Node.js http client with custom lookup logic

by Evan Lucas, 2015-10-27

A lot of interesting things have happened with the convergence of io.js and Node.js. The latest Node.js (v4.2.1 at the time of writing) is based on io.js v3.x. In the io.js v2.0.0 release, I added the ability to specify a custom lookup function to net.createConnection. It allows one to do fun things like cache a dns lookup or add certain tracing logic into lookups. This is a quite specific use case, but can really improve performance. Below is an example of implementing a custom lookup in Node's http client. This is done merely to show the new capabilities, but I would not recommend using this particular solution in a production environment without understanding the implications.

Disclaimer: DNS records can change and this example does not take that into account.

'use strict'
const http = require('http')
const net = require('net')
const dns = require('dns')
// this will hold our hosts 
// [ { family: 4, address: <ip address> } ] 
var hosts
// custom lookup function 
// the callback must be called with the following signature: 
// function(err, family, address) 
// where family is either 4 for IPv4 or 6 for IPv6 
function lookup(hostname, opts, cb) {
  console.log('lookup', hostname)
  opts = opts || {}
  // this fetches all of the records instead of a single one 
  opts.all = true
  if (hosts && hosts.length) {
    // make sure to defer it 
    // otherwise, there are some special cases where an error 
    // could be thrown before a listener is added 
    setImmediate(() => {
      const host = random()
      console.log('Already looked up...using', host)
      cb(null, host.address,
  // we don't have any records cached, so go ahead and do the actual 
  // dns lookup 
  dns.lookup(hostname, opts, function(err, results) {
    if (err) return cb(err)
    hosts = results
    // get random one 
    const host = random()
    console.log('fresh lookup...using', host)
    cb(null, host.address,
function random() {
  return hosts[Math.floor(Math.random() * hosts.length)]
function get() {
    hostname: ''
  , path: '/'
  // specify our custom lookup function here 
  , lookup: lookup
  }, function(res) {
    console.log('RESPONSE', res.statusCode)
  }).on('error', function(err) {
    console.error('ERROR', err)
setTimeout(get, 5000)