ASP.NET Core Module configuration reference

By Luke Latham, Rick Anderson, Sourabh Shirhatti, and Justin Kotalik

This document provides instructions on how to configure the ASP.NET Core Module for hosting ASP.NET Core apps. For an introduction to the ASP.NET Core Module and installation instructions, see the ASP.NET Core Module overview.

Hosting model

For apps running on .NET Core 2.2 or later, the module supports an in-process hosting model for improved performance compared to reverse-proxy (out-of-process) hosting. For more information, see ASP.NET Core Module.

In-procsess hosting is opt-in for existing apps, but dotnet new templates default to the in-process hosting model for all IIS and IIS Express scenarios.

To configure an app for in-process hosting, add the <AspNetCoreHostingModel> property to the app's project file (for example, MyApp.csproj) with a value of inprocess (out-of-process hosting is set with outofprocess):

<PropertyGroup>
  <AspNetCoreHostingModel>inprocess</AspNetCoreHostingModel>
</PropertyGroup>

The following characteristics apply when hosting in-process:

  • The Kestrel server isn't used. A custom IServer implementation, IISHttpServer acts as the app's server.

  • The requestTimeout attribute doesn't apply to in-process hosting.

  • Sharing an app pool among apps isn't supported. Use one app pool per app.

  • When using Web Deploy or manually placing an app_offline.htm file in the deployment, the app might not shut down immediately if there's an open connection. For example, a websocket connection may delay app shut down.

  • The architecture (bitness) of the app and installed runtime (x64 or x86) must match the architecture of the app pool.

  • If setting up the app's host manually with WebHostBuilder (not using CreateDefaultBuilder) and the app is ever run directly on the Kestrel server (self-hosted), call UseKestrel before calling UseIISIntegration. If the order is reversed, the host fails to start.

  • Client disconnects are detected. The HttpContext.RequestAborted cancellation token is cancelled when the client disconnects.

  • Directory.GetCurrentDirectory() returns the worker directory of the process started by IIS rather than the application directory (for example, C:\Windows\System32\inetsrv for w3wp.exe).

Hosting model changes

If the hostingModel setting is changed in the web.config file (explained in the Configuration with web.config section), the module recycles the worker process for IIS.

For IIS Express, the module doesn't recycle the worker process but instead triggers a graceful shutdown of the current IIS Express process. The next request to the app spawns a new IIS Express process.

Process name

Process.GetCurrentProcess().ProcessName reports w3wp/iisexpress (in-process) or dotnet (out-of-process).

Configuration with web.config

The ASP.NET Core Module is configured with the aspNetCore section of the system.webServer node in the site's web.config file.

The following web.config file is published for a framework-dependent deployment and configures the ASP.NET Core Module to handle site requests:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <location path="." inheritInChildApplications="false">
    <system.webServer>
      <handlers>
        <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" />
      </handlers>
      <aspNetCore processPath="dotnet" 
                  arguments=".\MyApp.dll" 
                  stdoutLogEnabled="false" 
                  stdoutLogFile=".\logs\stdout" 
                  hostingModel="inprocess" />
    </system.webServer>
  </location>
</configuration>
<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
    </handlers>
    <aspNetCore processPath="dotnet" 
                arguments=".\MyApp.dll" 
                stdoutLogEnabled="false" 
                stdoutLogFile=".\logs\stdout" />
  </system.webServer>
</configuration>

The following web.config is published for a self-contained deployment:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <location path="." inheritInChildApplications="false">
    <system.webServer>
      <handlers>
        <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" />
      </handlers>
      <aspNetCore processPath=".\MyApp.exe" 
                  stdoutLogEnabled="false" 
                  stdoutLogFile=".\logs\stdout" 
                  hostingModel="inprocess" />
    </system.webServer>
  </location>
</configuration>

The InheritInChildApplications property is set to false to indicate that the settings specified within the <location> element aren't inherited by apps that reside in a subdirectory of the app.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
    </handlers>
    <aspNetCore processPath=".\MyApp.exe" 
                stdoutLogEnabled="false" 
                stdoutLogFile=".\logs\stdout" />
  </system.webServer>
</configuration>

When an app is deployed to Azure App Service, the stdoutLogFile path is set to \\?\%home%\LogFiles\stdout. The path saves stdout logs to the LogFiles folder, which is a location automatically created by the service.

