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Networking overview

OpenShift Container Platform 4.17

Understanding fundamental networking concepts and general tasks in OpenShift Container Platform

Red Hat OpenShift Documentation Team

Abstract

This document provides an introduction to core networking concepts, basic architecture, and general networking tasks within OpenShift Container Platform.

Chapter 1. About networking
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Red Hat OpenShift Networking is an ecosystem of features, plugins and advanced networking capabilities that extend Kubernetes networking with the advanced networking-related features that your cluster needs to manage its network traffic for one or multiple hybrid clusters. This ecosystem of networking capabilities integrates ingress, egress, load balancing, high-performance throughput, security, inter- and intra-cluster traffic management and provides role-based observability tooling to reduce its natural complexities.

The following list highlights some of the most commonly used Red Hat OpenShift Networking features available on your cluster:

  • Primary cluster network provided by either of the following Container Network Interface (CNI) plugins:

  • Certified 3rd-party alternative primary network plugins
  • Cluster Network Operator for network plugin management
  • Ingress Operator for TLS encrypted web traffic
  • DNS Operator for name assignment
  • MetalLB Operator for traffic load balancing on bare metal clusters
  • IP failover support for high-availability
  • Additional hardware network support through multiple CNI plugins, including for macvlan, ipvlan, and SR-IOV hardware networks
  • IPv4, IPv6, and dual stack addressing
  • Hybrid Linux-Windows host clusters for Windows-based workloads
  • Red Hat OpenShift Service Mesh for discovery, load balancing, service-to-service authentication, failure recovery, metrics, and monitoring of services
  • Single-node OpenShift
  • Network Observability Operator for network debugging and insights
  • Submariner for inter-cluster networking
  • Red Hat Service Interconnect for layer 7 inter-cluster networking

Chapter 2. Understanding networking
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Cluster Administrators have several options for exposing applications that run inside a cluster to external traffic and securing network connections:

  • Service types, such as node ports or load balancers
  • API resources, such as Ingress and Route

By default, Kubernetes allocates each pod an internal IP address for applications running within the pod. Pods and their containers can network, but clients outside the cluster do not have networking access. When you expose your application to external traffic, giving each pod its own IP address means that pods can be treated like physical hosts or virtual machines in terms of port allocation, networking, naming, service discovery, load balancing, application configuration, and migration.

Note

Some cloud platforms offer metadata APIs that listen on the 169.254.169.254 IP address, a link-local IP address in the IPv4 169.254.0.0/16 CIDR block.

This CIDR block is not reachable from the pod network. Pods that need access to these IP addresses must be given host network access by setting the spec.hostNetwork field in the pod spec to true.

If you allow a pod host network access, you grant the pod privileged access to the underlying network infrastructure.

2.1. OpenShift Container Platform DNS
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If you are running multiple services, such as front-end and back-end services for use with multiple pods, environment variables are created for user names, service IPs, and more so the front-end pods can communicate with the back-end services. If the service is deleted and recreated, a new IP address can be assigned to the service, and requires the front-end pods to be recreated to pick up the updated values for the service IP environment variable. Additionally, the back-end service must be created before any of the front-end pods to ensure that the service IP is generated properly, and that it can be provided to the front-end pods as an environment variable.

For this reason, OpenShift Container Platform has a built-in DNS so that the services can be reached by the service DNS as well as the service IP/port.

2.2. OpenShift Container Platform Ingress Operator
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When you create your OpenShift Container Platform cluster, pods and services running on the cluster are each allocated their own IP addresses. The IP addresses are accessible to other pods and services running nearby but are not accessible to outside clients.

The Ingress Operator makes it possible for external clients to access your service by deploying and managing one or more HAProxy-based Ingress Controllers to handle routing. You can use the Ingress Operator to route traffic by specifying OpenShift Container Platform Route and Kubernetes Ingress resources. Configurations within the Ingress Controller, such as the ability to define endpointPublishingStrategy type and internal load balancing, provide ways to publish Ingress Controller endpoints.

2.2.1. Comparing routes and Ingress
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The Kubernetes Ingress resource in OpenShift Container Platform implements the Ingress Controller with a shared router service that runs as a pod inside the cluster. The most common way to manage Ingress traffic is with the Ingress Controller. You can scale and replicate this pod like any other regular pod. This router service is based on HAProxy, which is an open source load balancer solution.

