What does “trust proxy” actually do in express.js, and do I need to use it?

Question

I am writing an express app that sits behind an nginx server. I was reading through express\'s documentation and it mentioned the \'trust proxy\' setting. All it says is

<

Answer 1

Annotated code to explain use of trust proxy

    var express = require('express');

    var app = express();

    // Set the ip-address of your trusted reverse proxy server such as 
    // haproxy or Apache mod proxy or nginx configured as proxy or others.
    // The proxy server should insert the ip address of the remote client
    // through request header 'X-Forwarded-For' as
    // 'X-Forwarded-For: some.client.ip.address'
    // Insertion of the forward header is an option on most proxy software
    app.set('trust proxy', '127.0.0.1');


    app.get('/test', function(req, res){
      var ip = req.ip; // trust proxy sets ip to the remote client (not to the ip of the last reverse proxy server)
      if (ip.substr(0,7) == '::ffff:') { // fix for if you have both ipv4 and ipv6
        ip = ip.substr(7);
      }
      // req.ip and req.protocol are now set to ip and protocol of the client, not the ip and protocol of the reverse proxy server
      // req.headers['x-forwarded-for'] is not changed
      // req.headers['x-forwarded-for'] contains more than 1 forwarder when
      // there are more forwarders between the client and nodejs.
      // Forwarders can also be spoofed by the client, but 
      // app.set('trust proxy') selects the correct client ip from the list
      // if the nodejs server is called directly, bypassing the trusted proxies,
      // then 'trust proxy' ignores x-forwarded-for headers and
      // sets req.ip to the remote client ip address

      res.json({"ip": ip, "protocol": req.protocol, "headers": req.headers['x-forwarded-for']});
    });

// in this example the reverse proxy is expected to forward to port 3110
var port = 3110;
app.listen(port);
// test through proxy: http://yourproxyserver/test, req.ip should be your client ip
// test direct connection: http://yournodeserver:3110/test, req.ip should be your client ip even if you insert bogus x-forwarded-for request headers
console.log('Listening at http://localhost:' + port);