How to know which nodeport can be allocated from Kubernetes API Server? - go

Question

I want to figure out how does kubernetes knows which nodeport can be allocated when create a new service with nodeport type like this:
apiVersion: v1
kind: Service
metadata:
name: my-service
spec:
type: NodePort
selector:
app: MyApp
ports:
- port: 80
targetPort: 80
I had search google and find these kubernetes soure code, but I don't understand how does it works.
https://github.com/kubernetes/kubernetes/blob/master/pkg/registry/core/service/portallocator/allocator.go

Answer 1

The Nodeport is chosen randomly between 30000-32767. You can set it in the service definition.
apiVersion: v1
kind: Service
metadata:
name: my-service
spec:
type: NodePort
selector:
app: MyApp
ports:
# By default and for convenience, the `targetPort` is set to the same value as the `port` field.
- port: 80
targetPort: 80
# Optional field
# By default and for convenience, the Kubernetes control plane will allocate a port from a range (default: 30000-32767)
nodePort: 30007
From the documentation: https://kubernetes.io/docs/concepts/services-networking/service/#nodeport
Update
The classes placed in the package kubernetes/pkg/registry/core/service/portallocator are responsible for allocating a node port for a service.
This test documents the behavior: https://github.com/kubernetes/kubernetes/blob/master/pkg/registry/core/service/portallocator/operation_test.go
Kubernetes just takes a random port and if that one isn't free it takes the next one.
If you can read go the other classes in that package are a good starting point to understand the behavior.