See Sub-application configuration for an important note pertaining to the configuration of web.config files in sub-apps.

Attributes of the aspNetCore element

Attribute Description Default
arguments

Optional string attribute.

Arguments to the executable specified in processPath.

disableStartUpErrorPage

Optional Boolean attribute.

If true, the 502.5 - Process Failure page is suppressed, and the 502 status code page configured in the web.config takes precedence.

false
forwardWindowsAuthToken

Optional Boolean attribute.

If true, the token is forwarded to the child process listening on %ASPNETCORE_PORT% as a header 'MS-ASPNETCORE-WINAUTHTOKEN' per request. It's the responsibility of that process to call CloseHandle on this token per request.

true
hostingModel

Optional string attribute.

Specifies the hosting model as in-process (inprocess) or out-of-process (outofprocess).

outofprocess
processesPerApplication

Optional integer attribute.

Specifies the number of instances of the process specified in the processPath setting that can be spun up per app.

†For in-process hosting, the value is limited to 1.

Default: 1
Min: 1
Max: 100
processPath

Required string attribute.

Path to the executable that launches a process listening for HTTP requests. Relative paths are supported. If the path begins with ., the path is considered to be relative to the site root.

rapidFailsPerMinute

Optional integer attribute.

Specifies the number of times the process specified in processPath is allowed to crash per minute. If this limit is exceeded, the module stops launching the process for the remainder of the minute.

Not supported with in-process hosting.

Default: 10
Min: 0
Max: 100
requestTimeout

Optional timespan attribute.

Specifies the duration for which the ASP.NET Core Module waits for a response from the process listening on %ASPNETCORE_PORT%.

In versions of the ASP.NET Core Module that shipped with the release of ASP.NET Core 2.1 or later, the requestTimeout is specified in hours, minutes, and seconds.

Doesn't apply to in-process hosting. For in-process hosting, the module waits for the app to process the request.

Default: 00:02:00
Min: 00:00:00
Max: 360:00:00
shutdownTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to gracefully shutdown when the app_offline.htm file is detected.

Default: 10
Min: 0
Max: 600
startupTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to start a process listening on the port. If this time limit is exceeded, the module kills the process. The module attempts to relaunch the process when it receives a new request and continues to attempt to restart the process on subsequent incoming requests unless the app fails to start rapidFailsPerMinute number of times in the last rolling minute.

A value of 0 (zero) is not considered an infinite timeout.

Default: 120
Min: 0
Max: 3600
stdoutLogEnabled

Optional Boolean attribute.

If true, stdout and stderr for the process specified in processPath are redirected to the file specified in stdoutLogFile.

false
stdoutLogFile

Optional string attribute.

Specifies the relative or absolute file path for which stdout and stderr from the process specified in processPath are logged. Relative paths are relative to the root of the site. Any path starting with . are relative to the site root and all other paths are treated as absolute paths. Any folders provided in the path must exist in order for the module to create the log file. Using underscore delimiters, a timestamp, process ID, and file extension (.log) are added to the last segment of the stdoutLogFile path. If .\logs\stdout is supplied as a value, an example stdout log is saved as stdout_20180205194132_1934.log in the logs folder when saved on 2/5/2018 at 19:41:32 with a process ID of 1934.

aspnetcore-stdout
Attribute Description Default
arguments

Optional string attribute.

Arguments to the executable specified in processPath.

disableStartUpErrorPage

Optional Boolean attribute.

