How to get Node JS Express to listen only on localhost

Question

I have an application that I have behind a reverse proxy, I would like for it to only listen to localhost/127.0.0.1.

I expected this to work:

app.listen(3001, 'localhost');

or

app.listen(3001, '127.0.0.1');

but instead I get an error:

node.js:201
    throw e; // process.nextTick error, or 'error' event on first tick
          ^
TypeError: Cannot read property 'port' of null
    at Object.<anonymous> (/home/ctoledo/hive-go/go.js:204:76)
    at Module._compile (module.js:441:26)
    at Object..js (module.js:459:10)
    at Module.load (module.js:348:31)
    at Function._load (module.js:308:12)
    at Array.0 (module.js:479:10)
    at EventEmitter._tickCallback (node.js:192:40)

How to fix this?

Answer 1

Hello @kartik,

This is a bug in hive-go that only shows up when you add a host. The last lines of it are:

app.listen(3001);
console.log("... port %d in %s mode", app.address().port, app.settings.env);

When you add the host on the first line, it is crashing when it calls app.address().port.

The problem is the potentially asynchronous nature of .listen(). Really it should be doing that console.log call inside a callback passed to listen. When you add the host, it tries to do a DNS lookup, which is async. So when that line tries to fetch the address, there isn't one yet because the DNS request is running, so it crashes.

Try this:

app.listen(3001, 'localhost', function() {
  console.log("... port %d in %s mode", app.address().port, app.settings.env);
});

Hope it helps!!

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Thank You!!