Configure endpoints for the ASP.NET Core Kestrel web server

ASP.NET Core projects are configured to bind to a random HTTP port between 5000-5300 and a random HTTPS port between 7000-7300. This default configuration is specified in the generated Properties/launchSettings.json file and can be overridden. If no ports are specified, Kestrel binds to http://localhost:5000.

Specify URLs using the:

• ASPNETCORE_URLS environment variable.
• --urls command-line argument.
• urls host configuration key.
• UseUrls extension method.

The value provided using these approaches can be one or more HTTP and HTTPS endpoints (HTTPS if a default cert is available). Configure the value as a semicolon-separated list (for example, "Urls": "http://localhost:8000;http://localhost:8001").

For more information on these approaches, see Server URLs and Override configuration.

Apps and containers are often given only a port to listen on, like port 80, without additional constraints like host or path. HTTP_PORTS and HTTPS_PORTS are config keys that specify the listening ports for the Kestrel and HTTP.sys servers. These keys may be specified as environment variables defined with the DOTNET_ or ASPNETCORE_ prefixes, or specified directly through any other config input, such as appsettings.json. Each is a semicolon-delimited list of port values, as shown in the following example:

ASPNETCORE_HTTP_PORTS=80;8080
ASPNETCORE_HTTPS_PORTS=443;8081

The preceding example is shorthand for the following configuration, which specifies the scheme (HTTP or HTTPS) and any host or IP.

ASPNETCORE_URLS=http://*:80/;http://*:8080/;https://*:443/;https://*:8081/

The HTTP_PORTS and HTTPS_PORTS configuration keys are lower priority and are overridden by URLS or values provided directly in code. Certificates still need to be configured separately via server-specific mechanics for HTTPS.

A development certificate is created:

• When the .NET SDK is installed.
• The dev-certs tool is used to create a certificate.

The development certificate is available only for the user that generates the certificate. Some browsers require granting explicit permission to trust the local development certificate.

Project templates configure apps to run on HTTPS by default and include HTTPS redirection and HSTS support.

Call Listen or ListenUnixSocket methods on KestrelServerOptions to configure URL prefixes and ports for Kestrel.

UseUrls, the --urls command-line argument, urls host configuration key, and the ASPNETCORE_URLS environment variable also work but have the limitations noted later in this section (a default certificate must be available for HTTPS endpoint configuration).

KestrelServerOptions configuration:

ConfigureEndpointDefaults(Action<ListenOptions>)

Specifies a configuration Action to run for each specified endpoint. Calling ConfigureEndpointDefaults multiple times replaces prior Actions with the last Action specified:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureEndpointDefaults won't have the defaults applied.

Configure(IConfiguration)

Enables Kestrel to load endpoints from an IConfiguration. The configuration must be scoped to the configuration section for Kestrel. The Configure(IConfiguration, bool) overload can be used to enable reloading endpoints when the configuration source changes.

By default, Kestrel configuration is loaded from the Kestrel section and reloading changes is enabled:

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "Https": {
        "Url": "https://localhost:5001"
      }
    }
  }
}

If reloading configuration is enabled and a change is signaled then the following steps are taken:

• The new configuration is compared to the old one, any endpoint without configuration changes are not modified.
• Removed or modified endpoints are given 5 seconds to complete processing requests and shut down.
• New or modified endpoints are started.

Clients connecting to a modified endpoint may be disconnected or refused while the endpoint is restarted.

ConfigureHttpsDefaults(Action<HttpsConnectionAdapterOptions>)

Specifies a configuration Action to run for each HTTPS endpoint. Calling ConfigureHttpsDefaults multiple times replaces prior Actions with the last Action specified.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureHttpsDefaults won't have the defaults applied.

ListenOptions.UseHttps

Configure Kestrel to use HTTPS.

ListenOptions.UseHttps extensions:

• UseHttps: Configure Kestrel to use HTTPS with the default certificate. Throws an exception if no default certificate is configured.
• UseHttps(string fileName)
• UseHttps(string fileName, string password)
• UseHttps(string fileName, string password, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(StoreName storeName, string subject)
• UseHttps(StoreName storeName, string subject, bool allowInvalid)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(X509Certificate2 serverCertificate)
• UseHttps(X509Certificate2 serverCertificate, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(Action<HttpsConnectionAdapterOptions> configureOptions)

ListenOptions.UseHttps parameters:

• filename is the path and file name of a certificate file, relative to the directory that contains the app's content files.
• password is the password required to access the X.509 certificate data.
• configureOptions is an Action to configure the HttpsConnectionAdapterOptions. Returns the ListenOptions.
• storeName is the certificate store from which to load the certificate.
• subject is the subject name for the certificate.
• allowInvalid indicates if invalid certificates should be considered, such as self-signed certificates.
• location is the store location to load the certificate from.
• serverCertificate is the X.509 certificate.

In production, HTTPS must be explicitly configured. At a minimum, a default certificate must be provided.

Supported configurations described next:

• No configuration
• Replace the default certificate from configuration
• Change the defaults in code

No configuration

Kestrel listens on http://localhost:5000.

Replace the default certificate from configuration

A default HTTPS app settings configuration schema is available for Kestrel. Configure multiple endpoints, including the URLs and the certificates to use, either from a file on disk or from a certificate store.

In the following appsettings.json example:

• Set AllowInvalid to true to permit the use of invalid certificates (for example, self-signed certificates).
• Any HTTPS endpoint that doesn't specify a certificate (HttpsDefaultCert in the example that follows) falls back to the cert defined under Certificates:Default or the development certificate.

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "HttpsInlineCertFile": {
        "Url": "https://localhost:5001",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertAndKeyFile": {
        "Url": "https://localhost:5002",
        "Certificate": {
          "Path": "<path to .pem/.crt file>",
          "KeyPath": "<path to .key file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertStore": {
        "Url": "https://localhost:5003",
        "Certificate": {
          "Subject": "<subject; required>",
          "Store": "<certificate store; required>",
          "Location": "<location; defaults to CurrentUser>",
          "AllowInvalid": "<true or false; defaults to false>"
        }
      },
      "HttpsDefaultCert": {
        "Url": "https://localhost:5004"
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

Schema notes:

• Endpoints names are case-insensitive. For example, HTTPS and Https are equivalent.
• The Url parameter is required for each endpoint. The format for this parameter is the same as the top-level Urls configuration parameter except that it's limited to a single value.
• These endpoints replace those defined in the top-level Urls configuration rather than adding to them. Endpoints defined in code via Listen are cumulative with the endpoints defined in the configuration section.
• The Certificate section is optional. If the Certificate section isn't specified, the defaults defined in Certificates:Default are used. If no defaults are available, the development certificate is used. If there are no defaults and the development certificate isn't present, the server throws an exception and fails to start.
• The Certificate section supports multiple certificate sources.
• Any number of endpoints may be defined in Configuration as long as they don't cause port conflicts.

Certificate sources

Certificate nodes can be configured to load certificates from a number of sources:

• Path and Password to load .pfx files.
• Path, KeyPath and Password to load .pem/.crt and .key files.
• Subject and Store to load from the certificate store.

For example, the Certificates:Default certificate can be specified as:

"Default": {
  "Subject": "<subject; required>",
  "Store": "<cert store; required>",
  "Location": "<location; defaults to CurrentUser>",
  "AllowInvalid": "<true or false; defaults to false>"
}

ConfigurationLoader

Configure(IConfiguration) returns a KestrelConfigurationLoader with an Endpoint(String, Action<EndpointConfiguration>) method that can be used to supplement a configured endpoint's settings:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    var kestrelSection = context.Configuration.GetSection("Kestrel");

    serverOptions.Configure(kestrelSection)
        .Endpoint("HTTPS", listenOptions =>
        {
            // ...
        });
});

KestrelServerOptions.ConfigurationLoader can be directly accessed to continue iterating on the existing loader, such as the one provided by WebApplicationBuilder.WebHost.

