NAME

Apache::Access::Headers - mod_perl HTTP header authorization module

SYNOPSIS

 # in httpd.conf
 PerlSetVar HeadersAccessConf conf/headers_access.conf
        
 DocumentRoot /usr/local/apache/htdocs
 <Directory "/usr/local/apache/htdocs">
    PerlModule Apache::Access::Headers
    PerlAccessHandler Apache::Access::Headers
 </Directory>

DESCRIPTION

This module is intended to be used as a mod_perl PerlAccessHandler. It's function is to authorize requests for server resources based on the existence of and content of HTTP headers.

Authorizing HTTP headers may be be set by a web browser, a software agent, or an authenitcating proxy server. This module was originally written to work with the latter.

Note: The default reponse from the handler is currently FORBIDDEN. This behavior is not yet configurable.

CONFIGURING APACHE

Module configuration is simple ( read: limited ). Currently, the module only works with a single configuration file, and works best when configured for a server's document root. See the LIMITATIONS section for an explanation of the modules current short-comings.

Add the following line to httpd.conf outside all Directory, Location and VirtualHost blocks:

 PerlSetVar HeadersAccessConf /path/to/conf/headers_access.conf
 

And add the following lines to the DocumentRoot Directory block:

  PerlModule Apache::Access::Headers
  PerlAccessHandler Apache::Access::Headers 

CONFIGURATION FILE

General Options

Although the modules is currently limited to a single xml-based configuration file, this configuration file is quite flexible.

The shell of the conf file is:

 <headers_authz>
  <headers>
  [...]
  </headers>
 </headers_authz>  

The important part of the conf file is the <header> blocks within the <headers> block. Each <header> block must contain two items: an <id> tag and a <path> tag.

The <id> tag specifies the name of the HTTP header that that must be set to allow access to the urls matched by the <path> tags. <path> tags are treated as regular expressions ( i.e., m|^$k$| where $k is the value of the <path> tag ).

Using the Sample Configuration File below, a request for /secrets/index.html must contain an X-Can-View-Secret-Stuff header with a non-zero value in order to be successfully authorized.

Likewise, a request for /secrets.html requires that either an X-Can-View-Secret-Stuff header or an X-Can-View-Super-Secret-Stuff header is present and set to a non-zero value.

As mentioned above, <path> tags are treated as regular expressions. You'll notice, then, that the <path> tag for <id>X-Secret-User-I/id in the sample conf contains parantheses. Parentheses tells the module to require that the value assigned to the needed header ( i.e. X-Secret-User-ID ) equal $1.

For example, using the sample conf, a request for /users/jeffo/ must have an X-Secret-User-ID header set to 'jeffo'. If X-Secret-User-ID header is present but set to 'tori', the request will be denied.

Other Options

There are three special configuration options. They are outlined here:

ALL

If the <id> of a <header> block is 'ALL', then _all_ requests for resources matched by the attached <path> tags. This is useful for allowing access to <path>/</path> and <path>index.html</path>, etc.

REFERER

If the <id> of a <header> block is 'REFERER', then the referer header is checked against an array of referer values specified by <referer> tags in the <header> block.

For example, using the sample conf file, requests for /images/background.gif coming from a page on http://www.rulez.com/, or http://www.picnicman.com/ will be accepted.

<referer> tags are treated as regexes ( like <path> tags ). The regex used is 'm|^$ar|' where $ar is the contents of the <referer> tags.

One little trick is to use 'https?://' at the start of the regex to allow connections from either secure or insecure pages.

Note: If the referer is not matched, FORBIDDEN will not be automatically returned. In this case, the module continues to loop over <path> values, looking for a secondary match.

i.e. if a request for /images/button.gif does not come from a valid referer, but contains a X-Can-View-Secret-Stuff header, the image will be served.

This behavior is open to debate. Admittedly, it's a hack meant to overcome some problems with the original spec. If you don't like it, then don't use REFERER as an <id>. :)

<prefix></prefix>

You'll notice a commented-out <prefix> tag in the sample conf file. The prefix tag was added because some - *cough* - authenticating proxies prepend a string to header values defined in the authtencation database.

If the <prefix> tag is set, then all header checks will look for $PREFIX . $HEADER.

For example, if <prefix> is set to 'Rulez-', requests for /secret/index.html would require not a X-Can-View-Secret-Stuff header, but a Rulez-X-Can-View-Secret-Stuff header.

Sample Configuration File

 <header_access>
  <headers>
   <!-- <prefix>Rulez-</prefix> -->
   <header>
    <id>ALL</id>
    <path>/</path>
    <path>/index.html</path>
   </header>
   <header>
    <id>REFERER</id>
    <referer>https?://www.rulez.com/</referer>
    <referer>https?://ww.picnicman.com/</referer>
    <path>/images/.*</path>
    <path>/cgi/*.cgi</path>
   </header>
   <header>
    <id>X-Can-View-Secret-Stuff</id>
    <path>/secret/.*</path>
    <path>/secrets.html</path>
    <path>/images/.*</path>
   </header>
   <header>
    <id>X-Can-View-Super-Secret-Stuff</id>
    <path>/super-secret/.*</path>
    <path>/secrets.html</path>
   </header>
   <header>
    <id>X-Secret-User-ID</id>
    <path>/users/(.*?)/.*(</path>
   </header>
  </headers>
</header_access>

Configuration File DTD

 <?xml version="1.0" ?>
 <!DOCTYPE header_access [
  <!ELEMENT header_access ( headers+ ) >
  <!ELEMENT headers ( header+ ) >
  <!ELEMENT header ( id, path+, referer+ ) >
  <!ELEMENT id ( #PCDATA ) >
  <!ELEMENT path ( #PCDATA ) >
  <!ELEMENT referer ( #PCDATA ) >
 ]>

TESTING & DEBUGGING

If you set Apache's LogLevel to 'debug', the module will spit out a bunch of information regarding it's handling of the request.

 ErrorLog ./logs/error_log
 LogLevel debug

This is particularly useful if you're creating complicated <path> tag regexes, or if you really have no idea why a request is getting though. :)

LIMITATIONS & THE FUTURE

This module was originally written to cover the entire document root of a web server. And right now, that's all it does. This was a security-driven decision. The down-side is that the module is not VirtualHost or Directory friendly.

I wanted to release this first version before adding support for use in VirtualHosts and Directory blocks with separate conf files.

If interest - and time - allow, these will be the key features of the next release. ( Not to mention configuration of the default return policy and inversion of deny, allow rules, etc. )

RESOURCES

For an example of a commercial authentication proxy, see SecureComputing's PremierAccess product. http://securecomputing.com/. I don't necessarily recommend it; I just know it because I wrote this to work with it. :)

AUTHOR

Jeffrey O'Connell, Jr. <jeffo@rulez.com>

COPYRIGHT

Copyright (c) 2003 Jeffrey O'Connell, Jr., Rulez New Media

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm Apache::Access::Headers

perl -MCPAN -e shell
install Apache::Access::Headers