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The Postfix mysql map type allows you to hook up Postfix to a MySQL database.
This implementation allows for multiple mysql databases: you can use one for a
virtual(5) table, one for an access(5) table, and one for an aliases(5) table
if you want. You can specify multiple servers for the same database, so that
Postfix can switch to a good database server if one goes bad.

Busy mail servers using mysql maps will generate lots of concurrent mysql
clients, so the mysql server(s) should be run with this fact in mind. You can
reduce the number of concurrent mysql clients by using the Postfix proxymap(8)
service.

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Note: to use mysql with Debian GNU/Linux's Postfix, all you need is to install
the postfix-mysql package and you're done. There is no need to recompile
Postfix.

The Postfix MySQL client utilizes the mysql client library, which can be
obtained from:

    http://www.mysql.com/downloads/
    http://sourceforge.net/projects/mysql/

In order to build Postfix with mysql map support, you will need to add -
DHAS_MYSQL and -I for the directory containing the mysql headers, and the
mysqlclient library (and libm) to AUXLIBS, for example:

    make -f Makefile.init makefiles \
        'CCARGS=-DHAS_MYSQL -I/usr/local/mysql/include' \
        'AUXLIBS=-L/usr/local/mysql/lib -lmysqlclient -lz -lm'

Then, just run 'make'. This requires libz, the compression library. Older mysql
implementations build without libz.

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Once Postfix is built with mysql support, you can specify a map type in main.cf
like this:

    alias_maps = mysql:/etc/postfix/mysql-aliases.cf

The file /etc/postfix/mysql-aliases.cf specifies lots of information telling
Postfix how to reference the mysql database. For a complete description, see
the mysql_table(5) manual page.

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#
# mysql config file for local(8) aliases(5) lookups
#

# The user name and password to log into the mysql server.
user = someone
password = some_password

# The database name on the servers.
dbname = customer_database

# The table name.
table = mxaliases

# Query components, see below.
select_field = forw_addr
where_field = alias

# You may specify additional_conditions or leave this empty.
additional_conditions = and status = 'paid'

# The above variables will result in a query of the form:
#
# select forw_addr from mxaliases where alias = '$lookup' and status = 'paid'
#
# ($lookup is escaped so if it contains single quotes or other odd
# characters, it will not cause trouble).

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The MySQL configuration interface setup allows for multiple mysql databases:
you can use one for a virtual table, one for an access table, and one for an
aliases table if you want.

Since sites that have a need for multiple mail exchangers may enjoy the
convenience of using a networked mailer database, but do not want to introduce
a single point of failure to their system, we've included the ability to have
Postfix reference multiple hosts for access to a single mysql map. This will
work if sites set up mirrored mysql databases on two or more hosts. Whenever
queries fail with an error at one host, the rest of the hosts will be tried in
random order. If no mysql server hosts are reachable, then mail will be
deferred until at least one of those hosts is reachable.

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  * The initial version was contributed by Scott Cotton and Joshua Marcus, IC
    Group, Inc.
  * Liviu Daia revised the configuration interface and added the main.cf
    configuration feature.