• The configuration section for each endpoint is available on the options in the Endpoint method so that custom settings may be read.
• Multiple configurations may be loaded by calling Configure(IConfiguration) again with another section. Only the last configuration is used, unless Load is explicitly called on prior instances. The metapackage doesn't call Load so that its default configuration section may be replaced.
• KestrelConfigurationLoader mirrors the Listen family of APIs from KestrelServerOptions as Endpoint overloads, so code and config endpoints may be configured in the same place. These overloads don't use names and only consume default settings from configuration.

Change the defaults in code

ConfigureEndpointDefaults and ConfigureHttpsDefaults can be used to change default settings for ListenOptions and HttpsConnectionAdapterOptions, including overriding the default certificate specified in the prior scenario. ConfigureEndpointDefaults and ConfigureHttpsDefaults should be called before any endpoints are configured.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });

    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Configure endpoints using Server Name Indication

Server Name Indication (SNI) can be used to host multiple domains on the same IP address and port. For SNI to function, the client sends the host name for the secure session to the server during the TLS handshake so that the server can provide the correct certificate. The client uses the furnished certificate for encrypted communication with the server during the secure session that follows the TLS handshake.

SNI can be configured in two ways:

• Create an endpoint in code and select a certificate using the host name with the ServerCertificateSelector callback.
• Configure a mapping between host names and HTTPS options in Configuration. For example, JSON in the appsettings.json file.

SNI with ServerCertificateSelector

Kestrel supports SNI via the ServerCertificateSelector callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var subExampleCert = CertificateLoader.LoadFromStoreCert(
                "sub.example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var certs = new Dictionary<string, X509Certificate2>(
                StringComparer.OrdinalIgnoreCase)
            {
                ["localhost"] = localhostCert,
                ["example.com"] = exampleCert,
                ["sub.example.com"] = subExampleCert
            };

            httpsOptions.ServerCertificateSelector = (connectionContext, name) =>
            {
                if (name is not null && certs.TryGetValue(name, out var cert))
                {
                    return cert;
                }

                return exampleCert;
            };
        });
    });
});

SNI with ServerOptionsSelectionCallback

Kestrel supports additional dynamic TLS configuration via the ServerOptionsSelectionCallback callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate and TLS configuration. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps((stream, clientHelloInfo, state, cancellationToken) =>
            {
                if (string.Equals(clientHelloInfo.ServerName, "localhost",
                    StringComparison.OrdinalIgnoreCase))
                {
                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = localhostCert,
                            // Different TLS requirements for this host
                            ClientCertificateRequired = true
                        });
                }

                return new ValueTask<SslServerAuthenticationOptions>(
                    new SslServerAuthenticationOptions
                    {
                        ServerCertificate = exampleCert
                    });
            }, state: null!);
        });
    });
});

SNI with TlsHandshakeCallbackOptions

Kestrel supports additional dynamic TLS configuration via the TlsHandshakeCallbackOptions.OnConnection callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate, TLS configuration, and other server options. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps(new TlsHandshakeCallbackOptions
            {
                OnConnection = context =>
                {
                    if (string.Equals(context.ClientHelloInfo.ServerName, "localhost",
                        StringComparison.OrdinalIgnoreCase))
                    {
                        // Different TLS requirements for this host
                        context.AllowDelayedClientCertificateNegotation = true;

                        return new ValueTask<SslServerAuthenticationOptions>(
                            new SslServerAuthenticationOptions
                            {
                                ServerCertificate = localhostCert
                            });
                    }

                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = exampleCert
                        });
                }
            });
        });
    });
});

SNI in configuration

Kestrel supports SNI defined in configuration. An endpoint can be configured with an Sni object that contains a mapping between host names and HTTPS options. The connection host name is matched to the options and they are used for that connection.

The following configuration adds an endpoint named MySniEndpoint that uses SNI to select HTTPS options based on the host name:

{
  "Kestrel": {
    "Endpoints": {
      "MySniEndpoint": {
        "Url": "https://*",
        "SslProtocols": ["Tls11", "Tls12"],
        "Sni": {
          "a.example.org": {
            "Protocols": "Http1AndHttp2",
            "SslProtocols": ["Tls11", "Tls12", "Tls13"],
            "Certificate": {
              "Subject": "<subject; required>",
              "Store": "<certificate store; required>",
            },
            "ClientCertificateMode" : "NoCertificate"
          },
          "*.example.org": {
            "Certificate": {
              "Path": "<path to .pfx file>",
              "Password": "$CREDENTIAL_PLACEHOLDER$"
            }
          },
          "*": {
            // At least one subproperty needs to exist per SNI section or it
            // cannot be discovered via IConfiguration
            "Protocols": "Http1",
          }
        }
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

HTTPS options that can be overridden by SNI:

• Certificate configures the certificate source.
• Protocols configures the allowed HTTP protocols.
• SslProtocols configures the allowed SSL protocols.
• ClientCertificateMode configures the client certificate requirements.

The host name supports wildcard matching:

• Exact match. For example, a.example.org matches a.example.org.
• Wildcard prefix. If there are multiple wildcard matches then the longest pattern is chosen. For example, *.example.org matches b.example.org and c.example.org.
• Full wildcard. * matches everything else, including clients that aren't using SNI and don't send a host name.

The matched SNI configuration is applied to the endpoint for the connection, overriding values on the endpoint. If a connection doesn't match a configured SNI host name then the connection is refused.

SNI requirements

All websites must run on the same Kestrel instance. Kestrel doesn't support sharing an IP address and port across multiple instances without a reverse proxy.

SSL/TLS Protocols

SSL Protocols are protocols used for encrypting and decrypting traffic between two peers, traditionally a client and a server.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.SslProtocols = SslProtocols.Tls13;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "SslProtocols": ["Tls12", "Tls13"],
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

The default value, SslProtocols.None, causes Kestrel to use the operating system defaults to choose the best protocol. Unless you have a specific reason to select a protocol, use the default.

Client Certificates

ClientCertificateMode configures the client certificate requirements.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.ClientCertificateMode = ClientCertificateMode.AllowCertificate;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "ClientCertificateMode": "AllowCertificate",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider.

The default value is ClientCertificateMode.NoCertificate where Kestrel will not request or require a certificate from the client.

For more information, see Configure certificate authentication in ASP.NET Core.

Connection logging

Call UseConnectionLogging to emit Debug level logs for byte-level communication on a connection. Connection logging is helpful for troubleshooting problems in low-level communication, such as during TLS encryption and behind proxies. If UseConnectionLogging is placed before UseHttps, encrypted traffic is logged. If UseConnectionLogging is placed after UseHttps, decrypted traffic is logged. This is built-in Connection Middleware.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseConnectionLogging();
    });
});

Bind to a TCP socket

The Listen method binds to a TCP socket, and an options lambda permits X.509 certificate configuration:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Loopback, 5000);
    serverOptions.Listen(IPAddress.Loopback, 5001, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

The example configures HTTPS for an endpoint with ListenOptions. Use the same API to configure other Kestrel settings for specific endpoints.

On Windows, self-signed certificates can be created using the New-SelfSignedCertificate PowerShell cmdlet. For an unsupported example, see UpdateIISExpressSSLForChrome.ps1.

On macOS, Linux, and Windows, certificates can be created using OpenSSL.