If true, the 502.5 - Process Failure page is suppressed, and the 502 status code page configured in the web.config takes precedence.

false
forwardWindowsAuthToken

Optional Boolean attribute.

If true, the token is forwarded to the child process listening on %ASPNETCORE_PORT% as a header 'MS-ASPNETCORE-WINAUTHTOKEN' per request. It's the responsibility of that process to call CloseHandle on this token per request.

true
processesPerApplication

Optional integer attribute.

Specifies the number of instances of the process specified in the processPath setting that can be spun up per app.

Default: 1
Min: 1
Max: 100
processPath

Required string attribute.

Path to the executable that launches a process listening for HTTP requests. Relative paths are supported. If the path begins with ., the path is considered to be relative to the site root.

rapidFailsPerMinute

Optional integer attribute.

Specifies the number of times the process specified in processPath is allowed to crash per minute. If this limit is exceeded, the module stops launching the process for the remainder of the minute.

Default: 10
Min: 0
Max: 100
requestTimeout

Optional timespan attribute.

Specifies the duration for which the ASP.NET Core Module waits for a response from the process listening on %ASPNETCORE_PORT%.

In versions of the ASP.NET Core Module that shipped with the release of ASP.NET Core 2.1 or later, the requestTimeout is specified in hours, minutes, and seconds.

Default: 00:02:00
Min: 00:00:00
Max: 360:00:00
shutdownTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to gracefully shutdown when the app_offline.htm file is detected.

Default: 10
Min: 0
Max: 600
startupTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to start a process listening on the port. If this time limit is exceeded, the module kills the process. The module attempts to relaunch the process when it receives a new request and continues to attempt to restart the process on subsequent incoming requests unless the app fails to start rapidFailsPerMinute number of times in the last rolling minute.

A value of 0 (zero) is not considered an infinite timeout.

Default: 120
Min: 0
Max: 3600
stdoutLogEnabled

Optional Boolean attribute.

If true, stdout and stderr for the process specified in processPath are redirected to the file specified in stdoutLogFile.

false
stdoutLogFile

Optional string attribute.

Specifies the relative or absolute file path for which stdout and stderr from the process specified in processPath are logged. Relative paths are relative to the root of the site. Any path starting with . are relative to the site root and all other paths are treated as absolute paths. Any folders provided in the path must exist in order for the module to create the log file. Using underscore delimiters, a timestamp, process ID, and file extension (.log) are added to the last segment of the stdoutLogFile path. If .\logs\stdout is supplied as a value, an example stdout log is saved as stdout_20180205194132_1934.log in the logs folder when saved on 2/5/2018 at 19:41:32 with a process ID of 1934.

aspnetcore-stdout
Attribute Description Default
arguments

Optional string attribute.

Arguments to the executable specified in processPath.

disableStartUpErrorPage

Optional Boolean attribute.

If true, the 502.5 - Process Failure page is suppressed, and the 502 status code page configured in the web.config takes precedence.

false
forwardWindowsAuthToken

Optional Boolean attribute.

If true, the token is forwarded to the child process listening on %ASPNETCORE_PORT% as a header 'MS-ASPNETCORE-WINAUTHTOKEN' per request. It's the responsibility of that process to call CloseHandle on this token per request.

true
processesPerApplication

Optional integer attribute.

Specifies the number of instances of the process specified in the processPath setting that can be spun up per app.

Default: 1
Min: 1
Max: 100
processPath

Required string attribute.

Path to the executable that launches a process listening for HTTP requests. Relative paths are supported. If the path begins with ., the path is considered to be relative to the site root.

rapidFailsPerMinute

Optional integer attribute.

Specifies the number of times the process specified in processPath is allowed to crash per minute. If this limit is exceeded, the module stops launching the process for the remainder of the minute.

Default: 10
Min: 0
Max: 100
requestTimeout

Optional timespan attribute.

Specifies the duration for which the ASP.NET Core Module waits for a response from the process listening on %ASPNETCORE_PORT%.

