How to expose a service in Kubernetes

Question

I am trying to expose a service in a simple kubernetes cluster composed of a single worker and one master. In particular, I am using the descriptor below:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mongodb-deployment
  labels:
    app: mongodb
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mongodb
  template:
    metadata:
      labels:
        app: mongodb
    spec:
      containers:
      - name: mongodb
        image: mongo
        ports:
        - containerPort: 27017
---
apiVersion: v1
kind: Service
metadata:
  name: mongodb-service
spec:
  selector:
    app: mongodb
  ports:
    - protocol: TCP
      port: 27017
      targetPort: 27017

Then I try to use this service from another pod:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mongo-express
  labels:
    app: mongo-express
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mongo-express
  template:
    metadata:
      labels:
        app: mongo-express
    spec:
      containers:
      - name: mongo-express
        image: mongo-express
        ports:
        - containerPort: 8081
        env:
        - name: ME_CONFIG_MONGODB_SERVER
          value: mongodb-service

However, what I get in the other mongo-express pod is that mongodb-service cannot be resolved. In fact if I spin up a pod and I try a simple wget this is the output:

$ wget http://mongodb-service/ -O-
--2021-06-23 13:31:08--  http://mongodb-service/
Resolving mongodb-service (mongodb-service)... failed: Name or service not known.
wget: unable to resolve host address 'mongodb-service'

Instead nslookup mongodb-service works fine:

$ nslookup mongodb-service
Server:     10.96.0.10
Address:    10.96.0.10#53

However, if I try with netcat I get this:

$nc mongodb-service 27017
nc: getaddrinfo for host "mongodb-service" port 27017: Name or service not known

So it seems that it is getaddrinfo that is failing.

How could I debug the problem?

Answer 1

Go inside the pod and try following:

nslookup mongodb-service.default.svc.cluster.local

It will give you the clusterIP of service and then try to wget with this IP instead of service's DNS.

wget http://10.100.142.166 -O-

The reason for using IP over DNS: https://stackoverflow.com/questions/53921281/kubernetes-1-13-coredns-cluster-curl-service