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Introduce dns_mode and resolvconf_mode and implement docker_dns mode

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Also update reset.yml to do more dns/network related cleanup.
This commit is contained in:
Alexander Block
2016-12-21 17:18:11 +01:00
parent aa33613b98
commit 1d2a18b355
18 changed files with 258 additions and 107 deletions
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164
docs/dns-stack.md
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@@ -9,47 +9,14 @@ to serve as an authoritative DNS server for a given ``dns_domain`` and its
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.
Note, custom ``ndots`` values affect only the dnsmasq daemon set (explained below).
While the kubedns has the ``ndots=5`` hardcoded, which is not recommended due to
[DNS performance reasons](https://github.com/kubernetes/kubernetes/issues/14051).
You can use config maps for the kubedns app to workaround the issue, which is
yet in the Kargo scope.
Additional search (sub)domains may be defined in the ``searchdomains``
and ``ndots`` vars. And additional recursive DNS resolvers in the `` upstream_dns_servers``,
``nameservers`` vars. Intranet/cloud provider DNS resolvers should be specified
in the first place, followed by external resolvers, for example:
DNS variables
=============
```
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]
```
The vars are explained below. For the early cluster deployment stage, when there
is yet K8s cluster and apps exist, a user may expect local repos to be
accessible via authoritative intranet resolvers. For that case, if none custom vars
was specified, the default resolver is set to either the cloud provider default
or `8.8.8.8`. And domain is set to the default ``dns_domain`` value as well.
Later, the nameservers will be reconfigured to the DNS service IP that Kargo
configures for K8s cluster.
There are several global variables which can be used to modify DNS settings:
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.
This is required for hostnet pods networking and for [kubelet to not exceed search domains
limits](https://github.com/kubernetes/kubernetes/issues/9229).
Instead, new domain, search, nameserver records and options will be defined from the
aforementioned vars:
* Superseded via dhclient's DNS update hook.
* Generated via cloud-init (CoreOS only).
* Statically defined in the `/etc/resolv.conf`, if none of above is applicable.
* Resolvconf's head/base files are disabled from populating anything into the
`/etc/resolv.conf`.
#### 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.
@@ -58,48 +25,97 @@ replies for [bogus internal FQDNS](https://github.com/kubernetes/kubernetes/issu
before it even hits the kubedns app. This enables dnsmasq to serve as a
protective, but still recursive resolver in front of kubedns.
DNS configuration details
-------------------------
#### searchdomains
Custom search domains to be added in addition to the cluster search domains (``default.svc.{{ dns_domain }}, svc.{{ dns_domain }}``).
Here is an approximate picture of how DNS things working and
being configured by Kargo ansible playbooks:
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 limitted to less then the total limit.
![Image](figures/dns.jpeg?raw=true)
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.
Note that an additional dnsmasq daemon set is installed by Kargo
by default. Kubelet will configure DNS base of all pods to use the
given dnsmasq cluster IP, which is defined via the ``dns_server`` var.
The dnsmasq forwards requests for a given cluster ``dns_domain`` to
Kubedns's SkyDns service. The SkyDns server is configured to be an
authoritative DNS server for the given cluser domain (and its subdomains
up to ``ndots:5`` depth). Note: you should scale its replication controller
up, if SkyDns chokes. These two layered DNS forwarders provide HA for the
DNS cluster IP endpoint, which is a critical moving part for Kubernetes apps.
#### 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).
Nameservers are as well configured in the hosts' ``/etc/resolv.conf`` files,
as the given DNS cluster IP merged with ``nameservers`` values. While the
DNS cluster IP merged with the ``upstream_dns_servers`` defines additional
nameservers for the aforementioned nsmasq daemon set running on all hosts.
This mitigates existing Linux limitation of max 3 nameservers in the
``/etc/resolv.conf`` and also brings an additional caching layer for the
clustered DNS services.
#### 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. These are also added as upstream
DNS servers used by ``dnsmasq`` (when deployed with ``dns_mode: dnsmasq_kubedns``).
You can skip the dnsmasq daemon set install steps by setting the
``skip_dnsmasq: true``. This may be the case, if you're fine with
the nameservers limitation. Sadly, there is no way to work around the
search domain limitations of a 256 chars and 6 domains. Thus, you can
use the ``searchdomains`` var to define no more than a three custom domains.
Remaining three slots are reserved for K8s cluster default subdomains.
DNS modes supported by kargo
============================
You can modify how kargo sets up DNS for your cluster with the variables ``dns_mode`` and ``resolvconf_mode``.
## dns_mode
``dns_mode`` configures how kargo will setup cluster DNS. There are three modes available:
#### dnsmasq_kubedns (default)
This installs an additional dnsmasq DaemonSet which gives more flexibility and lifts some
limitations (e.g. number of nameservers). Kubelet is instructed to use dnsmasq instead of kubedns/skydns.
It is configured to forward all DNS queries belonging to cluster services to kubedns/skydns. All
other queries are forwardet to the nameservers found in ``upstream_dns_servers`` or ``default_resolver``
#### kubedns
This does not install the dnsmasq DaemonSet and instructs kubelet to directly use kubedns/skydns for
all queries.
#### none
This does not install any of dnsmasq and kubedns/skydns. This basically disables cluster DNS completely and
leaves you with a non functional cluster.
## resolvconf_mode
``resolvconf_mode`` configures how kargo will setup DNS for ``hostNetwork: true`` PODs and non-k8s containers.
There are three modes available:
#### 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 dnsmasq or kubedns, 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.
#### host_resolvconf
This activates the classic kargo behaviour that modifies the hosts ``/etc/resolv.conf`` file and dhclient
configuration to point to the cluster dns server (either dnsmasq or kubedns, 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.
#### 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.
When dnsmasq skipped, Kargo redefines the DNS cluster IP to point directly
to SkyDns cluster IP ``skydns_server`` and configures Kubelet's
``--dns_cluster`` to use that IP as well. While this greatly simplifies
things, it comes by the price of limited nameservers though. As you know now,
the DNS cluster IP takes a slot in the ``/etc/resolv.conf``, thus you can
specify no more than a two nameservers for infra and/or external use.
Those may be specified either in ``nameservers`` or ``upstream_dns_servers``
and will be merged together with the ``skydns_server`` IP into the hots'
``/etc/resolv.conf``.
Limitations
-----------