors.Is(err, os.ErrDeadlineExceeded) {
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f.logf("send: %v", err)
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}
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return
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}
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}
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}
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func (f *forwarder) recv(connIdx uint16, conn *net.UDPConn) {
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defer f.wg.Done()
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for {
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out := make([]byte, maxResponseBytes)
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n, err := conn.Read(out)
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if err != nil {
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// Do not log errors due to expired deadline.
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if !errors.Is(err, os.ErrDeadlineExceeded) {
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f.logf("recv: %v", err)
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}
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return
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}
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if n < headerBytes {
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f.logf("recv: packet too small (%d bytes)", n)
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}
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out = out[:n]
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txid := getTxID(out)
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f.mu.Lock()
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record, found := f.txMap[txid]
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// At most one nameserver will return a response:
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// the first one to do so will delete txid from the map.
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if !found {
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f.mu.Unlock()
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continue
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}
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delete(f.txMap, txid)
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f.mu.Unlock()
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packet := Packet{
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Payload: out,
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Addr: record.src,
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}
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select {
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case <-f.closed:
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return
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case f.responses <- packet:
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// continue
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}
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}
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}
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// cleanMap periodically deletes timed-out forwarding records from f.txMap to bound growth.
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func (f *forwarder) cleanMap() {
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defer f.wg.Done()
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t := time.NewTicker(cleanupInterval)
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defer t.Stop()
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var now time.Time
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for {
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select {
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case <-f.closed:
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return
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case now = <-t.C:
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// continue
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}
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f.mu.Lock()
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for k, v := range f.txMap {
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if now.Sub(v.createdAt) > responseTimeout {
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delete(f.txMap, k)
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}
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}
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f.mu.Unlock()
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}
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}
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// forward forwards the query to all upstream nameservers and returns the first response.
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func (f *forwarder) forward(query Packet) error {
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txid := getTxID(query.Payload)
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f.mu.Lock()
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upstreams := f.upstreams
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if len(upstreams) == 0 {
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f.mu.Unlock()
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return errNoUpstreams
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}
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f.txMap[txid] = forwardingRecord{
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src: query.Addr,
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createdAt: time.Now(),
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}
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f.mu.Unlock()
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packet := forwardedPacket{
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payload: query.Payload,
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}
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for _, upstream := range upstreams {
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packet.dst = upstream
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select {
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case <-f.closed:
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return ErrClosed
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case f.queue <- packet:
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// continue
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}
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}
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return nil
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}
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