Blog: Kubernetes 1.27: Avoid Collisions Assigning Ports to NodePort Services (Web and Cloud)

Author: Xu Zhenglun (Alibaba) In Kubernetes, a Service can be used to provide a unified traffic endpoint for applications running on a set of Pods. Clients can use the virtual IP address (or VIP) provided by the Service for access, and Kubernetes provides load balancing for traffic accessing different back-end Pods, but a ClusterIP type of Service is limited to providing access to nodes within the cluster, while traffic from outside the cluster cannot be routed. One way to solve this problem is to use a type: NodePort Service, which sets up a mapping to a specific port of all nodes in the cluster, thus redirecting traffic from the outside to the inside of the cluster. How Kubernetes allocates node ports to Services? When a type: NodePort Service is created, its corresponding port(s) are allocated in one of two ways: Dynamic : If the Service type is NodePort and you do not set a nodePort value explicitly in the spec for that Service, the Kubernetes control plane will automatically allocate an unused port to it at creation time. Static : In addition to the dynamic auto-assignment described above, you can also explicitly assign a port that is within the nodeport port range configuration. The value of nodePort that you manually assign must be unique across the whole cluster. Attempting to create a Service of type: NodePort where you explicitly specify a node port that was already allocated results in an error. Why do you need to reserve ports of NodePort Service? Sometimes, you may want to have a NodePort Service running on well-known ports so that other components and users inside o r outside the cluster can use them. In some complex cluster deployments with a mix of Kubernetes nodes and other servers on the same network, it may be necessary to use some pre-defined ports for communication. In particular, some fundamental components cannot rely on the VIPs that back type: LoadBalancer Services because the virtual IP address mapping implementation for that cluster also relies on these foundational components. Now suppose you need to expose a Minio object storage service on Kubernetes to clients running outside the Kubernetes cluster, and the agreed port is 30009, we need to create a Service as follows: apiVersion: v1 kind: Service metadata: name: minio spec: ports: - name: api nodePort: 30009 port: 9000 protocol: TCP targetPort: 9000 selector: app: minio type: NodePort However, as mentioned before, if the port (30009) required for the minio Service is not reserved, and another type: NodePort (or possibly type: LoadBalancer) Service is created and dynamically allocated before or concurrently with the minio Service, TCP port 30009 might be allocated to that other Service; if so, creation of the minio Service will fail due to a node port collision. How can you avoid NodePort Service port conflicts? Kubernetes 1.24 introduced changes for type: ClusterIP Services, dividing the CIDR range for cluster IP addresses into two blocks that use different allocation policies to reduce the risk of conflicts. In Kubernetes 1.27, as an alpha feature, you can adopt a similar policy for type: NodePort Services. You can enable a new feature gate ServiceNodePortStaticSubrange. Turning this on allows you to use a different port allocation strategy for type: NodePort Services, and reduce the risk of collision. The port range for NodePort will be divided, based on the formula min(max(16, nodeport-size / 32), 128). The outcome of the formula will be a number between 16 and 128, with a step size that increases as the size of the nodeport range increases. The outcome of the formula determine that the size of static port range. When the port range is less than 16, the size of static port range will be set to 0, which means that all ports will be dynamically allocated. Dynamic port assignment will use the upper band by default, once this has been exhausted it will use the lower range. This will allow users to use static allocations on the lower band with a low risk of collision. Examples default range: 30000-32767 Range properties Values service-node-port-range 30000-32767 Band Offset min(max(16, 2768/32), 128) = min(max(16, 86), 128) = min(86, 128) = 86 Static band start 30000 Static band end 30085 Dynamic band start 30086 Dynamic band end 32767 pie showData title 30000-32767 "Static" : 86 "Dynamic" : 2682 JavaScript must be enabled to view this content very small range: 30000-30015 Range properties Values service-node-port-range 30000-30015 Band Offset 0 Static band start - Static band end - Dynamic band start 30000 Dynamic band end 30015 pie showData title 30000-30015 "Static" : 0 "Dynamic" : 16 JavaScript must be enabled to view this content small(lower boundary) range: 30000-30127 Range proper

Web and Cloud LLC

May 11, 2023 - 06:04

Jan 13, 2024 - 09:19

Author: Xu Zhenglun (Alibaba)

In Kubernetes, a Service can be used to provide a unified traffic endpoint for applications running on a set of Pods. Clients can use the virtual IP address (or VIP) provided by the Service for access, and Kubernetes provides load balancing for traffic accessing different back-end Pods, but a ClusterIP type of Service is limited to providing access to nodes within the cluster, while traffic from outside the cluster cannot be routed. One way to solve this problem is to use a type: NodePort Service, which sets up a mapping to a specific port of all nodes in the cluster, thus redirecting traffic from the outside to the inside of the cluster.

How Kubernetes allocates node ports to Services?

When a type: NodePort Service is created, its corresponding port(s) are allocated in one of two ways:

Dynamic : If the Service type is NodePort and you do not set a nodePort value explicitly in the spec for that Service, the Kubernetes control plane will automatically allocate an unused port to it at creation time.
Static : In addition to the dynamic auto-assignment described above, you can also explicitly assign a port that is within the nodeport port range configuration.

The value of nodePort that you manually assign must be unique across the whole cluster. Attempting to create a Service of type: NodePort where you explicitly specify a node port that was already allocated results in an error.

Why do you need to reserve ports of NodePort Service?

Sometimes, you may want to have a NodePort Service running on well-known ports so that other components and users inside o r outside the cluster can use them.

In some complex cluster deployments with a mix of Kubernetes nodes and other servers on the same network, it may be necessary to use some pre-defined ports for communication. In particular, some fundamental components cannot rely on the VIPs that back type: LoadBalancer Services because the virtual IP address mapping implementation for that cluster also relies on these foundational components.

Now suppose you need to expose a Minio object storage service on Kubernetes to clients running outside the Kubernetes cluster, and the agreed port is 30009, we need to create a Service as follows:

apiVersion: v1
kind: Service
metadata:
 name: minio
spec:
 ports:
 - name: api
 nodePort: 30009
 port: 9000
 protocol: TCP
 targetPort: 9000
 selector:
 app: minio
 type: NodePort

However, as mentioned before, if the port (30009) required for the minio Service is not reserved, and another type: NodePort (or possibly type: LoadBalancer) Service is created and dynamically allocated before or concurrently with the minio Service, TCP port 30009 might be allocated to that other Service; if so, creation of the minio Service will fail due to a node port collision.

How can you avoid NodePort Service port conflicts?

Kubernetes 1.24 introduced changes for type: ClusterIP Services, dividing the CIDR range for cluster IP addresses into two blocks that use different allocation policies to reduce the risk of conflicts. In Kubernetes 1.27, as an alpha feature, you can adopt a similar policy for type: NodePort Services. You can enable a new feature gate ServiceNodePortStaticSubrange. Turning this on allows you to use a different port allocation strategy for type: NodePort Services, and reduce the risk of collision.

The port range for NodePort will be divided, based on the formula min(max(16, nodeport-size / 32), 128). The outcome of the formula will be a number between 16 and 128, with a step size that increases as the size of the nodeport range increases. The outcome of the formula determine that the size of static port range. When the port range is less than 16, the size of static port range will be set to 0, which means that all ports will be dynamically allocated.

Dynamic port assignment will use the upper band by default, once this has been exhausted it will use the lower range. This will allow users to use static allocations on the lower band with a low risk of collision.

Examples