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<sect1 id="dlz-info">

<title>DLZ (Dynamically Loadable Zones)</title>

<para>

DLZ (Dynamically Loadable Zones) is an extention 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.

</para>

<para>

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, <userinput>"configure --with-dlz-ldap"</userinput>).

Currently, the drivers provided in the BIND 9 tarball in

<filename>contrib/dlz/drivers</filename> are still linked this

way.

</para>

<para>

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

<filename>contrib/dlz/drivers</filename>.

</para>

<para>

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.)

</para>

<sect2>

<title>Configuring DLZ</title>

<para>

A DLZ database is configured with a <command>dlz</command>

statement in <filename>named.conf</filename>:

</para>

<screen>

dlz example {

database "dlopen driver.so <option>args</option>";

search yes;

};

</screen>

<para>

This specifies a DLZ module to search when answering queries; the

module is implemented in <filename>driver.so</filename> and is

loaded at runtime by the dlopen DLZ driver. Multiple

<command>dlz</command> statements can be specified; when

answering a query, all DLZ modules with <option>search</option>

set to <literal>yes</literal> 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.

</para>

<para>

The <option>search</option> option in the above example can be

omitted, because <literal>yes</literal> is the default value.

</para>

<para>

If <option>search</option> is set to <literal>no</literal>, then

this DLZ module is <emphasis>not</emphasis> 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:

</para>

<screen>

dlz other {

database "dlopen driver.so <option>args</option>";

search no;

};

zone "." {

type redirect;

dlz other;

};

</screen>

</sect2>

<sect2>

<title>Sample DLZ Driver</title>

<para>

For guidance in implementation of DLZ modules, the directory

<filename>contrib/dlz/example</filename> 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 <command>dlz</command>

statement:

</para>

<screen>

dlz other {

database "dlopen driver.so example.nil";

};

</screen>

<para>

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:

</para>

<screen>

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

</screen>

<para>

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.<option>zonename</option>>/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.

</para>

<para>

Documentation of the DLZ module API can be found in

<filename>contrib/dlz/example/README</filename>. This directory also

contains the header file <filename>dlz_minimal.h</filename>, which

defines the API and should be included by any dynamically-linkable

DLZ module.

</para>

</sect2>

</sect1>