Locking down external access with DNS-based policies

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Deploy the Demo Application

DNS-based policies are very useful for controlling access to services running outside the Kubernetes cluster. DNS acts as a persistent service identifier for both external services provided by AWS, Google, Twilio, Stripe, etc., and internal services such as database clusters running in private subnets outside Kubernetes. CIDR or IP-based policies are cumbersome and hard to maintain as the IPs associated with external services can change frequently. The Cilium DNS-based policies provide an easy mechanism to specify access control while Cilium manages the harder aspects of tracking DNS to IP mapping.

• Controlling egress access to services outside the cluster using DNS-based policies
• Using patterns (or wildcards) to whitelist a subset of DNS domains
• Combining DNS, port and L7 rules for restricting access to external service

In line with our Star Wars theme examples, we will use a simple scenario where the empire’s pods need access to Twitter for managing the empire’s tweets. The pods shouldn’t have access to any other external service.

$ kubectl create -f https://raw.githubusercontent.com/cilium/cilium/v1.8/examples/kubernetes-dns/dns-sw-app.yaml
$ kubectl get po
NAME                             READY   STATUS    RESTARTS   AGE
pod/mediabot                     1/1     Running   0          14s

The following Cilium network policy allows mediabot pods to only access api.twitter.com.

apiVersion: "cilium.io/v2"
kind: CiliumNetworkPolicy
metadata:
  name: "fqdn"
spec:
  endpointSelector:
    matchLabels:
      org: empire
      class: mediabot
  egress:
  - toFQDNs:
    - matchName: "api.twitter.com"  
    - matchLabels:
        "k8s:io.kubernetes.pod.namespace": kube-system
        "k8s:k8s-app": kube-dns
    toPorts:
    - ports:
      - port: "53"
        protocol: ANY
      rules:
        dns:
        - matchPattern: "*"

Let’s take a closer look at the policy:

• The first egress section uses toFQDNs: matchName specification to allow egress to api.twitter.com. The destination DNS should match exactly the name specified in the rule. The endpointSelector allows only pods with labels to have the egress access.
• The second egress section allows mediabot pods to access kube-dns service. Note that rules: dns instructs Cilium to inspect and allow DNS lookups matching specified patterns. In this case, inspect and allow all DNS queries.

Note that with this policy the mediabot doesn’t have access to any internal cluster service other than kube-dns. Refer to to learn more about policies for controlling access to internal cluster services.

$ kubectl create -f https://raw.githubusercontent.com/cilium/cilium/v1.8/examples/kubernetes-dns/dns-matchname.yaml

Testing the policy, we see that mediabot has access to api.twitter.com but doesn’t have access to any other external service, e.g., help.twitter.com.

DNS Policies Using Patterns

The above policy controlled DNS access based on exact match of the DNS domain name. Often, it is required to allow access to a subset of domains. Let’s say, in the above example, mediabot pods need access to any Twitter sub-domain, e.g., the pattern *.twitter.com. We can achieve this easily by changing the toFQDN rule to use matchPattern instead of matchName.

apiVersion: "cilium.io/v2"
kind: CiliumNetworkPolicy
metadata:
  name: "fqdn"
spec:
  endpointSelector:
    matchLabels:
      org: empire
      class: mediabot
  egress:
  - toFQDNs:
    - matchPattern: "*.twitter.com" 
    - matchLabels:
        "k8s:io.kubernetes.pod.namespace": kube-system
        "k8s:k8s-app": kube-dns
    toPorts:
    - ports:
        protocol: ANY
      rules:
        dns:
        - matchPattern: "*"

$ kubectl apply -f https://raw.githubusercontent.com/cilium/cilium/v1.8/examples/kubernetes-dns/dns-pattern.yaml

Test that mediabot has access to multiple Twitter services for which the DNS matches the pattern *.twitter.com. It is important to note and test that this doesn’t allow access to twitter.com because the *. in the pattern requires one subdomain to be present in the DNS name. You can simply add more matchName and matchPattern clauses to extend the access. (See policies to learn more about specifying DNS rules using patterns and names.)

$ kubectl exec -it mediabot -- curl -sL https://help.twitter.com
...
$ kubectl exec -it mediabot -- curl -sL https://about.twitter.com
...
$ kubectl exec -it mediabot -- curl -sL https://twitter.com
^C

The DNS-based policies can be combined with port (L4) and API (L7) rules to further restrict the access. In our example, we will restrict mediabot pods to access Twitter services only on ports 443. The toPorts section in the policy below achieves the port-based restrictions along with the DNS-based policies.

$ kubectl apply -f https://raw.githubusercontent.com/cilium/cilium/v1.8/examples/kubernetes-dns/dns-port.yaml

$ kubectl exec -it mediabot -- curl https://help.twitter.com
...
$ kubectl exec -it mediabot -- curl http://help.twitter.com
^C

Refer to Layer 4 Examples and to learn more about Cilium L4 and L7 network policies.

Clean-up

$ kubectl delete cnp fqdn