The OpenShift Container Platform route provides Ingress traffic to services in the cluster. Routes provide advanced features that might not be supported by standard Kubernetes Ingress Controllers, such as TLS re-encryption, TLS passthrough, and split traffic for blue-green deployments.

Ingress traffic accesses services in the cluster through a route. Routes and Ingress are the main resources for handling Ingress traffic. Ingress provides features similar to a route, such as accepting external requests and delegating them based on the route. However, with Ingress you can only allow certain types of connections: HTTP/2, HTTPS and server name identification (SNI), and TLS with certificate. In OpenShift Container Platform, routes are generated to meet the conditions specified by the Ingress resource.

This glossary defines common terms that are used in the networking content.

authentication
To control access to an OpenShift Container Platform cluster, a cluster administrator can configure user authentication and ensure only approved users access the cluster. To interact with an OpenShift Container Platform cluster, you must authenticate to the OpenShift Container Platform API. You can authenticate by providing an OAuth access token or an X.509 client certificate in your requests to the OpenShift Container Platform API.
AWS Load Balancer Operator
The AWS Load Balancer (ALB) Operator deploys and manages an instance of the aws-load-balancer-controller.
Cluster Network Operator
The Cluster Network Operator (CNO) deploys and manages the cluster network components in an OpenShift Container Platform cluster. This includes deployment of the Container Network Interface (CNI) network plugin selected for the cluster during installation.
config map
A config map provides a way to inject configuration data into pods. You can reference the data stored in a config map in a volume of type ConfigMap. Applications running in a pod can use this data.
custom resource (CR)
A CR is extension of the Kubernetes API. You can create custom resources.
DNS
Cluster DNS is a DNS server which serves DNS records for Kubernetes services. Containers started by Kubernetes automatically include this DNS server in their DNS searches.
DNS Operator
The DNS Operator deploys and manages CoreDNS to provide a name resolution service to pods. This enables DNS-based Kubernetes Service discovery in OpenShift Container Platform.
deployment
A Kubernetes resource object that maintains the life cycle of an application.
domain
Domain is a DNS name serviced by the Ingress Controller.
egress
The process of data sharing externally through a network’s outbound traffic from a pod.
External DNS Operator
The External DNS Operator deploys and manages ExternalDNS to provide the name resolution for services and routes from the external DNS provider to OpenShift Container Platform.
HTTP-based route
An HTTP-based route is an unsecured route that uses the basic HTTP routing protocol and exposes a service on an unsecured application port.
Ingress
The Kubernetes Ingress resource in OpenShift Container Platform implements the Ingress Controller with a shared router service that runs as a pod inside the cluster.
Ingress Controller
The Ingress Operator manages Ingress Controllers. Using an Ingress Controller is the most common way to allow external access to an OpenShift Container Platform cluster.
installer-provisioned infrastructure
The installation program deploys and configures the infrastructure that the cluster runs on.
kubelet
A primary node agent that runs on each node in the cluster to ensure that containers are running in a pod.
Kubernetes NMState Operator
The Kubernetes NMState Operator provides a Kubernetes API for performing state-driven network configuration across the OpenShift Container Platform cluster’s nodes with NMState.
kube-proxy
Kube-proxy is a proxy service which runs on each node and helps in making services available to the external host. It helps in forwarding the request to correct containers and is capable of performing primitive load balancing.
load balancers
OpenShift Container Platform uses load balancers for communicating from outside the cluster with services running in the cluster.
MetalLB Operator
As a cluster administrator, you can add the MetalLB Operator to your cluster so that when a service of type LoadBalancer is added to the cluster, MetalLB can add an external IP address for the service.
multicast
With IP multicast, data is broadcast to many IP addresses simultaneously.
namespaces
A namespace isolates specific system resources that are visible to all processes. Inside a namespace, only processes that are members of that namespace can see those resources.
networking
Network information of a OpenShift Container Platform cluster.
node
A worker machine in the OpenShift Container Platform cluster. A node is either a virtual machine (VM) or a physical machine.
OpenShift Container Platform Ingress Operator
The Ingress Operator implements the IngressController API and is the component responsible for enabling external access to OpenShift Container Platform services.
pod
One or more containers with shared resources, such as volume and IP addresses, running in your OpenShift Container Platform cluster. A pod is the smallest compute unit defined, deployed, and managed.
PTP Operator
The PTP Operator creates and manages the linuxptp services.
route
The OpenShift Container Platform route provides Ingress traffic to services in the cluster. Routes provide advanced features that might not be supported by standard Kubernetes Ingress Controllers, such as TLS re-encryption, TLS passthrough, and split traffic for blue-green deployments.
scaling
Increasing or decreasing the resource capacity.
service
Exposes a running application on a set of pods.
Single Root I/O Virtualization (SR-IOV) Network Operator
The Single Root I/O Virtualization (SR-IOV) Network Operator manages the SR-IOV network devices and network attachments in your cluster.
software-defined networking (SDN)
A software-defined networking (SDN) approach to provide a unified cluster network that enables communication between pods across the OpenShift Container Platform cluster.
Stream Control Transmission Protocol (SCTP)
SCTP is a reliable message based protocol that runs on top of an IP network.
taint
Taints and tolerations ensure that pods are scheduled onto appropriate nodes. You can apply one or more taints on a node.
toleration
You can apply tolerations to pods. Tolerations allow the scheduler to schedule pods with matching taints.
web console
A user interface (UI) to manage OpenShift Container Platform.

