IIS modules with ASP.NET Core

By Luke Latham

ASP.NET Core apps are hosted by IIS in a reverse proxy configuration. Some of the native IIS modules and all of the IIS managed modules aren’t available to process requests for ASP.NET Core apps. In many cases, ASP.NET Core offers an alternative to the features of IIS native and managed modules.

Native modules

The table indicates native IIS modules that are functional on reverse proxy requests to ASP.NET Core apps.

†The URL Rewrite Module’s isFile and isDirectory match types don’t work with ASP.NET Core apps due to the changes in directory structure.

Managed modules

Managed modules are not functional with hosted ASP.NET Core apps when the app pool’s .NET CLR version is set to No Managed Code. ASP.NET Core offers middleware alternatives in several cases.

IIS Manager application changes

When using IIS Manager to configure settings, the web.config file of the app is changed. If deploying an app and including web.config, any changes made with IIS Manager are overwritten by the deployed web.config file. If changes are made to the server’s web.config file, copy the updated web.config file on the server to the local project immediately.

Disabling IIS modules

If an IIS module is configured at the server level that must be disabled for an app, an addition to the app’s web.config file can disable the module. Either leave the module in place and deactivate it using a configuration setting (if available) or remove the module from the app.

Module deactivation

Many modules offer a configuration setting that allows them to be disabled without removing the module from the app. This is the simplest and quickest way to deactivate a module. For example, the HTTP Redirection Module can be disabled with the <httpRedirect> element in web.config:

     <httpRedirect enabled="false" />

For more information on disabling modules with configuration settings, follow the links in the Child Elements section of IIS <system.webServer>.

Module removal

If opting to remove a module with a setting in web.config, unlock the module and unlock the <modules> section of web.config first:

  1. Unlock the module at the server level. Select the IIS server in the IIS Manager Connections sidebar. Open the Modules in the IIS area. Select the module in the list. In the Actions sidebar on the right, select Unlock. Unlock as many modules as you plan to remove from web.config later.

  2. Deploy the app without a <modules> section in web.config. If an app is deployed with a web.config containing the <modules> section without having unlocked the section first in the IIS Manager, the Configuration Manager throws an exception when attempting to unlock the section. Therefore, deploy the app without a <modules> section.

  3. Unlock the <modules> section of web.config. In the Connections sidebar, select the website in Sites. In the Management area, open the Configuration Editor. Use the navigation controls to select the system.webServer/modules section. In the Actions sidebar on the right, select to Unlock the section.

  4. At this point, a <modules> section can be added to the web.config file with a <remove> element to remove the module from the app. Multiple <remove> elements can be added to remove multiple modules. If web.config changes are made on the server, immediately make the same changes to the project’s web.config file locally. Removing a module this way won’t affect the use of the module with other apps on the server.

         <remove name="MODULE_NAME" /> 

An IIS module can also be removed with Appcmd.exe. Provide the MODULE_NAME and APPLICATION_NAME in the command:

Appcmd.exe delete module MODULE_NAME /app.name:APPLICATION_NAME

For example, remove the DynamicCompressionModule from the Default Web Site:

%windir%system32inetsrvappcmd.exe delete module DynamicCompressionModule /app.name:"Default Web Site"

Minimum module configuration

The only modules required to run an ASP.NET Core app are the Anonymous Authentication Module and the ASP.NET Core Module.

IIS Manager open to Modules with the minimum module configuration shown

The URI Caching Module (UriCacheModule) allows IIS to cache website configuration at the URL level. Without this module, IIS must read and parse configuration on every request, even when the same URL is repeatedly requested. Parsing the configuration every request results in a significant performance penalty. Although the URI Caching Module isn’t strictly required for a hosted ASP.NET Core app to run, we recommend that the URI Caching Module be enabled for all ASP.NET Core deployments.

The HTTP Caching Module (HttpCacheModule) implements the IIS output cache and also the logic for caching items in the HTTP.sys cache. Without this module, content is no longer cached in kernel mode, and cache profiles are ignored. Removing the HTTP Caching Module usually has adverse effects on performance and resource usage. Although the HTTP Caching Module isn’t strictly required for a hosted ASP.NET Core app to run, we recommend that the HTTP Caching Module be enabled for all ASP.NET Core deployments.

Additional resources