156 lines
7.1 KiB
Markdown
156 lines
7.1 KiB
Markdown
# K8s DNS stack by Kubespray
|
|
|
|
For K8s cluster nodes, Kubespray configures a [Kubernetes DNS](http://kubernetes.io/docs/admin/dns/)
|
|
[cluster add-on](http://releases.k8s.io/master/cluster/addons/README.md)
|
|
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 limited to less than 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:
|
|
|
|
#### 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: 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.
|
|
|
|
#### resolvconf_mode: host_resolvconf
|
|
|
|
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: 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](https://github.com/kubernetes/enhancements/blob/master/keps/sig-network/0030-nodelocal-dns-cache.md).
|
|
|
|
**As per the 2.10 release, Nodelocal DNS cache is enabled by default.**
|
|
|
|
## 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](https://github.com/skynetservices/skydns)
|
|
for details.
|
|
|
|
* There is
|
|
[no way to specify a custom value](https://github.com/kubernetes/kubernetes/issues/33554)
|
|
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.
|
|
|
|
* 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.
|