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Use ASP.NET Core with HTTP/3 on IIS

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Note

This isn't the latest version of this article. For the current release, see the .NET 9 version of this article.

Warning

This version of ASP.NET Core is no longer supported. For more information, see the .NET and .NET Core Support Policy. For the current release, see the .NET 9 version of this article.

Important

This information relates to a pre-release product that may be substantially modified before it's commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

For the current release, see the .NET 9 version of this article.

By Chris Ross

HTTP/3 is fully supported with ASP.NET Core in the following IIS scenarios:

  • In-process
  • Out-of-Process. In Out-of-Process, IIS responds to the client using HTTP/3, but the reverse proxy connection to the Kestrel server uses HTTP/1.1.

For more information on the in-process and out-of-process hosting models, see ASP.NET Core Module (ANCM) for IIS.

The following requirements also need to be met:

For an in-process deployment when an HTTP/3 connection is established, HttpRequest.Protocol reports HTTP/3. For an out-of-process deployment when an HTTP/3 connection is established, HttpRequest.Protocol reports HTTP/1.1 because that is how IIS proxies the requests to Kestrel.

Alt-Svc

HTTP/3 is discovered as an upgrade from HTTP/1.1 or HTTP/2 via the alt-svc header. That means the first request will normally use HTTP/1.1 or HTTP/2 before switching to HTTP/3. IIS doesn't automatically add the alt-svc header, it must be added by the application. The following code is a middleware example that adds the alt-svc response header.

C# 
app.Use((context, next) =>
{
    context.Response.Headers.AltSvc = "h3=\":443\"";
    return next(context);
});

Place the preceding code early in the request pipeline.

IIS also supports sending an AltSvc HTTP/2 protocol message rather than a response header to notify the client that HTTP/3 is available. See the EnableAltSvc registry key. Note this requires netsh sslcert bindings that use host names rather than IP addresses.

HTTP/3 is supported with ASP.NET Core in the following IIS deployment scenarios:

  • In-process
  • Out-of-Process. In Out-of-Process, IIS responds to the client using HTTP/3, but the reverse proxy connection to the Kestrel server uses HTTP/1.1.

For more information on the in-process and out-of-process hosting models, see ASP.NET Core Module (ANCM) for IIS.

The following requirements also need to be met:

For an in-process deployment when an HTTP/3 connection is established, HttpRequest.Protocol reports HTTP/3. For an out-of-process deployment when an HTTP/3 connection is established, HttpRequest.Protocol reports HTTP/1.1 because that is how IIS proxies the requests to Kestrel.

Alt-Svc

HTTP/3 is discovered as an upgrade from HTTP/1.1 or HTTP/2 via the alt-svc header. That means the first request will normally use HTTP/1.1 or HTTP/2 before switching to HTTP/3. IIS doesn't automatically add the alt-svc header, it must be added by the application. The following code is a middleware example that adds the alt-svc response header.

C# 
app.Use((context, next) =>
{
    context.Response.Headers.AltSvc = "h3=\":443\"";
    return next(context);
});

Place the preceding code early in the request pipeline.

IIS also supports sending an AltSvc HTTP/2 protocol message rather than a response header to notify the client that HTTP/3 is available. See the EnableAltSvc registry key. Note this requires netsh sslcert bindings that use host names rather than IP addresses.