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Add more DNS docs

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Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>
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Bogdan Dobrelya 2016-09-26 13:47:34 +02:00
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docs/dns-stack.md
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K8s DNS stack by Kargo
======================
Kargo 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).
Note, additional search (sub)domains may be defined in the ``searchdomains``
var. And additional recursive DNS resolvers in the `` upstream_dns_servers``,
``nameservers`` vars. Intranet DNS resolvers should be specified in the first
place, followed by external resolvers, for example:
```
skip_dnsmasq: true
nameservers: [8.8.8.8]
upstream_dns_servers: [172.18.32.6]
```
or
```
skip_dnsmasq: false
upstream_dns_servers: [172.18.32.6, 172.18.32.7, 8.8.8.8, 8.8.8.4]
```
Remember the limitations (the vars are explained below):
* 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.
Here is an approximate picture of how DNS things working and
being configured by Kargo ansible playbooks: