url_rewrite_based_on_hostname.js

module.exports = function url_rewrite_based_on_hostname(hostname,prefix_path){
  if (!hostname) throw new Error('url rewrite hostname required');
  if (!prefix_path) throw new Error('url rewrite prefix path required');
  var regexp = new RegExp('^' + hostname.replace(/[*]/g, '(.*?)') + '$');


  // this is the actual middleware implementation, that is called when you get an HTTP request.
  return function fn(req, res, next){
    if (!req.headers.host) return next(); // skip if there is no HOST header.
    var host = req.headers.host.split(':')[0]; // get the HOST header
    if (req.subdomains = regexp.exec(host)) { // see if the hostname matches, and pull out subdomains.
      req.url = prefix_path + req.url; // REWRITE THE URL 
      req.subdomains = req.subdomains.slice(1); // GET SUBDOMAINS (probably USELESS)
    };
    next(); // Delegate to next middleware
  };
};


domain1.com => 127.0.0.1:9000/path1
domain2.com => 127.0.0.1:9000/path2
domain3.com => 127.0.0.1:9000/path3

var express = require('express');

var app = express.createServer(
    url_rewrite_based_on_hostname("domain1.com","/path1"),
    url_rewrite_based_on_hostname("domain2.com","/path2"),
    url_rewrite_based_on_hostname("domain2.com","/path3"),
  );