1 # K8s DNS stack by Kubespray
3 For K8s cluster nodes, Kubespray configures a [Kubernetes DNS](https://kubernetes.io/docs/admin/dns/)
4 [cluster add-on](https://releases.k8s.io/master/cluster/addons/README.md)
5 to serve as an authoritative DNS server for a given ``dns_domain`` and its
6 ``svc, default.svc`` default subdomains (a total of ``ndots: 5`` max levels).
8 Other nodes in the inventory, like external storage nodes or a separate etcd cluster
9 node group, considered non-cluster and left up to the user to configure DNS resolve.
13 There are several global variables which can be used to modify DNS settings:
17 ndots value to be used in ``/etc/resolv.conf``
19 It is important to note that multiple search domains combined with high ``ndots``
20 values lead to poor performance of DNS stack, so please choose it wisely.
24 Custom search domains to be added in addition to the cluster search domains (``default.svc.{{ dns_domain }}, svc.{{ dns_domain }}``).
26 Most Linux systems limit the total number of search domains to 6 and the total length of all search domains
27 to 256 characters. Depending on the length of ``dns_domain``, you're limited to less than the total limit.
29 Please note that ``resolvconf_mode: docker_dns`` will automatically add your systems search domains as
30 additional search domains. Please take this into the accounts for the limits.
34 This variable is only used by ``resolvconf_mode: host_resolvconf``. These nameservers are added to the hosts
35 ``/etc/resolv.conf`` *after* ``upstream_dns_servers`` and thus serve as backup nameservers. If this variable
36 is not set, a default resolver is chosen (depending on cloud provider or 8.8.8.8 when no cloud provider is specified).
38 ### upstream_dns_servers
40 DNS servers to be added *after* the cluster DNS. Used by all ``resolvconf_mode`` modes. These serve as backup
41 DNS servers in early cluster deployment when no cluster DNS is available yet.
43 ### coredns_external_zones
45 Array of optional external zones to coredns forward queries to. It's injected into
46 `coredns`' config file before default kubernetes zone. Use it as an optimization for well-known zones and/or internal-only
47 domains, i.e. VPN for internal networks (default is unset)
52 coredns_external_zones:
61 - https://mycompany.local:4453
70 coredns_external_zones='[{"cache": 30,"zones":["example.com","example.io:453"],"nameservers":["1.1.1.1","2.2.2.2"]}]'
73 ### dns_etchosts (coredns)
75 Optional hosts file content to coredns use as /etc/hosts file. This will also be used by nodelocaldns, if enabled.
81 192.168.0.100 api.example.com
82 192.168.0.200 ingress.example.com
85 ### enable_coredns_reverse_dns_lookups
87 Whether reverse DNS lookups are enabled in the coredns config. Defaults to `true`.
89 ### CoreDNS default zone cache plugin
91 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.
93 An example value (more information on the [plugin's documentation](https://coredns.io/plugins/cache/)) to:
95 * raise the max cache TTL to 3600 seconds
96 * raise the max amount of success responses to cache to 3000
97 * disable caching of denial responses altogether
98 * enable pre-fetching of lookups with at least 10 lookups per minute before they expire
103 coredns_default_zone_cache_block: |
111 ## DNS modes supported by Kubespray
113 You can modify how Kubespray sets up DNS for your cluster with the variables ``dns_mode`` and ``resolvconf_mode``.
117 ``dns_mode`` configures how Kubespray will setup cluster DNS. There are four modes available:
119 #### dns_mode: coredns (default)
121 This installs CoreDNS as the default cluster DNS for all queries.
123 #### dns_mode: coredns_dual
125 This installs CoreDNS as the default cluster DNS for all queries, plus a secondary CoreDNS stack.
127 #### dns_mode: manual
129 This does not install coredns, but allows you to specify
130 `manual_dns_server`, which will be configured on nodes for handling Pod DNS.
131 Use this method if you plan to install your own DNS server in the cluster after
136 This does not install any of DNS solution at all. This basically disables cluster DNS completely and
137 leaves you with a non functional cluster.
141 ``resolvconf_mode`` configures how Kubespray will setup DNS for ``hostNetwork: true`` PODs and non-k8s containers.
