KEP3070: Reserve Service IP Ranges For Dynamic and Static IP Allocation
Antonio Ojea
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layout: blog
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title: "Kubernetes 1.24: Avoid Collisions Assigning IP Addresses to Services"
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date: 2022-05-23
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slug: service-ip-dynamic-and-static-allocation
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---
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**Author:** Antonio Ojea (Red Hat)
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In Kubernetes, [Services](/docs/concepts/services-networking/service/) are an abstract way to expose
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an application running on a set of Pods. Services
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can have a cluster-scoped virtual IP address (using a Service of `type: ClusterIP`).
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Clients can connect using that virtual IP address, and Kubernetes then load-balances traffic to that
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Service across the different backing Pods.
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## How Service ClusterIPs are allocated?
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A Service `ClusterIP` can be assigned:
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_dynamically_
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: the cluster's control plane automatically picks a free IP address from within the configured IP range for `type: ClusterIP` Services.
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_statically_
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: you specify an IP address of your choice, from within the configured IP range for Services.
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Across your whole cluster, every Service `ClusterIP` must be unique.
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Trying to create a Service with a `ClusterIP` that has already
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been allocated will return an error.
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## Why do you need to reserve Service Cluster IPs?
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Because you may want to have Services running in well-known IP addresses, so other components and
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users in the cluster can use them.
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The best example is the DNS Service for the cluster. Some Kubernetes installers assign the 10th address from
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the Service IP range to the DNS service. Assuming you configured your cluster with Service IP range
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10.96.0.0/16 and you want your DNS Service IP to be 10.96.0.10, you'd have to create a Service like
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this:
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```yaml
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apiVersion: v1
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kind: Service
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metadata:
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labels:
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k8s-app: kube-dns
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kubernetes.io/cluster-service: "true"
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kubernetes.io/name: CoreDNS
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name: kube-dns
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namespace: kube-system
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spec:
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clusterIP: 10.96.0.10
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ports:
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- name: dns
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port: 53
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protocol: UDP
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targetPort: 53
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- name: dns-tcp
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port: 53
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protocol: TCP
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targetPort: 53
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selector:
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k8s-app: kube-dns
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type: ClusterIP
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```
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but as I explained before, the IP address 10.96.0.10 has not been reserved; if other Services are created
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before or in parallel with dynamic allocation, there is a chance they can allocate this IP, hence,
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you will not be able to create the DNS Service because it will fail with a conflict error.
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## How can you avoid Service ClusterIP conflicts? {#avoid-ClusterIP-conflict}
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In Kubernetes 1.24, you can enable a new feature gate `ServiceIPStaticSubrange`.
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Turning this on allows you to use a different IP
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allocation strategy for Services, reducing the risk of collision.
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The `ClusterIP` range will be divided, based on the formula `min(max(16, cidrSize / 16), 256)`,
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described as _never less than 16 or more than 256 with a graduated step between them_.
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Dynamically IP assignment will use the upper band by default, once this has been exhausted it will
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use the lower range. This will allow users to use static allocations on the lower band with a low
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risk of collision.
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Examples:
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#### Service IP CIDR block: 10.96.0.0/24
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Range Size: 2<sup>8</sup> - 2 = 254
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Band Offset: `min(max(16,256/16),256)` = `min(16,256)` = 16
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Static band start: 10.96.0.1
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Static band end: 10.96.0.16
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Range end: 10.96.0.254
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{{< mermaid >}}
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pie showData
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title 10.96.0.0/24
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"Static" : 16
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"Dynamic" : 238
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{{< /mermaid >}}
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#### Service IP CIDR block: 10.96.0.0/20
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Range Size: 2<sup>12</sup> - 2 = 4094
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Band Offset: `min(max(16,256/16),256)` = `min(256,256)` = 256
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Static band start: 10.96.0.1
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Static band end: 10.96.1.0
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Range end: 10.96.15.254
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{{< mermaid >}}
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pie showData
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title 10.96.0.0/20
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"Static" : 256
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"Dynamic" : 3838
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{{< /mermaid >}}
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#### Service IP CIDR block: 10.96.0.0/16
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Range Size: 2<sup>16</sup> - 2 = 65534
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Band Offset: `min(max(16,65536/16),256)` = `min(4096,256)` = 256
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Static band start: 10.96.0.1
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Static band ends: 10.96.1.0
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Range end: 10.96.255.254
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{{< mermaid >}}
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pie showData
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title 10.96.0.0/16
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"Static" : 256
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"Dynamic" : 65278
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{{< /mermaid >}}
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## Get involved with SIG Network
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The current SIG-Network [KEPs](https://github.com/orgs/kubernetes/projects/10) and [issues](https://github.com/kubernetes/kubernetes/issues?q=is%3Aopen+is%3Aissue+label%3Asig%2Fnetwork) on GitHub illustrate the SIG’s areas of emphasis.
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[SIG-Network meetings](https://github.com/kubernetes/community/tree/master/sig-network) are a friendly, welcoming venue for you to connect with the community and share your ideas.
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Looking forward to hearing from you!
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