# DLZ (Dynamically Loadable Zones)

DLZ (Dynamically Loadable Zones) is an extension to BIND 9 that allows zone data to be retrieved directly from an external database. There is no required format or schema. DLZ drivers exist for several different database backends including PostgreSQL, MySQL, and LDAP and can be written for any other.

Historically, DLZ drivers had to be statically linked with the `named` binary and were turned on via a configure option at compile time (for example, `"configure --with-dlz-ldap"`). Currently, the drivers provided in the BIND 9 tarball in `contrib/dlz/drivers` are still linked this way.

In BIND 9.8 and higher, it is possible to link some DLZ modules dynamically at runtime, via the DLZ "dlopen" driver, which acts as a generic wrapper around a shared object implementing the DLZ API. The "dlopen" driver is linked into `named` by default, so configure options are no longer necessary when using these dynamically linkable drivers, but are still needed for the older drivers in `contrib/dlz/drivers`.

When the DLZ module provides data to `named`, it does so in text format. The response is converted to DNS wire format by `named`. This conversion, and the lack of any internal caching, places significant limits on the query performance of DLZ modules. Consequently, DLZ is not recommended for use on high-volume servers. However, it can be used in a hidden master configuration, with slaves retrieving zone updates via AXFR. (Note, however, that DLZ has no built-in support for DNS notify; slaves are not automatically informed of changes to the zones in the database.)

## Configuring DLZ

A DLZ database is configured with a `dlz` statement in `named.conf`:

        dlz example {
    	database "dlopen driver.so args";
    	search yes;
        };

This specifies a DLZ module to search when answering queries; the module is implemented in `driver.so` and is loaded at runtime by the dlopen DLZ driver. Multiple `dlz` statements can be specified; when answering a query, all DLZ modules with `search` set to `yes` will be queried to find out if they contain an answer for the query name; the best available answer will be returned to the client.

The `search` option in the above example can be omitted, because `yes` is the default value.

If `search` is set to `no`, then this DLZ module is not searched for the best match when a query is received. Instead, zones in this DLZ must be separately specified in a zone statement. This allows you to configure a zone normally using standard zone option semantics, but specify a different database back-end for storage of the zone's data. For example, to implement NXDOMAIN redirection using a DLZ module for back-end storage of redirection rules:

        dlz other {
    	database "dlopen driver.so args";
    	search no;
        };

        zone "." {
    	type redirect;
    	dlz other;
        };

## Sample DLZ Driver

For guidance in implementation of DLZ modules, the directory `contrib/dlz/example` contains a basic dynamically-linkable DLZ module--i.e., one which can be loaded at runtime by the "dlopen" DLZ driver. The example sets up a single zone, whose name is passed to the module as an argument in the `dlz` statement:

        dlz other {
    	database "dlopen driver.so example.nil";
        };

In the above example, the module is configured to create a zone "example.nil", which can answer queries and AXFR requests, and accept DDNS updates. At runtime, prior to any updates, the zone contains an SOA, NS, and a single A record at the apex:

     example.nil.  3600    IN      SOA     example.nil. hostmaster.example.nil. (
    					       123 900 600 86400 3600
    				       )
     example.nil.  3600    IN      NS      example.nil.
     example.nil.  1800    IN      A       10.53.0.1

The sample driver is capable of retrieving information about the querying client, and altering its response on the basis of this information. To demonstrate this feature, the example driver responds to queries for "source-addr.`zonename`>/TXT" with the source address of the query. Note, however, that this record will *not* be included in AXFR or ANY responses. Normally, this feature would be used to alter responses in some other fashion, e.g., by providing different address records for a particular name depending on the network from which the query arrived.

Documentation of the DLZ module API can be found in `contrib/dlz/example/README`. This directory also contains the header file `dlz_minimal.h`, which defines the API and should be included by any dynamically-linkable DLZ module.