Document a simpler SSH SOCKS configuration.

[dnstt.git] / dnstt-client / dns.go

package main

import (
        "bytes"
        "crypto/rand"
        "encoding/base32"
        "encoding/binary"
        "fmt"
        "io"
        "log"
        "net"
        "time"

        "www.bamsoftware.com/git/dnstt.git/dns"
        "www.bamsoftware.com/git/dnstt.git/turbotunnel"
)

const (
        // How many bytes of random padding to insert into queries.
        numPadding = 3
        // In an otherwise empty polling query, insert even more random padding,
        // to reduce the chance of a cache hit. Cannot be greater than 31,
        // because the prefix codes indicating padding start at 224.
        numPaddingForPoll = 8

        // sendLoop has a poll timer that automatically sends an empty polling
        // query when a certain amount of time has elapsed without a send. The
        // poll timer is initially set to initPollDelay. It increases by a
        // factor of pollDelayMultiplier every time the poll timer expires, up
        // to a maximum of maxPollDelay. The poll timer is reset to
        // initPollDelay whenever an a send occurs that is not the result of the
        // poll timer expiring.
        initPollDelay       = 500 * time.Millisecond
        maxPollDelay        = 10 * time.Second
        pollDelayMultiplier = 2.0
)

// base32Encoding is a base32 encoding without padding.
var base32Encoding = base32.StdEncoding.WithPadding(base32.NoPadding)

// DNSPacketConn provides a packet-sending and -receiving interface over various
// forms of DNS. It handles the details of how packets and padding are encoded
// as a DNS name in the Question section of an upstream query, and as a TXT RR
// in downstream responses.
//
// DNSPacketConn does not handle the mechanics of actually sending and receiving
// encoded DNS messages. That is rather the responsibility of some other
// net.PacketConn such as net.UDPConn, HTTPPacketConn, or TLSPacketConn, one of
// which must be provided to NewDNSPacketConn.
//
// We don't have a need to match up a query and a response by ID. Queries and
// responses are vehicles for carrying data and for our purposes don't need to
// be correlated. When sending a query, we generate a random ID, and when
// receiving a response, we ignore the ID.
type DNSPacketConn struct {
        clientID turbotunnel.ClientID
        domain   dns.Name
        // Sending on pollChan permits sendLoop to send an empty polling query.
        // sendLoop also does its own polling according to a time schedule.
        pollChan chan struct{}
        // QueuePacketConn is the direct receiver of ReadFrom and WriteTo calls.
        // recvLoop and sendLoop take the messages out of the receive and send
        // queues and actually put them on the network.
        *turbotunnel.QueuePacketConn
}

// NewDNSPacketConn creates a new DNSPacketConn. transport, through its WriteTo
// and ReadFrom methods, handles the actual sending and receiving the DNS
// messages encoded by DNSPacketConn. addr is the address to be passed to
// transport.WriteTo whenever a message needs to be sent.
func NewDNSPacketConn(transport net.PacketConn, addr net.Addr, domain dns.Name) *DNSPacketConn {
        // Generate a new random ClientID.
        clientID := turbotunnel.NewClientID()
        c := &DNSPacketConn{
                clientID:        clientID,
                domain:          domain,
                pollChan:        make(chan struct{}),
                QueuePacketConn: turbotunnel.NewQueuePacketConn(clientID, 0),
        }
        go func() {
                err := c.recvLoop(transport)
                if err != nil {
                        log.Printf("recvLoop: %v", err)
                }
        }()
        go func() {
                err := c.sendLoop(transport, addr)
                if err != nil {
                        log.Printf("sendLoop: %v", err)
                }
        }()
        return c
}

// dnsResponsePayload extracts the downstream payload of a DNS response, encoded
// into the RDATA of a TXT RR. It returns nil if the message doesn't pass format
// checks, or if the name in its Question entry is not a subdomain of domain.
func dnsResponsePayload(resp *dns.Message, domain dns.Name) []byte {
        if resp.Flags&0x8000 != 0x8000 {
                // QR != 1, this is not a response.
                return nil
        }
        if resp.Flags&0x000f != dns.RcodeNoError {
                return nil
        }

        if len(resp.Answer) != 1 {
                return nil
        }
        answer := resp.Answer[0]

