Configuring ExternalIPs for services - Configuring ingress cluster traffic | Networking

Prerequisites
About ExternalIP
Additional resources
Configuration for ExternalIP
Restrictions on the assignment of an external IP address
Example policy objects
ExternalIP address block configuration
Configure external IP address blocks for your cluster
Next steps

As a cluster administrator, you can designate an IP address block that is external to the cluster that can send traffic to services in the cluster.

This functionality is generally most useful for clusters installed on bare-metal hardware.

Prerequisites

Your network infrastructure must route traffic for the external IP addresses to your cluster.

About ExternalIP

For non-cloud environments, OpenShift Container Platform supports the use of the ExternalIP facility to specify external IP addresses in the spec.externalIPs[] parameter of the Service object. A service configured with an ExternalIP functions similarly to a service with type=NodePort, whereby you traffic directs to a local node for load balancing.

For cloud environments, use the load balancer services for automatic deployment of a cloud load balancer to target the endpoints of a service.

After you specify a value for the parameter, OpenShift Container Platform assigns an additional virtual IP address to the service. The IP address can exist outside of the service network that you defined for your cluster.

Because ExternalIP is disabled by default, enabling the ExternalIP functionality might introduce security risks for the service, because in-cluster traffic to an external IP address is directed to that service. This configuration means that cluster users could intercept sensitive traffic destined for external resources.

You can use either a MetalLB implementation or an IP failover deployment to attach an ExternalIP resource to a service in the following ways:

Automatic assignment of an external IP: OpenShift Container Platform automatically assigns an IP address from the autoAssignCIDRs CIDR block to the spec.externalIPs[] array when you create a Service object with spec.type=LoadBalancer set. For this configuration, OpenShift Container Platform implements a cloud version of the load balancer service type and assigns IP addresses to the services. Automatic assignment is disabled by default and must be configured by a cluster administrator as described in the "Configuration for ExternalIP" section.
Manual assignment of an external IP: OpenShift Container Platform uses the IP addresses assigned to the spec.externalIPs[] array when you create a Service object. You cannot specify an IP address that is already in use by another service.

After using either the MetalLB implementation or an IP failover deployment to host external IP address blocks, you must configure your networking infrastructure to ensure that the external IP address blocks are routed to your cluster. This configuration means that the IP address is not configured in the network interfaces from nodes. To handle the traffic, you must configure the routing and access to the external IP by using a method, such as static Address Resolution Protocol (ARP) entries.

OpenShift Container Platform extends the ExternalIP functionality in Kubernetes by adding the following capabilities:

Restrictions on the use of external IP addresses by users through a configurable policy
Allocation of an external IP address automatically to a service upon request

Additional resources

Configuration for ExternalIP

Use of an external IP address in OpenShift Container Platform is governed by the following parameters in the Network.config.openshift.io custom resource (CR) that is named cluster:

spec.externalIP.autoAssignCIDRs defines an IP address block used by the load balancer when choosing an external IP address for the service. OpenShift Container Platform supports only a single IP address block for automatic assignment. This configuration requires less steps than manually assigning ExternalIPs to services, which requires managing the port space of a limited number of shared IP addresses. If you enable automatic assignment, a Service object with spec.type=LoadBalancer is allocated an external IP address.
spec.externalIP.policy defines the permissible IP address blocks when manually specifying an IP address. OpenShift Container Platform does not apply policy rules to IP address blocks that you defined in the spec.externalIP.autoAssignCIDRs parameter.

If routed correctly, external traffic from the configured external IP address block can reach service endpoints through any TCP or UDP port that the service exposes.

As a cluster administrator, you must configure routing to externalIPs. You must also ensure that the IP address block you assign terminates at one or more nodes in your cluster. For more information, see Kubernetes External IPs.

OpenShift Container Platform supports both the automatic and manual assignment of IP addresses, where each address is guaranteed to be assigned to a maximum of one service. This configuration ensures that each service can expose its chosen ports regardless of the ports exposed by other services.

To use IP address blocks defined by autoAssignCIDRs in OpenShift Container Platform, you must configure the necessary IP address assignment and routing for your host network.

The following YAML describes a service with an external IP address configured:

Example Service object with spec.externalIPs[] set

apiVersion: v1
kind: Service
metadata:
  name: http-service
spec:
  clusterIP: 172.30.163.110
  externalIPs:
  - 192.168.132.253
  externalTrafficPolicy: Cluster
  ports:
  - name: highport
    nodePort: 31903
    port: 30102
    protocol: TCP
    targetPort: 30102
  selector:
    app: web
  sessionAffinity: None
  type: LoadBalancer
status:
  loadBalancer:
    ingress:
    - ip: 192.168.132.253
# ...

Restrictions on the assignment of an external IP address

As a cluster administrator, you can specify IP address blocks to allow and to reject IP addresses for a service. Restrictions apply only to users without cluster-admin privileges. A cluster administrator can always set the service spec.externalIPs[] field to any IP address.

