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Postfix Documentation
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The Postfix lookup table model

Postfix uses lookup tables to store and look up information for access control, address rewriting and even for content filtering. All Postfix lookup tables are specified as " type:table", where "type" is one of the database types described under "Postfix lookup table types" at the end of this document, and where "table" is the lookup table name. The Postfix documentation uses the terms "database" and "lookup table" for the same thing.

Examples of lookup tables that appear often in the Postfix documentation:

alias_maps = hash:/etc/postfix/aliases            (local aliasing)
header_checks = 
regexp:/etc/postfix/header_checks (content filtering)
transport_maps = hash:/etc/postfix/transport      (routing table)
virtual_alias_maps = hash:/etc/postfix/virtual    (address rewriting)

All Postfix lookup tables store information as (key, value) pairs. This interface may seem simplistic at first, but it turns out to be very powerful. The (key, value) query interface completely hides the complexities of LDAP or SQL from Postfix. This is a good example of connecting complex systems with simple interfaces.

Benefits of the Postfix (key, value) query interface:

  • You can implement Postfix lookup tables first with local Berkeley DB files and then switch to LDAP or MySQL without any impact on the Postfix configuration itself, as described under "Preparing Postfix for LDAP or SQL lookups" below.
  • You can use Berkeley DB files with fixed lookup strings for simple address rewriting operations and you can use regular expression tables for the more complicated work.
Postfix Documentation
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