Bind to a Unix socket

Listen on a Unix socket with ListenUnixSocket for improved performance with Nginx, as shown in this example:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock");
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock", listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testpassword");
    });
});

• In the Nginx configuration file, set the server > location > proxy_pass entry to http://unix:/tmp/{KESTREL SOCKET}:/;. {KESTREL SOCKET} is the name of the socket provided to ListenUnixSocket (for example, kestrel-test.sock in the preceding example).
• Ensure that the socket is writeable by Nginx (for example, chmod go+w /tmp/kestrel-test.sock).

Port 0

When the port number 0 is specified, Kestrel dynamically binds to an available port. The following example shows how to determine which port Kestrel bound at runtime:

app.Run(async (context) =>
{
    var serverAddressFeature = context.Features.Get<IServerAddressesFeature>();

    if (serverAddressFeature is not null)
    {
        var listenAddresses = string.Join(", ", serverAddressFeature.Addresses);

        // ...
    }
});

Dynamically binding a port isn't available in some situations:

• ListenLocalhost
• Binding TCP-based HTTP/1.1 or HTTP/2, and QUIC-based HTTP/3 together.

Limitations

Configure endpoints with the following approaches:

• UseUrls
• --urls command-line argument
• urls host configuration key
• ASPNETCORE_URLS environment variable

These methods are useful for making code work with servers other than Kestrel. However, be aware of the following limitations:

• HTTPS can't be used with these approaches unless a default certificate is provided in the HTTPS endpoint configuration (for example, using KestrelServerOptions configuration or a configuration file as shown earlier in this article).
• When both the Listen and UseUrls approaches are used simultaneously, the Listen endpoints override the UseUrls endpoints.

IIS endpoint configuration

When using IIS, the URL bindings for IIS override bindings are set by either Listen or UseUrls. For more information, see ASP.NET Core Module.

ListenOptions.Protocols

The Protocols property establishes the HTTP protocols (HttpProtocols) enabled on a connection endpoint or for the server. Assign a value to the Protocols property from the HttpProtocols enum.

HttpProtocols enum value	Connection protocol permitted
Http1	HTTP/1.1 only. Can be used with or without TLS.
Http2	HTTP/2 only. May be used without TLS only if the client supports a Prior Knowledge mode.
Http3	HTTP/3 only. Requires TLS. The client may need to be configured to use HTTP/3 only.
Http1AndHttp2	HTTP/1.1 and HTTP/2. HTTP/2 requires the client to select HTTP/2 in the TLS Application-Layer Protocol Negotiation (ALPN) handshake; otherwise, the connection defaults to HTTP/1.1.
Http1AndHttp2AndHttp3	HTTP/1.1, HTTP/2 and HTTP/3. The first client request normally uses HTTP/1.1 or HTTP/2, and the alt-svc response header prompts the client to upgrade to HTTP/3. HTTP/2 and HTTP/3 requires TLS; otherwise, the connection defaults to HTTP/1.1.

The default ListenOptions.Protocols value for any endpoint is HttpProtocols.Http1AndHttp2.

TLS restrictions for HTTP/2:

• TLS version 1.2 or later
• Renegotiation disabled
• Compression disabled
• Minimum ephemeral key exchange sizes:
  • Elliptic curve Diffie-Hellman (ECDHE) [RFC4492]: 224 bits minimum
  • Finite field Diffie-Hellman (DHE) [TLS12]: 2048 bits minimum
• Cipher suite not prohibited.

TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186] is supported by default.

The following example permits HTTP/1.1 and HTTP/2 connections on port 8000. Connections are secured by TLS with a supplied certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

On Linux, CipherSuitesPolicy can be used to filter TLS handshakes on a per-connection basis:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.OnAuthenticate = (context, sslOptions) =>
        {
            sslOptions.CipherSuitesPolicy = new CipherSuitesPolicy(
                new[]
                {
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
                    // ...
                });
        };
    });
});

Connection Middleware

Custom connection middleware can filter TLS handshakes on a per-connection basis for specific ciphers if necessary.

The following example throws NotSupportedException for any cipher algorithm that the app doesn't support. Alternatively, define and compare ITlsHandshakeFeature.CipherAlgorithm to a list of acceptable cipher suites.

No encryption is used with a CipherAlgorithmType.Null cipher algorithm.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");

        listenOptions.Use((context, next) =>
        {
            var tlsFeature = context.Features.Get<ITlsHandshakeFeature>()!;

            if (tlsFeature.CipherAlgorithm == CipherAlgorithmType.Null)
            {
                throw new NotSupportedException(
                    $"Prohibited cipher: {tlsFeature.CipherAlgorithm}");
            }

            return next();
        });
    });
});

Set the HTTP protocol from configuration

By default, Kestrel configuration is loaded from the Kestrel section. The following appsettings.json example establishes HTTP/1.1 as the default connection protocol for all endpoints:

{
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

The following appsettings.json example establishes the HTTP/1.1 connection protocol for a specific endpoint:

{
  "Kestrel": {
    "Endpoints": {
      "HttpsDefaultCert": {
        "Url": "https://localhost:5001",
        "Protocols": "Http1"
      }
    }
  }
}

Protocols specified in code override values set by configuration.

URL prefixes

When using UseUrls, --urls command-line argument, urls host configuration key, or ASPNETCORE_URLS environment variable, the URL prefixes can be in any of the following formats.

Only HTTP URL prefixes are valid. Kestrel doesn't support HTTPS when configuring URL bindings using UseUrls.

• IPv4 address with port number

  http://65.55.39.10:80/
  

  0.0.0.0 is a special case that binds to all IPv4 addresses.

• IPv6 address with port number

  http://[0:0:0:0:0:ffff:4137:270a]:80/
  

  [::] is the IPv6 equivalent of IPv4 0.0.0.0.

• Host name with port number

  http://contoso.com:80/
  http://*:80/
  

  Host names, *, and +, aren't special. Anything not recognized as a valid IP address or localhost binds to all IPv4 and IPv6 IPs. To bind different host names to different ASP.NET Core apps on the same port, use HTTP.sys or a reverse proxy server. Reverse proxy server examples include IIS, Nginx, or Apache.

  Warning

  Hosting in a reverse proxy configuration requires host filtering.

• Host localhost name with port number or loopback IP with port number

  http://localhost:5000/
  http://127.0.0.1:5000/
  http://[::1]:5000/
  

  When localhost is specified, Kestrel attempts to bind to both IPv4 and IPv6 loopback interfaces. If the requested port is in use by another service on either loopback interface, Kestrel fails to start. If either loopback interface is unavailable for any other reason (most commonly because IPv6 isn't supported), Kestrel logs a warning.

ASP.NET Core projects are configured to bind to a random HTTP port between 5000-5300 and a random HTTPS port between 7000-7300. This default configuration is specified in the generated Properties/launchSettings.json file and can be overridden. If no ports are specified, Kestrel binds to http://localhost:5000.

Specify URLs using the:

• ASPNETCORE_URLS environment variable.
• --urls command-line argument.
• urls host configuration key.
• UseUrls extension method.

The value provided using these approaches can be one or more HTTP and HTTPS endpoints (HTTPS if a default cert is available). Configure the value as a semicolon-separated list (for example, "Urls": "http://localhost:8000;http://localhost:8001").

For more information on these approaches, see Server URLs and Override configuration.

A development certificate is created:

• When the .NET SDK is installed.
• The dev-certs tool is used to create a certificate.

The development certificate is available only for the user that generates the certificate. Some browsers require granting explicit permission to trust the local development certificate.

Project templates configure apps to run on HTTPS by default and include HTTPS redirection and HSTS support.

