Allow forwarding for x-forwarded-[for|port|proto] to enabled layering of http-proxies.

lib/node-http-proxy.js

@@ -328,6 +328,7 @@ HttpProxy.prototype.close = function () {
 //     options.host {string} Host of the proxy target.
 //     options.buffer {Object} Result from `httpProxy.buffer(req)`
 //     options.https {Object|boolean} Settings for https.
+//     options.allow_xforwarded_headers {boolean} Don't clobber x-forwarded headers to allow layered proxies.
 //
 HttpProxy.prototype.proxyRequest = function (req, res, options) {
   var self = this, errState = false, location, outgoing, protocol, reverseProxy;
@@ -340,6 +341,7 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
   options      = options || {};
   options.host = options.host || this.target.host;
   options.port = options.port || this.target.port;
+  options.allow_xforwarded_headers = options.allow_xforwarded_headers || false;
   
   //
   // Check the proxy table for this instance to see if we need
@@ -379,9 +381,11 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
   // * `x-forwarded-proto`: Protocol of the original request
   // * `x-forwarded-port`: Port of the original request. 
   //
-  req.headers['x-forwarded-for']   = req.connection.remoteAddress || req.connection.socket.remoteAddress;
-  req.headers['x-forwarded-port']  = req.connection.remotePort || req.connection.socket.remotePort;
-  req.headers['x-forwarded-proto'] = res.connection.pair ? 'https' : 'http';
+  if (options.allow_xforwarded_headers == true) {
+    req.headers['x-forwarded-for']   = req.connection.remoteAddress || req.connection.socket.remoteAddress;
+    req.headers['x-forwarded-port']  = req.connection.remotePort || req.connection.socket.remotePort;
+    req.headers['x-forwarded-proto'] = res.connection.pair ? 'https' : 'http';
+  }
   
   //
   // Emit the `start` event indicating that we have begun the proxy operation.