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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.

dns_timeout

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

dns_attempts

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

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 limited to less than the total limit.

remove_default_searchdomains: true will remove the default cluster search domains.

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.

coredns_external_zones

Array of optional external zones to coredns forward queries to. It's injected into coredns' config file before default kubernetes zone. Use it as an optimization for well-known zones and/or internal-only domains, i.e. VPN for internal networks (default is unset)

Example:

coredns_external_zones:
- zones:
  - example.com
  - example.io:1053
  nameservers:
  - 1.1.1.1
  - 2.2.2.2
  cache: 5
- zones:
  - https://mycompany.local:4453
  nameservers:
  - 192.168.0.53
  cache: 0
- zones:
  - mydomain.tld
  nameservers:
  - 10.233.0.3
  cache: 5
  rewrite:
  - name stop website.tld website.namespace.svc.cluster.local

or as INI

coredns_external_zones='[{"cache": 30,"zones":["example.com","example.io:453"],"nameservers":["1.1.1.1","2.2.2.2"]}]'

dns_etchosts (coredns)

Optional hosts file content to coredns use as /etc/hosts file. This will also be used by nodelocaldns, if enabled.

Example:

dns_etchosts: |
  192.168.0.100 api.example.com
  192.168.0.200 ingress.example.com

enable_coredns_reverse_dns_lookups

Whether reverse DNS lookups are enabled in the coredns config. Defaults to true.

CoreDNS default zone cache plugin

If you wish to configure the caching behaviour of CoreDNS on the default zone, you can do so using the coredns_default_zone_cache_block string block.

An example value (more information on the plugin's documentation) to:

  • raise the max cache TTL to 3600 seconds
  • raise the max amount of success responses to cache to 3000
  • disable caching of denial responses altogether
  • enable pre-fetching of lookups with at least 10 lookups per minute before they expire

Would be as follows:

coredns_default_zone_cache_block: |
  cache 3600 {
    success 3000
    denial 0
    prefetch 10 1m
  }

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:

dns_mode: coredns (default)

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

dns_mode: coredns_dual

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

dns_mode: 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.

dns_mode: 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:

resolvconf_mode: host_resolvconf (default)

This activates the classic Kubespray behavior 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.

resolvconf_mode: docker_dns

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

These dns options can be overridden by setting a different list:

docker_dns_options:
- ndots:{{ ndots }}
- timeout:2
- attempts:2
- rotate

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.

resolvconf_mode: 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 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.

As per the 2.10 release, Nodelocal DNS cache is enabled by default.

External zones

It's possible to extent the nodelocaldns' configuration by adding an array of external zones. For example:

nodelocaldns_external_zones:
- zones:
  - example.com
  - example.io:1053
  nameservers:
  - 1.1.1.1
  - 2.2.2.2
  cache: 5
- zones:
  - https://mycompany.local:4453
  nameservers:
  - 192.168.0.53

dns_etchosts (nodelocaldns)

See dns_etchosts above.

Nodelocal DNS HA

Under some circumstances the single POD nodelocaldns implementation may not be able to be replaced soon enough and a cluster upgrade or a nodelocaldns upgrade can cause DNS requests to time out for short intervals. If for any reason your applications cannot tollerate this behavior you can enable a redundant nodelocal DNS pod on each node:

enable_nodelocaldns_secondary: true

Note: when the nodelocaldns secondary is enabled, the primary is instructed to no longer tear down the iptables rules it sets up to direct traffic to itself. In case both daemonsets have failing pods on the same node, this can cause a DNS blackout with traffic no longer being forwarded to the coredns central service as a fallback. Please ensure you account for this also if you decide to disable the nodelocaldns cache.

There is a time delta (in seconds) allowed for the secondary nodelocaldns to survive in case both primary and secondary daemonsets are updated at the same time. It is advised to tune this variable after you have performed some tests in your own environment.

nodelocaldns_secondary_skew_seconds: 5

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.

  • 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. If remove_default_searchdomains: true added you are back to 6 names.

  • 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.