# Kubernetes Default Service CIDR Reconfiguration

Kubernetes Default Service CIDR ReconfigurationFEATURE STATE: Kubernetes v1.33 [stable] (enabled by default: true)This document shares how to reconfigure the default Service IP range(s) assigned to a cluster.Before you beginYou need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:iximiuz LabsKillercodaKodeKloudPlay with KubernetesYour Kubernetes server must be at or later than version v1.33.To check the version, enter kubectl version.Kubernetes Default Service CIDR ReconfigurationThis document explains how to manage the Service IP address range within a Kubernetes cluster, which also influences the cluster's supported IP families for Services.The IP families available for Service ClusterIPs are determined by the --service-cluster-ip-range flag to kube-apiserver. For a better understanding of Service IP address allocation, refer to the Services IP address allocation tracking documentation.Since Kubernetes 1.33, the Service IP families configured for the cluster are reflected by the ServiceCIDR object named kubernetes. The kubernetes ServiceCIDR object is created by the first kube-apiserver instance that starts, based on its configured --service-cluster-ip-range flag. To ensure consistent cluster behavior, all kube-apiserver instances must be configured with the same --service-cluster-ip-range values, which must match the default kubernetes ServiceCIDR object.Kubernetes Service CIDR Reconfiguration CategoriesWe can categorize Service CIDR reconfiguration into the following scenarios:Extending the existing Service CIDRs: This can be done dynamically by adding new ServiceCIDR objects without the need of reconfiguration of the kube-apiserver. Please refer to the dedicated documentation on Extending Service IP Ranges.Single-to-dual-stack conversion preserving the primary service CIDR: This involves introducing a secondary IP family (IPv6 to an IPv4-only cluster, or IPv4 to an IPv6-only cluster) while keeping the original IP family as primary. This requires an update to the kube-apiserver configuration and a corresponding modification of various cluster components that need to handle this additional IP family. These components include, but are not limited to, kube-proxy, the CNI or network plugin, service mesh implementations, and DNS services.Dual-to-single conversion preserving the primary service CIDR: This involves removing the secondary IP family from a dual-stack cluster, reverting to a single IP family while retaining the original primary IP family. In addition to the reconfiguration of the components to match the new IP family, you might need to address Services that were explicitly configured to use the removed IP family.Anything that results in changing the primary service CIDR: Completely replacing the default ServiceCIDR is a complex operation. If the new ServiceCIDR does not overlap with the existing one, it will require renumbering all existing Services and changing the kubernetes.default service. The case where the primary IP family also changes is even more complicated, and may require to change multiple cluster components (kubelet, network plugins, etc.) to match the new primary IP family.Manual Operations for Replacing the Default Service CIDRReconfiguring the default Service CIDR necessitates manual steps performed by the cluster operator, administrator, or the software managing the cluster lifecycle. These typically include:Updating the kube-apiserver configuration: Modify the --service-cluster-ip-range flag with the new IP range(s).Reconfiguring the network components: This is a critical step and the specific procedure depends on the different networking components in use. It might involve updating configuration files, restarting agent pods, or updating the components to manage the new Service CIDR(s) and the desired IP family configuration for Pods. Typical components can be the implementation of Kubernetes Services, such as kube-proxy, and the configured networking plugin, and potentially other networking components like service mesh controllers and DNS servers, to ensure they can correctly handle traffic and perform service discovery with the new IP family configuration.Managing existing Services: Services with IPs from the old CIDR need to be addressed if they are not within the new configured ranges. Options include recreation (leading to downtime and new IP assignments) or potentially more complex reconfiguration strategies.Recreating internal Kubernetes services: The kubernetes.default service must be deleted and recreated to obtain an IP address from the new Service CIDR if the primary IP family is changed or replaced by a different network.Illustrative Reconfiguration StepsThe following steps describe a controlled reconfiguration focusing on the completely replacement of the default Service CIDR and the recreation of the kubernetes.default Service:Start the kube-apiserver with the initial --service-cluster-ip-range.Create initial Services that obtain IPs from this range.Introduce a new Service CIDR as a temporary target for reconfiguration.Mark the kubernetes default Service CIDR for deletion (it will remain pending due to existing IPs and finalizers). This prevents new allocations from the old range.Recreate existing Services. They should now be allocated IPs from the new, temporary Service CIDR.Restart the kube-apiserver with the new Service CIDR(s) configured and shut down the old instance.Delete the kubernetes.default service. The new kube-apiserver will recreate it within the new Service CIDR.What's nextKubernetes Networking Concepts: https://kubernetes.io/docs/concepts/cluster-administration/networking/Kubernetes Dual-Stack Services: https://kubernetes.io/docs/concepts/services-networking/dual-stack/Extending Kubernetes Service IP Ranges: https://kubernetes.io/docs/tasks/network/extend-service-ip-ranges/