What is a services in kubernetes ?

 Services:-           

A Kubernetes service can easily expose an application deployed on a set of pods using a single endpoint.

i)Kubernetes Networking Concepts

1.     Pod-to-Pod Communication (Flat Network)

o   Every pod gets a unique IP address.

o   Pods can communicate without NAT inside the cluster.

o   Uses CNI (Container Network Interface) plugins like Flannel, Calico, Cilium.

2.     Pod-to-Service Communication (Stable Networking)

o   Kubernetes Services provide a stable IP and DNS name.

o   Traffic is load-balanced across multiple pod replicas.

3.     External Access (Ingress, LoadBalancer, NodePort)

o   Ingress: Routes external HTTP(S) traffic to services.

o   LoadBalancer: Exposes a service externally (cloud provider-dependent).

o   NodePort: Opens a high port (e.g., 30000-32767) on each node.

 

ii) Types of Kubernetes Services.

Type	Access	Example Use Case
ClusterIP	Internal Only	Pod-to-pod communication
NodePort	Exposes on Node IP & port (30000-32767)	Debugging or small-scale external access
LoadBalancer	Assigns external IP (Cloud provider)	Production apps on cloud
ExternalName	DNS CNAME alias	Redirect traffic to external services

iii) Kubernetes DNS (CoreDNS)

•   Every Service gets a DNS name (e.g., myservice.default.svc.cluster.local).
•   Uses CoreDNS for name resolution.

# kubectl logs -n kube-system -l k8s-app=kube-dns

 

iv) Network Policies (Calico, Cilium)

By default, all pods can communicate with each other.
Use Network Policies to restrict access.

 

v) Troubleshooting Kubernetes Networking.

Issue	Fix
Pods can’t talk to each other	Check CNI plugin (kubectl get pods -n kube-system)
DNS resolution fails	Restart CoreDNS (kubectl rollout restart deployment coredns -n kube-system)
Service unreachable	Check kubectl get svc -A and kubectl describe svc <name>
Ingress not working	Check Ingress Controller (kubectl get pods -n ingress-nginx)
Flannel subnet error	Restart Flannel (kubectl rollout restart ds kube-flannel -n kube-system)

 

1.There are many types of Services:

i) ClusterIP (default) :- Exposes a service which is only accessible from within the cluster.

ii) NodePort :- Exposes a service via a static port on each node’s IP.

iii) LoadBalancer :- It uses cloud providers’ load balancer. NodePort and ClusterIP services are created automatically to which the external load balancer will route.

iv) Ingress :- Ingress is actually not a type of service. It sits in front of multiple services and performs smart routing between them, providing access to your cluster. Several types of ingress controllers have different routing capabilities. In GKE, the ingress controller creates an HTTP Load Balancer, which can route traffic to services in the Kubernetes cluster based on path or subdomain.

v) ExternalName :-  Maps a service to a predefined externalName field by returning a value for the CNAME record.

vi) Headless :- Services that do not need load balancing and only expose a single IP can create a ‘headless’ service by specifying “none” as the clusterIP.

vii) External IPs :- If there are external IPs that route to one or more cluster nodes.

viii) Endpoint:-  An endpoint is an resource that gets IP addresses of one or more pods dynamically assigned to it, along with a port. 

ix) KubeDNS or Kubernetes DNS:- is a component within the Kubernetes ecosystem that provides Domain Name System (DNS) resolution services for applications running on Kubernetes clusters. It essentially enables the mapping of service names to their corresponding network endpoints within the Kubernetes environment.

2.Two CIDRs are available on a k8s cluster.

1. Pods CIDR :- This specifies the CIDR range allocated for pod IP addresses in the Kubernetes cluster. Pods in the cluster will be assigned IP addresses from this range.

2. Services CIDR :- This specifies the CIDR range allocated for Kubernetes service IP addresses. Services in the cluster will be assigned virtual IP addresses from this range.

--cluster-cidr=192.169.0.0/16

--service-cluster-ip-range=10.96.0.0/12

# kubectl describe pod kube-controller-manager-master -n kube-system

3.Each node in a Kubernetes cluster typically has its own CIDR block assigned for pod IP addresses.

# kubectl get nodes -o custom-columns=NAME:.metadata.name,CIDR:.spec.podCIDR

NAME     CIDR

master   192.169.0.0/24

node-1   192.169.2.0/24

node-2   192.169.1.0/24

4. How to check container IP.

#  kubectl describe pods   <Pods_name>

5. How to update pods subnet CDIR.

# kubectl get ippool -o wide

# curl -L https://github.com/projectcalico/calico/releases/download/v3.27.3/calicoctl-linux-amd64 -o calicoctl

# mv calicoctl /usr/bin/

# chmod +x /usr/bin/calicoctl

# vim ip-pool_change.yaml

apiVersion: projectcalico.org/v3

kind: IPPool

metadata:

  name: new-pool

spec:

  cidr: 172.17.0.0/20

  ipipMode: Always

  natOutgoing: true

# calicoctl apply -f ip-pool_change.yaml

# calicoctl get ippool -o wide

# calicoctl get ippool -o yaml > ippool_new.yaml

# vim  ippool_new.yaml

   disabled: true

# calicoctl apply -f  ippool_new.yaml

# calicoctl get ippool -o wide

# kubectl -n kube-system edit cm kubeadm-config

# x=$(kubectl get pods -n kube-system | awk -F " " '{print $1}')

# kubectl delete pods $x -n kube-system

Restart all worker nodes

# init 6

# kubectl get pods -A -o wide