        _, ok := answer.Name.TrimSuffix(domain)
        if !ok {
                // Not the name we are expecting.
                return nil
        }

        if answer.Type != dns.RRTypeTXT {
                // We only support TYPE == TXT.
                return nil
        }
        payload, err := dns.DecodeRDataTXT(answer.Data)
        if err != nil {
                return nil
        }

        return payload
}

// nextPacket reads the next length-prefixed packet from r. It returns a nil
// error only when a complete packet was read. It returns io.EOF only when there
// were 0 bytes remaining to read from r. It returns io.ErrUnexpectedEOF when
// EOF occurs in the middle of an encoded packet.
func nextPacket(r *bytes.Reader) ([]byte, error) {
        for {
                var n uint16
                err := binary.Read(r, binary.BigEndian, &n)
                if err != nil {
                        // We may return a real io.EOF only here.
                        return nil, err
                }
                p := make([]byte, n)
                _, err = io.ReadFull(r, p)
                // Here we must change io.EOF to io.ErrUnexpectedEOF.
                if err == io.EOF {
                        err = io.ErrUnexpectedEOF
                }
                return p, err
        }
}

// recvLoop repeatedly calls transport.ReadFrom to receive a DNS message,
// extracts its payload and breaks it into packets, and stores the packets in a
// queue to be returned from a future call to c.ReadFrom.
//
// Whenever we receive a response with a non-empty payload, we send twice on
// c.pollChan to permit sendLoop to send two immediate polling queries. The
// intuition behind polling immediately after receiving is that we know the
// server has just had something to send, it may need to send more, and the only
// way it can send is if we give it a query to respond to. The intuition behind
// doing *two* polls when we receive is similar to TCP slow start: we want to
// maintain some number of queries "in flight", and the faster the server is
// sending, the higher that number should be. If we polled only once in response
// to received data, we would tend to have only one query in flight at a time,
// ping-pong style. The first polling request replaces the in-flight request
// that has just finished in our receiving data; the second grows the effective
// in-flight window proportionally to the rate at which data-carrying responses
// are being received. Compare to Eq. (2) of
// https://tools.ietf.org/html/rfc5681#section-3.1; the differences are that we
// count messages, not bytes, and we don't maintain an explicit window. If a
// response comes back without data, or if a query or response is dropped by the
// network, then we don't poll again, which decreases the effective in-flight
// window.
func (c *DNSPacketConn) recvLoop(transport net.PacketConn) error {
        for {
                var buf [4096]byte
                n, addr, err := transport.ReadFrom(buf[:])
                if err != nil {
                        if err, ok := err.(net.Error); ok && err.Temporary() {
                                log.Printf("ReadFrom temporary error: %v", err)
                                continue
                        }
                        return err
                }

                // Got a response. Try to parse it as a DNS message.
                resp, err := dns.MessageFromWireFormat(buf[:n])
                if err != nil {
                        log.Printf("MessageFromWireFormat: %v", err)
                        continue
                }

                payload := dnsResponsePayload(&resp, c.domain)

                // Pull out the packets contained in the payload.
                r := bytes.NewReader(payload)
                any := false
                for {
                        p, err := nextPacket(r)
                        if err != nil {
                                break
                        }
                        any = true
                        c.QueuePacketConn.QueueIncoming(p, addr)
                }

                // If the payload contained one or more packets, permit sendLoop
                // to poll immediately.
                if any {
                        select {
                        case c.pollChan <- struct{}{}:
                        default:
                        }
                        select {
                        case c.pollChan <- struct{}{}:
                        default:
                        }
                }
        }
}

// chunks breaks p into non-empty subslices of at most n bytes, greedily so that
// only final subslice has length < n.
func chunks(p []byte, n int) [][]byte {
        var result [][]byte
        for len(p) > 0 {
                sz := len(p)
                if sz > n {
                        sz = n
                }
                result = append(result, p[:sz])
                p = p[sz:]
        }
        return result
}

