External Databases

Postfix map files provide an easy and efficient mechanism for the many lookup operations needed when handling email. In some situations, however, it can be more convenient to have the information in a database separate from Postfix. A database can provide a central repository available to many system or network services that need similar information, such as account names and passwords. A database can be useful when redundant systems running Postfix need to share the same configuration information. A central database might also be more convenient when you have multiple people who need access to edit information.

Databases can also slow Postfix performance compared to normal index files. In general, if you don't have a definite need for a database, you're better off with the standard Postfix maps. In many cases you can get the best of both options by storing information in a database and running regular scripts that update your Postfix files from the central data repository. But if your environment requires instant access to revised data, an external database configuration may be your only option.

In this chapter, we'll look at configuring Postfix to work with MySQL and LDAP. (Postfix also has support for PostgreSQL as of Version 2.1.) In either case, Postfix must be compiled with additional libraries to support the mysql and ldap map types. If you are using a prebuilt package, make sure that it has support for the type of database you plan to use. If you built your own Postfix, see Chapter 15 for information on compiling with the additional libraries.

You can easily check if your Postfix installation contains support for LDAP and MySQL with the postconf -m command:

$ postconf -m
static
pcre
regexp
mysql
environ
proxy
ldap
btree
unix
hash

You should see either ldap or mysql or both listed among the map types.

While the databases you use with Postfix may contain a variety of information, conceptually they work the same as Postfix maps. You have a key such as the recipient email address, and you expect to get back a value associated with the key such as a forwarding address. How to perform this with each type of database, MySQL and LDAP, is explained in the next sections.

It is a good practice to make sure your lookups work correctly with normal Postfix lookup tables. Then duplicate your configuration with MySQL or LDAP lookups. Make sure that you get the same results from both. In most cases, Postfix expects a lookup to return only one result. Make sure that your database queries do not return multiple result values.