After migration of your website (from ServerA to ServerB) to a new Webserver when you try to access the website you get “503 Error – Service Unavailable” Error on your website and IIS Management Console shows that application pool of the site is crashed or stopped. Even after you start the application pool of the site it is getting crashed when someone try to access website.

Event Viewer shows following error

The Module DLL C:\\Windows\\system32\\inetsrv\\rewrite.dll failed to load. The data is the error.

This error shows that the your site is trying to access IIS rewrite module which is not installed or configured on your New Server. If you check the IIS Roles and Features you will find that both the server(A & B) have same roles and features installed but still IIS of ServerB don’t have Rewrite module. To fix this problem you have to install IIS Rewrite modules separately. IIS URL Rewrite module enables Web administrators to create rules to implement URLs that are easier to remember and easier to find by the search engines. You can see more information about this module on IIS.NET site or you can directly download them using following URL:

• Download IIS Rewrite Module (x86)
• Download IIS Rewrite Module (x64)

After installation of the module don’t forget to restart IIS Web Server.

Recently while working I faced an issue where the website (IIS 7) was showing 503-Internal Server Error. Checking Event viewer shown following Events for IIS

Complete Event in Event Viewer was

Both the DLL’s which were mentioned in the Event we present in respective Directories but still IIS was not able to detect them. So, to fix the issue

1) Copy “C:/Program_Files/IIS\Advanced_Logging/ClientLoggingHandler.dll” and “C:/Program_Files/IIS/Advanced_Logging/AdvancedLoggingModule.dll” to %windir%/System32/

2) Edit C:/Windows/System32/netsrv/config/applicationHost.config and Search for following Lines

and change it to

3) Restart IIS

Sometime when you try to setup a application for your website on IIS7 you get following error when you load your website.

HTTP Error 500.24 – Internal Server Error
An ASP.NET setting has been detected that does not apply in Integrated managed pipeline mode.

This happens when ASP.NET modules and handlers should be specified in the IIS and configuration sections in Integrated mode. This problem can be fixed using following workaround

1) Change application configuration: First way to fix this issue is to migrate the application configuration to work properly in Integrated mode. You can use “AppCmd“ command to migrate the application configuration:

%windir%/system32/inetsrv/>Appcmd migrate config “<AppPath>”

Where “<AppPath>“ is the virtual path of the application, such as “Default Web Site/virtualapp1“.

2) Modify web.config: You have to manually move the customer entries in the <system.web>/<httpModules> and configuration to the <system.web>/<httpHandlers><system.webServer>/<handlers> and <system.webServer>/<modules> configuration sections, and either remove the <httpHandlers> and <httpModules> configuration OR add the following to your application’s or websites web.config:

<system.webServer>
<validation validateIntegratedModeConfiguration=”false”/>
</system.webServer>

3) Switch to Classic ASP.NET: Switch the website or application to an application pool that is configured to run in Classic ASP.NET mode. To move back to classic mode go to IIS7 Manager >> Select the site (which you want to switch) >> edit settings and you will get following screen

Press “Select” button which will open a window “Select Application Pool” (check below image); Select “Classic .NET AppPool” from the drop-down and press OK.

Now try to Access your website/application.