mdns/mdns.go

// Copyright 2019 The Fuchsia 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 mdns

import (
	"bytes"
	"context"
	"encoding/binary"
	"fmt"
	"io"
	"net"
	"os"
	"strings"
	"syscall"
	"unicode/utf8"

	"golang.org/x/net/ipv4"
)

type Header struct {
	ID      uint16
	Flags   uint16
	QDCount uint16
	ANCount uint16
	NSCount uint16
	ARCount uint16
}

type Record struct {
	Domain string
	Type   uint16
	Class  uint16
	Flush  bool
	TTL    uint32
	Data   []byte
}

type Question struct {
	Domain  string
	Type    uint16
	Class   uint16
	Unicast bool
}

type Packet struct {
	Header     Header
	Questions  []Question
	Answers    []Record
	Authority  []Record
	Additional []Record
}

func writeUint16(out io.Writer, val uint16) error {
	buf := make([]byte, 2)
	binary.BigEndian.PutUint16(buf, val)
	_, err := out.Write(buf)
	return err
}

func (h Header) serialize(out io.Writer) error {
	if err := writeUint16(out, h.ID); err != nil {
		return err
	}
	if err := writeUint16(out, h.Flags); err != nil {
		return err
	}
	if err := writeUint16(out, h.QDCount); err != nil {
		return err
	}
	if err := writeUint16(out, h.ANCount); err != nil {
		return err
	}
	if err := writeUint16(out, h.NSCount); err != nil {
		return err
	}
	if err := writeUint16(out, h.ARCount); err != nil {
		return err
	}
	return nil
}

func writeDomain(out io.Writer, domain string) error {
	domain = strings.TrimSuffix(domain, ".")
	parts := strings.Split(domain, ".")
	// TODO(jakehehrlich): Add check that each label is ASCII.
	// TODO(jakehehrlich): Add check that each label is <= 63 in length.
	// TODO(jakehehrlich): Add support for compression.
	for _, dpart := range parts {
		ascii := []byte(dpart)
		if _, err := out.Write([]byte{byte(len(ascii))}); err != nil {
			return err
		}
		if _, err := out.Write(ascii); err != nil {
			return err
		}
	}
	_, err := out.Write([]byte{0})
	return err
}

func (q Question) serialize(out io.Writer) error {
	if err := writeDomain(out, q.Domain); err != nil {
		return err
	}
	if err := writeUint16(out, q.Type); err != nil {
		return err
	}
	var unicast uint16
	if q.Unicast {
		unicast = 1 << 15
	}
	if err := writeUint16(out, unicast|q.Class); err != nil {
		return err
	}
	return nil
}

func writeUint32(out io.Writer, val uint32) error {
	buf := make([]byte, 4)
	binary.BigEndian.PutUint32(buf, val)
	_, err := out.Write(buf)
	return err
}

func (r Record) serialize(out io.Writer) error {
	if err := writeDomain(out, r.Domain); err != nil {
		return err
	}
	if err := writeUint16(out, r.Type); err != nil {
		return err
	}
	var flush uint16
	if r.Flush {
		flush = 1 << 15
	}
	if err := writeUint16(out, flush|r.Class); err != nil {
		return err
	}
	if err := writeUint32(out, r.TTL); err != nil {
		return err
	}
	if err := writeUint16(out, uint16(len(r.Data))); err != nil {
		return err
	}
	if _, err := out.Write(r.Data); err != nil {
		return err
	}
	return nil
}

func (p Packet) serialize(out io.Writer) error {
	if err := p.Header.serialize(out); err != nil {
		return err
	}
	for _, question := range p.Questions {
		if err := question.serialize(out); err != nil {
			return err
		}
	}
	for _, answer := range p.Answers {
		if err := answer.serialize(out); err != nil {
			return err
		}
	}
	for _, authority := range p.Authority {
		if err := authority.serialize(out); err != nil {
			return err
		}
	}
	for _, addon := range p.Additional {
		if err := addon.serialize(out); err != nil {
			return err
		}
	}
	return nil
}

func readUint16(in io.Reader, out *uint16) error {
	buf := make([]byte, 2)
	_, err := in.Read(buf)
	if err != nil {
		return err
	}
	*out = binary.BigEndian.Uint16(buf)
	return nil
}