142 There are three modes available:
144 ### resolvconf_mode: host_resolvconf (default)
146 This activates the classic Kubespray behavior that modifies the hosts ``/etc/resolv.conf`` file and dhclient
147 configuration to point to the cluster dns server (either coredns or coredns_dual, depending on dns_mode).
149 As cluster DNS is not available on early deployment stage, this mode is split into 2 stages. In the first
150 stage (``dns_early: true``), ``/etc/resolv.conf`` is configured to use the DNS servers found in ``upstream_dns_servers``
151 and ``nameservers``. Later, ``/etc/resolv.conf`` is reconfigured to use the cluster DNS server first, leaving
152 the other nameservers as backups.
154 Also note, existing records will be purged from the `/etc/resolv.conf`,
155 including resolvconf's base/head/cloud-init config files and those that come from dhclient.
157 ### resolvconf_mode: docker_dns
159 This sets up the docker daemon with additional --dns/--dns-search/--dns-opt flags.
161 The following nameservers are added to the docker daemon (in the same order as listed here):
163 * cluster nameserver (depends on dns_mode)
164 * content of optional upstream_dns_servers variable
165 * host system nameservers (read from hosts /etc/resolv.conf)
167 The following search domains are added to the docker daemon (in the same order as listed here):
169 * cluster domains (``default.svc.{{ dns_domain }}``, ``svc.{{ dns_domain }}``)
170 * content of optional searchdomains variable
171 * host system search domains (read from hosts /etc/resolv.conf)
173 The following dns options are added to the docker daemon
179 These dns options can be overridden by setting a different list:
189 For normal PODs, k8s will ignore these options and setup its own DNS settings for the PODs, taking
190 the --cluster_dns (either coredns or coredns_dual, depending on dns_mode) kubelet option into account.
191 For ``hostNetwork: true`` PODs however, k8s will let docker setup DNS settings. Docker containers which
192 are not started/managed by k8s will also use these docker options.
194 The host system name servers are added to ensure name resolution is also working while cluster DNS is not
195 running yet. This is especially important in early stages of cluster deployment. In this early stage,
196 DNS queries to the cluster DNS will timeout after a few seconds, resulting in the system nameserver being
197 used as a backup nameserver. After cluster DNS is running, all queries will be answered by the cluster DNS
198 servers, which in turn will forward queries to the system nameserver if required.
200 ### resolvconf_mode: none
202 Does nothing regarding ``/etc/resolv.conf``. This leaves you with a cluster that works as expected in most cases.
203 The only exception is that ``hostNetwork: true`` PODs and non-k8s managed containers will not be able to resolve
204 cluster service names.
206 ## Nodelocal DNS cache
208 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).
210 More information on the rationale behind this implementation can be found [here](https://github.com/kubernetes/enhancements/blob/master/keps/sig-network/0030-nodelocal-dns-cache.md).
212 **As per the 2.10 release, Nodelocal DNS cache is enabled by default.**
216 It's possible to extent the `nodelocaldns`' configuration by adding an array of external zones. For example:
219 nodelocaldns_external_zones:
228 - https://mycompany.local:4453
233 ### dns_etchosts (nodelocaldns)
235 See [dns_etchosts](#dns_etchosts-coredns) above.
239 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:
242 enable_nodelocaldns_secondary: true
245 **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.
247 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.
250 nodelocaldns_secondary_skew_seconds: 5
255 * Kubespray has yet ways to configure Kubedns addon to forward requests SkyDns can
256 not answer with authority to arbitrary recursive resolvers. This task is left
257 for future. See [official SkyDns docs](https://github.com/skynetservices/skydns)
261 [no way to specify a custom value](https://github.com/kubernetes/kubernetes/issues/33554)
262 for the SkyDNS ``ndots`` param.
264 * the ``searchdomains`` have a limitation of a 6 names and 256 chars
265 length. Due to default ``svc, default.svc`` subdomains, the actual
266 limits are a 4 names and 239 chars respectively.
268 * the ``nameservers`` have a limitation of a 3 servers, although there
269 is a way to mitigate that with the ``upstream_dns_servers``,
270 see below. Anyway, the ``nameservers`` can take no more than a two
271 custom DNS servers because of one slot is reserved for a Kubernetes