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K8s DNS stack by Kubespray

For K8s cluster nodes, Kubespray configures a Kubernetes DNS cluster add-on to serve as an authoritative DNS server for a given dns_domain and its svc, default.svc default subdomains (a total of ndots: 5 max levels).

Other nodes in the inventory, like external storage nodes or a separate etcd cluster node group, considered non-cluster and left up to the user to configure DNS resolve.

DNS variables

There are several global variables which can be used to modify DNS settings:

ndots

ndots value to be used in /etc/resolv.conf

It is important to note that multiple search domains combined with high ndots values lead to poor performance of DNS stack, so please choose it wisely.

searchdomains

Custom search domains to be added in addition to the cluster search domains (default.svc.{{ dns_domain }}, svc.{{ dns_domain }}).

Most Linux systems limit the total number of search domains to 6 and the total length of all search domains to 256 characters. Depending on the length of dns_domain, you're limitted to less then the total limit.

Please note that resolvconf_mode: docker_dns will automatically add your systems search domains as additional search domains. Please take this into the accounts for the limits.

nameservers

This variable is only used by resolvconf_mode: host_resolvconf. These nameservers are added to the hosts /etc/resolv.conf after upstream_dns_servers and thus serve as backup nameservers. If this variable is not set, a default resolver is chosen (depending on cloud provider or 8.8.8.8 when no cloud provider is specified).

upstream_dns_servers

DNS servers to be added after the cluster DNS. Used by all resolvconf_mode modes. These serve as backup DNS servers in early cluster deployment when no cluster DNS is available yet.

DNS modes supported by Kubespray

You can modify how Kubespray sets up DNS for your cluster with the variables dns_mode and resolvconf_mode.

dns_mode

dns_mode configures how Kubespray will setup cluster DNS. There are four modes available:

coredns (default)

This installs CoreDNS as the default cluster DNS for all queries.

coredns_dual

This installs CoreDNS as the default cluster DNS for all queries, plus a secondary CoreDNS stack.

manual

This does not install coredns, but allows you to specify manual_dns_server, which will be configured on nodes for handling Pod DNS. Use this method if you plan to install your own DNS server in the cluster after initial deployment.

none

This does not install any of DNS solution at all. This basically disables cluster DNS completely and leaves you with a non functional cluster.

resolvconf_mode

resolvconf_mode configures how Kubespray will setup DNS for hostNetwork: true PODs and non-k8s containers. There are three modes available:

docker_dns (default)

This sets up the docker daemon with additional --dns/--dns-search/--dns-opt flags.

The following nameservers are added to the docker daemon (in the same order as listed here):

  • cluster nameserver (depends on dns_mode)
  • content of optional upstream_dns_servers variable
  • host system nameservers (read from hosts /etc/resolv.conf)

The following search domains are added to the docker daemon (in the same order as listed here):

  • cluster domains (default.svc.{{ dns_domain }}, svc.{{ dns_domain }})
  • content of optional searchdomains variable
  • host system search domains (read from hosts /etc/resolv.conf)

The following dns options are added to the docker daemon

  • ndots:{{ ndots }}
  • timeout:2
  • attempts:2

For normal PODs, k8s will ignore these options and setup its own DNS settings for the PODs, taking the --cluster_dns (either coredns or coredns_dual, depending on dns_mode) kubelet option into account. For hostNetwork: true PODs however, k8s will let docker setup DNS settings. Docker containers which are not started/managed by k8s will also use these docker options.

The host system name servers are added to ensure name resolution is also working while cluster DNS is not running yet. This is especially important in early stages of cluster deployment. In this early stage, DNS queries to the cluster DNS will timeout after a few seconds, resulting in the system nameserver being used as a backup nameserver. After cluster DNS is running, all queries will be answered by the cluster DNS servers, which in turn will forward queries to the system nameserver if required.

host_resolvconf

This activates the classic Kubespray behaviour that modifies the hosts /etc/resolv.conf file and dhclient configuration to point to the cluster dns server (either coredns or coredns_dual, depending on dns_mode).

As cluster DNS is not available on early deployment stage, this mode is split into 2 stages. In the first stage (dns_early: true), /etc/resolv.conf is configured to use the DNS servers found in upstream_dns_servers and nameservers. Later, /etc/resolv.conf is reconfigured to use the cluster DNS server first, leaving the other nameservers as backups.

Also note, existing records will be purged from the /etc/resolv.conf, including resolvconf's base/head/cloud-init config files and those that come from dhclient.

none

Does nothing regarding /etc/resolv.conf. This leaves you with a cluster that works as expected in most cases. The only exception is that hostNetwork: true PODs and non-k8s managed containers will not be able to resolve cluster service names.

Nodelocal DNS cache

Setting enable_nodelocaldns to true will make pods reach out to the dns (core-dns) caching agent running on the same node, thereby avoiding iptables DNAT rules and connection tracking. The local caching agent will query kube-dns / core-dns (depending on what main DNS plugin is configured in your cluster) for cache misses of cluster hostnames(cluster.local suffix by default).

More information on the rationale behind this implementation can be found here.

Limitations

  • Kubespray has yet ways to configure Kubedns addon to forward requests SkyDns can not answer with authority to arbitrary recursive resolvers. This task is left for future. See official SkyDns docs for details.

  • There is no way to specify a custom value for the SkyDNS ndots param via an option for KubeDNS add-on, while SkyDNS supports it though.

  • the searchdomains have a limitation of a 6 names and 256 chars length. Due to default svc, default.svc subdomains, the actual limits are a 4 names and 239 chars respectively.

  • the nameservers have a limitation of a 3 servers, although there is a way to mitigate that with the upstream_dns_servers, see below. Anyway, the nameservers can take no more than a two custom DNS servers because of one slot is reserved for a Kubernetes cluster needs.