Securing a service network using Skupper policies

By default, Skupper includes many security features, including using mutual TLS for all service network communication between sites. You can add extra security features by installing the Skupper policy CRD. By default, applying a Skupper policy CRD to a cluster prevents all service network communication to and from that cluster. You specify granular Skupper policies CRs to permit only the service network communication you require.

📌 NOTE
A Skupper policy is distinct from the Kubernetes network policy, that is the network-policy option, which restricts access to Skupper services to the current namespace as described in Using the Skupper CLI.

Each site in a service network runs a Skupper router and has a private, dedicated certificate authority (CA). Communication between sites is secured with mutual TLS, so the service network is isolated from external access, preventing security risks such as lateral attacks, malware infestations, and data exfiltration. A set of Skupper policies adds another layer at a cluster level to help a cluster administrator control access to a service network.

This guide assumes that you understand the following Skupper concepts:

  • site
    A namespace in which Skupper is installed.
  • token
    A token is required to establish a link between two sites.
  • service network
    After exposing services using Skupper, you have created a service network.

About Skupper policies

After a cluster administrator installs a Skupper policy Custom Resource Definition (CRD), the cluster administrator needs to configure one or more policies to allow developers create and use services on the service network.

📌 NOTE
In this guide, developers refers to users of a cluster who have access to a namespace, but do not have administrator privileges.

A cluster administrator configures one or more of following items using custom resources (CRs) to enable communication:

  • Allow incoming links
    Use allowIncomingLinks to enable developers create tokens and configure incoming links.
  • Allow outgoing links to specific hosts
    Use allowedOutgoingLinksHostnames to specify hosts that developers can create links to.
  • Allow services
    Use allowedServices to specify which services developers can create or use on the service network.
  • Allow resources to be exposed
    Use allowedExposedResources to specify which resources a developer can expose on the service network.

📌 NOTE
A cluster administrator can apply each policy CR setting to one or more namespaces.

For example, the following policy CR fully allows all Skupper capabilities on all namespaces, except for:

  • only allows outgoing links to any domain ending in .example.com.
  • only allows 'deployment/nginx' resources to be exposed on the service network.
apiVersion: skupper.io/v1alpha1
kind: SkupperClusterPolicy
metadata:
  name: cluster-policy-sample-01
spec:
  namespaces:
    - "*"
  allowIncomingLinks: true
  allowedExposedResources:
    - "deployment/nginx"
  allowedOutgoingLinksHostnames: [".*\.example.com$"]
  allowedServices:
    - "*"
📌 NOTE

You can apply many policy CRs, and if there are conflicts in the items allowed, the most permissive policy is applied. For example, if you apply an additional policy CR with the line allowedOutgoingLinksHostnames: [], which does not list any hostnames, outgoing links to *.example.com are still permitted because that is permitted in the original CR.

  • namespaces
    One or more patterns to specify the namespaces that this policy applies to. Note that you can use Label selectors to match the namespaces.
  • allowIncomingLinks
    Specify true to allow other sites create links to the specified namespaces.
  • allowedOutgoingLinksHostnames
    Specify one or more patterns to determine which hosts you can create links to from the specified namespaces.
  • allowedServices
    Specify one or more patterns to determine the permitted names of services allowed on the service network from the specified namespaces.
  • allowedExposedResources
    Specify one or more permitted names of resources allowed on the service network from the specified namespaces. Note that patterns are not supported.
💡 TIP

Use regular expressions to create pattern matches, for example:

  • .*\.com$ matches any string ending in .com. A double backslash is required to avoid issues in YAML.
  • ^abc$ matches the string abc.

If you create another Skupper policy CR that allows outgoing links for a specific namespace, a user can create a link from that namespace to join a service network. That is, the logic for multiple policy CRs is OR. An operation is permitted if any single policy CR permits the operation.

Installing the Skupper policy CRD

Installing the Skupper policy CRD enables a cluster administrator to enforce policies for service networks.

📌 NOTE
If there are existing sites on the cluster, see Installing a Skupper policy CRD on a cluster with existing sites to avoid service network disruption.

  • Access to a cluster using a cluster-admin account
  • The Skupper operator is installed

  1. Log in to the cluster using a cluster-admin account.

  2. Download the CRD:

    $ wget https://raw.githubusercontent.com/skupperproject/skupper/{skupper-version}/api/types/crds/skupper_cluster_policy_crd.yaml

  3. Apply the CRD:

    $ kubectl apply -f skupper_cluster_policy_crd.yaml

customresourcedefinition.apiextensions.k8s.io/skupperclusterpolicies.skupper.io created
clusterrole.rbac.authorization.k8s.io/skupper-service-controller created

  4. To verify that a Skupper policy is active, use the skupper status command and check that the output includes the following line:

    Skupper is enabled for namespace "<namespace>" in interior mode (with policies).