You configure an IP address policy by specifying Classless Inter-Domain Routing (CIDR) address blocks for the spec.ExternalIP.policy parameter in the policy object.

Example in JSON form of a policy object and its CIDR parameters

{
  "policy": {
    "allowedCIDRs": [],
    "rejectedCIDRs": []
  }
}

When configuring policy restrictions, the following rules apply:

If policy is set to {}, creating a Service object with spec.ExternalIPs[] results in a failed service. This setting is the default for OpenShift Container Platform. The same behavior exists for policy: null.
If policy is set and either policy.allowedCIDRs[] or policy.rejectedCIDRs[] is set, the following rules apply:
- If allowedCIDRs[] and rejectedCIDRs[] are both set, rejectedCIDRs[] has precedence over allowedCIDRs[].
- If allowedCIDRs[] is set, creating a Service object with spec.ExternalIPs[] succeeds only if the specified IP addresses are allowed.
- If rejectedCIDRs[] is set, creating a Service object with spec.ExternalIPs[] succeeds only if the specified IP addresses are not rejected.

Example policy objects

The examples in this section show different spec.externalIP.policy configurations.

In the following example, the policy prevents OpenShift Container Platform from creating any service with a specified external IP address.
Example policy to reject any value specified for Service object spec.externalIPs[]
```
apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  externalIP:
    policy: {}
# ...
```

In the following example, both the allowedCIDRs and rejectedCIDRs fields are set.

Example policy that includes both allowed and rejected CIDR blocks

apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  externalIP:
    policy:
      allowedCIDRs:
      - 172.16.66.10/23
      rejectedCIDRs:
      - 172.16.66.10/24
# ...

In the following example, policy is set to {}. With this configuration, using the oc get networks.config.openshift.io -o yaml command to view the configuration means policy parameter does not show on the command output. The same behavior exists for policy: null.
Example policy to allow any value specified for Service object spec.externalIPs[]
```
apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  clusterNetwork:
  - cidr: 10.128.0.0/14
    hostPrefix: 23
  externalIP:
    policy: {}
# ...
```

ExternalIP address block configuration

The configuration for ExternalIP address blocks is defined by a Network custom resource (CR) named cluster. The Network CR is part of the config.openshift.io API group.

During cluster installation, the Cluster Version Operator (CVO) automatically creates a Network CR named cluster. Creating any other CR objects of this type is not supported.

The following YAML describes the ExternalIP configuration:

Network.config.openshift.io CR named cluster

apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  externalIP:
    autoAssignCIDRs: [] (1)
    policy: (2)
      ...

1	Defines the IP address block in CIDR format that is available for automatic assignment of external IP addresses to a service. Only a single IP address range is allowed.
2	Defines restrictions on manual assignment of an IP address to a service. If no restrictions are defined, specifying the `spec.externalIP` field in a `Service` object is not allowed. By default, no restrictions are defined.

The following YAML describes the fields for the policy stanza:

Network.config.openshift.io policy stanza

policy:
  allowedCIDRs: [] (1)
  rejectedCIDRs: [] (2)

1	A list of allowed IP address ranges in CIDR format.
2	A list of rejected IP address ranges in CIDR format.

Example external IP configurations

Several possible configurations for external IP address pools are displayed in the following examples:

The following YAML describes a configuration that enables automatically assigned external IP addresses:

Example configuration with spec.externalIP.autoAssignCIDRs set

apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  ...
  externalIP:
    autoAssignCIDRs:
    - 192.168.132.254/29

The following YAML configures policy rules for the allowed and rejected CIDR ranges:

Example configuration with spec.externalIP.policy set

apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  ...
  externalIP:
    policy:
      allowedCIDRs:
      - 192.168.132.0/29
      - 192.168.132.8/29
      rejectedCIDRs:
      - 192.168.132.7/32

Configure external IP address blocks for your cluster

As a cluster administrator, you can configure the following ExternalIP settings:

An ExternalIP address block used by OpenShift Container Platform to automatically populate the spec.clusterIP field for a Service object.
A policy object to restrict what IP addresses may be manually assigned to the spec.clusterIP array of a Service object.

Prerequisites

Install the OpenShift CLI (oc).
Access to the cluster as a user with the cluster-admin role.

Procedure

Optional: To display the current external IP configuration, enter the following command:
```
$ oc describe networks.config cluster
```
To edit the configuration, enter the following command:
```
$ oc edit networks.config cluster
```

Modify the ExternalIP configuration, as in the following example:

apiVersion: config.openshift.io/v1
kind: Network
metadata:
  name: cluster
spec:
  ...
  externalIP: (1)
  ...

1	Specify the configuration for the `externalIP` stanza.

To confirm the updated ExternalIP configuration, enter the following command:

$ oc get networks.config cluster -o go-template='{{.spec.externalIP}}{{"\n"}}'

Next steps

Configuring ingress cluster traffic for a service external IP