index.js

var packet = require('dns-packet')
var dgram = require('dgram')
var thunky = require('thunky')
var events = require('events')
var os = require('os')

var noop = function () {}

module.exports = function (opts) {
  if (!opts) opts = {}

  var that = new events.EventEmitter()
  var port = typeof opts.port === 'number' ? opts.port : 5353
  var type = opts.type || 'udp4'
  var ip = opts.ip || opts.host || (type === 'udp4' ? '224.0.0.251' : null)
  var me = {address: ip, port: port}
  var memberships = {}
  var destroyed = false
  var interval = null

  if (type === 'udp6' && (!ip || !opts.interface)) {
    throw new Error('For IPv6 multicast you must specify `ip` and `interface`')
  }

  var socket = opts.socket || dgram.createSocket({
    type: type,
    reuseAddr: opts.reuseAddr !== false,
    toString: function () {
      return type
    }
  })

  socket.on('error', function (err) {
    if (err.code === 'EACCES' || err.code === 'EADDRINUSE') that.emit('error', err)
    else that.emit('warning', err)
  })

  socket.on('message', function (message, rinfo) {
    try {
      message = packet.decode(message)
    } catch (err) {
      that.emit('warning', err)
      return
    }

    that.emit('packet', message, rinfo)

    if (message.type === 'query') that.emit('query', message, rinfo)
    if (message.type === 'response') that.emit('response', message, rinfo)
  })

  socket.on('listening', function () {
    if (!port) port = me.port = socket.address().port
    if (opts.multicast !== false) {
      that.update()
      interval = setInterval(that.update, 5000)
      socket.setMulticastTTL(opts.ttl || 255)
      socket.setMulticastLoopback(opts.loopback !== false)
    }
  })

  var bind = thunky(function (cb) {
    if (!port || opts.bind === false) return cb(null)
    socket.once('error', cb)
    socket.bind(port, opts.bind || opts.interface, function () {
      socket.removeListener('error', cb)
      cb(null)
    })
  })

  bind(function (err) {
    if (err) return that.emit('error', err)
    that.emit('ready')
  })

  that.loopInterface = (interfaces, index, message, rinfo, cb) => {
    var int = interfaces[index];
    if (int) {
      try {
        socket.setMulticastInterface(int)
        socket.send(
          message,
          0,
          message.length,
          rinfo.port,
          rinfo.address || rinfo.host,
          () => {
            setTimeout(() => that.loopInterface(interfaces, index + 1, message, rinfo, cb), 10)
          })
      } catch (err) {
        that.emit('warning', err)
      }
    }
    else {
      cb()
    }
  }

  that.send = function (value, rinfo, cb) {
    if (typeof rinfo === 'function') return that.send(value, null, rinfo)
    if (!cb) cb = noop
    if (!rinfo) rinfo = me
    else if (!rinfo.host && !rinfo.address) rinfo.address = me.address

    bind(onbind)

    function onbind (err) {
      if (destroyed) return cb()
      if (err) return cb(err)
      var message = packet.encode(value)
      if (socket.setMulticastInterface) {
        if (opts.interface) {
          try {  
            socket.setMulticastInterface(opts.interface)
          } catch (err) {
            that.emit('warning', err)
          }
          socket.send(message, 0, message.length, rinfo.port, rinfo.address || rinfo.host, cb)
        }
        else {
          var interfaces = getExternalInterfaces();
          if (interfaces.length > 0) {
            that.loopInterface(interfaces, 0, message, rinfo, cb);
          }
          else {
            try {  
              socket.setMulticastInterface("0.0.0.0")
            } catch (err) {
              that.emit('warning', err)
            }
            socket.send(message, 0, message.length, rinfo.port, rinfo.address || rinfo.host, cb)
          }
        }
      }
    }
  }

  that.response =
  that.respond = function (res, rinfo, cb) {
    if (Array.isArray(res)) res = {answers: res}

    res.type = 'response'
    res.flags = (res.flags || 0) | packet.AUTHORITATIVE_ANSWER
    that.send(res, rinfo, cb)
  }

  that.query = function (q, type, rinfo, cb) {
    if (typeof type === 'function') return that.query(q, null, null, type)
    if (typeof type === 'object' && type && type.port) return that.query(q, null, type, rinfo)
    if (typeof rinfo === 'function') return that.query(q, type, null, rinfo)
    if (!cb) cb = noop

    if (typeof q === 'string') q = [{name: q, type: type || 'ANY'}]
    if (Array.isArray(q)) q = {type: 'query', questions: q}

    q.type = 'query'
    that.send(q, rinfo, cb)
  }

  that.destroy = function (cb) {
    if (!cb) cb = noop
    if (destroyed) return process.nextTick(cb)
    destroyed = true
    clearInterval(interval)

    // Need to drop memberships by hand and ignore errors.
    // socket.close() does not cope with errors.
    for (var iface in memberships) {
      try {
        socket.dropMembership(ip, iface)
      } catch (e) {
        // eat it
      }
    }
    memberships = {}
    socket.close(cb)
  }

  that.update = function () {
    var ifaces = opts.interface ? [].concat(opts.interface) : allInterfaces()
    var updated = false

    for (var i = 0; i < ifaces.length; i++) {
      var addr = ifaces[i]
      if (memberships[addr]) continue

      try {
        socket.addMembership(ip, addr)
        memberships[addr] = true
        updated = true
      } catch (err) {
        that.emit('warning', err)
      }
    }
  }

  return that
}

function getExternalInterfaces () {
  var networks = os.networkInterfaces()
  var names = Object.keys(networks)
  var interfaceList = [];
  for (var i = 0; i < names.length; i++) {
    var net = networks[names[i]]
    for (var j = 0; j < net.length; j++) {
      var iface = net[j]
      if (isIPv4(iface.family) && !iface.internal) {
        interfaceList.push(iface.address);
      }
    }
  }

  return interfaceList
}

function allInterfaces () {
  var networks = os.networkInterfaces()
  var names = Object.keys(networks)
  var res = []

  for (var i = 0; i < names.length; i++) {
    var net = networks[names[i]]
    for (var j = 0; j < net.length; j++) {
      var iface = net[j]
      if (isIPv4(iface.family)) {
        res.push(iface.address)
        // could only addMembership once per interface (https://nodejs.org/api/dgram.html#dgram_socket_addmembership_multicastaddress_multicastinterface)
        break
      }
    }
  }

  return res
}

function isIPv4 (family) { // for backwards compat
  return family === 4 || family === 'IPv4'
}