120 lines
5.9 KiB
Markdown
120 lines
5.9 KiB
Markdown
K8s DNS stack by Kargo
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======================
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For K8s cluster nodes, kargo configures a [Kubernetes DNS](http://kubernetes.io/docs/admin/dns/)
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[cluster add-on](http://releases.k8s.io/master/cluster/addons/README.md)
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to serve as an authoritative DNS server for a given ``dns_domain`` and its
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``svc, default.svc`` default subdomains (a total of ``ndots: 5`` max levels).
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Other nodes in the inventory, like external storage nodes or a separate etcd cluster
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node group, considered non-cluster and left up to the user to configure DNS resolve.
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Note, custom ``ndots`` values affect only the dnsmasq daemon set (explained below).
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While the kubedns has the ``ndots=5`` hardcoded, which is not recommended due to
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[DNS performance reasons](https://github.com/kubernetes/kubernetes/issues/14051).
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You can use config maps for the kubedns app to workaround the issue, which is
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yet in the Kargo scope.
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Additional search (sub)domains may be defined in the ``searchdomains``
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and ``ndots`` vars. And additional recursive DNS resolvers in the `` upstream_dns_servers``,
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``nameservers`` vars. Intranet/cloud provider DNS resolvers should be specified
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in the first place, followed by external resolvers, for example:
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```
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skip_dnsmasq: true
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nameservers: [8.8.8.8]
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upstream_dns_servers: [172.18.32.6]
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```
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or
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```
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skip_dnsmasq: false
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upstream_dns_servers: [172.18.32.6, 172.18.32.7, 8.8.8.8, 8.8.8.4]
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```
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The vars are explained below. For the early cluster deployment stage, when there
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is yet K8s cluster and apps exist, a user may expect local repos to be
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accessible via authoritative intranet resolvers. For that case, if none custom vars
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was specified, the default resolver is set to either the cloud provider default
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or `8.8.8.8`. And domain is set to the default ``dns_domain`` value as well.
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Later, the nameservers will be reconfigured to the DNS service IP that Kargo
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configures for K8s cluster.
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Also note, existing records will be purged from the `/etc/resolv.conf`,
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including resolvconf's base/head/cloud-init config files and those that come from dhclient.
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This is required for hostnet pods networking and for [kubelet to not exceed search domains
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limits](https://github.com/kubernetes/kubernetes/issues/9229).
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Instead, new domain, search, nameserver records and options will be defined from the
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aforementioned vars:
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* Superseded via dhclient's DNS update hook.
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* Generated via cloud-init (CoreOS only).
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* Statically defined in the `/etc/resolv.conf`, if none of above is applicable.
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* Resolvconf's head/base files are disabled from populating anything into the
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`/etc/resolv.conf`.
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DNS configuration details
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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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Note that an additional dnsmasq daemon set is installed by Kargo
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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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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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Limitations
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-----------
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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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* There is
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[no way to specify a custom value](https://github.com/kubernetes/kubernetes/issues/33554)
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for the SkyDNS ``ndots`` param via an
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[option for KubeDNS](https://github.com/kubernetes/kubernetes/blob/master/cmd/kube-dns/app/options/options.go)
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add-on, while SkyDNS supports it though. Thus, DNS SRV records may not work
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as expected as they require the ``ndots:7``.
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* the ``searchdomains`` have a limitation of a 6 names and 256 chars
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length. Due to default ``svc, default.svc`` subdomains, the actual
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limits are a 4 names and 239 chars respectively.
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* the ``nameservers`` have a limitation of a 3 servers, although there
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is a way to mitigate that with the ``upstream_dns_servers``,
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see below. Anyway, the ``nameservers`` can take no more than a two
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custom DNS servers because of one slot is reserved for a Kubernetes
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cluster needs.
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