In versions of the ASP.NET Core Module that shipped with the release of ASP.NET Core 2.0 or earlier, the requestTimeout must be specified in whole minutes only, otherwise it defaults to 2 minutes.

Default: 00:02:00
Min: 00:00:00
Max: 360:00:00
shutdownTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to gracefully shutdown when the app_offline.htm file is detected.

Default: 10
Min: 0
Max: 600
startupTimeLimit

Optional integer attribute.

Duration in seconds that the module waits for the executable to start a process listening on the port. If this time limit is exceeded, the module kills the process. The module attempts to relaunch the process when it receives a new request and continues to attempt to restart the process on subsequent incoming requests unless the app fails to start rapidFailsPerMinute number of times in the last rolling minute.

Default: 120
Min: 0
Max: 3600
stdoutLogEnabled

Optional Boolean attribute.

If true, stdout and stderr for the process specified in processPath are redirected to the file specified in stdoutLogFile.

false
stdoutLogFile

Optional string attribute.

Specifies the relative or absolute file path for which stdout and stderr from the process specified in processPath are logged. Relative paths are relative to the root of the site. Any path starting with . are relative to the site root and all other paths are treated as absolute paths. Any folders provided in the path must exist in order for the module to create the log file. Using underscore delimiters, a timestamp, process ID, and file extension (.log) are added to the last segment of the stdoutLogFile path. If .\logs\stdout is supplied as a value, an example stdout log is saved as stdout_20180205194132_1934.log in the logs folder when saved on 2/5/2018 at 19:41:32 with a process ID of 1934.

aspnetcore-stdout

Setting environment variables

Environment variables can be specified for the process in the processPath attribute. Specify an environment variable with the environmentVariable child element of an environmentVariables collection element. Environment variables set in this section take precedence over system environment variables.

The following example sets two environment variables. ASPNETCORE_ENVIRONMENT configures the app's environment to Development. A developer may temporarily set this value in the web.config file in order to force the Developer Exception Page to load when debugging an app exception. CONFIG_DIR is an example of a user-defined environment variable, where the developer has written code that reads the value on startup to form a path for loading the app's configuration file.

<aspNetCore processPath="dotnet"
      arguments=".\MyApp.dll"
      stdoutLogEnabled="false"
      stdoutLogFile="\\?\%home%\LogFiles\stdout"
      hostingModel="inprocess">
  <environmentVariables>
    <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />
    <environmentVariable name="CONFIG_DIR" value="f:\application_config" />
  </environmentVariables>
</aspNetCore>
<aspNetCore processPath="dotnet"
      arguments=".\MyApp.dll"
      stdoutLogEnabled="false"
      stdoutLogFile="\\?\%home%\LogFiles\stdout">
  <environmentVariables>
    <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />
    <environmentVariable name="CONFIG_DIR" value="f:\application_config" />
  </environmentVariables>
</aspNetCore>

Warning

Only set the ASPNETCORE_ENVIRONMENT environment variable to Development on staging and testing servers that aren't accessible to untrusted networks, such as the Internet.

app_offline.htm

If a file with the name app_offline.htm is detected in the root directory of an app, the ASP.NET Core Module attempts to gracefully shutdown the app and stop processing incoming requests. If the app is still running after the number of seconds defined in shutdownTimeLimit, the ASP.NET Core Module kills the running process.

While the app_offline.htm file is present, the ASP.NET Core Module responds to requests by sending back the contents of the app_offline.htm file. When the app_offline.htm file is removed, the next request starts the app.

When using the out-of-process hosting model, the app might not shut down immediately if there's an open connection. For example, a websocket connection may delay app shut down.

Start-up error page

Both in-process and out-of-process hosting produce custom error pages when they fail to start the app.

If the ASP.NET Core Module fails to find either the in-process or out-of-process request handler, a 500.0 - In-Process/Out-Of-Process Handler Load Failure status code page appears.