func (h *Header) deserialize(data []byte, in io.Reader) error {
	if err := readUint16(in, &h.ID); err != nil {
		return err
	}
	if err := readUint16(in, &h.Flags); err != nil {
		return err
	}
	if err := readUint16(in, &h.QDCount); err != nil {
		return err
	}
	if err := readUint16(in, &h.ANCount); err != nil {
		return err
	}
	if err := readUint16(in, &h.NSCount); err != nil {
		return err
	}
	if err := readUint16(in, &h.ARCount); err != nil {
		return err
	}
	return nil
}

func readDomain(data []byte, in io.Reader, domain *string) error {
	// TODO(jakehehrlich): Don't stack overflow when domain contains cycle.

	var d bytes.Buffer
	for {
		sizeBuf := make([]byte, 1)
		if _, err := in.Read(sizeBuf); err != nil {
			return err
		}
		size := sizeBuf[0]
		// A size of zero indicates that we're done.
		if size == 0 {
			break
		}
		// We don't support compressed domains right now.
		if size > 63 {
			if size < 192 {
				return fmt.Errorf("invalid size for label")
			}
			if _, err := in.Read(sizeBuf); err != nil {
				return err
			}
			offset := ((size & 0x3f) << 8) | sizeBuf[0]
			var pDomain string
			readDomain(data, bytes.NewBuffer(data[offset:]), &pDomain)
			if _, err := d.WriteString(pDomain); err != nil {
				return err
			}
			if err := d.WriteByte(byte('.')); err != nil {
				return err
			}
			break
		}
		// Read in the specified bytes (max length 256)
		buf := make([]byte, size)
		if _, err := in.Read(buf); err != nil {
			return err
		}
		// Make sure the string is ASCII
		for _, b := range buf {
			if b >= utf8.RuneSelf {
				return fmt.Errorf("Found non-ASCII byte %v in domain", b)
			}
		}
		// Now add this to a temporary domain
		if _, err := d.Write(buf); err != nil {
			return err
		}
		// Add the trailing "." as seen in the RFC.
		if err := d.WriteByte(byte('.')); err != nil {
			return err
		}
	}
	*domain = string(d.Bytes())
	// Remove the trailing '.' to canonicalize.
	*domain = strings.TrimSuffix(*domain, ".")
	return nil
}

func (q *Question) deserialize(data []byte, in io.Reader) error {
	if err := readDomain(data, in, &q.Domain); err != nil {
		return fmt.Errorf("reading domain: %v", err)
	}
	if err := readUint16(in, &q.Type); err != nil {
		return err
	}
	var tmp uint16
	if err := readUint16(in, &tmp); err != nil {
		return err
	}
	// Extract class and unicast bit.
	q.Unicast = (tmp >> 15) != 0
	q.Class = (tmp << 1) >> 1
	return nil
}

func readUint32(in io.Reader, out *uint32) error {
	buf := make([]byte, 4)
	_, err := in.Read(buf)
	if err != nil {
		return err
	}
	*out = binary.BigEndian.Uint32(buf)
	return nil
}

func (r *Record) deserialize(data []byte, in io.Reader) error {
	if err := readDomain(data, in, &r.Domain); err != nil {
		return err
	}
	if err := readUint16(in, &r.Type); err != nil {
		return err
	}
	var tmp uint16
	if err := readUint16(in, &tmp); err != nil {
		return err
	}
	// Extract class and flush bit.
	r.Flush = (tmp >> 15) != 0
	r.Class = (tmp << 1) >> 1
	if err := readUint32(in, &r.TTL); err != nil {
		return err
	}

	var dataLength uint16
	if err := readUint16(in, &dataLength); err != nil {
		return err
	}
	// Now read the data (max allocation size of 64k)
	r.Data = make([]byte, dataLength)
	if _, err := in.Read(r.Data); err != nil {
		return err
	}
	return nil
}

