kube-dns
Kubernetes has a built-in DNS service that helps in resolving the DNS names. This service is called kube-dns.
There is a DNS record for each service and pod created in the cluster. The DNS server is responsible for resolving the DNS names to the IP addresses.
Internally, Kubernetes uses CoreDNS as the DNS server. Let's see the components that make up the kube-dns service:
CoreDNS Deployment: This is the deployment for the CoreDNS server. It manages the CoreDNS replica sets.
$ kubectl get deployments -n kube-system -l k8s-app=kube-dns -o wide
NAME READY UP-TO-DATE AVAILABLE AGE CONTAINERS IMAGES SELECTOR
coredns 2/2 2 2 21m coredns registry.k8s.io/coredns/coredns:v1.11.1 k8s-app=kube-dns
CoreDNS Replica Set: This is the replica set for the CoreDNS server. It manages the CoreDNS pods.
kubectl get replicaset -n kube-system -l k8s-app=kube-dns -o wide
NAME DESIRED CURRENT READY AGE CONTAINERS IMAGES SELECTOR
coredns-7db6d8ff4d 2 2 2 21m coredns registry.k8s.io/coredns/coredns:v1.11.1 k8s-app=kube-dns,pod-template-hash=7db6d8ff4d
CoreDNS Pods: These are the pods that run the CoreDNS server. There are usually two pods running in the cluster. These pods are labeled with k8s-app=kube-dns.
$ kubectl get pods -n kube-system -l k8s-app=kube-dns -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES LABELS
coredns-7db6d8ff4d-792wd 1/1 Running 0 157m 192.168.219.65 master <none> <none> k8s-app=kube-dns,pod-template-hash=7db6d8ff4d
coredns-7db6d8ff4d-nvxsf 1/1 Running 0 157m 192.168.219.68 master <none> <none> k8s-app=kube-dns,pod-template-hash=7db6d8ff4d
The Service kube-dns is a ClusterIP service that exposes the CoreDNS server to the cluster. A ClusterIP Service is only accessible within the cluster.
$ kubectl get svc -n kube-system -l k8s-app=kube-dns -o wide
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 22m k8s-app=kube-dns
As shown above, this service uses the kube-dns label selector, which is the same label used by the CoreDNS pods.
DNS Resolution in Kubernetes
The DNS Server, i.e. the codedns pods, always run in the kube-system namespace in the master node. The DNS server is responsible for resolving the DNS names to the IP addresses. Whenever kubelet creates a pod, it injects a DNS configuration file, /etc/resolv.conf, into the pod. This file contains the IP address of the DNS server and the search domains.
Run the following command to create a simple pod and ssh into it:
kubectl run -i --tty alpine --image=alpine --restart=Never -- sh
Once you're inside the pod, view the /etc/resolv.conf file:
cat /etc/resolv.conf
You'll see the IP address of the DNS server and the search domains.
Let's test the DNS resolution. Run the following command to resolve the IP address of the kube-dns service:
nslookup kube-dns.kube-system.svc.cluster.local
Here's the sample output:
$ kubectl run -i --tty alpine --image=alpine --restart=Never -- sh
If you don't see a command prompt, try pressing enter.
/ #
/ #
/ # cat /etc/resolv.conf
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
/ #
/ #
/ # nslookup kube-dns.kube-system.svc.cluster.local
Server: 10.96.0.10
Address: 10.96.0.10:53
Name: kube-dns.kube-system.svc.cluster.local
Address: 10.96.0.10
From the /etc/resolv.conf file, you can see that the DNS Server is located at 10.96.0.10 on port 53, which is the kube-dns service. When you run the nslookup command, the request is sent to the DNS server. The search field in the /etc/resolv.conf file specifies the search domains to append to the DNS query.
Putting it all together
Kubernetes has an internal DNS service, called kube-dns. This service is responsible for resolving the DNS names to the IP addresses.
Kubernetes creates DNS records for:
- Services
- Pods
Kubernetes uses CoreDNS as the DNS server. The CoreDNS server is run as a deployment and exposed via a ClusterIP service called kube-dns. The DNS server pods are scheduled on the Master node.
Kubelet is responsible for injecting the DNS configuration file into the pod. The /etc/resolve.conf file contains the IP address of the Master node having the DNS server and the search domains. Whenever a request is made, the pods append the search domains to the DNS query till the DNS server resolves the IP address.
Whenever a pod has to make a request, it looks up the /etc/resolve.conf file to find the IP of the DNS server. It then sends the DNS query to the DNS server. The DNS server resolves the IP address and sends it back to the pod. The pod then makes the request to the resolved IP address.