For in-process hosting if the ASP.NET Core Module fails to start the app, a 500.30 - Start Failure status code page appears.

For out-of-process hosting if the ASP.NET Core Module fails to launch the backend process or the backend process starts but fails to listen on the configured port, a 502.5 - Process Failure status code page appears.

To suppress this page and revert to the default IIS 5xx status code page, use the disableStartUpErrorPage attribute. For more information on configuring custom error messages, see HTTP Errors <httpErrors>.

If the ASP.NET Core Module fails to launch the backend process or the backend process starts but fails to listen on the configured port, a 502.5 - Process Failure status code page appears. To suppress this page and revert to the default IIS 502 status code page, use the disableStartUpErrorPage attribute. For more information on configuring custom error messages, see HTTP Errors <httpErrors>.

502.5 Process Failure Status Code Page

Log creation and redirection

The ASP.NET Core Module redirects stdout and stderr console output to disk if the stdoutLogEnabled and stdoutLogFile attributes of the aspNetCore element are set. Any folders in the stdoutLogFile path must exist in order for the module to create the log file. The app pool must have write access to the location where the logs are written (use IIS AppPool\<app_pool_name> to provide write permission).

Logs aren't rotated, unless process recycling/restart occurs. It's the responsibility of the hoster to limit the disk space the logs consume.

Using the stdout log is only recommended for troubleshooting app startup issues. Don't use the stdout log for general app logging purposes. For routine logging in an ASP.NET Core app, use a logging library that limits log file size and rotates logs. For more information, see third-party logging providers.

A timestamp and file extension are added automatically when the log file is created. The log file name is composed by appending the timestamp, process ID, and file extension (.log) to the last segment of the stdoutLogFile path (typically stdout) delimited by underscores. If the stdoutLogFile path ends with stdout, a log for an app with a PID of 1934 created on 2/5/2018 at 19:42:32 has the file name stdout_20180205194132_1934.log.

If stdoutLogEnabled is false, errors that occur on app startup are captured and emitted to the event log up to 30 KB. After startup, all additional logs are discarded.

The following sample aspNetCore element configures stdout logging for an app hosted in Azure App Service. A local path or network share path is acceptable for local logging. Confirm that the AppPool user identity has permission to write to the path provided.

<aspNetCore processPath="dotnet"
    arguments=".\MyApp.dll"
    stdoutLogEnabled="true"
    stdoutLogFile="\\?\%home%\LogFiles\stdout"
    hostingModel="inprocess">
</aspNetCore>
<aspNetCore processPath="dotnet"
    arguments=".\MyApp.dll"
    stdoutLogEnabled="true"
    stdoutLogFile="\\?\%home%\LogFiles\stdout">
</aspNetCore>

Enhanced diagnostic logs

The ASP.NET Core Module provides is configurable to provide enhanced diagnostics logs. Add the <handlerSettings> element to the <aspNetCore> element in web.config. Setting the debugLevel to TRACE exposes a higher fidelity of diagnostic information:

<aspNetCore processPath="dotnet"
    arguments=".\MyApp.dll"
    stdoutLogEnabled="false"
    stdoutLogFile="\\?\%home%\LogFiles\stdout"
    hostingModel="inprocess">
  <handlerSettings>
    <handlerSetting name="debugFile" value="aspnetcore-debug.log" />
    <handlerSetting name="debugLevel" value="FILE,TRACE" />
  </handlerSettings>
</aspNetCore>

Debug level (debugLevel) values can include both the level and the location.

Levels (in order from least to most verbose):

  • ERROR
  • WARNING
  • INFO
  • TRACE

Locations (multiple locations are permitted):

  • CONSOLE
  • EVENTLOG
  • FILE

The handler settings can also be provided via environment variables:

  • ASPNETCORE_MODULE_DEBUG_FILE – Path to the debug log file. (Default: aspnetcore-debug.log)
  • ASPNETCORE_MODULE_DEBUG – Debug level setting.

