Azure Firewall – Where to Start?

About a year ago, Microsoft introduced the first release of Azure Firewall. Since then, and since its general release the service has grown and the features have matured.

To begin, let’s understand what Azure Firewall is? At its core it’s a managed, network security service that protects your Azure Virtual Network resources. It functions as a stateful firewall-as-a-service and offers built-in high availability and scalability. This means you can centrally control, enforce and log all of your network traffic. It fully integrates with Azure Monitor too which means all of the usual logging and analytical goodness.

If the above sounds like something you’d like to use, or at least try, in your Azure environment, read on! To start, let’s break out what can be configured within Azure Firewall and which features could be useful for you.

When deploying an Azure Firewall, you need a couple of things in advance. It needs a dedicated subnet, specifically named “AzureFirewallSubnet” and the minimum size it can be is a /26. It also needs at least one Static Public IP. The Public IP must be on the Standard tier. My recommendation here is to look at creating a Public IP Prefix in advance of creating your Azure Firewall. That way, if you need to delete it and redeploy, you can continue to use the same Public IP again and again. If you want to use multiple Public IPs, it supports up to 100.

So, let’s look at what Azure Firewall (AFW) can do for you on your Virtual Network and then consider some deployment options.

Access

Using your single, or multiple Public IP addresses, AFW allows both source and destination NATing. Meaning it can support multiple inbound ports, such as HTTPS over 443 to different resources. Outbound SNAT helps greatly with services that require white-listing. If you are using multiple Public IPs, AFW randomly picks one for SNAT, so ensure you include all of them in your white-listing requirements.

Protection

AFW uses a Microsoft service called Threat Intelligence filtering. This allows Azure Firewall to alert and deny traffic to and from known malicious IPs and domains. You can turn this setting off, set it to just alert or to both alert and deny. All of the actions are logged.

Filtering

Finally, for filtering, AFW can use both Network Traffic and Application FQDN rules. This means that you can limit traffic to only those explicitly listed within the rule collections. For example, an application rule that only allows traffic to the FQDN – www.wedoazure.ie

A visual representation of the above features is below:

Firewall overview

Now that you understand AFW, let’s look at how to configure to your needs. Normally I would go into the deployment aspect, but it is excellently documented already and relatively easy to follow. However, there are some aspects of the configuration that warrant further detail.

Once deployed, you must create a Custom Route Table to force traffic to your AFW. In the tutorial, it shows you how to create a route for Internet traffic (0.0.0.0/0), however you may want the AFW to be your central control point for your vnet traffic too. Don’t forget, traffic between subnets is not filtered by default. Routing all traffic for each subnet to AFW could allow you to manage which subnet can route where centrally. For example, if we have three subnets, Web, App and DB. A single route table applied to each subnet can tunnel all traffic to AFW. On the AFW you can then allow Web to the Internet and the App subnet. The App subnet can access Web and DB but not Internet and finally the DB subnet can only access the App subnet. This would all be achieved with a single Network Rule collection.

Similarly you can allow/block specific FQDNs with an Application Rule collection. In the tutorial, a single FQDN is allowed. This means that all others are blocked as that is the default behaviour. This might not be practical for your environment and the good news is, you can implement the reverse. With the right priority order, you can allow all traffic except for blocked FQDNs.

A feature you may also want to consider trying is destination NATing. This thankfully has another well documented tutorial on Docs.

Finally, and in some cases most importantly, let’s look at price. You are charged in two ways for AFW. There is a price per-hour-per-instance. That means if you deploy and don’t use it for anything, you will pay approx. €770 per-month (PAYG Calculator). On top of that, you will pay for both data inbound and outbound that is filtered by AFW. You’re charged the same price either direction and that’s approx. €14 per-Tb-per-month. Depending on your environment and/or requirements this price could be OK or too steep. My main advice is to ensure you understand it before deploying!

As always, if there are any questions please get in touch!

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