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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.

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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

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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.

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1.1 Limitations Across Namespaces

Environment variables are only injected into pods in the same namespace as the Service.
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.

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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

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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

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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.

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Comparison of Discovery Methods

MethodScopeExample Usage
Environment VariablesSame namespace onlyecho $NGINX_SERVICE_SERVICE_HOST
Cluster DNSCluster-widecurl http://nginx-service.default.svc.cluster.local

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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.

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