proxy multicast from one interface to another (node.js)

index.js

// proxies 239.255.42.42:5004 from eth0 239.1.2.3:6666 wlan0

var dgram = require('dgram')
var os = require('os')

var socket = dgram.createSocket({ type: 'udp4', reuseAddr: true })
socket.bind(5004, function () {
  socket.addMembership('239.255.42.42', getIpForInterface('eth0'))
  socket.setMulticastTTL(255)
})

var proxy = dgram.createSocket({type: 'udp4', reuseAddr: true})
proxy.bind(6666, '192.168.1.35', function () {
  proxy.addMembership('239.1.2.3', '192.168.1.35')
})

socket.on('message', function (m) {
  if (!m.length) return
  proxy.send(m, 0, m.length, '6666', '239.1.2.3', function (err) {
    if (err) throw err
  })
})

function getIpForInterface (ifname) {
  return os.networkInterfaces()[ifname].reduce(function (i, j) {
    if (i.family === 'IPv4') return i
    return j
  }).address  
}