coredns/plugin/kubernetes/README.md

# kubernetes

## Name

*kubernetes* - enables the reading zone data from a Kubernetes cluster.

## Description

This plugin implements the [Kubernetes DNS-Based Service Discovery
Specification](https://github.com/kubernetes/dns/blob/master/docs/specification.md).

CoreDNS running the kubernetes plugin can be used as a replacement for kube-dns in a kubernetes
cluster.  See the [deployment](https://github.com/coredns/deployment) repository for details on [how
to deploy CoreDNS in Kubernetes](https://github.com/coredns/deployment/tree/master/kubernetes).

[stubDomains and upstreamNameservers](https://kubernetes.io/blog/2017/04/configuring-private-dns-zones-upstream-nameservers-kubernetes/)
are implemented via the *forward* plugin and kubernetes *upstream*. See the examples below.

This plugin can only be used once per Server Block.

## Syntax

~~~
kubernetes [ZONES...]
~~~

With only the directive specified, the *kubernetes* plugin will default to the zone specified in
the server's block. It will handle all queries in that zone and connect to Kubernetes in-cluster. It
will not provide PTR records for services or A records for pods. If **ZONES** is used it specifies
all the zones the plugin should be authoritative for.

```
kubernetes [ZONES...] {
    resyncperiod DURATION
    endpoint URL
    tls CERT KEY CACERT
    kubeconfig KUBECONFIG CONTEXT
    namespaces NAMESPACE...
    labels EXPRESSION
    pods POD-MODE
    endpoint_pod_names
    upstream [ADDRESS...]
    ttl TTL
    noendpoints
    transfer to ADDRESS...
    fallthrough [ZONES...]
    ignore empty_service
}
```

* `resyncperiod` specifies the Kubernetes data API **DURATION** period.
* `endpoint` specifies the **URL** for a remote k8s API endpoint.
   If omitted, it will connect to k8s in-cluster using the cluster service account.
* `tls` **CERT** **KEY** **CACERT** are the TLS cert, key and the CA cert file names for remote k8s connection.
   This option is ignored if connecting in-cluster (i.e. endpoint is not specified).
* `kubeconfig` **KUBECONFIG** **CONTEXT** authenticates the connection to a remote k8s cluster using a kubeconfig file. It supports TLS, username and password, or token-based authentication. This option is ignored if connecting in-cluster (i.e., the endpoint is not specified).
* `namespaces` **NAMESPACE [NAMESPACE...]** only exposes the k8s namespaces listed.
   If this option is omitted all namespaces are exposed
* `labels` **EXPRESSION** only exposes the records for Kubernetes objects that match this label selector.
   The label selector syntax is described in the
   [Kubernetes User Guide - Labels](https://kubernetes.io/docs/user-guide/labels/). An example that
   only exposes objects labeled as "application=nginx" in the "staging" or "qa" environments, would
   use: `labels environment in (staging, qa),application=nginx`.
* `pods` **POD-MODE** sets the mode for handling IP-based pod A records, e.g.
   `1-2-3-4.ns.pod.cluster.local. in A 1.2.3.4`.
   This option is provided to facilitate use of SSL certs when connecting directly to pods. Valid
   values for **POD-MODE**:

   * `disabled`: Default. Do not process pod requests, always returning `NXDOMAIN`
   * `insecure`: Always return an A record with IP from request (without checking k8s).  This option
     is vulnerable to abuse if used maliciously in conjunction with wildcard SSL certs.  This
     option is provided for backward compatibility with kube-dns.
   * `verified`: Return an A record if there exists a pod in same namespace with matching IP.  This
     option requires substantially more memory than in insecure mode, since it will maintain a watch
     on all pods.

