Postfix Lookup Table Overview

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Overview

This document covers the following topics:

  * The Postfix lookup table model
  * Postfix lists versus tables
  * Preparing Postfix for LDAP or SQL lookups
  * Maintaining Postfix lookup table files
  * Updating Berkeley DB files safely
  * Postfix lookup table types

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:

    /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.

Postfix lists versus tables

Most Postfix lookup tables are used to look up information. Examples are
address rewriting (the lookup string is the old address, and the result is the
new address) or access control (the lookup string is the client, sender or
recipient, and the result is an action such as "reject").

With some tables, however, Postfix needs to know only if the lookup key exists.
The lookup result itself is not used. Examples are the local_recipient_maps
that determine what local recipients Postfix accepts in mail from the network,
the mydestination parameter that specifies what domains Postfix delivers
locally, or the mynetworks parameter that specifies the IP addresses of trusted
clients or client networks. Technically, these are lists, not tables. Despite
the difference, Postfix lists are described here because they use the same
underlying infrastructure as Postfix lookup tables.

Preparing Postfix for LDAP or SQL lookups

LDAP and SQL are complex systems. Trying to set up both Postfix and LDAP or SQL
at the same time is definitely not a good idea. You can save yourself a lot of
time by implementing Postfix first with local files such as Berkeley DB. Local
files have few surprises, and are easy to debug with the postmap(1) command:

    % postmap -q info@example.com hash:/etc/postfix/virtual

Once you have local files working properly you can follow the instructions in
ldap_table(5), mysql_table(5) or pgsql_table(5) and replace local file lookups
with LDAP or SQL lookups. When you do this, you should use the postmap(1)
command again, to verify that database lookups still produce the exact same
results as local file lookup:

    % postmap -q info@example.com ldap:/etc/postfix/virtual.cf

Be sure to exercise all the partial address or parent domain queries that are
documented under "table search order" in the relevant manual page: access(5),
canonical(5), virtual(5), transport(5), or under the relevant configuration
parameter: mynetworks, relay_domains, parent_domain_matches_subdomains.

Maintaining Postfix lookup table files

When you make changes to a database while the mail system is running, it would
be desirable if Postfix avoids reading information while that information is
being changed. It would also be nice if you can change a database without
having to execute "postfix reload", in order to force Postfix to use the new
information. Each time you do "postfix reload" Postfix loses a lot of
performance.

  * If you change a network database such as LDAP, NIS or SQL, there is no need
    to execute "postfix reload". The LDAP, NIS or SQL server takes care of
    read/write access conflicts and gives the new data to Postfix once that
    data is available.

  * If you change a regexp: or pcre: file then Postfix may or may not pick up
    the file changes immediately. This is because a Postfix process reads the
    entire file into memory once and never examines the file again.

      o If the file is used by a short-running process such as smtpd(8),
        cleanup(8) or local(8), there is no need to execute "postfix reload"
        after making a change.

      o If the file is being used by a long-running process such as trivial-
        rewrite(8) on a busy server it may be necessary to execute "postfix
        reload".

  * If you change a local file based database such as DBM or Berkeley DB, there
    is no need to execute "postfix reload". Postfix uses file locking to avoid
    read/write access conflicts, and whenever a Postfix daemon process notices
    that a file has changed it will terminate before handling the next client
    request, so that a new process can initialize with the new database.

Updating Berkeley DB files safely

Although Postfix uses file locking to avoid access conflicts while updating
Berkeley DB or other local database files, you still have a problem when the
update fails because the disk is full or because something else happens. This
is because commands such as postmap(1) or postalias(1) overwrite existing
files. If the update fails in the middle then you have no usable database, and
Postfix will stop working. This is not an issue with the CDB database type
available with Postfix 2.2 and later, because CDB database rebuilds are atomic.

With multi-file databases such as DBM, there is no simple solution. With
Berkeley DB and other "one file" databases, it is possible to add some extra
robustness by using "mv" to REPLACE an existing database file instead of
overwriting it:

    # postmap access.in && mv access.in.db access.db

This converts the input file "access.in" into the output file "access.in.db",
and replaces the file "access.db" only when the postmap(1) command was
successful. Of course typing such commands becomes boring quickly, and this is
why people use "make" instead, as shown below. User input is shown in bold
font.