Call Listen or ListenUnixSocket methods on KestrelServerOptions to configure URL prefixes and ports for Kestrel.

UseUrls, the --urls command-line argument, urls host configuration key, and the ASPNETCORE_URLS environment variable also work but have the limitations noted later in this section (a default certificate must be available for HTTPS endpoint configuration).

KestrelServerOptions configuration:

ConfigureEndpointDefaults(Action<ListenOptions>)

Specifies a configuration Action to run for each specified endpoint. Calling ConfigureEndpointDefaults multiple times replaces prior Actions with the last Action specified:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureEndpointDefaults won't have the defaults applied.

Configure(IConfiguration)

Enables Kestrel to load endpoints from an IConfiguration. The configuration must be scoped to the configuration section for Kestrel. The Configure(IConfiguration, bool) overload can be used to enable reloading endpoints when the configuration source changes.

By default, Kestrel configuration is loaded from the Kestrel section and reloading changes is enabled:

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "Https": {
        "Url": "https://localhost:5001"
      }
    }
  }
}

If reloading configuration is enabled and a change is signaled then the following steps are taken:

• The new configuration is compared to the old one, any endpoint without configuration changes are not modified.
• Removed or modified endpoints are given 5 seconds to complete processing requests and shut down.
• New or modified endpoints are started.

Clients connecting to a modified endpoint may be disconnected or refused while the endpoint is restarted.

ConfigureHttpsDefaults(Action<HttpsConnectionAdapterOptions>)

Specifies a configuration Action to run for each HTTPS endpoint. Calling ConfigureHttpsDefaults multiple times replaces prior Actions with the last Action specified.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureHttpsDefaults won't have the defaults applied.

ListenOptions.UseHttps

Configure Kestrel to use HTTPS.

ListenOptions.UseHttps extensions:

• UseHttps: Configure Kestrel to use HTTPS with the default certificate. Throws an exception if no default certificate is configured.
• UseHttps(string fileName)
• UseHttps(string fileName, string password)
• UseHttps(string fileName, string password, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(StoreName storeName, string subject)
• UseHttps(StoreName storeName, string subject, bool allowInvalid)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(X509Certificate2 serverCertificate)
• UseHttps(X509Certificate2 serverCertificate, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(Action<HttpsConnectionAdapterOptions> configureOptions)

ListenOptions.UseHttps parameters:

• filename is the path and file name of a certificate file, relative to the directory that contains the app's content files.
• password is the password required to access the X.509 certificate data.
• configureOptions is an Action to configure the HttpsConnectionAdapterOptions. Returns the ListenOptions.
• storeName is the certificate store from which to load the certificate.
• subject is the subject name for the certificate.
• allowInvalid indicates if invalid certificates should be considered, such as self-signed certificates.
• location is the store location to load the certificate from.
• serverCertificate is the X.509 certificate.

In production, HTTPS must be explicitly configured. At a minimum, a default certificate must be provided.

Supported configurations described next:

• No configuration
• Replace the default certificate from configuration
• Change the defaults in code

No configuration

Kestrel listens on http://localhost:5000.

Replace the default certificate from configuration

A default HTTPS app settings configuration schema is available for Kestrel. Configure multiple endpoints, including the URLs and the certificates to use, either from a file on disk or from a certificate store.

In the following appsettings.json example:

• Set AllowInvalid to true to permit the use of invalid certificates (for example, self-signed certificates).
• Any HTTPS endpoint that doesn't specify a certificate (HttpsDefaultCert in the example that follows) falls back to the cert defined under Certificates:Default or the development certificate.

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "HttpsInlineCertFile": {
        "Url": "https://localhost:5001",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertAndKeyFile": {
        "Url": "https://localhost:5002",
        "Certificate": {
          "Path": "<path to .pem/.crt file>",
          "KeyPath": "<path to .key file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertStore": {
        "Url": "https://localhost:5003",
        "Certificate": {
          "Subject": "<subject; required>",
          "Store": "<certificate store; required>",
          "Location": "<location; defaults to CurrentUser>",
          "AllowInvalid": "<true or false; defaults to false>"
        }
      },
      "HttpsDefaultCert": {
        "Url": "https://localhost:5004"
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

Schema notes:

• Endpoints names are case-insensitive. For example, HTTPS and Https are equivalent.
• The Url parameter is required for each endpoint. The format for this parameter is the same as the top-level Urls configuration parameter except that it's limited to a single value.
• These endpoints replace those defined in the top-level Urls configuration rather than adding to them. Endpoints defined in code via Listen are cumulative with the endpoints defined in the configuration section.
• The Certificate section is optional. If the Certificate section isn't specified, the defaults defined in Certificates:Default are used. If no defaults are available, the development certificate is used. If there are no defaults and the development certificate isn't present, the server throws an exception and fails to start.
• The Certificate section supports multiple certificate sources.
• Any number of endpoints may be defined in Configuration as long as they don't cause port conflicts.

Certificate sources

Certificate nodes can be configured to load certificates from a number of sources:

• Path and Password to load .pfx files.
• Path, KeyPath and Password to load .pem/.crt and .key files.
• Subject and Store to load from the certificate store.

For example, the Certificates:Default certificate can be specified as:

"Default": {
  "Subject": "<subject; required>",
  "Store": "<cert store; required>",
  "Location": "<location; defaults to CurrentUser>",
  "AllowInvalid": "<true or false; defaults to false>"
}

ConfigurationLoader

Configure(IConfiguration) returns a KestrelConfigurationLoader with an Endpoint(String, Action<EndpointConfiguration>) method that can be used to supplement a configured endpoint's settings:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    var kestrelSection = context.Configuration.GetSection("Kestrel");

    serverOptions.Configure(kestrelSection)
        .Endpoint("HTTPS", listenOptions =>
        {
            // ...
        });
});

KestrelServerOptions.ConfigurationLoader can be directly accessed to continue iterating on the existing loader, such as the one provided by WebApplicationBuilder.WebHost.

• The configuration section for each endpoint is available on the options in the Endpoint method so that custom settings may be read.
• Multiple configurations may be loaded by calling Configure(IConfiguration) again with another section. Only the last configuration is used, unless Load is explicitly called on prior instances. The metapackage doesn't call Load so that its default configuration section may be replaced.
• KestrelConfigurationLoader mirrors the Listen family of APIs from KestrelServerOptions as Endpoint overloads, so code and config endpoints may be configured in the same place. These overloads don't use names and only consume default settings from configuration.

Change the defaults in code

ConfigureEndpointDefaults and ConfigureHttpsDefaults can be used to change default settings for ListenOptions and HttpsConnectionAdapterOptions, including overriding the default certificate specified in the prior scenario. ConfigureEndpointDefaults and ConfigureHttpsDefaults should be called before any endpoints are configured.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });

    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Configure endpoints using Server Name Indication

Server Name Indication (SNI) can be used to host multiple domains on the same IP address and port. For SNI to function, the client sends the host name for the secure session to the server during the TLS handshake so that the server can provide the correct certificate. The client uses the furnished certificate for encrypted communication with the server during the secure session that follows the TLS handshake.

SNI can be configured in two ways:

• Create an endpoint in code and select a certificate using the host name with the ServerCertificateSelector callback.
• Configure a mapping between host names and HTTPS options in Configuration. For example, JSON in the appsettings.json file.

