coredns/plugin/hosts/README.md

# hosts

## Name

*hosts* - enables serving zone data from a `/etc/hosts` style file.

## Description

The *hosts* plugin is useful for serving zones from a `/etc/hosts` file. It serves from a preloaded
file that exists on disk. It checks the file for changes and updates the zones accordingly. This
plugin only supports A, AAAA, and PTR records. The hosts plugin can be used with readily
available hosts files that block access to advertising servers.

The plugin reloads the content of the hosts file every 5 seconds. Upon reload, CoreDNS will use the
new definitions. Should the file be deleted, any inlined content will continue to be served. When
the file is restored, it will then again be used.

If you want to pass the request to the rest of the plugin chain if there is no match in the *hosts*
plugin, you must specify the `fallthrough` option.

This plugin can only be used once per Server Block.

## The hosts file

Commonly the entries are of the form `IP_address canonical_hostname [aliases...]` as explained by
the hosts(5) man page.

Examples:

~~~
# The following lines are desirable for IPv4 capable hosts
127.0.0.1       localhost
192.168.1.10    example.com            example

# The following lines are desirable for IPv6 capable hosts
::1                     localhost ip6-localhost ip6-loopback
fdfc:a744:27b5:3b0e::1  example.com example
~~~

### Wildcard records

Owner names may use a `*` as the leftmost label to match one additional label below
that name. This follows the same wildcard semantics as the *file* plugin.

Examples:

~~~
192.168.1.10    *.example.com
192.168.1.11    a.example.com
192.168.1.12    b.example.com
~~~

With the entries above:

* `a.example.com` and `b.example.com` resolve to their explicit addresses.
* `apps.example.com` resolves to `192.168.1.10`.
* `example.com` does not match the wildcard (the zone apex is excluded).
* `deep.apps.example.com` does not match `*.example.com` (only one label is matched).

Wildcard entries do not generate PTR records.

### PTR records

PTR records for reverse lookups are generated automatically by CoreDNS (based on the hosts file
entries) and cannot be created manually.

## Syntax

~~~
hosts [FILE [ZONES...]] {
    [INLINE]
    ttl SECONDS
    no_reverse
    reload DURATION
    fallthrough [ZONES...]
}
~~~

* **FILE** the hosts file to read and parse. If the path is relative the path from the *root*
  plugin will be prepended to it. Defaults to /etc/hosts if omitted. We scan the file for changes
  every 5 seconds.
* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
   are used.
* **INLINE** the hosts file contents inlined in Corefile. If there are any lines before fallthrough
   then all of them will be treated as the additional content for hosts file. The specified hosts
   file path will still be read but entries will be overridden.
* `ttl` change the DNS TTL of the records generated (forward and reverse). The default is 3600 seconds (1 hour).
* `reload` change the period between each hostsfile reload. A time of zero seconds disables the
  feature. Examples of valid durations: "300ms", "1.5h" or "2h45m". See Go's
  [time](https://godoc.org/time). package.
* `no_reverse` disable the automatic generation of the `in-addr.arpa` or `ip6.arpa` entries for the hosts
* `fallthrough` If zone matches and no record can be generated, pass request to the next plugin.
  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.

## Metrics

If monitoring is enabled (via the *prometheus* plugin) then the following metrics are exported:

- `coredns_hosts_entries{}` - The combined number of entries in hosts and Corefile.
- `coredns_hosts_reload_timestamp_seconds{}` - The timestamp of the last reload of hosts file.

## Examples

Load `/etc/hosts` file.

~~~ corefile
. {
    hosts
}
~~~

Load `example.hosts` file in the current directory.

~~~
. {
    hosts example.hosts
}
~~~

Load example.hosts file and only serve example.org and example.net from it and fall through to the
next plugin if query doesn't match.

~~~
. {
    hosts example.hosts example.org example.net {
        fallthrough
    }
}
~~~

Load hosts file inlined in Corefile.

~~~
example.hosts example.org {
    hosts {
        10.0.0.1 example.org
        fallthrough
    }
    whoami
}
~~~

Resolve all single-label subdomains of `example.com` to one address, with explicit
exceptions, and fall through for everything else under `example.com`.

~~~
. {
    hosts example.hosts example.com {
        192.168.1.10 *.example.com
        192.168.1.11 www.example.com
        fallthrough example.com
    }
    forward . 8.8.8.8
}
~~~

## See also

The form of the entries in the `/etc/hosts` file are based on IETF [RFC 952](https://tools.ietf.org/html/rfc952) which was updated by IETF [RFC 1123](https://tools.ietf.org/html/rfc1123).