How to know your on k8s master node?

Question

I'm writing some scripts that check the system to make sure of some cluster characteristics. Things running on private IP address spaces, etc. These checks are just a manual step when setting up a cluster, and used just for sanity checking.

They'll be run on each node, but I'd like a set of them to run when on the master node. Is there a bash, curl, kubectl, or another command that has information indicating the current node is a master node?

Phonemic

Answer 1

The master(s) usually has the 'master' role associated with it. For example:

$ kubectl get nodes
NAME                                         STATUS    ROLES     AGE       VERSION
ip-x-x-x-x.us-west-2.compute.internal        Ready     <none>    7d        v1.11.2
ip-x-x-x-x.us-west-2.compute.internal        Ready     master    78d       v1.11.2
ip-x-x-x-x.us-west-2.compute.internal        Ready     <none>    7d        v1.11.2
ip-x-x-x-x.us-west-2.compute.internal        Ready     <none>    7d        v1.11.2
ip-x-x-x-x.us-west-2.compute.internal        Ready     <none>    7d        v1.11.2

It also has a label node-role.kubernetes.io/master associated with it. For example:

$ kubectl get node ip-x-x-x-x.us-west-2.compute.internal -o=yaml
apiVersion: v1
kind: Node
metadata:
  annotations:
    kubeadm.alpha.kubernetes.io/cri-socket: /var/run/dockershim.sock
    node.alpha.kubernetes.io/ttl: "0"
    projectcalico.org/IPv4Address: x.x.x.x/20
    volumes.kubernetes.io/controller-managed-attach-detach: "true"
  creationTimestamp: 2018-07-23T21:10:22Z
  labels:
    beta.kubernetes.io/arch: amd64
    beta.kubernetes.io/instance-type: t3.medium
    beta.kubernetes.io/os: linux
    failure-domain.beta.kubernetes.io/region: us-west-2
    failure-domain.beta.kubernetes.io/zone: us-west-2c
    kubernetes.io/hostname: ip-x-x-x-x.us-west-2.compute.internal
    node-role.kubernetes.io/master: ""

Kovacev

Answer 2

Some more ways:

$ kubectl cluster-info 
Kubernetes master is running at https://node1.example.com:8443
...

You can use kubectl with label selector:

$ kubectl get nodes -l node-role.kubernetes.io/master=true
NAME                STATUS    ROLES     AGE       VERSION
node1.example.com   Ready     master    1d        v1.10.5
node2.example.com   Ready     master    1d        v1.10.5

And you can get specific data via jsonpath, e.g master IPs/hostnames:

$ kubectl get nodes -l node-role.kubernetes.io/master=true -o 'jsonpath={.items[*].status.addresses[?(@.type=="InternalIP")].address}'
192.168.168.197 192.168.168.198

$ kubectl get nodes -l node-role.kubernetes.io/master=true -o 'jsonpath={.items[*].status.addresses[?(@.type=="Hostname")].address}'
node1.example.com node2.example.com 

Cowpea