SNI with ServerCertificateSelector

Kestrel supports SNI via the ServerCertificateSelector callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var subExampleCert = CertificateLoader.LoadFromStoreCert(
                "sub.example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var certs = new Dictionary<string, X509Certificate2>(
                StringComparer.OrdinalIgnoreCase)
            {
                ["localhost"] = localhostCert,
                ["example.com"] = exampleCert,
                ["sub.example.com"] = subExampleCert
            };

            httpsOptions.ServerCertificateSelector = (connectionContext, name) =>
            {
                if (name is not null && certs.TryGetValue(name, out var cert))
                {
                    return cert;
                }

                return exampleCert;
            };
        });
    });
});

SNI with ServerOptionsSelectionCallback

Kestrel supports additional dynamic TLS configuration via the ServerOptionsSelectionCallback callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate and TLS configuration. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps((stream, clientHelloInfo, state, cancellationToken) =>
            {
                if (string.Equals(clientHelloInfo.ServerName, "localhost",
                    StringComparison.OrdinalIgnoreCase))
                {
                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = localhostCert,
                            // Different TLS requirements for this host
                            ClientCertificateRequired = true
                        });
                }

                return new ValueTask<SslServerAuthenticationOptions>(
                    new SslServerAuthenticationOptions
                    {
                        ServerCertificate = exampleCert
                    });
            }, state: null!);
        });
    });
});

SNI with TlsHandshakeCallbackOptions

Kestrel supports additional dynamic TLS configuration via the TlsHandshakeCallbackOptions.OnConnection callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate, TLS configuration, and other server options. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps(new TlsHandshakeCallbackOptions
            {
                OnConnection = context =>
                {
                    if (string.Equals(context.ClientHelloInfo.ServerName, "localhost",
                        StringComparison.OrdinalIgnoreCase))
                    {
                        // Different TLS requirements for this host
                        context.AllowDelayedClientCertificateNegotation = true;

                        return new ValueTask<SslServerAuthenticationOptions>(
                            new SslServerAuthenticationOptions
                            {
                                ServerCertificate = localhostCert
                            });
                    }

                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = exampleCert
                        });
                }
            });
        });
    });
});

SNI in configuration

Kestrel supports SNI defined in configuration. An endpoint can be configured with an Sni object that contains a mapping between host names and HTTPS options. The connection host name is matched to the options and they are used for that connection.

The following configuration adds an endpoint named MySniEndpoint that uses SNI to select HTTPS options based on the host name:

{
  "Kestrel": {
    "Endpoints": {
      "MySniEndpoint": {
        "Url": "https://*",
        "SslProtocols": ["Tls11", "Tls12"],
        "Sni": {
          "a.example.org": {
            "Protocols": "Http1AndHttp2",
            "SslProtocols": ["Tls11", "Tls12", "Tls13"],
            "Certificate": {
              "Subject": "<subject; required>",
              "Store": "<certificate store; required>",
            },
            "ClientCertificateMode" : "NoCertificate"
          },
          "*.example.org": {
            "Certificate": {
              "Path": "<path to .pfx file>",
              "Password": "$CREDENTIAL_PLACEHOLDER$"
            }
          },
          "*": {
            // At least one subproperty needs to exist per SNI section or it
            // cannot be discovered via IConfiguration
            "Protocols": "Http1",
          }
        }
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

HTTPS options that can be overridden by SNI:

• Certificate configures the certificate source.
• Protocols configures the allowed HTTP protocols.
• SslProtocols configures the allowed SSL protocols.
• ClientCertificateMode configures the client certificate requirements.

The host name supports wildcard matching:

• Exact match. For example, a.example.org matches a.example.org.
• Wildcard prefix. If there are multiple wildcard matches then the longest pattern is chosen. For example, *.example.org matches b.example.org and c.example.org.
• Full wildcard. * matches everything else, including clients that aren't using SNI and don't send a host name.

The matched SNI configuration is applied to the endpoint for the connection, overriding values on the endpoint. If a connection doesn't match a configured SNI host name then the connection is refused.

SNI requirements

All websites must run on the same Kestrel instance. Kestrel doesn't support sharing an IP address and port across multiple instances without a reverse proxy.

SSL/TLS Protocols

SSL Protocols are protocols used for encrypting and decrypting traffic between two peers, traditionally a client and a server.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.SslProtocols = SslProtocols.Tls13;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "SslProtocols": ["Tls12", "Tls13"],
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

The default value, SslProtocols.None, causes Kestrel to use the operating system defaults to choose the best protocol. Unless you have a specific reason to select a protocol, use the default.

Client Certificates

ClientCertificateMode configures the client certificate requirements.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.ClientCertificateMode = ClientCertificateMode.AllowCertificate;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "ClientCertificateMode": "AllowCertificate",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider.

The default value is ClientCertificateMode.NoCertificate where Kestrel will not request or require a certificate from the client.

For more information, see Configure certificate authentication in ASP.NET Core.

Connection logging

Call UseConnectionLogging to emit Debug level logs for byte-level communication on a connection. Connection logging is helpful for troubleshooting problems in low-level communication, such as during TLS encryption and behind proxies. If UseConnectionLogging is placed before UseHttps, encrypted traffic is logged. If UseConnectionLogging is placed after UseHttps, decrypted traffic is logged. This is built-in Connection Middleware.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseConnectionLogging();
    });
});

Bind to a TCP socket

The Listen method binds to a TCP socket, and an options lambda permits X.509 certificate configuration:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Loopback, 5000);
    serverOptions.Listen(IPAddress.Loopback, 5001, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

The example configures HTTPS for an endpoint with ListenOptions. Use the same API to configure other Kestrel settings for specific endpoints.

On Windows, self-signed certificates can be created using the New-SelfSignedCertificate PowerShell cmdlet. For an unsupported example, see UpdateIISExpressSSLForChrome.ps1.

On macOS, Linux, and Windows, certificates can be created using OpenSSL.

Bind to a Unix socket

Listen on a Unix socket with ListenUnixSocket for improved performance with Nginx, as shown in this example:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock");
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock", listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testpassword");
    });
});

• In the Nginx configuration file, set the server > location > proxy_pass entry to http://unix:/tmp/{KESTREL SOCKET}:/;. {KESTREL SOCKET} is the name of the socket provided to ListenUnixSocket (for example, kestrel-test.sock in the preceding example).
• Ensure that the socket is writeable by Nginx (for example, chmod go+w /tmp/kestrel-test.sock).

Port 0

When the port number 0 is specified, Kestrel dynamically binds to an available port. The following example shows how to determine which port Kestrel bound at runtime:

app.Run(async (context) =>
{
    var serverAddressFeature = context.Features.Get<IServerAddressesFeature>();

    if (serverAddressFeature is not null)
    {
        var listenAddresses = string.Join(", ", serverAddressFeature.Addresses);

        // ...
    }
});

Dynamically binding a port isn't available in some situations:

• ListenLocalhost
• Binding TCP-based HTTP/1.1 or HTTP/2, and QUIC-based HTTP/3 together.

Limitations

Configure endpoints with the following approaches:

• UseUrls
• --urls command-line argument
• urls host configuration key
• ASPNETCORE_URLS environment variable

These methods are useful for making code work with servers other than Kestrel. However, be aware of the following limitations:

• HTTPS can't be used with these approaches unless a default certificate is provided in the HTTPS endpoint configuration (for example, using KestrelServerOptions configuration or a configuration file as shown earlier in this article).
• When both the Listen and UseUrls approaches are used simultaneously, the Listen endpoints override the UseUrls endpoints.

IIS endpoint configuration

When using IIS, the URL bindings for IIS override bindings are set by either Listen or UseUrls. For more information, see ASP.NET Core Module.

ListenOptions.Protocols

The Protocols property establishes the HTTP protocols (HttpProtocols) enabled on a connection endpoint or for the server. Assign a value to the Protocols property from the HttpProtocols enum.

