Azure Kubernetes Service and Azure Firewall

Deploying Azure Kubernetes Service (AKS) is, like most other Kubernetes-as-a-service offerings such as those from DigitalOcean and Google, very straightforward. It’s either a few clicks in the portal or one or two command lines and you are finished.

Using these services properly and in a secure fashion is another matter though. I am often asked how to secure access to the cluster and its applications. In addition, customers also want visibility and control of incoming and outgoing traffic. Combining Azure Firewall with AKS is one way of achieving those objectives.

This post will take a look at the combination of Azure Firewall and AKS. It is inspired by this post by Dennis Zielke. In that post, Dennis provides all the necessary Azure CLI commands to get to the following setup:

AKS and Azure Firewall (from
https://medium.com/@denniszielke/setting-up-azure-firewall-for-analysing-outgoing-traffic-in-aks-55759d188039 by Dennis Zielke)

In what follows, I will keep referring to the subnet names and IP addresses as in the above diagram.

Azure Firewall

Azure Firewall is a stateful firewall, provided as a service with built-in high availability. You deploy it in a subnet of a virtual network. The subnet should have the name AzureFirewallSubnet. The firewall will get two IP addresses:

  • Internal IP: the first IP address in the subnet (here 10.0.3.4)
  • Public IP: a public IP address; in the above setup we will use it to provide access to a Kubernetes Ingress controller via a DNAT rule

As in the physical world, you will need to instruct systems to route traffic through the firewall. In Azure, this is done via a route table. The following route table was created:

Route table

In (1) a route to 0.0.0.0/0 is defined that routes to the private IP of the firewall. The route will be used when no other route applies! The route table is associated with just the aks-5-subnet (2), which is the subnet where AKS (with advanced networking) is deployed. It’s important to note that now, all external traffic originating from the Kubernetes cluster passes through the firewall.

When you compare Azure Firewall to the Network Virtual Appliances (NVAs) from vendors such as CheckPoint, you will notice that the capabilities are somewhat limited. On the flip side though, Azure Firewall is super simple to deploy when compared with a highly available NVA setup.

Before we look at the firewall rules, let’s take a look at the Kubernetes Ingress Controller.

Kubernetes Ingress Controller

In this example, I will deploy nginx-ingress as an Ingress Controller. It will provide access to HTTP-based workloads running in the cluster and it can route to various workloads based on the URL. I will deploy the nginx-ingress with Helm.

Think of an nginx-ingress as a reverse proxy. It receives http requests, looks at the hostname and path (e.g. mydomain.com/api/user) and routes the request to the appropriate Kubernetes service (e.g. the user service).

Diagram showing Ingress traffic flow in an AKS cluster
Ingress in Kubernetes (from Microsoft:
https://docs.microsoft.com/en-us/azure/aks/operator-best-practices-network#distribute-ingress-traffic )

Normally, the nginx-ingress service is accessed via an Azure external load balancer. Behind the scenes, this is the result of the service object having spec.type set to the value LoadBalancer. If we want external traffic to nginx-ingress to pass through the firewall, we will need to tell Kubernetes to create an internal load balancer via an annotation. Let’s do that with Helm. First, you will need to install tiller, the server-side component of Helm. Use the following procedure from the Microsoft documentation:

  • Create a service account for tiller: link
  • Configure tiller: link

With tiller installed, issue the following two commands:

kubectl create ns ingress 

helm install stable/nginx-ingress --namespace ingress --set controller.replicaCount=2 --set controller.service.annotations."service\.beta\.kubernetes\.io/azure-load-balancer-internal"=true --set controller.service.annotations."service\.beta\.kubernetes\.io/azure-load-balancer-internal-subnet"=ing-4-subnet

The second command installs nginx-ingress in the ingress namespace. The two –set parameters add the following annotations to the service object (yes I know, the Helm annotation parameters are ugly 🤢):

service.beta.kubernetes.io/azure-load-balancer-internal: "true"
service.beta.kubernetes.io/azure-load-balancer-internal-subnet: ing-4-subnet

This ensures an internal load balancer gets created. It gets created in the mc-* resource group that backs your AKS deployment:

Internal load balancer created by the Kubernetes cloud integration components

Note that Kubernetes creates the load balancer, including the rules and probes for port 80 and 443 as defined in the service object that comes with the Helm chart. The load balancer is created in the ing-4-subnet as instructed by the service annotation. Its private IP address is 10.0.4.4 as in the diagram at the top of this post

DNAT Rule to Load Balancer

To provide access to internal resources, Azure Firewall uses DNAT rules which stands for destination network address translation. The concept is simple: traffic to the firewall’s public IP on some port can be forwarded to an internal IP on the same or another port. In our case, traffic to the firewall’s public IP on port 80 and 443 is forwarded to the internal load balancer’s private IP on port 80 and 443. The load balancer will forward the request to nginx-ingress:

DNAT rule forwarding port 80 and 443 traffic to the internal load balancer

If the installation of nginx-ingress was successful, you should end up at the default back-end when you go to http://firewallPublicIP.

nginx-ingress default backend when browsing to public IP of firewall

If you configured Log Analytics and installed the Azure Firewall solution, you can look at the firewall logs. DNAT actions are logged and can be inspected:

Firewall logs via Log Analytics

Application and Network Rules

Azure Firewall application rules are rules that allow or deny outgoing HTTP/HTTPS traffic based on the URL. The following rules were defined:

Application rules

The above rules allow http and https traffic to destinations such as docker.io, cloudflare and more.

Note that another Azure Firewall rule type, network rules, are evaluated first. If a match is found, rule evaluation is stopped. Suppose you have these network rules:

Network rules

The above network rule allows port 22 and 443 for all sources and destinations. This means that Kubernetes can actually connect to any https-enabled site on the default port, regardless of the defined application rules. See rule processing for more information.

Threat Intelligence

This feature alerts on and/or denies network traffic coming from known bad IP addresses or domains. You can track this via Log Analytics:

Threat Intelligence Alerts and Denies on Azure Firewall

Above, you see denied port scans, traffic from botnets or brute force credentials attacks all being blocked by Azure Firewall. This feature is currently in preview.

Best Practices

The AKS documentation has a best practices section that discusses networking. It contains useful information about the networking model (Kubenet vs Azure CNI), ingresses and WAF. It does not, at this point in time (May 2019), desicribe how to use Azure Firewall with AKS. It would be great if that were added in the near future.

Here are a couple of key points to think about:

  • WAF (Web Application Firewall): Azure Firewall threat intelligence is not WAF; to enable WAF, there are several options:
    • you can enable mod_security in nginx_ingress
    • you can use Azure WAF or a 3rd party WAF
    • you can use cloud-native WAFs such as TwistLock (WAF is one of the features of this product; it also provides firewall and vulnerability assessment)
  • remote access to Kubernetes API: today, the API server is exposed via a public IP address; having the API server on a local IP will be available soon
  • remote access to Kubernetes hosts using SSH: only allow SSH on the private IP addresses; use a bastion host to enable connectivity

Conclusion

Azure Kubernetes Service (AKS) can be combined with Azure Firewall to control network traffic to and from your Kubernetes cluster. Log Analytics provides the dashboard and logs to report and alert on traffic patterns. Features such as threat intelligence provide an extra layer of defense. For HTTP/HTTPS workloads (so most workloads), you should complement the deployment with a WAF such as Azure Application Gateway or 3rd party.