// send sends p as a single packet encoded into a DNS query, using
// transport.WriteTo(query, addr). The length of p must be less than 224 bytes.
//
// Here is an example of how a packet is encoded into a DNS name, using
//     p = "supercalifragilisticexpialidocious"
//     c.clientID = "CLIENTID"
//     domain = "t.example.com"
//
// 0. Start with the raw packet contents.
//     supercalifragilisticexpialidocious
// 1. Length-prefix the packet and add random padding. A length prefix L < 0xe0
// means a data packet of L bytes. A length prefix L >= 0xe0 means padding of L -
// 0xe0 bytes (not counting the length of the length prefix itself).
//     \xe3\xd9\xa3\x15\x22supercalifragilisticexpialidocious
// 2. Prefix the ClientID.
//     CLIENTID\xe3\xd9\xa3\x15\x22supercalifragilisticexpialidocious
// 3. Base32-encode, without padding and in lower case.
//     ingesrkokreujy6zumkse43vobsxey3bnruwm4tbm5uwy2ltoruwgzlyobuwc3djmrxwg2lpovzq
// 4. Break into labels of at most 63 octets.
//     ingesrkokreujy6zumkse43vobsxey3bnruwm4tbm5uwy2ltoruwgzlyobuwc3d.jmrxwg2lpovzq
// 5. Append the domain.
//     ingesrkokreujy6zumkse43vobsxey3bnruwm4tbm5uwy2ltoruwgzlyobuwc3d.jmrxwg2lpovzq.t.example.com
func (c *DNSPacketConn) send(transport net.PacketConn, p []byte, addr net.Addr) error {
        var decoded []byte
        {
                if len(p) >= 224 {
                        return fmt.Errorf("too long")
                }
                var buf bytes.Buffer
                // ClientID
                buf.Write(c.clientID[:])
                n := numPadding
                if len(p) == 0 {
                        n = numPaddingForPoll
                }
                // Padding / cache inhibition
                buf.WriteByte(byte(224 + n))
                io.CopyN(&buf, rand.Reader, int64(n))
                // Packet contents
                if len(p) > 0 {
                        buf.WriteByte(byte(len(p)))
                        buf.Write(p)
                }
                decoded = buf.Bytes()
        }

        encoded := make([]byte, base32Encoding.EncodedLen(len(decoded)))
        base32Encoding.Encode(encoded, decoded)
        encoded = bytes.ToLower(encoded)
        labels := chunks(encoded, 63)
        labels = append(labels, c.domain...)
        name, err := dns.NewName(labels)
        if err != nil {
                return err
        }

        var id uint16
        binary.Read(rand.Reader, binary.BigEndian, &id)
        query := &dns.Message{
                ID:    id,
                Flags: 0x0100, // QR = 0, RD = 1
                Question: []dns.Question{
                        {
                                Name:  name,
                                Type:  dns.RRTypeTXT,
                                Class: dns.ClassIN,
                        },
                },
                // EDNS(0)
                Additional: []dns.RR{
                        {
                                Name:  dns.Name{},
                                Type:  dns.RRTypeOPT,
                                Class: 4096, // requester's UDP payload size
                                TTL:   0,    // extended RCODE and flags
                                Data:  []byte{},
                        },
                },
        }
        buf, err := query.WireFormat()
        if err != nil {
                return err
        }

        _, err = transport.WriteTo(buf, addr)
        return err
}

// sendLoop takes packets that have been written using c.WriteTo, and sends them
// on the network using send. It also does polling with empty packets when
// requested by pollChan or after a timeout.
func (c *DNSPacketConn) sendLoop(transport net.PacketConn, addr net.Addr) error {
        pollDelay := initPollDelay
        pollTimer := time.NewTimer(pollDelay)
        for {
                var p []byte
                outgoingQueue := c.QueuePacketConn.OutgoingQueue(addr)
                pollTimerExpired := false
                // Prioritize sending an actual data packet from OutgoingQueue.
                // Only consider a poll when OutgoingQueue is empty.
                select {
                case p = <-outgoingQueue:
                default:
                        select {
                        case p = <-outgoingQueue:
                        case <-c.pollChan:
                                p = nil
                        case <-pollTimer.C:
                                p = nil
                                pollTimerExpired = true
                        }
                }

                if len(p) > 0 {
                        // A data-carrying packet displaces one pending poll
                        // opportunity, if any.
                        select {
                        case <-c.pollChan:
                        default:
                        }
                }

                if pollTimerExpired {
                        // We're polling because it's been a while since we last
                        // polled. Increase the poll delay.
                        pollDelay = time.Duration(float64(pollDelay) * pollDelayMultiplier)
                        if pollDelay > maxPollDelay {
                                pollDelay = maxPollDelay
                        }
                } else {
                        // We're sending an actual data packet, or we're polling
                        // in response to a received packet. Reset the poll
                        // delay to initial.
                        if !pollTimer.Stop() {
                                <-pollTimer.C
                        }
                        pollDelay = initPollDelay
                }
                pollTimer.Reset(pollDelay)

                // Unlike in the server, in the client we assume that because
                // the data capacity of queries is so limited, it's not worth
                // trying to send more than one packet per query.
                err := c.send(transport, p, addr)
                if err != nil {
                        log.Printf("send: %v", err)
                        continue
                }
        }
}