HttpProtocols enum value	Connection protocol permitted
Http1	HTTP/1.1 only. Can be used with or without TLS.
Http2	HTTP/2 only. May be used without TLS only if the client supports a Prior Knowledge mode.
Http3	HTTP/3 only. Requires TLS. The client may need to be configured to use HTTP/3 only.
Http1AndHttp2	HTTP/1.1 and HTTP/2. HTTP/2 requires the client to select HTTP/2 in the TLS Application-Layer Protocol Negotiation (ALPN) handshake; otherwise, the connection defaults to HTTP/1.1.
Http1AndHttp2AndHttp3	HTTP/1.1, HTTP/2 and HTTP/3. The first client request normally uses HTTP/1.1 or HTTP/2, and the alt-svc response header prompts the client to upgrade to HTTP/3. HTTP/2 and HTTP/3 requires TLS; otherwise, the connection defaults to HTTP/1.1.

The default ListenOptions.Protocols value for any endpoint is HttpProtocols.Http1AndHttp2.

TLS restrictions for HTTP/2:

• TLS version 1.2 or later
• Renegotiation disabled
• Compression disabled
• Minimum ephemeral key exchange sizes:
  • Elliptic curve Diffie-Hellman (ECDHE) [RFC4492]: 224 bits minimum
  • Finite field Diffie-Hellman (DHE) [TLS12]: 2048 bits minimum
• Cipher suite not prohibited.

TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186] is supported by default.

The following example permits HTTP/1.1 and HTTP/2 connections on port 8000. Connections are secured by TLS with a supplied certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

On Linux, CipherSuitesPolicy can be used to filter TLS handshakes on a per-connection basis:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.OnAuthenticate = (context, sslOptions) =>
        {
            sslOptions.CipherSuitesPolicy = new CipherSuitesPolicy(
                new[]
                {
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
                    // ...
                });
        };
    });
});

Connection Middleware

Custom connection middleware can filter TLS handshakes on a per-connection basis for specific ciphers if necessary.

The following example throws NotSupportedException for any cipher algorithm that the app doesn't support. Alternatively, define and compare ITlsHandshakeFeature.CipherAlgorithm to a list of acceptable cipher suites.

No encryption is used with a CipherAlgorithmType.Null cipher algorithm.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");

        listenOptions.Use((context, next) =>
        {
            var tlsFeature = context.Features.Get<ITlsHandshakeFeature>()!;

            if (tlsFeature.CipherAlgorithm == CipherAlgorithmType.Null)
            {
                throw new NotSupportedException(
                    $"Prohibited cipher: {tlsFeature.CipherAlgorithm}");
            }

            return next();
        });
    });
});

Set the HTTP protocol from configuration

By default, Kestrel configuration is loaded from the Kestrel section. The following appsettings.json example establishes HTTP/1.1 as the default connection protocol for all endpoints:

{
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

The following appsettings.json example establishes the HTTP/1.1 connection protocol for a specific endpoint:

{
  "Kestrel": {
    "Endpoints": {
      "HttpsDefaultCert": {
        "Url": "https://localhost:5001",
        "Protocols": "Http1"
      }
    }
  }
}

Protocols specified in code override values set by configuration.

URL prefixes

When using UseUrls, --urls command-line argument, urls host configuration key, or ASPNETCORE_URLS environment variable, the URL prefixes can be in any of the following formats.

Only HTTP URL prefixes are valid. Kestrel doesn't support HTTPS when configuring URL bindings using UseUrls.

• IPv4 address with port number

  http://65.55.39.10:80/
  

  0.0.0.0 is a special case that binds to all IPv4 addresses.

• IPv6 address with port number

  http://[0:0:0:0:0:ffff:4137:270a]:80/
  

  [::] is the IPv6 equivalent of IPv4 0.0.0.0.

• Host name with port number

  http://contoso.com:80/
  http://*:80/
  

  Host names, *, and +, aren't special. Anything not recognized as a valid IP address or localhost binds to all IPv4 and IPv6 IPs. To bind different host names to different ASP.NET Core apps on the same port, use HTTP.sys or a reverse proxy server. Reverse proxy server examples include IIS, Nginx, or Apache.

  Warning

  Hosting in a reverse proxy configuration requires host filtering.

• Host localhost name with port number or loopback IP with port number

  http://localhost:5000/
  http://127.0.0.1:5000/
  http://[::1]:5000/
  

  When localhost is specified, Kestrel attempts to bind to both IPv4 and IPv6 loopback interfaces. If the requested port is in use by another service on either loopback interface, Kestrel fails to start. If either loopback interface is unavailable for any other reason (most commonly because IPv6 isn't supported), Kestrel logs a warning.

ASP.NET Core projects are configured to bind to a random HTTP port between 5000-5300 and a random HTTPS port between 7000-7300. This default configuration is specified in the generated Properties/launchSettings.json file and can be overridden. If no ports are specified, Kestrel binds to:

• http://localhost:5000
• https://localhost:5001 (when a local development certificate is present)

Specify URLs using the:

• ASPNETCORE_URLS environment variable.
• --urls command-line argument.
• urls host configuration key.
• UseUrls extension method.

The value provided using these approaches can be one or more HTTP and HTTPS endpoints (HTTPS if a default cert is available). Configure the value as a semicolon-separated list (for example, "Urls": "http://localhost:8000;http://localhost:8001").

For more information on these approaches, see Server URLs and Override configuration.

A development certificate is created:

• When the .NET SDK is installed.
• The dev-certs tool is used to create a certificate.

The development certificate is available only for the user that generates the certificate. Some browsers require granting explicit permission to trust the local development certificate.

Project templates configure apps to run on HTTPS by default and include HTTPS redirection and HSTS support.

Call Listen or ListenUnixSocket methods on KestrelServerOptions to configure URL prefixes and ports for Kestrel.

UseUrls, the --urls command-line argument, urls host configuration key, and the ASPNETCORE_URLS environment variable also work but have the limitations noted later in this section (a default certificate must be available for HTTPS endpoint configuration).

KestrelServerOptions configuration:

ConfigureEndpointDefaults(Action<ListenOptions>)

Specifies a configuration Action to run for each specified endpoint. Calling ConfigureEndpointDefaults multiple times replaces prior Actions with the last Action specified:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureEndpointDefaults won't have the defaults applied.

Configure(IConfiguration)

Enables Kestrel to load endpoints from an IConfiguration. The configuration must be scoped to the configuration section for Kestrel. The Configure(IConfiguration, bool) overload can be used to enable reloading endpoints when the configuration source changes.

By default, Kestrel configuration is loaded from the Kestrel section and reloading changes is enabled:

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "Https": {
        "Url": "https://localhost:5001"
      }
    }
  }
}

If reloading configuration is enabled and a change is signaled then the following steps are taken:

• The new configuration is compared to the old one, any endpoint without configuration changes are not modified.
• Removed or modified endpoints are given 5 seconds to complete processing requests and shut down.
• New or modified endpoints are started.

Clients connecting to a modified endpoint may be disconnected or refused while the endpoint is restarted.

ConfigureHttpsDefaults(Action<HttpsConnectionAdapterOptions>)

Specifies a configuration Action to run for each HTTPS endpoint. Calling ConfigureHttpsDefaults multiple times replaces prior Actions with the last Action specified.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Note

Endpoints created by calling Listen before calling ConfigureHttpsDefaults won't have the defaults applied.

ListenOptions.UseHttps

Configure Kestrel to use HTTPS.

