We get a ton of inquiries along the lines of “I want to program my firewall using IP ranges to allow outbound access only to my cloud-based apps”. If you (or the IT department) insist on doing this with Windows Azure, there is even a downloadable and fairly regularly updated list of the IP ranges on the Microsoft Download Center in a straightforward XML format.

Now, we do know that there are a lot of customers who keep insisting on using IP address ranges for that purpose, but that strategy is not a recipe for success.

The IP ranges shift and expand on a very frequent basis and cover all of the Windows Azure services. Thus, a customer will open their firewall for traffic for the entire multitenant range of Azure, which means that the customer’s environment can reach their own apps and the backend services for the “Whack A Panda” game just the same. With apps in the cloud, there is no actual security gain from these sorts of constraints; pretty much all the advantages of automated, self-service cloud environments stem from shared resources including shared networking and shared gateways and the ability to do dynamic failover including cross-DC failover and the like that means that there aren’t any reservations at the IP level that last forever.

The best way to handle this is to do the exact inverse of what’s being tried with these rules, and rather limit access to outside resources to a constrained set of services based on the services’ or users’ identity as it is done on our Microsoft corporate network. At Microsoft, you can’t get out through the NAT/Proxy unless you have an account that has external network privileges. If you are worried about a service or user abusing access to the Internet, don’t give them Internet. If you think you need to have tight control, make a DMZ – in the opposite direction of how you usually think about a DMZ.

Using IP-address based outbound firewall access rules constraining access to public cloud computing resources is probably getting a box on a check-list ticked, but it doesn’t add anything from a security perspective. It’s theater. IMHO.