Warning

Do not leave debug logging enabled in the deployment for longer than required to troubleshoot an issue. The size of the log isn't limited. Leaving the debug log enabled can exhaust the available disk space and crash the server or app service.

See Configuration with web.config for an example of the aspNetCore element in the web.config file.

Proxy configuration uses HTTP protocol and a pairing token

Only applies to out-of-process hosting.

The proxy created between the ASP.NET Core Module and Kestrel uses the HTTP protocol. Using HTTP is a performance optimization, where the traffic between the module and Kestrel takes place on a loopback address off of the network interface. There's no risk of eavesdropping the traffic between the module and Kestrel from a location off of the server.

A pairing token is used to guarantee that the requests received by Kestrel were proxied by IIS and didn't come from some other source. The pairing token is created and set into an environment variable (ASPNETCORE_TOKEN) by the module. The pairing token is also set into a header (MSAspNetCoreToken) on every proxied request. IIS Middleware checks each request it receives to confirm that the pairing token header value matches the environment variable value. If the token values are mismatched, the request is logged and rejected. The pairing token environment variable and the traffic between the module and Kestrel aren't accessible from a location off of the server. Without knowing the pairing token value, an attacker can't submit requests that bypass the check in the IIS Middleware.

ASP.NET Core Module with an IIS Shared Configuration

The ASP.NET Core Module installer runs with the privileges of the SYSTEM account. Because the local system account doesn't have modify permission for the share path used by the IIS Shared Configuration, the installer hits an access denied error when attempting to configure the module settings in applicationHost.config on the share. When using an IIS Shared Configuration, follow these steps:

  1. Disable the IIS Shared Configuration.
  2. Run the installer.
  3. Export the updated applicationHost.config file to the share.
  4. Re-enable the IIS Shared Configuration.

Module version and Hosting Bundle installer logs

To determine the version of the installed ASP.NET Core Module:

  1. On the hosting system, navigate to %windir%\System32\inetsrv.
  2. Locate the aspnetcore.dll file.
  3. Right-click the file and select Properties from the contextual menu.
  4. Select the Details tab. The File version and Product version represent the installed version of the module.

The Hosting Bundle installer logs for the module are found at C:\Users\%UserName%\AppData\Local\Temp. The file is named dd_DotNetCoreWinSvrHosting__<timestamp>_000_AspNetCoreModule_x64.log.

Module, schema, and configuration file locations

Module

IIS (x86/amd64):

  • %windir%\System32\inetsrv\aspnetcore.dll

  • %windir%\SysWOW64\inetsrv\aspnetcore.dll

  • %ProgramFiles%\IIS\Asp.Net Core Module\V2\aspnetcorev2.dll

  • %ProgramFiles(x86)%\IIS\Asp.Net Core Module\V2\aspnetcorev2.dll

IIS Express (x86/amd64):

  • %ProgramFiles%\IIS Express\aspnetcore.dll

  • %ProgramFiles(x86)%\IIS Express\aspnetcore.dll

  • %ProgramFiles%\IIS Express\Asp.Net Core Module\V2\aspnetcorev2.dll

  • %ProgramFiles(x86)%\IIS Express\Asp.Net Core Module\V2\aspnetcorev2.dll

Schema

IIS

  • %windir%\System32\inetsrv\config\schema\aspnetcore_schema.xml
  • %windir%\System32\inetsrv\config\schema\aspnetcore_schema_v2.xml

IIS Express

  • %ProgramFiles%\IIS Express\config\schema\aspnetcore_schema.xml
  • %ProgramFiles%\IIS Express\config\schema\aspnetcore_schema_v2.xml

Configuration

IIS

  • %windir%\System32\inetsrv\config\applicationHost.config

IIS Express

  • %ProgramFiles%\IIS Express\config\templates\PersonalWebServer\applicationHost.config

The files can be found by searching for aspnetcore in the applicationHost.config file.