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:
/etc/postfix/main.cf:
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.