ListenOptions.UseHttps extensions:

• UseHttps: Configure Kestrel to use HTTPS with the default certificate. Throws an exception if no default certificate is configured.
• UseHttps(string fileName)
• UseHttps(string fileName, string password)
• UseHttps(string fileName, string password, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(StoreName storeName, string subject)
• UseHttps(StoreName storeName, string subject, bool allowInvalid)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(X509Certificate2 serverCertificate)
• UseHttps(X509Certificate2 serverCertificate, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(Action<HttpsConnectionAdapterOptions> configureOptions)

ListenOptions.UseHttps parameters:

• filename is the path and file name of a certificate file, relative to the directory that contains the app's content files.
• password is the password required to access the X.509 certificate data.
• configureOptions is an Action to configure the HttpsConnectionAdapterOptions. Returns the ListenOptions.
• storeName is the certificate store from which to load the certificate.
• subject is the subject name for the certificate.
• allowInvalid indicates if invalid certificates should be considered, such as self-signed certificates.
• location is the store location to load the certificate from.
• serverCertificate is the X.509 certificate.

In production, HTTPS must be explicitly configured. At a minimum, a default certificate must be provided.

Supported configurations described next:

• No configuration
• Replace the default certificate from configuration
• Change the defaults in code

No configuration

Kestrel listens on http://localhost:5000 and https://localhost:5001 (if a default cert is available).

Replace the default certificate from configuration

A default HTTPS app settings configuration schema is available for Kestrel. Configure multiple endpoints, including the URLs and the certificates to use, either from a file on disk or from a certificate store.

In the following appsettings.json example:

• Set AllowInvalid to true to permit the use of invalid certificates (for example, self-signed certificates).
• Any HTTPS endpoint that doesn't specify a certificate (HttpsDefaultCert in the example that follows) falls back to the cert defined under Certificates:Default or the development certificate.

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "HttpsInlineCertFile": {
        "Url": "https://localhost:5001",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertAndKeyFile": {
        "Url": "https://localhost:5002",
        "Certificate": {
          "Path": "<path to .pem/.crt file>",
          "KeyPath": "<path to .key file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertStore": {
        "Url": "https://localhost:5003",
        "Certificate": {
          "Subject": "<subject; required>",
          "Store": "<certificate store; required>",
          "Location": "<location; defaults to CurrentUser>",
          "AllowInvalid": "<true or false; defaults to false>"
        }
      },
      "HttpsDefaultCert": {
        "Url": "https://localhost:5004"
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

Schema notes:

• Endpoints names are case-insensitive. For example, HTTPS and Https are equivalent.
• The Url parameter is required for each endpoint. The format for this parameter is the same as the top-level Urls configuration parameter except that it's limited to a single value.
• These endpoints replace those defined in the top-level Urls configuration rather than adding to them. Endpoints defined in code via Listen are cumulative with the endpoints defined in the configuration section.
• The Certificate section is optional. If the Certificate section isn't specified, the defaults defined in Certificates:Default are used. If no defaults are available, the development certificate is used. If there are no defaults and the development certificate isn't present, the server throws an exception and fails to start.
• The Certificate section supports multiple certificate sources.
• Any number of endpoints may be defined in Configuration as long as they don't cause port conflicts.

Certificate sources

Certificate nodes can be configured to load certificates from a number of sources:

• Path and Password to load .pfx files.
• Path, KeyPath and Password to load .pem/.crt and .key files.
• Subject and Store to load from the certificate store.

For example, the Certificates:Default certificate can be specified as:

"Default": {
  "Subject": "<subject; required>",
  "Store": "<cert store; required>",
  "Location": "<location; defaults to CurrentUser>",
  "AllowInvalid": "<true or false; defaults to false>"
}

ConfigurationLoader

Configure(IConfiguration) returns a KestrelConfigurationLoader with an Endpoint(String, Action<EndpointConfiguration>) method that can be used to supplement a configured endpoint's settings:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    var kestrelSection = context.Configuration.GetSection("Kestrel");

    serverOptions.Configure(kestrelSection)
        .Endpoint("HTTPS", listenOptions =>
        {
            // ...
        });
});

KestrelServerOptions.ConfigurationLoader can be directly accessed to continue iterating on the existing loader, such as the one provided by WebApplicationBuilder.WebHost.

• The configuration section for each endpoint is available on the options in the Endpoint method so that custom settings may be read.
• Multiple configurations may be loaded by calling Configure(IConfiguration) again with another section. Only the last configuration is used, unless Load is explicitly called on prior instances. The metapackage doesn't call Load so that its default configuration section may be replaced.
• KestrelConfigurationLoader mirrors the Listen family of APIs from KestrelServerOptions as Endpoint overloads, so code and config endpoints may be configured in the same place. These overloads don't use names and only consume default settings from configuration.

Change the defaults in code

ConfigureEndpointDefaults and ConfigureHttpsDefaults can be used to change default settings for ListenOptions and HttpsConnectionAdapterOptions, including overriding the default certificate specified in the prior scenario. ConfigureEndpointDefaults and ConfigureHttpsDefaults should be called before any endpoints are configured.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // ...
    });

    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // ...
    });
});

Configure endpoints using Server Name Indication

Server Name Indication (SNI) can be used to host multiple domains on the same IP address and port. For SNI to function, the client sends the host name for the secure session to the server during the TLS handshake so that the server can provide the correct certificate. The client uses the furnished certificate for encrypted communication with the server during the secure session that follows the TLS handshake.

SNI can be configured in two ways:

• Create an endpoint in code and select a certificate using the host name with the ServerCertificateSelector callback.
• Configure a mapping between host names and HTTPS options in Configuration. For example, JSON in the appsettings.json file.

SNI with ServerCertificateSelector

Kestrel supports SNI via the ServerCertificateSelector callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var subExampleCert = CertificateLoader.LoadFromStoreCert(
                "sub.example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var certs = new Dictionary<string, X509Certificate2>(
                StringComparer.OrdinalIgnoreCase)
            {
                ["localhost"] = localhostCert,
                ["example.com"] = exampleCert,
                ["sub.example.com"] = subExampleCert
            };

            httpsOptions.ServerCertificateSelector = (connectionContext, name) =>
            {
                if (name is not null && certs.TryGetValue(name, out var cert))
                {
                    return cert;
                }

                return exampleCert;
            };
        });
    });
});

SNI with ServerOptionsSelectionCallback

Kestrel supports additional dynamic TLS configuration via the ServerOptionsSelectionCallback callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate and TLS configuration. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps((stream, clientHelloInfo, state, cancellationToken) =>
            {
                if (string.Equals(clientHelloInfo.ServerName, "localhost",
                    StringComparison.OrdinalIgnoreCase))
                {
                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = localhostCert,
                            // Different TLS requirements for this host
                            ClientCertificateRequired = true
                        });
                }

                return new ValueTask<SslServerAuthenticationOptions>(
                    new SslServerAuthenticationOptions
                    {
                        ServerCertificate = exampleCert
                    });
            }, state: null!);
        });
    });
});

SNI with TlsHandshakeCallbackOptions

Kestrel supports additional dynamic TLS configuration via the TlsHandshakeCallbackOptions.OnConnection callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate, TLS configuration, and other server options. Default certificates and ConfigureHttpsDefaults are not used with this callback.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps(new TlsHandshakeCallbackOptions
            {
                OnConnection = context =>
                {
                    if (string.Equals(context.ClientHelloInfo.ServerName, "localhost",
                        StringComparison.OrdinalIgnoreCase))
                    {
                        // Different TLS requirements for this host
                        context.AllowDelayedClientCertificateNegotation = true;

                        return new ValueTask<SslServerAuthenticationOptions>(
                            new SslServerAuthenticationOptions
                            {
                                ServerCertificate = localhostCert
                            });
                    }

                    return new ValueTask<SslServerAuthenticationOptions>(
                        new SslServerAuthenticationOptions
                        {
                            ServerCertificate = exampleCert
                        });
                }
            });
        });
    });
});

SNI in configuration

Kestrel supports SNI defined in configuration. An endpoint can be configured with an Sni object that contains a mapping between host names and HTTPS options. The connection host name is matched to the options and they are used for that connection.

