Fix "Server Error in '/' Application" When ASP.NET Is Missing

What does this error mean?

ASP.NET is Microsoft's system for running dynamic websites. IIS is the Windows program that serves websites. If IIS does not have ASP.NET, it cannot run your pages. You then see an error page. The text may also say "HTTP Error 404.17" or "handler not found". The fix is to add ASP.NET and register it with IIS.

You need admin rights on the server. Take a backup of your site first. This guide uses Windows Server with Server Manager.

Step 1: Install ASP.NET features

1. Open Server Manager.
2. Click Manage, then Add Roles and Features.
3. Click Next until you reach Server Roles.
4. Expand Web Server (IIS), then Web Server, then Application Development.
5. Tick ASP.NET 4.x (use the version your site needs). Click Add Features if asked.
6. Also tick .NET Extensibility 4.x, ISAPI Extensions and ISAPI Filters.
7. Click Next and then Install.
8. Click Close when done.

Step 2: Register ASP.NET with IIS

This step is needed if IIS was installed before .NET.

1. Open Command Prompt as administrator.
2. Go to the .NET folder:

   cd %windir%\Microsoft.NET\Framework64\v4.0.30319

   This command moves you into the folder for 64-bit .NET 4.
3. Run:

   aspnet_regiis.exe -i

   This command registers ASP.NET with IIS.
4. Wait for the message that it finished.

Step 3: Check the application pool

An application pool is a work area that IIS gives your site.

1. Open IIS Manager.
2. Click Application Pools.
3. Click your site's pool.
4. Check that .NET CLR Version matches your site (for example v4.0). Use Basic Settings on the right to change it.
5. Make sure the pool is Started.

Step 4: Restart IIS

iisreset

This command restarts IIS. Visitors may see a short pause.

Quick recap

• The error often means IIS cannot run ASP.NET.
• Add the ASP.NET feature in Server Manager.
• Run aspnet_regiis.exe -i and check the application pool version.
• Run iisreset and test again.