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Customizing DNS Service

This page provides hints on configuring DNS Pod and guidance on customizing the DNS resolution process.

Before you begin

To check the version, enter kubectl version.

Introduction

Starting from Kubernetes v1.3, DNS is a built-in service launched automatically using the addon manager cluster add-on.

The running Kubernetes DNS pod holds 3 containers:

The DNS pod is exposed as a Kubernetes Service with a static IP. Once assigned the kubelet passes DNS configured using the --cluster-dns=<dns-service-ip> 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 is based off the SkyDNS library. It supports forward lookups (A records), service lookups (SRV records) and reverse IP address lookups (PTR records).

Inheriting DNS from the node

When running a pod, kubelet will prepend the cluster DNS server and search paths to the node’s own DNS settings. If the node is able to resolve DNS names specific to the larger environment, pods should be able to, also. See Known issues below for a caveat.

If you don’t want this, or if you want a different DNS config for pods, you can use the kubelet’s --resolv-conf flag. Setting it to “” means that pods will not inherit DNS. Setting it to a valid file path means that kubelet will use this file instead of /etc/resolv.conf for DNS inheritance.

Configure stub-domain and upstream DNS servers

Cluster administrators can specify custom stub domains and upstream nameservers by providing a ConfigMap for kube-dns (kube-system:kube-dns).

For example, the following ConfigMap sets up a DNS configuration with a single stub domain and two upstream nameservers.

apiVersion: v1
kind: ConfigMap
metadata:
  name: kube-dns
  namespace: kube-system
data:
  stubDomains: |
    {"acme.local": ["1.2.3.4"]}
  upstreamNameservers: |
    ["8.8.8.8", "8.8.4.4"]

As specified, DNS requests with the “.acme.local” suffix are forwarded to a DNS listening at 1.2.3.4. Google Public DNS serves the upstream queries.

The table below describes how queries with certain domain names would map to their destination DNS servers:

Domain name Server answering the query
kubernetes.default.svc.cluster.local kube-dns
foo.acme.local custom DNS (1.2.3.4)
widget.com upstream DNS (one of 8.8.8.8, 8.8.4.4)

See ConfigMap options for details about the configuration option format.

Impacts on Pods

Custom upstream nameservers and stub domains won’t impact Pods that have their dnsPolicy set to “Default” or “None”.

If a Pod’s dnsPolicy is set to “ClusterFirst”, its name resolution is handled differently, depending on whether stub-domain and upstream DNS servers are configured.

Without custom configurations: Any 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.

With custom configurations: If stub domains and upstream DNS servers are configured (as in the previous example), DNS queries will be routed according to the following flow:

  1. The query is first sent to the DNS caching layer in kube-dns.

  2. From the caching layer, the suffix of the request is examined and then forwarded to the appropriate DNS, based on the following cases:

    • Names with the cluster suffix (e.g.”.cluster.local”): The request is sent to kube-dns.

    • Names with the stub domain suffix (e.g. “.acme.local”): The request is sent to the configured custom DNS resolver (e.g. listening at 1.2.3.4).

    • Names without a matching suffix (e.g.”widget.com”): The request is forwarded to the upstream DNS (e.g. Google public DNS servers at 8.8.8.8 and 8.8.4.4).

DNS lookup flow

ConfigMap options

Options for the kube-dns kube-system:kube-dns ConfigMap:

Field Format Description
stubDomains (optional) A JSON map using a DNS suffix key (e.g. “acme.local”) and a value consisting of a JSON array of DNS IPs. The target nameserver may itself be a Kubernetes service. For instance, you can run your own copy of dnsmasq to export custom DNS names into the ClusterDNS namespace.
upstreamNameservers (optional) A JSON array of DNS IPs. Note: If specified, then the values specified replace the nameservers taken by default from the node’s /etc/resolv.conf. Limits: a maximum of three upstream nameservers can be specified.

Examples

Example: Stub domain

In this example, the user has a Consul DNS service discovery system that they wish to integrate with kube-dns. The consul domain server is located at 10.150.0.1, and all consul names have the suffix “.consul.local”. To configure Kubernetes, the cluster administrator simply creates a ConfigMap object as shown below.

apiVersion: v1
kind: ConfigMap
metadata:
  name: kube-dns
  namespace: kube-system
data:
  stubDomains: |
    {"consul.local": ["10.150.0.1"]}

Note that the cluster administrator did not wish to override the node’s upstream nameservers, so they did not specify the optional upstreamNameservers field.

Example: Upstream nameserver

In this example the cluster administrator wants to explicitly force all non-cluster DNS lookups to go through their own nameserver at 172.16.0.1. Again, this is easy to accomplish; they just need to create a ConfigMap with the upstreamNameservers field specifying the desired nameserver.

apiVersion: v1
kind: ConfigMap
metadata:
  name: kube-dns
  namespace: kube-system
data:
  upstreamNameservers: |
    ["172.16.0.1"]

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