// TODO(jakehehrlich): Handle truncation.
func (p *Packet) deserialize(data []byte, in io.Reader) error {
	if err := p.Header.deserialize(data, in); err != nil {
		return err
	}
	p.Questions = make([]Question, p.Header.QDCount)
	for i := uint16(0); i < p.Header.QDCount; i++ {
		if err := p.Questions[i].deserialize(data, in); err != nil {
			return err
		}
	}
	p.Answers = make([]Record, p.Header.ANCount)
	for i := uint16(0); i < p.Header.ANCount; i++ {
		if err := p.Answers[i].deserialize(data, in); err != nil {
			return err
		}
	}
	p.Authority = make([]Record, p.Header.NSCount)
	for i := uint16(0); i < p.Header.NSCount; i++ {
		if err := p.Authority[i].deserialize(data, in); err != nil {
			return err
		}
	}
	p.Additional = make([]Record, p.Header.ARCount)
	for i := uint16(0); i < p.Header.ARCount; i++ {
		if err := p.Additional[i].deserialize(data, in); err != nil {
			return err
		}
	}
	return nil
}

// getFlag constructs the flag field of a header for the tiny subset of
// flag options that we need.
// TODO(jakehehrlich): Implement response code error handling.
// TODO(jakehehrlich): Implement truncation.
func getFlag(query bool, authority bool) uint16 {
	var out uint16
	if !query {
		out |= 1
	}
	if authority {
		out |= 1 << 5
	}
	return out
}

const (
	// A is the DNS Type for ipv4
	A = 1
	// AAAA is the DNS Type for ipv6
	AAAA = 28
	// PTR is the DNS Type for domain name pointers
	PTR = 12
	// SRV is the DNS Type for services
	SRV = 33
	// IN is the Internet DNS Class
	IN = 1
)

// MDNS is the central type though which requests are sent and received.
// This implementation is agnostic to use case and asynchronous.
// To handle various responses add Handlers. To send a packet you may use
// either SendTo (generally used for unicast) or Send (generally used for
// multicast).
type MDNS struct {
	conn      *ipv4.PacketConn
	senders   []net.PacketConn
	port      int
	pHandlers []func(net.Interface, net.Addr, Packet)
	wHandlers []func(net.Addr, error)
	eHandlers []func(error)
}

// AddHandler calls f on every Packet received.
func (m *MDNS) AddHandler(f func(net.Interface, net.Addr, Packet)) {
	m.pHandlers = append(m.pHandlers, f)
}

// AddWarningHandler calls f on every non-fatal error.
func (m *MDNS) AddWarningHandler(f func(net.Addr, error)) {
	m.wHandlers = append(m.wHandlers, f)
}

// AddErrorHandler calls f on every fatal error. After
// all active handlers are called, m will stop listening and
// close it's connection so this function will not be called twice.
func (m *MDNS) AddErrorHandler(f func(error)) {
	m.eHandlers = append(m.eHandlers, f)
}

// SendTo serializes and sends packet to dst. If dst is a multicast
// address then packet is multicast to the corresponding group on
// all interfaces. Note that start must be called prior to making this
// call.
func (m *MDNS) SendTo(packet Packet, dst *net.UDPAddr) error {
	var buf bytes.Buffer
	// TODO(jakehehrlich): Add checking that the packet is well formed.
	if err := packet.serialize(&buf); err != nil {
		return err
	}
	for _, sender := range m.senders {
		if _, err := sender.WriteTo(buf.Bytes(), dst); err != nil {
			return err
		}
	}
	return nil
}

var mdnsMulticastIPv4 = net.ParseIP("224.0.0.251")

// Send serializes and sends packet out as a multicast to all interfaces
// using the port that m is listening on. Note that Start must be
// called prior to making this call.
func (m *MDNS) Send(packet Packet) error {
	dst := net.UDPAddr{IP: mdnsMulticastIPv4, Port: m.port}
	return m.SendTo(packet, &dst)
}

