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Introduction
As of Kubernetes 1.3, DNS is a built-in service launched automatically using the addon manager cluster add-on.
Kubernetes DNS schedules a DNS Pod and Service on the cluster, and configures the kubelets to tell individual containers to use the DNS Service's IP to resolve DNS names.
What things get DNS names?
Every Service defined in the cluster (including the DNS server itself) is assigned a DNS name. By default, a client Pod's DNS search list will include the Pod's own namespace and the cluster's default domain. This is best illustrated by example:
Assume a Service named foo in the Kubernetes namespace bar. A Pod running
in namespace bar can look up this service by simply doing a DNS query for
foo. A Pod running in namespace quux can look up this service by doing a
DNS query for foo.bar.
Supported DNS schema
The following sections detail the supported record types and layout that is supported. Any other layout or names or queries that happen to work are considered implementation details and are subject to change without warning.
Services
A records
"Normal" (not headless) Services are assigned a DNS A record for a name of the
form my-svc.my-namespace.svc.cluster.local. This resolves to the cluster IP
of the Service.
"Headless" (without a cluster IP) Services are also assigned a DNS A record for
a name of the form my-svc.my-namespace.svc.cluster.local. Unlike normal
Services, this resolves to the set of IPs of the pods selected by the Service.
Clients are expected to consume the set or else use standard round-robin
selection from the set.
SRV records
SRV Records are created for named ports that are part of normal or Headless
Services.
For each named port, the SRV record would have the form
_my-port-name._my-port-protocol.my-svc.my-namespace.svc.cluster.local.
For a regular service, this resolves to the port number and the CNAME:
my-svc.my-namespace.svc.cluster.local.
For a headless service, this resolves to multiple answers, one for each pod
that is backing the service, and contains the port number and a CNAME of the pod
of the form auto-generated-name.my-svc.my-namespace.svc.cluster.local.
Backwards compatibility
Previous versions of kube-dns made names of the for
my-svc.my-namespace.cluster.local (the 'svc' level was added later). This
is no longer supported.
Pods
A Records
When enabled, pods are assigned a DNS A record in the form of pod-ip-address.my-namespace.pod.cluster.local.
For example, a pod with ip 1.2.3.4 in the namespace default with a dns name of cluster.local would have an entry: 1-2-3-4.default.pod.cluster.local.
A Records and hostname based on Pod's hostname and subdomain fields
Currently when a pod is created, its hostname is the Pod's metadata.name value.
With v1.2, users can specify a Pod annotation, pod.beta.kubernetes.io/hostname, to specify what the Pod's hostname should be.
The Pod annotation, if specified, takes precendence over the Pod's name, to be the hostname of the pod.
For example, given a Pod with annotation pod.beta.kubernetes.io/hostname: my-pod-name, the Pod will have its hostname set to "my-pod-name".
With v1.3, the PodSpec has a hostname field, which can be used to specify the Pod's hostname. This field value takes precedence over the
pod.beta.kubernetes.io/hostname annotation value.
v1.2 introduces a beta feature where the user can specify a Pod annotation, pod.beta.kubernetes.io/subdomain, to specify the Pod's subdomain.
The final domain will be "...svc.".
For example, a Pod with the hostname annotation set to "foo", and the subdomain annotation set to "bar", in namespace "my-namespace", will have the FQDN "foo.bar.my-namespace.svc.cluster.local"
With v1.3, the PodSpec has a subdomain field, which can be used to specify the Pod's subdomain. This field value takes precedence over the
pod.beta.kubernetes.io/subdomain annotation value.
Example:
apiVersion: v1
kind: Pod
metadata:
name: busybox
namespace: default
spec:
hostname: busybox-1
subdomain: default
containers:
- image: busybox
command:
- sleep
- "3600"
name: busybox
If there exists a headless service in the same namespace as the pod and with the same name as the subdomain, the cluster's KubeDNS Server also returns an A record for the Pod's fully qualified hostname. Given a Pod with the hostname set to "foo" and the subdomain set to "bar", and a headless Service named "bar" in the same namespace, the pod will see it's own FQDN as "foo.bar.my-namespace.svc.cluster.local". DNS serves an A record at that name, pointing to the Pod's IP.
With v1.2, the Endpoints object also has a new annotation endpoints.beta.kubernetes.io/hostnames-map. Its value is the json representation of map[string(IP)][endpoints.HostRecord], for example: '{"10.245.1.6":{HostName: "my-webserver"}}'.
If the Endpoints are for a headless service, an A record is created with the format ...svc.
For the example json, if endpoints are for a headless service named "bar", and one of the endpoints has IP "10.245.1.6", an A is created with the name "my-webserver.bar.my-namespace.svc.cluster.local" and the A record lookup would return "10.245.1.6".
This endpoints annotation generally does not need to be specified by end-users, but can used by the internal service controller to deliver the aforementioned feature.
With v1.3, The Endpoints object can specify the hostname for any endpoint, along with its IP. The hostname field takes precedence over the hostname value
that might have been specified via the endpoints.beta.kubernetes.io/hostnames-map annotation.
With v1.3, the following annotations are deprecated: pod.beta.kubernetes.io/hostname, pod.beta.kubernetes.io/subdomain, endpoints.beta.kubernetes.io/hostnames-map
How do I test if it is working?
Create a simple Pod to use as a test environment.
Create a file named busybox.yaml with the following contents:
apiVersion: v1
kind: Pod
metadata:
name: busybox
namespace: default
spec:
containers:
- image: busybox
command:
- sleep
- "3600"
imagePullPolicy: IfNotPresent
name: busybox
restartPolicy: Always
Then create a pod using this file:
kubectl create -f busybox.yaml
Wait for this pod to go into the running state.
You can get its status with:
kubectl get pods busybox
You should see:
NAME READY STATUS RESTARTS AGE
busybox 1/1 Running 0 <some-time>
Validate DNS works
Once that pod is running, you can exec nslookup in that environment:
kubectl exec busybox -- nslookup kubernetes.default
You should see something like:
Server: 10.0.0.10
Address 1: 10.0.0.10
Name: kubernetes.default
Address 1: 10.0.0.1
If you see that, DNS is working correctly.
Kubernetes Federation (Multiple Zone support)
Release 1.3 introduced Cluster Federation support for multi-site Kubernetes installations. This required some minor (backward-compatible) changes to the way the Kubernetes cluster DNS server processes DNS queries, to facilitate the lookup of federated services (which span multiple Kubernetes clusters). See the Cluster Federation Administrators' Guide for more details on Cluster Federation and multi-site support.
How it Works
The running Kubernetes DNS pod holds 3 containers - kubedns, dnsmasq and a health check called healthz. The kubedns process watches the Kubernetes master for changes in Services and Endpoints, and maintains in-memory lookup structures to service DNS requests. The dnsmasq container adds DNS caching to improve performance. The healthz container provides a single health check endpoint while performing dual healthchecks (for dnsmasq and kubedns).
The DNS pod is exposed as a Kubernetes Service with a static IP. Once assigned the
kubelet passes DNS configured using the --cluster-dns=10.0.0.10 flag to each
container.
DNS names also need domains. The local domain is configurable, in the kubelet using
the flag --cluster-domain=<default local domain>
The Kubernetes cluster DNS server (based off the SkyDNS library) supports forward lookups (A records), service lookups (SRV records) and reverse IP address lookups (PTR records).