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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:
- Windows Server 2022 / Windows 11 or later
- An
httpsurl binding is used. - The EnableHttp3 registry key is set.
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.
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:
- Windows Server 2022 / Windows 11 or later
- An
httpsurl binding is used. - The EnableHttp3 registry key is set.
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.
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.
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- Last updated on 2025-04-04