Enabling multicast for a project - OpenShift SDN network plugin | Networking

About multicast
Enabling multicast between pods

OpenShift SDN CNI is deprecated as of OpenShift Container Platform 4.14. As of OpenShift Container Platform 4.15, the network plugin is not an option for new installations. In a subsequent future release, the OpenShift SDN network plugin is planned to be removed and no longer supported. Red Hat will provide bug fixes and support for this feature until it is removed, but this feature will no longer receive enhancements. As an alternative to OpenShift SDN CNI, you can use OVN Kubernetes CNI instead. For more information, see OpenShift SDN CNI removal.

About multicast

With IP multicast, data is broadcast to many IP addresses simultaneously.

At this time, multicast is best used for low-bandwidth coordination or service discovery and not a high-bandwidth solution.
By default, network policies affect all connections in a namespace. However, multicast is unaffected by network policies. If multicast is enabled in the same namespace as your network policies, it is always allowed, even if there is a deny-all network policy. Cluster administrators should consider the implications to the exemption of multicast from network policies before enabling it.

Multicast traffic between OpenShift Container Platform pods is disabled by default. If you are using the OpenShift SDN network plugin, you can enable multicast on a per-project basis.

When using the OpenShift SDN network plugin in networkpolicy isolation mode:

Multicast packets sent by a pod will be delivered to all other pods in the project, regardless of NetworkPolicy objects. Pods might be able to communicate over multicast even when they cannot communicate over unicast.
Multicast packets sent by a pod in one project will never be delivered to pods in any other project, even if there are NetworkPolicy objects that allow communication between the projects.

When using the OpenShift SDN network plugin in multitenant isolation mode:

Multicast packets sent by a pod will be delivered to all other pods in the project.
Multicast packets sent by a pod in one project will be delivered to pods in other projects only if each project is joined together and multicast is enabled in each joined project.

Enabling multicast between pods

You can enable multicast between pods for your project.

Prerequisites

Install the OpenShift CLI (oc).
You must log in to the cluster with a user that has the cluster-admin role.

Procedure

Run the following command to enable multicast for a project. Replace <namespace> with the namespace for the project you want to enable multicast for.
```
$ oc annotate netnamespace <namespace> \
    netnamespace.network.openshift.io/multicast-enabled=true
```

Verification

To verify that multicast is enabled for a project, complete the following procedure:

Change your current project to the project that you enabled multicast for. Replace <project> with the project name.
```
$ oc project <project>
```

Create a pod to act as a multicast receiver:

$ cat <<EOF| oc create -f -
apiVersion: v1
kind: Pod
metadata:
  name: mlistener
  labels:
    app: multicast-verify
spec:
  containers:
    - name: mlistener
      image: registry.access.redhat.com/ubi9
      command: ["/bin/sh", "-c"]
      args:
        ["dnf -y install socat hostname && sleep inf"]
      ports:
        - containerPort: 30102
          name: mlistener
          protocol: UDP
EOF

Create a pod to act as a multicast sender:

$ cat <<EOF| oc create -f -
apiVersion: v1
kind: Pod
metadata:
  name: msender
  labels:
    app: multicast-verify
spec:
  containers:
    - name: msender
      image: registry.access.redhat.com/ubi9
      command: ["/bin/sh", "-c"]
      args:
        ["dnf -y install socat && sleep inf"]
EOF

In a new terminal window or tab, start the multicast listener.

Get the IP address for the Pod:

$ POD_IP=$(oc get pods mlistener -o jsonpath='{.status.podIP}')

Start the multicast listener by entering the following command:

$ oc exec mlistener -i -t -- \
    socat UDP4-RECVFROM:30102,ip-add-membership=224.1.0.1:$POD_IP,fork EXEC:hostname

Start the multicast transmitter.

Get the pod network IP address range:

$ CIDR=$(oc get Network.config.openshift.io cluster \
    -o jsonpath='{.status.clusterNetwork[0].cidr}')

To send a multicast message, enter the following command:

$ oc exec msender -i -t -- \
    /bin/bash -c "echo | socat STDIO UDP4-DATAGRAM:224.1.0.1:30102,range=$CIDR,ip-multicast-ttl=64"

If multicast is working, the previous command returns the following output:

mlistener