143 lines
7.1 KiB
Markdown
143 lines
7.1 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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DNS variables
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=============
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There are several global variables which can be used to modify DNS settings:
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#### ndots
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ndots value to be used in ``/etc/resolv.conf``
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It is important to note that multiple search domains combined with high ``ndots``
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values lead to poor performance of DNS stack, so please choose it wisely.
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The dnsmasq DaemonSet can accept lower ``ndots`` values and return NXDOMAIN
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replies for [bogus internal FQDNS](https://github.com/kubernetes/kubernetes/issues/19634#issuecomment-253948954)
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before it even hits the kubedns app. This enables dnsmasq to serve as a
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protective, but still recursive resolver in front of kubedns.
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#### searchdomains
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Custom search domains to be added in addition to the cluster search domains (``default.svc.{{ dns_domain }}, svc.{{ dns_domain }}``).
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Most Linux systems limit the total number of search domains to 6 and the total length of all search domains
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to 256 characters. Depending on the length of ``dns_domain``, you're limitted to less then the total limit.
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Please note that ``resolvconf_mode: docker_dns`` will automatically add your systems search domains as
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additional search domains. Please take this into the accounts for the limits.
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#### nameservers
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This variable is only used by ``resolvconf_mode: host_resolvconf``. These nameservers are added to the hosts
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``/etc/resolv.conf`` *after* ``upstream_dns_servers`` and thus serve as backup nameservers. If this variable
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is not set, a default resolver is chosen (depending on cloud provider or 8.8.8.8 when no cloud provider is specified).
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#### upstream_dns_servers
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DNS servers to be added *after* the cluster DNS. Used by all ``resolvconf_mode`` modes. These serve as backup
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DNS servers in early cluster deployment when no cluster DNS is available yet. These are also added as upstream
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DNS servers used by ``dnsmasq`` (when deployed with ``dns_mode: dnsmasq_kubedns``).
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DNS modes supported by kargo
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============================
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You can modify how kargo sets up DNS for your cluster with the variables ``dns_mode`` and ``resolvconf_mode``.
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## dns_mode
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``dns_mode`` configures how kargo will setup cluster DNS. There are three modes available:
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#### dnsmasq_kubedns (default)
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This installs an additional dnsmasq DaemonSet which gives more flexibility and lifts some
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limitations (e.g. number of nameservers). Kubelet is instructed to use dnsmasq instead of kubedns/skydns.
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It is configured to forward all DNS queries belonging to cluster services to kubedns/skydns. All
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other queries are forwardet to the nameservers found in ``upstream_dns_servers`` or ``default_resolver``
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#### kubedns
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This does not install the dnsmasq DaemonSet and instructs kubelet to directly use kubedns/skydns for
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all queries.
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#### none
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This does not install any of dnsmasq and kubedns/skydns. This basically disables cluster DNS completely and
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leaves you with a non functional cluster.
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## resolvconf_mode
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``resolvconf_mode`` configures how kargo will setup DNS for ``hostNetwork: true`` PODs and non-k8s containers.
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There are three modes available:
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#### docker_dns (default)
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This sets up the docker daemon with additional --dns/--dns-search/--dns-opt flags.
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The following nameservers are added to the docker daemon (in the same order as listed here):
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* cluster nameserver (depends on dns_mode)
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* content of optional upstream_dns_servers variable
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* host system nameservers (read from hosts /etc/resolv.conf)
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The following search domains are added to the docker daemon (in the same order as listed here):
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* cluster domains (``default.svc.{{ dns_domain }}``, ``svc.{{ dns_domain }}``)
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* content of optional searchdomains variable
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* host system search domains (read from hosts /etc/resolv.conf)
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The following dns options are added to the docker daemon
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* ndots:{{ ndots }}
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* timeout:2
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* attempts:2
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For normal PODs, k8s will ignore these options and setup its own DNS settings for the PODs, taking
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the --cluster_dns (either dnsmasq or kubedns, depending on dns_mode) kubelet option into account.
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For ``hostNetwork: true`` PODs however, k8s will let docker setup DNS settings. Docker containers which
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are not started/managed by k8s will also use these docker options.
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The host system name servers are added to ensure name resolution is also working while cluster DNS is not
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running yet. This is especially important in early stages of cluster deployment. In this early stage,
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DNS queries to the cluster DNS will timeout after a few seconds, resulting in the system nameserver being
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used as a backup nameserver. After cluster DNS is running, all queries will be answered by the cluster DNS
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servers, which in turn will forward queries to the system nameserver if required.
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#### host_resolvconf
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This activates the classic kargo behaviour that modifies the hosts ``/etc/resolv.conf`` file and dhclient
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configuration to point to the cluster dns server (either dnsmasq or kubedns, depending on dns_mode).
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As cluster DNS is not available on early deployment stage, this mode is split into 2 stages. In the first
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stage (``dns_early: true``), ``/etc/resolv.conf`` is configured to use the DNS servers found in ``upstream_dns_servers``
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and ``nameservers``. Later, ``/etc/resolv.conf`` is reconfigured to use the cluster DNS server first, leaving
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the other nameservers as backups.
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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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#### none
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Does nothing regarding ``/etc/resolv.conf``. This leaves you with a cluster that works as expected in most cases.
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The only exception is that ``hostNetwork: true`` PODs and non-k8s managed containers will not be able to resolve
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cluster service names.
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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.
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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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