Blog: Kubernetes 1.27: Avoid Collisions Assigning Ports to NodePort Services

sorangutan

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

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

small(lower boundary) range: 30000-30127

Range properties	Values
service-node-port-range	30000-30127
Band Offset	`min(max(16, 128/32), 128)` = `min(max(16, 4), 128)` = `min(16, 128)` = 16
Static band start	30000
Static band end	30015
Dynamic band start	30016
Dynamic band end	30127

pie showData title 30000-30127 "Static" : 16 "Dynamic" : 112

large(upper boundary) range: 30000-34095

Range properties	Values
service-node-port-range	30000-34095
Band Offset	`min(max(16, 4096/32), 128)` = `min(max(16, 128), 128)` = `min(128, 128)` = 128
Static band start	30000
Static band end	30127
Dynamic band start	30128
Dynamic band end	34095

pie showData title 30000-34095 "Static" : 128 "Dynamic" : 3968

very large range: 30000-38191

Range properties	Values
service-node-port-range	30000-38191
Band Offset	`min(max(16, 8192/32), 128)` = `min(max(16, 256), 128)` = `min(256, 128)` = 128
Static band start	30000
Static band end	30127
Dynamic band start	30128
Dynamic band end	38191

pie showData title 30000-38191 "Static" : 128 "Dynamic" : 8064

https://kubernetes.io/blog/2023/05/11/nodeport-dynamic-and-static-allocation/