Chapter 3. Accessing hosts
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Learn how to create a bastion host to access OpenShift Container Platform instances and access the control plane nodes with secure shell (SSH) access.

The OpenShift Container Platform installer does not create any public IP addresses for any of the Amazon Elastic Compute Cloud (Amazon EC2) instances that it provisions for your OpenShift Container Platform cluster. To be able to SSH to your OpenShift Container Platform hosts, you must follow this procedure.

Procedure

  1. Create a security group that allows SSH access into the virtual private cloud (VPC) created by the openshift-install command.
  2. Create an Amazon EC2 instance on one of the public subnets the installer created.

  3. Associate a public IP address with the Amazon EC2 instance that you created.

    Unlike with the OpenShift Container Platform installation, you should associate the Amazon EC2 instance you created with an SSH keypair. It does not matter what operating system you choose for this instance, as it will simply serve as an SSH bastion to bridge the internet into your OpenShift Container Platform cluster’s VPC. The Amazon Machine Image (AMI) you use does matter. With Red Hat Enterprise Linux CoreOS (RHCOS), for example, you can provide keys via Ignition, like the installer does.

  4. After you provisioned your Amazon EC2 instance and can SSH into it, you must add the SSH key that you associated with your OpenShift Container Platform installation. This key can be different from the key for the bastion instance, but does not have to be.

    Note

    Direct SSH access is only recommended for disaster recovery. When the Kubernetes API is responsive, run privileged pods instead.

  5. Run oc get nodes, inspect the output, and choose one of the nodes that is a master. The hostname looks similar to ip-10-0-1-163.ec2.internal.

  6. From the bastion SSH host you manually deployed into Amazon EC2, SSH into that control plane host. Ensure that you use the same SSH key you specified during the installation: 

    $ ssh -i <ssh-key-path> core@<master-hostname>
    Copy to Clipboard Toggle word wrap

Chapter 4. Networking dashboards
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Networking metrics are viewable in dashboards within the OpenShift Container Platform web console, under Observe → Dashboards.

4.1. Network Observability Operator
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If you have the Network Observability Operator installed, you can view network traffic metrics dashboards by selecting the Netobserv dashboard from the Dashboards drop-down list. For more information about metrics available in this Dashboard, see Network Observability metrics dashboards.

4.2. Networking and OVN-Kubernetes dashboard
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You can view both general networking metrics as well as OVN-Kubernetes metrics from the dashboard.

To view general networking metrics, select Networking/Linux Subsystem Stats from the Dashboards drop-down list. You can view the following networking metrics from the dashboard: Network Utilisation, Network Saturation, and Network Errors.

To view OVN-Kubernetes metrics select Networking/Infrastructure from the Dashboards drop-down list. You can view the following OVN-Kuberenetes metrics: Networking Configuration, TCP Latency Probes, Control Plane Resources, and Worker Resources.

4.3. Ingress Operator dashboard
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You can view networking metrics handled by the Ingress Operator from the dashboard. This includes metrics like the following:

  • Incoming and outgoing bandwidth
  • HTTP error rates
  • HTTP server response latency

To view these Ingress metrics, select Networking/Ingress from the Dashboards drop-down list. You can view Ingress metrics for the following categories: Top 10 Per Route, Top 10 Per Namespace, and Top 10 Per Shard.

