Node.js request forwarder

passthrough.js

/*
node.js request forwarder

Usage:

const express = require('express');
const passthrough = require('./passthrough');

const app = express();
app.enable('trust proxy');
app.use('/proxy', passthrough);
*/

const url = require('url'),
  http = require('http'),
  https = require('https');

module.exports = function(req, res, next) {
  req.pause();
  const options = makeOptions(req),
    proxyUrl = `${req.protocol}://${req.headers['x-forwarded-host']}${req.baseUrl}/`,  // requires app.enable('trust proxy'); to work properly
    respHandler = function(serverResponse) { 
      return serverRespHandler(serverResponse, res, proxyUrl); 
    },
    connector = (options.protocol == 'https:' ? https : http).request(options, respHandler);

  req.pipe(connector, {end:true});
  req.resume();
}


function makeOptions(req) {
  const reqUrl = req.url.substr(1),
    options = url.parse(reqUrl);

  options.headers = req.headers;
  options.method = req.method;
  options.agent = false;
  options.headers['host'] = options.host;
  
  return options;
}

function absLocation(loc, req) {
  if(loc.toLowerCase().substr(0,4) == 'http') {
    return loc;
  }
  
  return new url.URL(loc, `${req.url.substr(1)}`).href;
}

function serverRespHandler(serverResponse, res, proxyUrl) {
    serverResponse.pause();
    serverResponse.headers['access-control-allow-origin'] = '*';

    switch (serverResponse.statusCode) {
      // pass through.  we're not too smart here...
      case 200: case 201: case 202: case 203: case 204: case 205: case 206:
      case 304:
      case 400: case 401: case 402: case 403: case 404: case 405:
      case 406: case 407: case 408: case 409: case 410: case 411:
      case 412: case 413: case 414: case 415: case 416: case 417: case 418:
        res.writeHeader(serverResponse.statusCode, serverResponse.headers);
        serverResponse.pipe(res, {end:true});
        serverResponse.resume();
        break;

      // fix host and pass through
      case 301:
      case 302:
      case 303:
      case 307:
      case 308:        
        const redirectTo = absLocation(serverResponse.headers['location'], res.req);
        serverResponse.headers['location'] = proxyUrl + redirectTo;
        console.log('\t-> Redirecting to ', serverResponse.headers['location']);
        res.writeHeader(serverResponse.statusCode, serverResponse.headers);
        serverResponse.pipe(res, {end:true});
        serverResponse.resume();
        break;

      // error everything else
      default:
        let stringifiedHeaders = JSON.stringify(serverResponse.headers, null, 4);
        serverResponse.resume();
        res.writeHeader(500, {'content-type': 'text/plain'});
        res.end(`Error ${serverResponse.statusCode}\n${stringifiedHeaders}`);
      break;
    }
}