FIRST – I am stealing code here and re-sharing (with very little modification). All credit goes the fine gentleman that wrote these two articles, I would urge you to read them:

Bulk Add IP Access Restrictions to Azure App Service Using AZ Powershell

Bulk Add Cloudflares IPs to Azure App Service Access Restrictions Using AZ Powershell

I made a few minor modifications the provided code. First, I like to just run a lot of my Azure Powershell stuff from an ISE session and don’t like encapsulating everything in new commands. Partly because I am not all that familiar with working that way even though it is probably a MUCH better way of doing things.

Before we get to the code though, what is this for exactly?

If you use cloudflare as a protection and CDN layer for a website it works by acting as a reverse proxy for your site. I.E. client connects to your site by a cloudflare hosted DNS record… instead of connecting directly to your server, their connection terminates at Cloudflare, they do things, then the pass the connection along to your actual service. ‘Nuff said, google if you need more info.

In the case of an Azure Web App (or any other web server I supposed), the app is hosted/available on some public IP and/or azure domain name that azure provides when you create the app…

What this means is that someone can easily bypass your cloudflare layer (and the associated performance enhancements and protections like web application firewalls) if they know your source systems IP address and in the case of azure, your azure provided domain name for your app.

So what that means is that you need to setup an ACL (Access Control List) on your source system to say “Allow traffic from all Cloudflare IP ranges and block everyone else”.

Cloudflare has like 20 IP ranges… And setting up that ACL by hand on a web app in Azure is arduous at best. But that is why we have scripting… to make things that are generally a pain in the rear… NOT a pain in the rear. (more…)