    # cat Makefile
    all: aliases.db access.db virtual.db ...etcetera...

    # Note 1: commands are specified after a TAB character.
    # Note 2: use postalias(1) for local aliases, postmap(1) for the rest.
    aliases.db: aliases.in
    	postalias aliases.in
    	mv aliases.in.db aliases.db

    access.db: access.in
    	postmap access.in
    	mv access.in.db access.db

    virtual.db: virtual.in
    	postmap virtual.in
    	mv virtual.in.db virtual.db

    ...etcetera...
    # vi access.in
    ...editing session not shown...
    # make
    postmap access.in
    mv access.in.db access.db
    #

The "make" command updates only the files that have changed. In case of error,
the "make" command will stop and will not invoke the "mv" command, so that
Postfix will keep using the existing database file as if nothing happened.

Postfix lookup table types

To find out what database types your Postfix system supports, use the "postconf
-m" command. Here is a list of database types that are often supported:

    btree
        A sorted, balanced tree structure. This is available only on systems
        with support for Berkeley DB databases. Database files are created with
        the postmap(1) or postalias(1) command. The lookup table name as used
        in "btree:table" is the database file name without the ".db" suffix.
    cdb
        A read-optimized structure with no support for incremental updates.
        Database files are created with the postmap(1) or postalias(1) command.
        The lookup table name as used in "cdb:table" is the database file name
        without the ".cdb" suffix. This feature is available with Postfix 2.2
        and later.
    cidr
        A table that associates values with Classless Inter-Domain Routing
        (CIDR) patterns. The table format is described in cidr_table(5).
    dbm
        An indexed file type based on hashing. This is available only on
        systems with support for DBM databases. Database files are created with
        the postmap(1) or postalias(1) command. The lookup table name as used
        in "dbm:table" is the database file name without the ".dir" or ".pag"
        suffix.
    environ
        The UNIX process environment array. The lookup key is the variable
        name. The lookup table name in "environ:table" is ignored.
    hash
        An indexed file type based on hashing. This is available only on
        systems with support for Berkeley DB databases. Database files are
        created with the postmap(1) or postalias(1) command. The database name
        as used in "hash:table" is the database file name without the ".db"
        suffix.
    ldap (read-only)
        Perform lookups using the LDAP protocol. Configuration details are
        given in the ldap_table(5).
    mysql (read-only)
        Perform MySQL database lookups. Configuration details are given in
        mysql_table(5).
    netinfo (read-only)
        Perform Netinfo database lookups.
    nis (read-only)
        Perform NIS database lookups.
    nisplus (read-only)
        Perform NIS+ database lookups. Configuration details are given in
        nisplus_table(5).
    pcre (read-only)
        A lookup table based on Perl Compatible Regular Expressions. The file
        format is described in pcre_table(5). The lookup table name as used in
        "pcre:table" is the name of the regular expression file.
    pgsql (read-only)
        Perform PostgreSQL database lookups. Configuration details are given in
        pgsql_table(5).
    proxy (read-only)
        Access information via the Postfix proxymap(8) service. The lookup
        table name syntax is "proxy:type:table".
    regexp (read-only)
        A lookup table based on regular expressions. The file format is
        described in regexp_table(5). The lookup table name as used in "regexp:
        table" is the name of the regular expression file.
    sdbm
        An indexed file type based on hashing. This is available only on
        systems with support for SDBM databases. Database files are created
        with the postmap(1) or postalias(1) command. The lookup table name as
        used in "sdbm:table" is the database file name without the ".dir" or
        ".pag" suffix.
    static (read-only)
        Always returns its lookup table name as lookup result. For example, the
        lookup table "static:foobar" always returns the string "foobar" as
        lookup result.
    tcp
        Access information through a TCP/IP server. The protocol is described
        in tcp_table(5). The lookup table name is "tcp:host:port" where "host"
        specifies a symbolic hostname or a numeric IP address, and "port"
        specifies a symbolic service name or a numeric port number. This
        protocol is not available up to and including Postfix version 2.2.
    unix (read-only)
        A limited way to query the UNIX authentication database. The following
        tables are implemented:
        unix:passwd.byname
            The table is the UNIX password database. The key is a login name.
            The result is a password file entry in passwd(5) format.
        unix:group.byname
            The table is the UNIX group database. The key is a group name. The
            result is a group file entry in group(5) format.

Other lookup table types may be available depending on how Postfix was built.
With some Postfix distributions the list is dynamically extensible as support
for lookup tables is dynamically linked into Postfix.

