Kubernetes Service

Services create rules for accessing Pods
Services route traffic to the ready Pods only
Services matches Pods using selectors
Services expose DNS name that is available across the cluster.
The full service name: name.namespace.svc.cluster.local
Default resolution policy in Kubernetes is ClusterFirst:

• DNS query is routed to dnsmasq (running in kube-dns pod)
• dnsmasq routes the request to:
  • kube-dns if the name ends with a cluster suffix
  • to the upstream DNS server otherwise

Basics

Name resolution is configured in /etc/resolv.conf.
By default pods contain resolv.conf with roughly:

nameserver 10.10.10.10
search namespace.svc.cluster.local svc.cluster.local cluster.local something.cloud.provider.specific
options ndots:5

If the requested DNS name contains fewer than 5 dots then the search domains are checked. If there is no match then the name is treated as an absolute name
The FQDN domains (the ones ending with .) are always treated as absolute. Thus if the pod needs to resolve the name that is known to be external to the cluster: it is good to configure that name as FQDN in the application. The pod will make fewer (number of search option entries) DNS queries
It is also possible to customize resolv.conf using dnsConfig pod's section

Headless

type: ClusterIP
clusterIP: None

No IP is allocated. DNS is configured:

• if selectors are used then DNS returns all matching Pods
• if selectors are not configured then:
  • DNS returns CNAME records for ExternalName -type Services
  • DNS returns records for and Endpoints that share a name with this Service, for all other types

Using headless service it is possible to expose pods hostname for cluster availability (required: hostname and subdomain on the pod)

ClusterIP

type: ClusterIP

Default Service type, guarantees unique IP across the cluster. This IP 'lives' only in iptables and is maintained by kube-proxy.
By default uses round-robin to pick pods.

NodePort

type: NodePort

Gives access from outside of the cluster. Opens socket with high port on every node thus allowing to access the service from outside of the cluster using node IP (with that high port)

LoadBalancer

type: LoadBalancer

Gives access from outside of the cluster. This is cloud-specific. Creates the load-balancer-type resource in cloud provider