When deploying an ASP.NET application under Mono/Apache on Ubuntu, I encountered peculiar behavior where requests to /icons returned a directory listing instead of being processed by my ASP.NET handlers. This didn't occur with other similar paths like /images. Here's what I discovered:

Apache HTTP Server comes with several predefined aliases in its default configuration. The /icons/ path is one such special case. In the default apache2.conf or httpd.conf, you'll often find:

Alias /icons/ "/usr/share/apache2/icons/"
<Directory "/usr/share/apache2/icons">
    Options Indexes MultiViews
    AllowOverride None
    Require all granted
</Directory>

To verify if this is affecting your setup, check:

# Check loaded modules
apache2ctl -M | grep alias

# Check active aliases
grep -r "Alias /icons/" /etc/apache2/

For ASP.NET applications, you'll want to disable or override this alias. Here are two approaches:

Method 1: Remove the Alias Completely

# In your apache configuration:
<IfModule alias_module>
    # Comment out or remove the icons alias
    # Alias /icons/ "/usr/share/apache2/icons/"
</IfModule>

Method 2: Create Custom Location

# In your virtual host configuration:
Alias /icons/ "/var/www/your_asp_app/icons/"
<Directory "/var/www/your_asp_app/icons">
    Options -Indexes
    AllowOverride None
    Require all granted
    SetHandler mono
    AddType application/x-asp-net .aspx .ashx .asmx .ascx .asax .config .ascx
</Directory>

After making changes, test with:

sudo apache2ctl configtest
sudo systemctl restart apache2

Then access /icons in your browser - it should now be processed by your ASP.NET application rather than showing a directory listing.

The /images path typically doesn't have a predefined alias in Apache's default configuration, which explains why it behaves as expected. This difference highlights why special paths like /icons, /manual, or /cgi-bin require extra attention in Apache configurations.

When deploying ASP.NET applications via Mono on Apache2/Ubuntu, you might encounter unexpected behavior with specific paths like /icons. While most routes correctly trigger your ASP.NET handlers, some special paths default to Apache's built-in directory listings.

Apache HTTP Server comes with several preconfigured aliases for common directories. One such default alias is:

Alias /icons/ "/usr/share/apache2/icons/"

This explains why /icons shows a directory listing while your custom /images directory works normally. The /icons path is being intercepted by Apache before reaching your Mono handlers.

To check if this is the case in your setup:

grep -r "Alias /icons" /etc/apache2/

There are two effective approaches to resolve this:

Option 1: Remove the Alias Completely

# In your apache config or virtual host:
<Directory "/usr/share/apache2/icons">
    Options -Indexes
</Directory>
Alias /icons "/path/to/your/actual/icons"

Option 2: Use Location Directive

<Location "/icons">
    SetHandler mono
    DirectoryIndex default.aspx
</Location>

If you can modify your application, consider renaming the problematic path:

// In your ASP.NET routing configuration
routes.MapRoute(
    name: "Icons",
    url: "assets/icons/{filename}",
    defaults: new { controller = "Resources", action = "GetIcon" }
);

To verify which paths are being intercepted by Apache before reaching Mono:

apache2ctl -S 2>&1 | grep Alias