For nearly all HTTP-serving related work, our software of choice is the Apache web server. Lately, we have been moving some old setups which were still running Apache 1.3 to the latest version in the 2.x series. This may be a lot of work when the number of virtual hosts you are serving is far from low, or if you have some lengthy, repetitive configuration blocks which must be modified everytime you want introduce some change. One may be tempted to auto-generate configuration snippets from something else using some quick Bash/Perl/Python/Whatever script, but there is a more elegant solution: meet mod_macro.

As the name suggests, it is an Apache 2.x module which allows for defining snippets with customizable optional parameters. Take this real-world example from /etc/apache2/mods-available/macro.conf:

AuthType basic
AuthBasicProvider ldap
AuthLDAPUrl ldap://ldap.local/dc=yoursite,dc=com?uid
AuthLDAPGroupAttribute uniqueMember
AuthLDAPGroupAttributeIsDN On
Require ldap-group cn=$group,ou=Group,dc=yoursite,dc=com

This is a common setup in most of our services, so now everytime one wants to add autentication against the LDAP server, it is only a matter of adding an Use clause in the proper place:

# Put this into the virtual host configuration block:
Use AuthLDAPSetup wwwusers

Ta-da! This is 5 lines shorter, and less error-prone. Also, if some day some extra configuration is needed to authenticate using LDAP, it is enough to change the macro definition, and changes will be automatically propagated to all places where it is used.

Wrapping up, you may consider installing mod_macro in your Apache installs for the following reasons:

• Greatly simplifies repeated code snippets in configuration files.
• It is integrated with the web server: no external tools are needed.
• Is especially useful to make changes to big sites in a single shot.
• Does not add overhead to request processing, only to initial configuration file parsing at server startup.
• It is simple enough to learn and use in a couple of minutes.