2016-09-22 17:14:51 +00:00
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K8s DNS stack by Kargo
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======================
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Here is an approximate picture of how DNS things working and
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being configured by Kargo ansible playbooks:
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2016-09-22 17:14:51 +00:00
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2016-09-22 17:14:51 +00:00
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Note that an additional dnsmasq daemon set is installed by Kargo
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2016-09-22 17:14:51 +00:00
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by default. Kubelet will configure DNS base of all pods to use the
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given dnsmasq cluster IP, which is defined via the ``dns_server`` var.
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The dnsmasq forwards requests for a given cluster ``dns_domain`` to
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Kubedns's SkyDns service. The SkyDns server is configured to be an
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authoritative DNS server for the given cluser domain (and its subdomains
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up to ``ndots:5`` depth). Note: you should scale its replication controller
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up, if SkyDns chokes. These two layered DNS forwarders provide HA for the
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DNS cluster IP endpoint, which is a critical moving part for Kubernetes apps.
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2016-09-22 17:14:51 +00:00
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2016-09-22 17:14:51 +00:00
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Nameservers are as well configured in the hosts' ``/etc/resolv.conf`` files,
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as the given DNS cluster IP merged with ``nameservers`` values. While the
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DNS cluster IP merged with the ``upstream_dns_servers`` defines additional
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nameservers for the aforementioned nsmasq daemon set running on all hosts.
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This mitigates existing Linux limitation of max 3 nameservers in the
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``/etc/resolv.conf`` and also brings an additional caching layer for the
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clustered DNS services.
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You can skip the dnsmasq daemon set install steps by setting the
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``skip_dnsmasq: true``. This may be the case, if you're fine with
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the nameservers limitation. Sadly, there is no way to work around the
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search domain limitations of a 256 chars and 6 domains. Thus, you can
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use the ``searchdomains`` var to define no more than a three custom domains.
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Remaining three slots are reserved for K8s cluster default subdomains.
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When dnsmasq skipped, Kargo redefines the DNS cluster IP to point directly
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to SkyDns cluster IP ``skydns_server`` and configures Kubelet's
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``--dns_cluster`` to use that IP as well. While this greatly simplifies
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things, it comes by the price of limited nameservers though. As you know now,
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the DNS cluster IP takes a slot in the ``/etc/resolv.conf``, thus you can
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specify no more than a two nameservers for infra and/or external use.
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Those may be specified either in ``nameservers`` or ``upstream_dns_servers``
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and will be merged together with the ``skydns_server`` IP into the hots'
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``/etc/resolv.conf``.
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Kargo has yet ways to configure Kubedns addon to forward requests SkyDns can
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not answer with authority to arbitrary recursive resolvers. This task is left
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for future. See [official SkyDns docs](https://github.com/skynetservices/skydns)
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for details.
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