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

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

very small range: 30000-30015

small(lower boundary) range: 30000-30127

large(upper boundary) range: 30000-34095

very large range: 30000-38191