Installing a Skupper policy CRD on a cluster with existing sites

If the cluster already hosts Skupper sites, note the following before installing the CRD:

  • All existing connections are closed. You must apply a policy CR to reopen connections.
  • All existing service network services and exposed resources are removed. You must create those resources again.

To avoid disruption:

  1. Plan the CRD deployment for an appropriate time.

  2. Search your cluster for sites:

    $ kubectl get pods --all-namespaces --selector=app=skupper

  3. Document each service and resource exposed on the service network.

  4. Install the CRD as described in Installing the Skupper policy CRD. This step closes connections and removes all service network services and exposed resources.

  5. If Skupper sites that were not created by cluster-admin exist in the cluster, you must grant permissions to read Skupper policies to avoid that site being blocked from the service network.

    For each site namespace:

    $ kubectl create clusterrolebinding skupper-service-controller-<namespace> --clusterrole=skupper-service-controller --serviceaccount=<namespace>:skupper-service-controller

    where <namespace> is the site namespace.

  6. Create Skupper policy CRs as described in Creating Skupper policy CRs

  7. Recreate any services and exposed resources as required.

Creating Skupper policy CRs

Skupper Policy CRs allow a cluster administrator to control communication across the service network from a cluster.

  • Access to a cluster using a cluster-admin account.
  • The Skupper policy CRD is installed on the cluster.

📌 NOTE
Typically, you create a Skupper policy CR that combines many elements from the steps below. See About Skupper policies for an example CR.

  1. Implement a policy to allow incoming links
  2. Implement a policy to allow outgoing links to specific hosts
  3. Implement a policy to allow specific services
  4. Implement a policy to allow specific resources

Use allowIncomingLinks to enable developers create tokens and configure incoming links.

  1. Determine which namespaces you want to apply this policy to.
  2. Create a CR with allowIncomingLinks set to true or false.
  3. Create and apply the CR.

For example, the following CR allows incoming links for all namespaces:

apiVersion: skupper.io/v1alpha1
kind: SkupperClusterPolicy
metadata:
  name: allowincominglinks
spec:
  namespaces:
    - "*"
  allowIncomingLinks: true

Use allowedOutgoingLinksHostnames to specify hosts that developers can create links to. You cannot create a allowedOutgoingLinksHostnames policy to disallow a specific host that was previously allowed.

  1. Determine which namespaces you want to apply this policy to.
  2. Create a CR with allowedOutgoingLinksHostnames set to a pattern of allowed hosts.
  3. Create and apply the CR.

For example, the following CR allows links to all subdomains of example.com for all namespaces:

apiVersion: skupper.io/v1alpha1
kind: SkupperClusterPolicy
metadata:
  name: allowedoutgoinglinkshostnames
spec:
  namespaces:
    - "*"
  allowedOutgoingLinksHostnames: ['.*\.example\.com']

Implement a policy to allow specific services

Use allowedServices to specify which services a developer can create or use on the service network. You cannot create a allowedServices policy to disallow a specific service that was previously allowed.

  1. Determine which namespaces you want to apply this policy to.
  2. Create a CR with allowedServices set to specify the services allowed on the service network.
  3. Create and apply the CR.

For example, the following CR allows users to expose and consume services with the prefix backend- for all namespaces:

apiVersion: skupper.io/v1alpha1
kind: SkupperClusterPolicy
metadata:
  name: allowedservices
spec:
  namespaces:
    - "*"
  allowedServices: ['^backend-']

📌 NOTE
When exposing services, you can use the --address <name> parameter of the skupper CLI to name services to match your policy.

Implement a policy to allow specific resources

Use allowedExposedResources to specify which resources a developer can expose on the service network. You cannot create a allowedExposedResources policy to disallow a specific resource that was previously allowed.

  1. Determine which namespaces you want to apply this policy to.
  2. Create a CR with allowedExposedResources set to specify resources that a developer can expose on the service network.
  3. Create and apply the CR.

For example, the following CR allows you to expose an nginx deployment for all namespaces:

apiVersion: skupper.io/v1alpha1
kind: SkupperClusterPolicy
metadata:
  name: allowedexposedresources
spec:
  namespaces:
    - "*"
  allowedExposedResources: ['deployment/nginx']

📌 NOTE
For allowedExposedResources, each entry must conform to the type/name syntax.