* `endpoint_pod_names` uses the pod name of the pod targeted by the endpoint as
   the endpoint name in A records, e.g.,
   `endpoint-name.my-service.namespace.svc.cluster.local. in A 1.2.3.4`
   By default, the endpoint-name name selection is as follows: Use the hostname
   of the endpoint, or if hostname is not set, use the dashed form of the endpoint
   IP address (e.g., `1-2-3-4.my-service.namespace.svc.cluster.local.`)
   If this directive is included, then name selection for endpoints changes as
   follows: Use the hostname of the endpoint, or if hostname is not set, use the
   pod name of the pod targeted by the endpoint. If there is no pod targeted by
   the endpoint, use the dashed IP address form.
* `upstream` [**ADDRESS**...] defines the upstream resolvers used for resolving services
  that point to external hosts (aka External Services, aka CNAMEs).  If no **ADDRESS** is given, CoreDNS
  will resolve External Services against itself. **ADDRESS** can be an IP, an IP:port, or a path
  to a file structured like resolv.conf.
* `ttl` allows you to set a custom TTL for responses. The default is 5 seconds.  The minimum TTL allowed is
  0 seconds, and the maximum is capped at 3600 seconds. Setting TTL to 0 will prevent records from being cached.
* `noendpoints` will turn off the serving of endpoint records by disabling the watch on endpoints.
  All endpoint queries and headless service queries will result in an NXDOMAIN.
* `transfer` enables zone transfers. It may be specified multiples times. `To` signals the direction
  (only `to` is allowed). **ADDRESS** must be denoted in CIDR notation (127.0.0.1/32 etc.) or just as
  plain addresses. The special wildcard `*` means: the entire internet.
  Sending DNS notifies is not supported.
  [Deprecated](https://github.com/kubernetes/dns/blob/master/docs/specification.md#26---deprecated-records) pod records in the subdomain `pod.cluster.local` are not transferred.
* `fallthrough` **[ZONES...]** If a query for a record in the zones for which the plugin is authoritative
  results in NXDOMAIN, normally that is what the response will be. However, if you specify this option,
  the query will instead be passed on down the plugin chain, which can include another plugin to handle
  the query. If **[ZONES...]** is omitted, then fallthrough happens for all zones for which the plugin
  is authoritative. If specific zones are listed (for example `in-addr.arpa` and `ip6.arpa`), then only
  queries for those zones will be subject to fallthrough.
* `ignore empty_service` returns NXDOMAIN for services without any ready endpoint addresses (e.g., ready pods).
  This allows the querying pod to continue searching for the service in the search path.
  The search path could, for example, include another Kubernetes cluster.

## Examples

Handle all queries in the `cluster.local` zone. Connect to Kubernetes in-cluster. Also handle all
`in-addr.arpa` `PTR` requests for `10.0.0.0/17` . Verify the existence of pods when answering pod
requests. Resolve upstream records against `10.102.3.10`. Note we show the entire server block here:

~~~ txt
10.0.0.0/17 cluster.local {
    kubernetes {
        pods verified
        upstream 10.102.3.10:53
    }
}
~~~

Or you can selectively expose some namespaces:

~~~ txt
kubernetes cluster.local {
    namespaces test staging
}
~~~

Connect to Kubernetes with CoreDNS running outside the cluster:

~~~ txt
kubernetes cluster.local {
    endpoint https://k8s-endpoint:8443
    tls cert key cacert
}
~~~

## stubDomains and upstreamNameservers

Here we use the *forward* plugin to implement a stubDomain that forwards `example.local` to the nameserver `10.100.0.10:53`.
The *upstream* option in the *kubernetes* plugin means that ExternalName services (CNAMEs) will be resolved using the respective proxy.
Also configured is an upstreamNameserver `8.8.8.8:53` that will be used for resolving names that do not fall in `cluster.local`
or `example.local`.

~~~ txt
cluster.local:53 {
    kubernetes cluster.local {
        upstream
    }
}
example.local {
    forward . 10.100.0.10:53
}

. {
    forward . 8.8.8.8:53
}
~~~

The configuration above represents the following Kube-DNS stubDomains and upstreamNameservers configuration.

~~~ txt
stubDomains: |
   {“example.local”: [“10.100.0.10:53”]}
upstreamNameservers: |
   [“8.8.8.8:53”]
~~~

## AutoPath

The *kubernetes* plugin can be used in conjunction with the *autopath* plugin.  Using this
feature enables server-side domain search path completion in Kubernetes clusters.  Note: `pods` must
be set to `verified` for this to function properly.

    cluster.local {
        autopath @kubernetes
        kubernetes {
            pods verified
        }
    }

## Federation

The *kubernetes* plugin can be used in conjunction with the *federation* plugin.  Using this
feature enables serving federated domains from the Kubernetes clusters.

    cluster.local {
        federation {
            prod prod.example.org
            staging staging.example.org
        }
        kubernetes
    }


## Wildcards

Some query labels accept a wildcard value to match any value.  If a label is a valid wildcard (\*,
or the word "any"), then that label will match all values.  The labels that accept wildcards are:

 * _endpoint_ in an `A` record request: _endpoint_.service.namespace.svc.zone, e.g., `*.nginx.ns.svc.cluster.local`
 * _service_ in an `A` record request: _service_.namespace.svc.zone, e.g., `*.ns.svc.cluster.local`
 * _namespace_ in an `A` record request: service._namespace_.svc.zone, e.g., `nginx.*.svc.cluster.local`
 * _port and/or protocol_ in an `SRV` request: __port_.__protocol_.service.namespace.svc.zone.,
   e.g., `_http.*.service.ns.svc.cluster.local`
 * multiple wildcards are allowed in a single query, e.g., `A` Request `*.*.svc.zone.` or `SRV` request `*.*.*.*.svc.zone.`

 For example, wildcards can be used to resolve all Endpoints for a Service as `A` records. e.g.: `*.service.ns.svc.myzone.local` will return the Endpoint IPs in the Service `service` in namespace `default`:
 ```
*.service.default.svc.cluster.local. 5	IN A	192.168.10.10
*.service.default.svc.cluster.local. 5	IN A	192.168.25.15
```
 This response can be randomized using the `loadbalance` plugin