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Chapter 14. Enabling multicast for a project

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

Important
  • 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 OVN-Kubernetes network plugin, you can enable multicast on a per-project basis.

14.2. Enabling multicast between pods
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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 namespace <namespace> \
    k8s.ovn.org/multicast-enabled=true
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    Tip

    You can alternatively apply the following YAML to add the annotation: 

    apiVersion: v1
kind: Namespace
metadata:
  name: <namespace>
  annotations:
    k8s.ovn.org/multicast-enabled: "true"
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Verification

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

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

    $ oc project <project>
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  2. 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
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  3. 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
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  4. In a new terminal window or tab, start the multicast listener.

    1. Get the IP address for the Pod: 

      $ POD_IP=$(oc get pods mlistener -o jsonpath='{.status.podIP}')
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    2. 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
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  5. Start the multicast transmitter.

    1. Get the pod network IP address range: 

      $ CIDR=$(oc get Network.config.openshift.io cluster \
    -o jsonpath='{.status.clusterNetwork[0].cidr}')
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    2. 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"
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      If multicast is working, the previous command returns the following output: 

      mlistener
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