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DNS for Services and Pods templates/concept

{{% capture overview %}} This page provides an overview of DNS support by Kubernetes. {{% /capture %}}

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Introduction

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.

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. For more up-to-date specification, see Kubernetes DNS-Based Service Discovery.

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.

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.

Pod's hostname and subdomain fields

Currently when a pod is created, its hostname is the Pod's metadata.name value.

The Pod spec has an optional hostname field, which can be used to specify the Pod's hostname. When specified, it takes precedence over the Pod's name to be the hostname of the pod. For example, given a Pod with hostname set to "my-host", the Pod will have its hostname set to "my-host".

The Pod spec also has an optional subdomain field which can be used to specify its subdomain. For example, a Pod with hostname set to "foo", and subdomain set to "bar", in namespace "my-namespace", will have the fully qualified domain name (FQDN) "foo.bar.my-namespace.svc.cluster.local".

Example:

apiVersion: v1
kind: Service
metadata:
  name: default-subdomain
spec:
  selector:
    name: busybox
  clusterIP: None
  ports:
  - name: foo # Actually, no port is needed.
    port: 1234
    targetPort: 1234
---
apiVersion: v1
kind: Pod
metadata:
  name: busybox1
  labels:
    name: busybox
spec:
  hostname: busybox-1
  subdomain: default-subdomain
  containers:
  - image: busybox
    command:
      - sleep
      - "3600"
    name: busybox
---
apiVersion: v1
kind: Pod
metadata:
  name: busybox2
  labels:
    name: busybox
spec:
  hostname: busybox-2
  subdomain: default-subdomain
  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. For example, given a Pod with the hostname set to "busybox-1" and the subdomain set to "default-subdomain", and a headless Service named "default-subdomain" in the same namespace, the pod will see its own FQDN as "busybox-1.default-subdomain.my-namespace.svc.cluster.local". DNS serves an A record at that name, pointing to the Pod's IP. Both pods "busybox1" and "busybox2" can have their distinct A records.

The Endpoints object can specify the hostname for any endpoint addresses, along with its IP.

Pod's DNS Policy

DNS policies can be set on a per-pod basis. Currently Kubernetes supports the following pod-specific DNS policies. These policies are specified in the dnsPolicy field of a Pod Spec.

  • "Default": The Pod inherits the name resolution configuration from the node that the pods run on. See related discussion for more details.
  • "ClusterFirst": Any DNS query that does not match the configured cluster domain suffix, such as "www.kubernetes.io", is forwarded to the upstream nameserver inherited from the node. Cluster administrators may have extra stub-domain and upstream DNS servers configured. See related discussion for details on how DNS queries are handled in those cases.
  • "ClusterFirstWithHostNet": For Pods running with hostNetwork, you should explicitly set its DNS policy "ClusterFirstWithHostNet".
  • "None": A new option value introduced in Kubernetes v1.9 (Beta in v1.10). It allows a Pod to ignore DNS settings from the Kubernetes environment. All DNS settings are supposed to be provided using the dnsConfig field in the Pod Spec. See DNS config subsection below.

{{< note >}} NOTE: "Default" is not the default DNS policy. If dnsPolicy is not explicitly specified, then “ClusterFirst” is used. {{< /note >}}

The example below shows a Pod with its DNS policy set to "ClusterFirstWithHostNet" because it has hostNetwork set to true.

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  namespace: default
spec:
  containers:
  - image: busybox
    command:
      - sleep
      - "3600"
    imagePullPolicy: IfNotPresent
    name: busybox
  restartPolicy: Always
  hostNetwork: true
  dnsPolicy: ClusterFirstWithHostNet

Pod's DNS Config

Kubernetes v1.9 introduces an Alpha feature (Beta in v1.10) that allows users more control on the DNS settings for a Pod. This feature is enabled by default in v1.10. To enable this feature in v1.9, the cluster administrator needs to enable the CustomPodDNS feature gate on the apiserver and the kubelet, for example, "--feature-gates=CustomPodDNS=true,...". When the feature gate is enabled, users can set the dnsPolicy field of a Pod to "None" and they can add a new field dnsConfig to a Pod Spec.

The dnsConfig field is optional and it can work with any dnsPolicy settings. However, when a Pod's dnsPolicy is set to "None", the dnsConfig field has to be specified.

Below are the properties a user can specify in the dnsConfig field:

  • nameservers: a list of IP addresses that will be used as DNS servers for the Pod. There can be at most 3 IP addresses specified. When the Pod's dnsPolicy is set to "None", the list must contain at least one IP address, otherwise this property is optional. The servers listed will be combined to the base nameservers generated from the specified DNS policy with duplicate addresses removed.
  • searches: a list of DNS search domains for hostname lookup in the Pod. This property is optional. When specified, the provided list will be merged into the base search domain names generated from the chosen DNS policy. Duplicate domain names are removed. Kubernetes allows for at most 6 search domains.
  • options: an optional list of objects where each object may have a name property (required) and a value property (optional). The contents in this property will be merged to the options generated from the specified DNS policy. Duplicate entries are removed.

The following is an example Pod with custom DNS settings:

{{< code file="custom-dns.yaml" >}}

When the Pod above is created, the container test gets the following contents in its /etc/resolv.conf file:

nameserver 1.2.3.4
search ns1.svc.cluster.local my.dns.search.suffix
options ndots:2 edns0

For IPv6 setup, search path and name server should be setup like this:

$ kubectl exec -it busybox -- cat /etc/resolv.conf
nameserver fd00:79:30::a
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5

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For guidance on administering DNS configurations, check Configure DNS Service

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