Set node label to the pod environment variable - kubernetes

Question

How to set Node label to Pod environment variable? I need to know the label topology.kubernetes.io/zone value inside the pod.

Answer 1

The Downward API currently does not support exposing node labels to pods/containers. There is an open issue about that on GitHib, but it is unclear when it will be implemented if at all.
That leaves the only option to get node labels from Kubernetes API, just as kubectl does. It is not easy to implement, especially if you want labels as environment variables. I'll give you an example how it can be done with an initContainer, curl, and jq but if possible, I suggest you rather implement this in your application, for it will be easier and cleaner.
To make a request for labels you need permissions to do that. Therefore, the example below creates a service account with permissions to get (describe) nodes. Then, the script in the initContainer uses the service account to make a request and extract labels from json. The test container reads environment variables from the file and echoes one.
Example:
# Create a service account
apiVersion: v1
kind: ServiceAccount
metadata:
name: describe-nodes
namespace: <insert-namespace-name-where-the-app-is>
---
# Create a cluster role that allowed to perform describe ("get") over ["nodes"]
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
name: describe-nodes
rules:
- apiGroups: [""]
resources: ["nodes"]
verbs: ["get"]
---
# Associate the cluster role with the service account
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
name: describe-nodes
roleRef:
apiGroup: rbac.authorization.k8s.io
kind: ClusterRole
name: describe-nodes
subjects:
- kind: ServiceAccount
name: describe-nodes
namespace: <insert-namespace-name-where-the-app-is>
---
# Proof of concept pod
apiVersion: v1
kind: Pod
metadata:
name: get-node-labels
spec:
# Service account to get node labels from Kubernetes API
serviceAccountName: describe-nodes
# A volume to keep the extracted labels
volumes:
- name: node-info
emptyDir: {}
initContainers:
# The container that extracts the labels
- name: get-node-labels
# The image needs 'curl' and 'jq' apps in it
# I used curl image and run it as root to install 'jq'
# during runtime
# THIS IS A BAD PRACTICE UNSUITABLE FOR PRODUCTION
# Make an image where both present.
image: curlimages/curl
# Remove securityContext if you have an image with both curl and jq
securityContext:
runAsUser: 0
# It'll put labels here
volumeMounts:
- mountPath: /node
name: node-info
env:
# pass node name to the environment
- name: NODENAME
valueFrom:
fieldRef:
fieldPath: spec.nodeName
- name: APISERVER
value: https://kubernetes.default.svc
- name: SERVICEACCOUNT
value: /var/run/secrets/kubernetes.io/serviceaccount
- name: SCRIPT
value: |
set -eo pipefail
# install jq; you don't need this line if the image has it
apk add jq
TOKEN=$(cat ${SERVICEACCOUNT}/token)
CACERT=${SERVICEACCOUNT}/ca.crt
# Get node labels into a json
curl --cacert ${CACERT} \
--header "Authorization: Bearer ${TOKEN}" \
-X GET ${APISERVER}/api/v1/nodes/${NODENAME} | jq .metadata.labels > /node/labels.json
# Extract 'topology.kubernetes.io/zone' from json
NODE_ZONE=$(jq '."topology.kubernetes.io/zone"' -r /node/labels.json)
# and save it into a file in the format suitable for sourcing
echo "export NODE_ZONE=${NODE_ZONE}" > /node/zone
command: ["/bin/ash", "-c"]
args:
- 'echo "$$SCRIPT" > /tmp/script && ash /tmp/script'
containers:
# A container that needs the label value
- name: test
image: debian:buster
command: ["/bin/bash", "-c"]
# source ENV variable from file, echo NODE_ZONE, and keep running doing nothing
args: ["source /node/zone && echo $$NODE_ZONE && cat /dev/stdout"]
volumeMounts:
- mountPath: /node
name: node-info

Answer 2

You could use InitContainer
...
spec:
initContainers:
- name: node2pod
image: <image-with-k8s-access>
env:
- name: NODE_NAME
valueFrom:
fieldRef:
fieldPath: spec.nodeName
...
Ref: Node Label to Pod
Edit Update
A similar Solution could be Inject node labels into Kubernetes pod