Chapter 5. CIDR range definitions
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If your cluster uses OVN-Kubernetes, you must specify non-overlapping ranges for Classless Inter-Domain Routing (CIDR) subnet ranges.

Important

For OpenShift Container Platform 4.17 and later versions, clusters use 169.254.0.0/17 for IPv4 and fd69::/112 for IPv6 as the default masquerade subnet. These ranges should also be avoided by users. For upgraded clusters, there is no change to the default masquerade subnet.

The following subnet types and are mandatory for a cluster that uses OVN-Kubernetes:

  • Join: Uses a join switch to connect gateway routers to distributed routers. A join switch reduces the number of IP addresses for a distributed router. For a cluster that uses the OVN-Kubernetes plugin, an IP address from a dedicated subnet is assigned to any logical port that attaches to the join switch.
  • Masquerade: Prevents collisions for identical source and destination IP addresses that are sent from a node as hairpin traffic to the same node after a load balancer makes a routing decision.
  • Transit: A transit switch is a type of distributed switch that spans across all nodes in the cluster. A transit switch routes traffic between different zones. For a cluster that uses the OVN-Kubernetes plugin, an IP address from a dedicated subnet is assigned to any logical port that attaches to the transit switch.
Note

You can change the join, masquerade, and transit CIDR ranges for your cluster as a post-installation task.

OVN-Kubernetes, the default network provider in OpenShift Container Platform 4.14 and later versions, internally uses the following IP address subnet ranges:

  • V4JoinSubnet: 100.64.0.0/16
  • V6JoinSubnet: fd98::/64
  • V4TransitSwitchSubnet: 100.88.0.0/16
  • V6TransitSwitchSubnet: fd97::/64
  • defaultV4MasqueradeSubnet: 169.254.0.0/17
  • defaultV6MasqueradeSubnet: fd69::/112
Important

The previous list includes join, transit, and masquerade IPv4 and IPv6 address subnets. If your cluster uses OVN-Kubernetes, do not include any of these IP address subnet ranges in any other CIDR definitions in your cluster or infrastructure.

5.1. Machine CIDR
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In the Machine classless inter-domain routing (CIDR) field, you must specify the IP address range for machines or cluster nodes.

Note

Machine CIDR ranges cannot be changed after creating your cluster.

The default is 10.0.0.0/16. This range must not conflict with any connected networks.

5.2. Service CIDR
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In the Service CIDR field, you must specify the IP address range for services. The range must be large enough to accommodate your workload. The address block must not overlap with any external service accessed from within the cluster. The default is 172.30.0.0/16.

5.3. Pod CIDR
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In the pod CIDR field, you must specify the IP address range for pods.

The pod CIDR is the same as the clusterNetwork CIDR and the cluster CIDR. The range must be large enough to accommodate your workload. The address block must not overlap with any external service accessed from within the cluster. The default is 10.128.0.0/14. You can expand the range after cluster installation.

5.4. Host Prefix
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In the Host Prefix field, you must specify the subnet prefix length assigned to pods scheduled to individual machines. The host prefix determines the pod IP address pool for each machine.

For example, if the host prefix is set to /23, each machine is assigned a /23 subnet from the pod CIDR address range. The default is /23, allowing 510 cluster nodes, and 510 pod IP addresses per node.

5.5. CIDR ranges for hosted control planes
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For deploying hosted control planes on OpenShift Container Platform, use the following required Classless Inter-Domain Routing (CIDR) subnet ranges:

  • v4InternalSubnet: 100.65.0.0/16 (OVN-Kubernetes)
  • clusterNetwork: 10.132.0.0/14 (pod network)
  • serviceNetwork: 172.31.0.0/16

For more information about OpenShift Container Platform CIDR range definitions, see "CIDR range definitions".

Legal Notice
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Copyright © 2025 Red Hat

OpenShift documentation is licensed under the Apache License 2.0 (https://www.apache.org/licenses/LICENSE-2.0).

Modified versions must remove all Red Hat trademarks.

Portions adapted from https://github.com/kubernetes-incubator/service-catalog/ with modifications by Red Hat.

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