Blog: Kubernetes 1.27: Avoid Collisions Assigning Ports to NodePort Services
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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 anodePort
value explicitly in thespec
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 oftype: 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 anothertype: NodePort
(or possiblytype: LoadBalancer
) Service is created and dynamically allocated before or concurrently with theminio
Service, TCP port 30009 might be allocated to that other Service; if so, creation of theminio
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 fortype: NodePort
Services. You can enable a new feature gateServiceNodePortStaticSubrange
. Turning this on allows you to use a different port allocation strategy fortype: NodePort
Services, and reduce the risk of collision.The port range for
NodePort
will be divided, based on the formulamin(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)
= 86Static band start 30000 Static band end 30085 Dynamic band start 30086 Dynamic band end 32767 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 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)
= 16Static band start 30000 Static band end 30015 Dynamic band start 30016 Dynamic band end 30127 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)
= 128Static band start 30000 Static band end 30127 Dynamic band start 30128 Dynamic band end 34095 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)
= 128Static band start 30000 Static band end 30127 Dynamic band start 30128 Dynamic band end 38191
https://kubernetes.io/blog/2023/05/11/nodeport-dynamic-and-static-allocation/
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