# Catalog Zones

A "catalog zone" is a special DNS zone that contains a list of other zones to be served, along with their configuration parameters. Zones listed in a catalog zone are called "member zones". When a catalog zone is loaded or transferred to a slave server which supports this functionality, the slave server will create the member zones automatically. When the catalog zone is updated (for example, to add or delete member zones, or change their configuration parameters) those changes are immediately put into effect. Because the catalog zone is a normal DNS zone, these configuration changes can be propagated using the standard AXFR/IXFR zone transfer mechanism.

Catalog zones' format and behavior are specified as an internet draft for interoperability among DNS implementations. As of this release, the latest revision of the DNS catalog zones draft can be found here: https://datatracker.ietf.org/doc/draft-muks-dnsop-dns-catalog-zones/

## Principle of Operation

Normally, if a zone is to be served by a slave server, the `named.conf` file on the server must list the zone, or the zone must be added using `rndc addzone`. In environments with a large number of slave servers and/or where the zones being served are changing frequently, the overhead involved in maintaining consistent zone configuration on all the slave servers can be significant.

A catalog zone is a way to ease this administrative burden. It is a DNS zone that lists member zones that should be served by slave servers. When a slave server receives an update to the catalog zone, it adds, removes, or reconfigures member zones based on the data received.

To use a catalog zone, it must first be set up as a normal zone on the master and the on slave servers that will be configured to use it. It must also be added to a `catalog-zones` list in the `options` or `view` statement in `named.conf`. (This is comparable to the way a policy zone is configured as a normal zone and also listed in a `response-policy` statement.)

To use the catalog zone feature to serve a new member zone:

- Set up the the member zone to be served on the master as normal. This could be done by editing `named.conf`, or by running `rndc addzone`.

- Add an entry to the catalog zone for the new member zone. This could be done by editing the catalog zone's master file and running `rndc reload`, or by updating the zone using `nsupdate`.

The change to the catalog zone will be propagated from the master to all slaves using the normal AXFR/IXFR mechanism. When the slave receives the update to the catalog zone, it will detect the entry for the new member zone, create an instance of of that zone on the slave server, and point that instance to the `masters` specified in the catalog zone data. The newly created member zone is a normal slave zone, so BIND will immediately initiate a transfer of zone contents from the master. Once complete, the slave will start serving the member zone.

Removing a member zone from a slave server requires nothing more than deleting the member zone's entry in the catalog zone. The change to the catalog zone is propagated to the slave server using the normal AXFR/IXFR transfer mechanism. The slave server, on processing the update, will notice that the member zone has been removed. It will stop serving the zone and remove it from its list of configured zones. (Removing the member zone from the master server has to be done in the normal way, by editing the configuration file or running `rndc delzone`.)

## Configuring Catalog Zones

Catalog zones are configured with a `catalog-zones` statement in the `options` or `view` section of `named.conf`. For example,

    catalog-zones {
    	zone "catalog.example"
    	     default-masters { 10.53.0.1; }
    	     in-memory no
    	     zone-directory "catzones"
    	     min-update-interval 10;
    };

This statement specifies that the zone `catalog.example` is a catalog zone. This zone must be properly configured in the same view. In most configurations, it would be a slave zone.

The options following the zone name are not required, and may be specified in any order:

The `default-masters` option defines the default masters for member zones listed in a catalog zone. This can be overridden by options within a catalog zone. If no such options are included, then member zones will transfer their contents from the servers listed in this option.

The `in-memory` option, if set to `yes`, causes member zones to be stored only in memory. This is functionally equivalent to configuring a slave zone without a `file`. option. The default is `no`; member zones' content will be stored locally in a file whose name is automatically generated from the view name, catalog zone name, and member zone name.

The `zone-directory` option causes local copies of member zones' master files (if `in-memory` is not set to `yes`) to be stored in the specified directory. The default is to store zone files in the server's working directory. A non-absolute pathname in `zone-directory` is assumed to be relative to the working directory.

The `min-update-interval` option sets the minimum interval between processing of updates to catalog zones, in seconds. If an update to a catalog zone (for example, via IXFR) happens less than `min-update-interval` seconds after the most recent update, then the changes will not be carried out until this interval has elapsed. The default is `5` seconds.

Catalog zones are defined on a per-view basis. Configuring a non-empty `catalog-zones` statement in a view will automatically turn on `allow-new-zones` for that view. (Note: this means `rndc addzone` and `rndc delzone` will also work in any view that supports catalog zones.)

## Catalog Zone format

A catalog zone is a regular DNS zone; therefore, it has to have a single `SOA` and at least one `NS` record.

A record stating the version of the catalog zone format is also required. If the version number listed is not supported by the server, then a catalog zone may not be used by that server.

    catalog.example.    IN SOA . . 2016022901 900 600 86400 1
    catalog.example.    IN NS nsexample.
    version.catalog.example.    IN TXT "1"

Note that this record must have the domain name version.catalog-zone-name. This illustrates how the meaning of data stored in a catalog zone is indicated by the the domain name label immediately before the catalog zone domain.

Catalog zone options can be set either globally for the whole catalog zone or for a single member zone. Global options override the settings in the configuration file and member zone options override global options.

Global options are set at the apex of the catalog zone, e.g.:

     masters.catalog.example.    IN AAAA 2001:db8::1

BIND currently supports the following options:

- A simple `masters` definition:

      	 masters.catalog.example.    IN A 192.0.2.1

  This option defines a master server for the member zones - it can be either an A or AAAA record. If multiple masters are set the order in which they are used is random.

- A `masters` with a TSIG key defined:

               label.masters.catalog.example.     IN A 192.0.2.2
               label.masters.catalog.example.	    IN TXT "tsig_key_name"

  This option defines a master server for the member zone with a TSIG key set. The TSIG key must be configured in the configuration file. `label` can be any valid DNS label.

- `allow-query` and `allow-transfer` ACLs:

               allow-query.catalog.example.	IN APL 1:10.0.0.1/24
               allow-transfer.catalog.example.	IN APL !1:10.0.0.1/32 1:10.0.0.0/24

  These options are the equivalents of `allow-query` and `allow-transfer` in a zone declaration in the `named.conf` configuration file. The ACL is processed in order - if there's no match to any rule the default policy is to deny access. For the syntax of the APL RR see RFC 3123

A member zone is added by including a `PTR` resource record in the `zones` sub-domain of the catalog zone. The record label is a `SHA-1` hash of the member zone name in wire format. The target of the PTR record is the member zone name. For example, to add the member zone `domain.example`:

    5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN PTR domain.example.

The hash is necessary to identify options for a specific member zone. The member zone-specific options are defined the same way as global options, but in the member zone subdomain:

    masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN A 192.0.2.2
    label.masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN AAAA 2001:db8::2
    label.masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN TXT "tsig_key"
    allow-query.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN APL 1:10.0.0.0/24

As would be expected, options defined for a specific zone override the global options defined in the catalog zone. These in turn override the global options defined in the `catalog-zones` statement in the configuration file.

(Note that none of the global records an option will be inherited if any records are defined for that option for the specific zone. For example, if the zone had a `masters` record of type A but not AAAA, then it would not inherit the type AAAA record from the global option.)