Demo Service Discovery and DNS

In this lesson, you’ll learn how Kubernetes implements stable service discovery for your applications. We’ll demonstrate two primary mechanisms:

1. Environment Variables injected into pods at launch
2. Cluster DNS names resolving service endpoints

Throughout this guide, we’ll use an nginx-service deployed in the default namespace as our example.

Service Overview

First, confirm that the nginx-service exists in the default namespace:

kubectl get svc nginx-service -n default

Expected output:

NAME            TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)   AGE
nginx-service   ClusterIP   10.103.206.194   <none>        80/TCP    10m

1. Environment Variable–Based Discovery

When a pod starts, the kubelet injects environment variables for each Service in the same namespace. Let’s verify this with a temporary pod:

kubectl run -i --tty --rm test-conn \
  --image=centos --restart=Never -- bash

Inside the pod shell, list variables related to nginx-service:

env | grep -i nginx

You should see:

NGINX_SERVICE_PORT=tcp://10.103.206.194:80
NGINX_SERVICE_PORT_80_TCP_ADDR=10.103.206.194
NGINX_SERVICE_PORT_80_TCP_PORT=80
NGINX_SERVICE_SERVICE_HOST=10.103.206.194
NGINX_SERVICE_SERVICE_PORT=80

Use these variables to curl the service:

curl http://$NGINX_SERVICE_SERVICE_HOST:$NGINX_SERVICE_SERVICE_PORT

You’ll receive the standard NGINX welcome page HTML.

1.1 Limitations Across Namespaces

To see this in action, launch a pod in the kube-system namespace:

kubectl run -i --tty --rm test-conn \
  --image=centos --restart=Never \
  --namespace=kube-system -- bash
env | grep -i nginx

No output appears, since nginx-service resides in default.

2. Cluster DNS–Based Discovery

Kubernetes also runs a DNS server (CoreDNS or kube-dns) that resolves Service names cluster-wide. Pods automatically get DNS settings in /etc/resolv.conf:

cat /etc/resolv.conf

Typical output:

search kube-system.svc.cluster.local svc.cluster.local cluster.local
nameserver 10.96.0.10
options ndots:5

2.1 Verifying DNS Resolution

Install DNS utilities and lookup the Service FQDN:

yum install -y bind-utils
nslookup nginx-service.default.svc.cluster.local

Expected response:

Name:   nginx-service.default.svc.cluster.local
Address: 10.103.206.194

You can curl using the full DNS name:

curl http://nginx-service.default.svc.cluster.local

2.2 Shortened DNS Names

Since svc.cluster.local is in your search domains, you can use a shorter name:

nslookup nginx-service.default
curl http://nginx-service.default

Both will resolve to 10.103.206.194.

Comparison of Discovery Methods

Summary

• Environment Variables
  Injected per namespace by the kubelet. Simple but limited to in-namespace communication.
• Cluster DNS
  Provides cross-namespace, cluster-wide name resolution. Requires CoreDNS or kube-dns running.

Links and References

• Kubernetes Service Discovery
• CoreDNS Documentation
• Kubernetes DNS Overview