> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Demo Service Discovery and DNS > This article explains Kubernetes service discovery using environment variables and cluster DNS for application communication. 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: ```shell theme={null} kubectl get svc nginx-service -n default ``` Expected output: ```text theme={null} NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE nginx-service ClusterIP 10.103.206.194 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: ```shell theme={null} kubectl run -i --tty --rm test-conn \ --image=centos --restart=Never -- bash ``` Inside the pod shell, list variables related to `nginx-service`: ```bash theme={null} env | grep -i nginx ``` You should see: ```text theme={null} 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: ```bash theme={null} curl http://$NGINX_SERVICE_SERVICE_HOST:$NGINX_SERVICE_SERVICE_PORT ``` You’ll receive the standard NGINX welcome page HTML. ### 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: ```shell theme={null} 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`: ```bash theme={null} cat /etc/resolv.conf ``` Typical output: ```text theme={null} 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: ```bash theme={null} yum install -y bind-utils nslookup nginx-service.default.svc.cluster.local ``` Expected response: ```text theme={null} Name: nginx-service.default.svc.cluster.local Address: 10.103.206.194 ``` You can curl using the full DNS name: ```bash theme={null} 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: ```bash theme={null} nslookup nginx-service.default curl http://nginx-service.default ``` Both will resolve to `10.103.206.194`. *** ## Comparison of Discovery Methods | Method | Scope | Example Usage | | --------------------- | ------------------- | ----------------------------------------------------- | | Environment Variables | Same namespace only | `echo $NGINX_SERVICE_SERVICE_HOST` | | Cluster DNS | Cluster-wide | `curl http://nginx-service.default.svc.cluster.local` | *** ## 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](https://kubernetes.io/docs/concepts/services-networking/service/) * [CoreDNS Documentation](https://coredns.io/manual/) * [Kubernetes DNS Overview](https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/)