This page describes how Google Kubernetes Engine (GKE) implements service discovery and managed DNS. For a general overview of how DNS is used in Kubernetes clusters, see DNS for Services and Pods.
Service discovery and the kube-dns add-on
In Kubernetes, service discovery is implemented with autogenerated service names that
map to the service's IP address. Service names
follow a
standard specification:
my-svc.my-namespace.svc.my-zone. Pods can also access external services, like
example.com, through their names. See DNS for Services and
Pods
for more information on the behaviour of DNS in Kubernetes.
GKE provides managed DNS for resolving service
names and for resolving external names. This is implemented by kube-dns, a
cluster add-on that is deployed by default in all GKE
clusters. kube-dns runs as a
Deployment that schedules
redundant kube-dns Pods to nodes in the cluster. The kube-dns Pods are in
the kube-system namespace. The kube-dns deployment is accessed through a corresponding
Service that groups the kube-dns
Pods and gives them a single IP address. By default, all Pods in a cluster use this service
to resolve DNS queries.
kube-dns scales to serve the DNS demands of the cluster. This scaling is
controlled by the kube-dns-autoscaler which is deployed by default in all
GKE clusters. kube-dns-autoscaler adjusts the number of
replicas in the kube-dns deployment based on the number of nodes and cores in
the cluster.
How Pod DNS is configured
The kubelet agent running on each Node configures the Pod's etc/resolv.conf to
use the kube-dns service's ClusterIP. An example of this configuration is shown
below, in this example the IP address of the kube-dns service is 10.0.0.10
(this IP address will be different in other clusters):
# cat /etc/resolv.conf nameserver 10.0.0.10 search default.svc.cluster.local svc.cluster.local cluster.local c.my-project-id.internal google.internal options ndots:5
kube-dns is the authoritative name server for the cluster domain (cluster.local)
and it recursively resolves external names. Short names that are not fully
qualified, like myservice, are completed first with local search paths. For
example, myservice.default.svc.cluster.local, myservice.svc.cluster.local,
myservice.cluster.local, myservice.c.my-project-id.internal, and myservice.google.internal.
What's next
- Learn how to provide scalable DNS resolution using NodeLocal DNSCache for clusters requiring high volumes of DNS queries.