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About network policy - Network policy | Networking | OpenShift Container Platform 4.5
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About network policy

As a cluster administrator, you can define network policies that restrict traffic to pods in your cluster.

About network policy

In a cluster using a Kubernetes Container Network Interface (CNI) plug-in that supports Kubernetes network policy, network isolation is controlled entirely by NetworkPolicy objects. In OpenShift Container Platform 4.5, OpenShift SDN supports using network policy in its default network isolation mode.

When using the OpenShift SDN cluster network provider, the following limitations apply regarding network policies:

  • Egress network policy as specified by the egress field is not supported.

  • IPBlock is supported by network policy, but without support for except clauses. If you create a policy with an IPBlock section that includes an except clause, the SDN pods log warnings and the entire IPBlock section of that policy is ignored.

Network policy does not apply to the host network namespace. Pods with host networking enabled are unaffected by network policy rules.

By default, all pods in a project are accessible from other pods and network endpoints. To isolate one or more pods in a project, you can create NetworkPolicy objects in that project to indicate the allowed incoming connections. Project administrators can create and delete NetworkPolicy objects within their own project.

If a pod is matched by selectors in one or more NetworkPolicy objects, then the pod will accept only connections that are allowed by at least one of those NetworkPolicy objects. A pod that is not selected by any NetworkPolicy objects is fully accessible.

The following example NetworkPolicy objects demonstrate supporting different scenarios:

  • Deny all traffic:

    To make a project deny by default, add a NetworkPolicy object that matches all pods but accepts no traffic:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: deny-by-default
spec:
  podSelector:
  ingress: []

  • Only allow connections from the OpenShift Container Platform ingress Controller:

    To make a project allow only connections from the OpenShift Container Platform ingress Controller, add the following NetworkPolicy object.

    For the OVN-Kubernetes network provider plug-in, when the ingress Controller is configured to use the HostNetwork endpoint publishing strategy, there is no supported way to apply network policy so that ingress traffic is allowed and all other traffic is denied.

    		 
    apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-from-openshift-ingress
spec:
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          network.openshift.io/policy-group: ingress
  podSelector: {}
  policyTypes:
  - ingress

    If the ingress Controller is configured with endpointPublishingStrategy: HostNetwork, then the ingress Controller pod runs on the host network. When running on the host network, the traffic from the ingress Controller is assigned the netid:0 Virtual Network ID (VNID). The netid for the namespace that is associated with the ingress Operator is different, so the matchLabel in the allow-from-openshift-ingress network policy does not match traffic from the default ingress Controller. With OpenShift SDN, the default namespace is assigned the netid:0 VNID and you can allow traffic from the default ingress Controller by labeling your default namespace with network.openshift.io/policy-group: ingress.

  • Only accept connections from pods within a project:

    To make pods accept connections from other pods in the same project, but reject all other connections from pods in other projects, add the following NetworkPolicy object:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: allow-same-namespace
spec:
  podSelector:
  ingress:
  - from:
    - podSelector: {}

  • Only allow HTTP and HTTPS traffic based on pod labels:

    To enable only HTTP and HTTPS access to the pods with a specific label (role=frontend in following example), add a NetworkPolicy object similar to the following:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: allow-http-and-https
spec:
  podSelector:
    matchLabels:
      role: frontend
  ingress:
  - ports:
    - protocol: TCP
      port: 80
    - protocol: TCP
      port: 443

  • Accept connections by using both namespace and pod selectors:

    To match network traffic by combining namespace and pod selectors, you can use a NetworkPolicy object similar to the following:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: allow-pod-and-namespace-both
spec:
  podSelector:
    matchLabels:
      name: test-pods
  ingress:
    - from:
      - namespaceSelector:
          matchLabels:
            project: project_name
        podSelector:
          matchLabels:
            name: test-pods

NetworkPolicy objects are additive, which means you can combine multiple NetworkPolicy objects together to satisfy complex network requirements.

For example, for the NetworkPolicy objects defined in previous samples, you can define both allow-same-namespace and allow-http-and-https policies within the same project. Thus allowing the pods with the label role=frontend, to accept any connection allowed by each policy. That is, connections on any port from pods in the same namespace, and connections on ports 80 and 443 from pods in any namespace.

Optimizations for network policy

Use a network policy to isolate pods that are differentiated from one another by labels within a namespace.

The guidelines for efficient use of network policy rules applies to only the OpenShift SDN cluster network provider.

It is inefficient to apply NetworkPolicy objects to large numbers of individual pods in a single namespace. Pod labels do not exist at the IP address level, so a network policy generates a separate Open vSwitch (OVS) flow rule for every possible link between every pod selected with a podSelector.

For example, if the spec podSelector and the ingress podSelector within a NetworkPolicy object each match 200 pods, then 40,000 (200*200) OVS flow rules are generated. This might slow down a node.

When designing your network policy, refer to the following guidelines:

  • Reduce the number of OVS flow rules by using namespaces to contain groups of pods that need to be isolated.

    NetworkPolicy objects that select a whole namespace, by using the namespaceSelector or an empty podSelector, generate only a single OVS flow rule that matches the VXLAN virtual network ID (VNID) of the namespace.

  • Keep the pods that do not need to be isolated in their original namespace, and move the pods that require isolation into one or more different namespaces.

  • Create additional targeted cross-namespace network policies to allow the specific traffic that you do want to allow from the isolated pods.

Next steps

Additional resources