server.js

var httpProxy = require('http-proxy')
  , url = require('url')
  , http = require('http')

exports.handleRequest = function(
  subProxyHost, // e.g., localhost
  port, // port to host on 
  req, 
  res, 
  proxy) { // http-proxy's proxy argument

  if(port != 80)
    subProxyHost += ':' + port

  if(req.headers.host.indexOf(subProxyHost) ===
    req.headers.host.length - subProxyHost.length) {

    // infer the proxy domain
    var proxyDomain = req.headers.host.
      slice(0, req.headers.host.length - subProxyHost.length -1)
    var originalHost = req.headers.host

    // replace the host in the request
    req.headers.host = proxyDomain

    // intercept calls to writeHead, so that we write the new location there
    var oldWriteHead = res.writeHead
    res.writeHead = function(statusCode, headers) {

      if(headers.location) {

        // replace the host in the response
        var parsedUrl = url.parse(headers.location)
        if(parsedUrl.host === req.headers.host) {
          parsedUrl.host = originalHost + (port === 80 ? '' : ':' + port)
          headers.location = url.format(parsedUrl)
        }
      }

      // now that we've overwritten the header the node-http-proxy passes,
      // we pass it along
      oldWriteHead.apply(res, arguments)
    }

    proxy.proxyRequest(req, res, {
      host: proxyDomain,
      port: 80
    })
  }

  else {

    res.statusCode = 500
    res.setHeader('Content-Type', 'text/plain')
    res.end('Internal Server Error: failed to identify host "' +
      req.headers.host + '"; expecting something ending with "' +
      subProxyHost +'"')
  }
}