Managing Resources with Apache mod_alias

In many cases, all of the resources served by an Apache host are located in that host’s DocumentRoot. The DocumentRoot is a directory specified in the <VirtualHost> configuration block. This directory is intended to represent the various files, directories, and resources that users access over HTTP on the file system. However, it is common for administrators to provide HTTP access to a resource on the file system which is not located in the DocumentRoot. While Apache will follow symbolic links in some situations, this can be difficult to maintain. As a result Apache makes it possible to specify an Alias that connects a location in the request to an alternate location.

Creating Aliases

Typically, Virtual Host configurations specify a DocumentRoot which specifies a directory named, by convention, public_html/ or public/. If the document root for the example.com virtual host is /srv/www/example.com/public_html/, then a request for http://www.example.com/index.htm will return the file located at /srv/www/example.com/public_html/index.htm.

If the administrator needed to maintain the code/ resource on the file system at /srv/git/public/ but have it be accessible at http://example.com/code/, an alias would be required. This is accomplished in the following example:

Apache Configuration
 
DocumentRoot /srv/www/example.com/public_html/
Alias /code /srv/git/public
<Directory /srv/git/public>
    Order allow,deny
    Allow from all
</Directory>

Without the Alias directive, a request for http://example.com/code/ would return resources available in the folder /srv/www/example.com/public_html/code/. However, the Alias would direct Apache to serve content from the /srv/git/public directory. The <Directory> section permits remote users to access this directory.

There are a couple of important factors to consider when using Alias directives:

• Directory blocks need to be created after Alias directives are asserted for the destination of the Alias. This makes it possible to permit access and otherwise control the behavior of those sections. In the example above that would be /srv/git/public.
• In general trailing slashes are to be avoided in Alias directives. If the above had read Alias /code/ /srv/git/public/ a request for http://example.com/code, without a trailing slash, would be served from the DocumentRoot.
• Alias directives need to be created either in the root-level server config (e.g. httpd.conf) or inside of a <VirtualHost> configuration block.

In addition to Alias, Apache provides an AliasMatch directive that offers similar functionality. AlaisMatch provides the additional ability to alias a class of requests for a given resource to a location outside of the DocumentRoot. Let us consider another fictive example.com virtual host configuration:

Apache Configuration
 
DocumentRoot /srv/www/example.com/public_html/
AliasMatch /code/projects/(.+) /srv/git/projects/$1
<DirectoryMatch "^/srv/git/projects/.+$">
    Order allow,deny
    Allow from all
</Directory>

In this example, requests for URLs such as http://example.com/code/projects/my_app and http://example.com/code/projects/my_app2 will be served resources in /srv/git/projects/my_app and /srv/git/projects/my_app2 respectively. However, http://example.com/code/projects would be served from /srv/www/example.com/public_html/code/projects/ rather than /srv/git/projects/, because of the trailing slash in the alias to /code/projects/(.+).

Although the potential use case for Alias is somewhat narrow, the functionality is very powerful for maintaining a secure and well-organized web server.