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Customise Apache to do what you want it to do by adding in
extra modules, or remove modules you do not need.
First published in Apache Week issue 32 (13th September 1996). Last updated 20th September 1998
Module Soup
Apache's 'modular' architecture makes is possible for anyone to add
new functions to the server. In fact, most of the code that comes as
part of the Apache distribution is in the form of modules, and can be
removed or replaced. For example, if the 'asis' function is never
needed, the asis module (mod_asis) can be removed, making the server
executable smaller and potentially reducing the load on the server
host.
There are a large number of modules now written for Apache. Besides
those included with the distribution, modules are also written to add
functions not already in the code, or to do things which are needed on
some sites but are not of widespread use. Some of these modules are
written by Apache developers. Most of them, however, are written by
other users of Apache who want to adapt its functionality for their
needs.
In this article, we will look at a range of Apache modules which can
be added to the server. First though, we show how to add a new
module.
Adding a Module
It is easy to add a module to Apache:
- Obtain the module source code file and place in the Apache src
directory
- Add the module definition to the Apache 'Configuration'
- Re-compile Apache
- Install the server executable and re-start the server
So first you need to download the new module. Most modules come as a
single source file, called mod_something.c. Place this file in
Apache's src directory. If the module comes as more than one
file (for example, the PHP/FI module) follow the instructions that
come with the module.
Having got the module source, Apache needs to be configured so that it
will compile this code. To do this, edit the Configuration
file in the src directory, and add a suitable Module
line. This will have the format
Module name_module mod_something.o
The first argument, name_module, must match the name
given in the module's source code - look for the 'module
definition' near the end of the file, which will look like this:
module name_module = {
NULL,
...
};
The name_module text in the Configuration file must match
the name_module text in the module source exactly. The second
argument on the Module line is the filename of the module, with
the final .c replaced by .o.
After editing Configuration, re-compile Apache by running
./Configure
make
Finally, stop your current server (with kill -TERM
pid), install the new httpd executable, and start it
running (e.g. ./httpd -d /usr/local/httpd).
The Standard Modules
If you have not looked at the standard modules which come with Apache,
you might be missing some functions you could find useful. In
addition, you might be compiling in some things you never use. All the
standard Apache modules are listed in the Configuration file.
The next release of Apache will come with a few more standard modules,
such as a module to rewrite URLs on the fly, and a module to add PICS
content-rating labels to responses.
Finding More Modules
Modules can be found in several different places:
- In the Apache 'src' directory
- In the Apache 'contrib/modules' directory
- In the 'Module Registry'
- Other sites (try a search engine and look for "Apache Module").
A Selection of Modules
To simplify finding modules to do what you want, here is the Apache
Week guide to add-on modules by function. These are taken from all the
above sources, and are presented as an example of what is
available. We cannot guarantee that these modules with do what they
say they do, or even that they work with all versions of Apache. If a
module named below is not a link, then that module is distributed with
Apache 1.1.1. Otherwise the link will take you to that module (if the
link is to a .c or .tar file, save it to a file,
else the link goes to an HTML page or FTP directory).
- Authentication
There are a whole range of options for different authentication
schemes. The usernames and passwords can be stored in
flat files (with the standard mod_auth), or in DBM or
Berkeley-DB files (with mod_auth_dbm or mod_auth_db respectively).
For more complex applications, usernames and password can
be stored in mSQL, Postgres95 or DBI-compatible databases,
using mod_auth_msql, mod_auth_pg95 or
mod_auth_dbi.
If passwords cannot be stored in a file or database (perhaps because
they are obtained at run-time from another network service), the
mod_auth_external.c
module lets you call an external program to check whether the given username
and password is valid.
If your site uses Kerebos,
mod_auth_kerb
allows Kerebos-based authentication. For LDAP authentication, see
mod_auth_ldap.
The mod_auth_anon module can be used to allow an 'anonymous-ftp'
style access to authenticated areas, where users give an
anonymous username and a real email address as password.
There are also modules to hold authentication information in cookies,
and to authenticate against standard /etc/passwd and NIS password
services. See the Module Registry.
- Blocking Access
mod_block.c
blocks access to pages based on the 'referer' field. This can be used to
help prevent (for example) your images being used on other people's pages.
For more complex cases, mod_rewrite can be used to implement blocking
based on arbitrary headers (e.g. referer and user-agent), as well as
on the URL itself.
- Counters
There are a number of counter modules available, including mod_counter.c
and mod_cntr.
Some server-side scripting languages, such as PHP/FI can also provide access
counters.
- Faster CGI Programs
Perl CGIs can be sped up considerably by using the
mod_perl
modules,
which build a perl interpreter into the Apache executable,
and optionally allows scripts to start up when the server starts.
Alternatively, the mod_fastcgi module
implements FastCGI on Apache, giving much better performance from
a CGI-like protocol.
- Languages and Internationalisation
The Russian Character
Set (RCS) module provides support for Russian character sets,
while mod_fontxlate can
translate characters in single-byte character sets, for countries with
multiple non-standard character sets.
- Miscellaneous
mod_speling.c
attempts to fix mis-capitalised URLs, by comparing with files and
directories in a case-insensitive manner. A module which makes your
ftp archive into web pages is available at mod_conv.tar.gz.
- Server-Side Scripting
There are several different modules which allow simple (or
not so simple) scripts to be embedded into HTML pages.
XSSI
is
an extended version of standard SSI commands, while
PHP and
NeoScript
are more powerful scripting languages.
- Throttling connections
mod_simultaneous.c
limits the number of simultaneous accesses to particular directories,
which could be a way of implementing limits for images directories. mod_bandwidth
provides a similar service. mod_throttle can
be used to slow down responses for users who exceed a given "bytes per
second" download rate.
- URL rewriting
The mod_rewrite module is a powerful (and complex) way of mapping the
request URL onto a new URL on the fly, using regular expressions and
optionally mapping files in text or DBM format. It can also implement
conditional rewrites based on other request headers (e.g. User-Agent).
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