func makeUnixIpv4Socket(port int, ip net.IP) (net.PacketConn, error) {
	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, syscall.IPPROTO_UDP)
	if err != nil {
		return nil, fmt.Errorf("creating socket: %v", err)
	}
	// SO_REUSEADDR and SO_REUSEPORT allows binding to the same port multiple
	// times which is necessary in the case when there are multiple instances.
	if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, 0x2 /*SO_REUSEADDR*/, 1); err != nil {
		syscall.Close(fd)
		return nil, fmt.Errorf("setting reuse addr: %v", err)
	}
	if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, 0xf /*SO_REUSEPORT*/, 1); err != nil {
		syscall.Close(fd)
		return nil, fmt.Errorf("setting reuse port: %v", err)
	}
	// Bind the socket to the specified port.
	var ipArray [4]byte
	copy(ipArray[:], []byte(ip))
	if err := syscall.Bind(fd, &syscall.SockaddrInet4{Addr: ipArray, Port: port}); err != nil {
		syscall.Close(fd)
		return nil, fmt.Errorf("binding to %v: %v", ip, err)
	}
	// Now make a socket.
	f := os.NewFile(uintptr(fd), "")
	conn, err := net.FilePacketConn(f)
	f.Close()
	if err != nil {
		return nil, fmt.Errorf("creating packet conn: %v", err)
	}
	return conn, nil
}

// Start causes m to start listening for MDNS packets on all interfaces on
// the specified port. Listening will stop if ctx is done.
func (m *MDNS) Start(ctx context.Context, port int) error {
	dst := &net.UDPAddr{IP: mdnsMulticastIPv4, Port: port}
	conn, err := net.ListenUDP("udp4", dst)
	if err != nil {
		return err
	}
	// Now we need a low level ipv4 packet connection.
	m.conn = ipv4.NewPacketConn(conn)
	m.conn.SetControlMessage(ipv4.FlagDst|ipv4.FlagInterface, true)
	m.port = port
	// Now we need to join this connection to every interface that supports
	// Multicast.
	ifaces, err := net.Interfaces()
	if err != nil {
		conn.Close()
		return fmt.Errorf("listing interfaces: %v", err)
	}
	// We need to make sure to handle each interface.
	for _, iface := range ifaces {
		if iface.Flags&net.FlagMulticast == 0 || iface.Flags&net.FlagUp == 0 {
			continue
		}
		// This allows us to listen on this specific interface.
		if err := m.conn.JoinGroup(&iface, dst); err != nil {
			conn.Close()
			return fmt.Errorf("joining %v%%%v: %v", iface, dst, err)
		}
		addrs, err := iface.Addrs()
		if err != nil {
			return fmt.Errorf("getting addresses of %v: %v", iface, err)
		}
		for _, addr := range addrs {
			var ip net.IP
			switch v := addr.(type) {
			case *net.IPNet:
				ip = v.IP
			case *net.IPAddr:
				ip = v.IP
			}
			if ip == nil || ip.To4() == nil {
				continue
			}
			conn, err := makeUnixIpv4Socket(port, ip.To4())
			if err != nil {
				return fmt.Errorf("creating socket for %v via %v: %v", iface, ip, err)
			}
			m.senders = append(m.senders, conn)
			break
		}
	}
	go func() {
		defer conn.Close()
		// Now that we've joined every possible interface we can handle the main loop.
		payloadBuf := make([]byte, 1<<16)
		for {
			select {
			case <-ctx.Done():
				return
			default:
			}
			size, cm, src, err := m.conn.ReadFrom(payloadBuf)
			if err != nil {
				for _, e := range m.eHandlers {
					go e(err)
				}
				return
			}
			iface, err := net.InterfaceByIndex(cm.IfIndex)
			if err != nil {
				for _, e := range m.eHandlers {
					go e(err)
				}
				return
			}
			var packet Packet
			data := payloadBuf[:size]
			if err := packet.deserialize(data, bytes.NewBuffer(data)); err != nil {
				for _, w := range m.wHandlers {
					go w(src, err)
				}
				continue
			}
			for _, p := range m.pHandlers {
				go p(*iface, src, packet)
			}
		}
	}()
	return nil
}

// QuestionPacket constructs and returns a packet that
// requests the ip address associated with domain.
func QuestionPacket(domain string) Packet {
	return Packet{
		Header: Header{QDCount: 1},
		Questions: []Question{
			Question{
				Domain:  domain,
				Type:    A,
				Class:   IN,
				Unicast: false,
			},
		},
	}
}

// AnswerPacket constructs and returns a packet that
// gives a response to the
func AnswerPacket(domain string, ip net.IP) Packet {
	return Packet{
		Header: Header{ANCount: 1},
		Answers: []Record{
			Record{
				Domain: domain,
				Type:   A,
				Class:  IN,
				Flush:  false,
				Data:   []byte(ip),
			},
		},
	}
}