.. Copyright (C) Internet Systems Consortium, Inc. ("ISC") .. .. SPDX-License-Identifier: MPL-2.0 .. .. This Source Code Form is subject to the terms of the Mozilla Public .. License, v. 2.0. If a copy of the MPL was not distributed with this .. file, you can obtain one at https://mozilla.org/MPL/2.0/. .. .. See the COPYRIGHT file distributed with this work for additional .. information regarding copyright ownership. .. _dlz-info: Dynamically Loadable Zones (DLZ) -------------------------------- Dynamically Loadable Zones (DLZ) are 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``). 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; they are still needed for the older drivers in ``contrib/dlz/drivers``. The DLZ module provides data to ``named`` in text format, which is then 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 primary configuration, with secondaries retrieving zone updates via AXFR. Note, however, that DLZ has no built-in support for DNS notify; secondary servers 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`` are queried to see whether they contain an answer for the query name. The best available answer is 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``, 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 users to configure a zone normally using standard zone-option semantics, but specify a different database backend for storage of the zone's data. For example, to implement NXDOMAIN redirection using a DLZ module for backend storage of redirection rules: :: dlz other { database "dlopen driver.so args"; search no; }; zone "." { type redirect; dlz other; }; Sample DLZ Driver ~~~~~~~~~~~~~~~~~ For guidance in the 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 can retrieve information about the querying client and alter 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 is 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.