Edit File: sdb
Copyright (C) 2004 Internet Systems Consortium, Inc. ("ISC") Copyright (C) 2000, 2001 Internet Software Consortium. See COPYRIGHT in the source root or http://isc.org/copyright.html for terms. Using the BIND 9 Simplified Database Interface This document describes the care and feeding of the BIND 9 Simplified Database Interface, which allows you to extend BIND 9 with new ways of obtaining the data that is published as DNS zones. The Original BIND 9 Database Interface BIND 9 has a well-defined "back-end database interface" that makes it possible to replace the component of the name server responsible for the storage and retrieval of zone data, called the "database", on a per-zone basis. The default database is an in-memory, red-black-tree data structure commonly referred to as "rbtdb", but it is possible to write drivers to support any number of alternative database technologies such as in-memory hash tables, application specific persistent on-disk databases, object databases, or relational databases. The original BIND 9 database interface defined in <dns/db.h> is designed to efficiently support the full set of database functionality needed by a name server that implements the complete DNS protocols, including features such as zone transfers, dynamic update, and DNSSEC. Each of these aspects of name server operations places its own set of demands on the data store, with the result that the database API is quite complex and contains operations that are highly specific to the DNS. For example, data are stored in a binary format, the name space is tree structured, and sets of data records are conceptually associated with DNSSEC signature sets. For these reasons, writing a driver using this interface is a highly nontrivial undertaking. The Simplified Database Interface Many BIND users wish to provide access to various data sources through the DNS, but are not necessarily interested in completely replacing the in-memory "rbt" database or in supporting features like dynamic update, DNSSEC, or even zone transfers. Often, all you want is limited, read-only DNS access to an existing system. For example, you may have an existing relational database containing hostname/address mappings and wish to provide forvard and reverse DNS lookups based on this information. Or perhaps you want to set up a simple DNS-based load balancing system where the name server answers queries about a single DNS name with a dynamically changing set of A records. BIND 9.1 introduced a new, simplified database interface, or "sdb", which greatly simplifies the writing of drivers for these kinds of applications. The sdb Driver An sdb driver is an object module, typically written in C, which is linked into the name server and registers itself with the sdb subsystem. It provides a set of callback functions, which also serve to advertise its capabilities. When the name server receives DNS queries, invokes the callback functions to obtain the data to respond with. Unlike the full database interface, the sdb interface represents all domain names and resource records as ASCII text. Writing an sdb Driver When a driver is registered, it specifies its name, a list of callback functions, and flags. The flags specify whether the driver wants to use relative domain names where possible. The callback functions are as follows. The only one that must be defined is lookup(). - create(zone, argc, argv, driverdata, dbdata) Create a database object for "zone". - destroy(zone, driverdata, dbdata) Destroy the database object for "zone". - lookup(zone, name, dbdata, lookup) Return all the records at the domain name "name". - authority(zone, dbdata, lookup) Return the SOA and NS records at the zone apex. - allnodes(zone, dbdata, allnodes) Return all data in the zone, for zone transfers. For more detail about these functions and their parameters, see bind9/lib/dns/include/dns/sdb.h. For example drivers, see bind9/contrib/sdb. Rebuilding the Server The driver module and header file must be copied to (or linked into) the bind9/bin/named and bind9/bin/named/include directories respectively, and must be added to the DBDRIVER_OBJS and DBDRIVER_SRCS lines in bin/named/Makefile.in (e.g. for the timedb sample sdb driver, add timedb.c to DBDRIVER_SRCS and timedb.@O@ to DBDRIVER_OBJS). If the driver needs additional header files or libraries in nonstandard places, the DBDRIVER_INCLUDES and DBDRIVER_LIBS lines should also be updated. Calls to dns_sdb_register() and dns_sdb_unregister() (or wrappers, e.g. timedb_init() and timedb_clear() for the timedb sample sdb driver) must be inserted into the server, in bind9/bin/named/main.c. Registration should be in setup(), before the call to ns_server_create(). Unregistration should be in cleanup(), after the call to ns_server_destroy(). A #include should be added corresponding to the driver header file. You should try doing this with one or more of the sample drivers before attempting to write a driver of your own. Configuring the Server To make a zone use a new database driver, specify a "database" option in its "zone" statement in named.conf. For example, if the driver registers itself under the name "acmedb", you might say zone "foo.com" { database "acmedb"; }; You can pass arbitrary arguments to the create() function of the driver by adding any number of whitespace-separated words after the driver name: zone "foo.com" { database "acmedb -mode sql -connect 10.0.0.1"; }; Hints for Driver Writers - If a driver is generating data on the fly, it probably should not implement the allnodes() function, since a zone transfer will not be meaningful. The allnodes() function is more relevant with data from a database. - The authority() function is necessary if and only if the lookup() function will not add SOA and NS records at the zone apex. If SOA and NS records are provided by the lookup() function, the authority() function should be NULL. - When a driver is registered, an opaque object can be provided. This object is passed into the database create() and destroy() functions. - When a database is created, an opaque object can be created that is associated with that database. This object is passed into the lookup(), authority(), and allnodes() functions, and is destroyed by the destroy() function. Future Directions A future release may support dynamic loading of sdb drivers. $Id: sdb,v 1.6 2004-03-05 05:04:54 marka Exp $
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