This was something that I meant to write about a while ago but never got around to it. I recieved a question recently by email and wanted to provide some tips and tricks when using ISAPI Rewrite in a multi-tiered load balanced environment. I wrote an article with Frank DeRienzo several years ago about using a multi-tiered load balanced enviroment. I followed this up later with a blog entry on my site for Using ISAPI Rewrite with ColdFusion.

If you are using ISAPI Rewrite to proxy requests from your webserver (IIS) to you ColdFusion server or through a load balancer you will run into issues when accessing various CGI variables. When proxying the ColdFusion server sees the web server as the end client and therefore variables like CGI.HTTP_HOST are set to the IP or hostname of the webserver. To get around this I inject custom headers with ISAPI rewrite, parse them in CF and then stored them in the request scope.

First I start by adding the headers to the main httpd.ini Start->All Programs->Helicon->ISAPI_Rewrite->httpd.ini (you may need to change the read only flag in order to save the file)

#Due to the proxy we add these headers to use in cf through
#GetHttpRequestData().headers.HEADERNAME
RewriteCond %REMOTE_ADDR (.*)
RewriteHeader CLIENT_REMOTE_ADDR: ^$ $1

RewriteCond %SCRIPT_NAME (.*)
RewriteHeader CLIENT_SCRIPT_NAME: ^$ $1

RewriteCond %SERVER_NAME (.*)
RewriteHeader CLIENT_SERVER_NAME: ^$ $1

RewriteCond %SERVER_PORT (.*)
RewriteHeader CLIENT_SERVER_PORT: ^$ $1

RewriteCond %HTTP_HOST (.*)
RewriteHeader CLIENT_HTTP_HOST: ^$ $1

RewriteCond %SERVER_PROTOCOL (.*)
RewriteHeader CLIENT_SERVER_PROTOCOL: ^$ $1

#stores the original URL before any rewriting in a custom header
RewriteCond URL (.*)
RewriteHeader x_rewrite_url_original: ^$ $1

RewriteCond X-Rewrite-Proxy (.*)
RewriteHeader x_rewrite_proxy: ^$ $1

I then create a page in CF called httpheaders.cfm and include it early on in my CF code within Appplication.cfm or within onRequest of Application.cfc

<cfsilent>
<!--- this page is added to handle the custom headers we write with 
ISAPI Rewrite due to the issues we see with proxying requests.
 --->
<cfset httpheaders=GetHttpRequestData().headers>
<cfset REQUEST.proxyheaders=structNew()>
<cfif isdefined("httpheaders.CLIENT_REMOTE_ADDR")>
    <cfset REQUEST.proxyheaders.REMOTE_ADDR=httpheaders.CLIENT_REMOTE_ADDR>
<cfelse>
    <cfset REQUEST.proxyheaders.REMOTE_ADDR=CGI.REMOTE_ADDR>
</cfif>
<!--- I need to find a way to handle SCRIPT_NAME better, with ISAPI 
Rewrite and JRun ones are different --->
<cfif isdefined("httpheaders.CLIENT_SCRIPT_NAME")>
    <!--- <cfset 
REQUEST.proxyheaders.SCRIPT_NAME=httpheaders.CLIENT_SCRIPT_NAME> --->
    <cfset REQUEST.proxyheaders.SCRIPT_NAME=CGI.SCRIPT_NAME>
<cfelse>
    <cfset REQUEST.proxyheaders.SCRIPT_NAME=CGI.SCRIPT_NAME>
</cfif>
<cfif isdefined("httpheaders.CLIENT_SERVER_NAME")>
    <cfset REQUEST.proxyheaders.SERVER_NAME=httpheaders.CLIENT_SERVER_NAME>
<cfelse>
    <cfset REQUEST.proxyheaders.SERVER_NAME=CGI.SERVER_NAME>
</cfif>
<cfif isdefined("httpheaders.CLIENT_SERVER_PORT")>
    <cfset REQUEST.proxyheaders.SERVER_PORT=httpheaders.CLIENT_SERVER_PORT>
<cfelse>
    <cfset REQUEST.proxyheaders.SERVER_PORT=CGI.SERVER_PORT>
</cfif>
<cfif isdefined("httpheaders.CLIENT_SERVER_PROTOCOL")>
    <cfset 
REQUEST.proxyheaders.SERVER_PROTOCOL=httpheaders.CLIENT_SERVER_PROTOCOL>
<cfelse>
    <cfset REQUEST.proxyheaders.SERVER_PROTOCOL=CGI.SERVER_PROTOCOL>
</cfif>
<cfif isdefined("httpheaders.CLIENT_HTTP_HOST")>
    <cfset REQUEST.proxyheaders.HTTP_HOST=httpheaders.CLIENT_HTTP_HOST>
<cfelse>
    <cfset REQUEST.proxyheaders.HTTP_HOST=CGI.HTTP_HOST>
</cfif>
</cfsilent>

After including this code you can access the client's IP with "REQUEST.proxyheaders.REMOTE_ADDR".

One other challenge is detecting if a user is using HTTPS if you have front ended your site with a load balancer and use the SSL accelerator of BIG-IP or Cisco CSS. To the app it looks exactlyt he same. I will save that discussion for another blog posting. Let me know if you have any questions.

There should be a similar solution with mod_proxy in apache by adding headers I just have not researched it. If you have the information post your findings and I will add it to the blog.

Can you give me some insight into how you handle this?

Something like: RewriteCond %HTTPS (.*) RewriteHeader CLIENT_HTTPS: ^$ $1

Tony