The following configuration adds an endpoint named MySniEndpoint that uses SNI to select HTTPS options based on the host name:

{
  "Kestrel": {
    "Endpoints": {
      "MySniEndpoint": {
        "Url": "https://*",
        "SslProtocols": ["Tls11", "Tls12"],
        "Sni": {
          "a.example.org": {
            "Protocols": "Http1AndHttp2",
            "SslProtocols": ["Tls11", "Tls12", "Tls13"],
            "Certificate": {
              "Subject": "<subject; required>",
              "Store": "<certificate store; required>",
            },
            "ClientCertificateMode" : "NoCertificate"
          },
          "*.example.org": {
            "Certificate": {
              "Path": "<path to .pfx file>",
              "Password": "$CREDENTIAL_PLACEHOLDER$"
            }
          },
          "*": {
            // At least one subproperty needs to exist per SNI section or it
            // cannot be discovered via IConfiguration
            "Protocols": "Http1",
          }
        }
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

HTTPS options that can be overridden by SNI:

• Certificate configures the certificate source.
• Protocols configures the allowed HTTP protocols.
• SslProtocols configures the allowed SSL protocols.
• ClientCertificateMode configures the client certificate requirements.

The host name supports wildcard matching:

• Exact match. For example, a.example.org matches a.example.org.
• Wildcard prefix. If there are multiple wildcard matches then the longest pattern is chosen. For example, *.example.org matches b.example.org and c.example.org.
• Full wildcard. * matches everything else, including clients that aren't using SNI and don't send a host name.

The matched SNI configuration is applied to the endpoint for the connection, overriding values on the endpoint. If a connection doesn't match a configured SNI host name then the connection is refused.

SNI requirements

All websites must run on the same Kestrel instance. Kestrel doesn't support sharing an IP address and port across multiple instances without a reverse proxy.

SSL/TLS Protocols

SSL Protocols are protocols used for encrypting and decrypting traffic between two peers, traditionally a client and a server.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.SslProtocols = SslProtocols.Tls13;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "SslProtocols": ["Tls12", "Tls13"],
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

The default value, SslProtocols.None, causes Kestrel to use the operating system defaults to choose the best protocol. Unless you have a specific reason to select a protocol, use the default.

Client Certificates

ClientCertificateMode configures the client certificate requirements.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.ClientCertificateMode = ClientCertificateMode.AllowCertificate;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "ClientCertificateMode": "AllowCertificate",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider.

The default value is ClientCertificateMode.NoCertificate where Kestrel will not request or require a certificate from the client.

For more information, see Configure certificate authentication in ASP.NET Core.

Connection logging

Call UseConnectionLogging to emit Debug level logs for byte-level communication on a connection. Connection logging is helpful for troubleshooting problems in low-level communication, such as during TLS encryption and behind proxies. If UseConnectionLogging is placed before UseHttps, encrypted traffic is logged. If UseConnectionLogging is placed after UseHttps, decrypted traffic is logged. This is built-in Connection Middleware.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseConnectionLogging();
    });
});

Bind to a TCP socket

The Listen method binds to a TCP socket, and an options lambda permits X.509 certificate configuration:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Loopback, 5000);
    serverOptions.Listen(IPAddress.Loopback, 5001, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

The example configures HTTPS for an endpoint with ListenOptions. Use the same API to configure other Kestrel settings for specific endpoints.

On Windows, self-signed certificates can be created using the New-SelfSignedCertificate PowerShell cmdlet. For an unsupported example, see UpdateIISExpressSSLForChrome.ps1.

On macOS, Linux, and Windows, certificates can be created using OpenSSL.

Bind to a Unix socket

Listen on a Unix socket with ListenUnixSocket for improved performance with Nginx, as shown in this example:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock");
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock", listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testpassword");
    });
});

• In the Nginx configuration file, set the server > location > proxy_pass entry to http://unix:/tmp/{KESTREL SOCKET}:/;. {KESTREL SOCKET} is the name of the socket provided to ListenUnixSocket (for example, kestrel-test.sock in the preceding example).
• Ensure that the socket is writeable by Nginx (for example, chmod go+w /tmp/kestrel-test.sock).

Port 0

When the port number 0 is specified, Kestrel dynamically binds to an available port. The following example shows how to determine which port Kestrel bound at runtime:

app.Run(async (context) =>
{
    var serverAddressFeature = context.Features.Get<IServerAddressesFeature>();

    if (serverAddressFeature is not null)
    {
        var listenAddresses = string.Join(", ", serverAddressFeature.Addresses);

        // ...
    }
});

Limitations

Configure endpoints with the following approaches:

• UseUrls
• --urls command-line argument
• urls host configuration key
• ASPNETCORE_URLS environment variable

These methods are useful for making code work with servers other than Kestrel. However, be aware of the following limitations:

• HTTPS can't be used with these approaches unless a default certificate is provided in the HTTPS endpoint configuration (for example, using KestrelServerOptions configuration or a configuration file as shown earlier in this article).
• When both the Listen and UseUrls approaches are used simultaneously, the Listen endpoints override the UseUrls endpoints.

IIS endpoint configuration

When using IIS, the URL bindings for IIS override bindings are set by either Listen or UseUrls. For more information, see ASP.NET Core Module.

ListenOptions.Protocols

The Protocols property establishes the HTTP protocols (HttpProtocols) enabled on a connection endpoint or for the server. Assign a value to the Protocols property from the HttpProtocols enum.

HttpProtocols enum value	Connection protocol permitted
Http1	HTTP/1.1 only. Can be used with or without TLS.
Http2	HTTP/2 only. May be used without TLS only if the client supports a Prior Knowledge mode.
Http1AndHttp2	HTTP/1.1 and HTTP/2. HTTP/2 requires the client to select HTTP/2 in the TLS Application-Layer Protocol Negotiation (ALPN) handshake; otherwise, the connection defaults to HTTP/1.1.

The default ListenOptions.Protocols value for any endpoint is HttpProtocols.Http1AndHttp2.

TLS restrictions for HTTP/2:

• TLS version 1.2 or later
• Renegotiation disabled
• Compression disabled
• Minimum ephemeral key exchange sizes:
  • Elliptic curve Diffie-Hellman (ECDHE) [RFC4492]: 224 bits minimum
  • Finite field Diffie-Hellman (DHE) [TLS12]: 2048 bits minimum
• Cipher suite not prohibited.

TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186] is supported by default.

The following example permits HTTP/1.1 and HTTP/2 connections on port 8000. Connections are secured by TLS with a supplied certificate:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

On Linux, CipherSuitesPolicy can be used to filter TLS handshakes on a per-connection basis:

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.OnAuthenticate = (context, sslOptions) =>
        {
            sslOptions.CipherSuitesPolicy = new CipherSuitesPolicy(
                new[]
                {
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
                    // ...
                });
        };
    });
});

Connection Middleware

Custom connection middleware can filter TLS handshakes on a per-connection basis for specific ciphers if necessary.

The following example throws NotSupportedException for any cipher algorithm that the app doesn't support. Alternatively, define and compare ITlsHandshakeFeature.CipherAlgorithm to a list of acceptable cipher suites.

No encryption is used with a CipherAlgorithmType.Null cipher algorithm.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, serverOptions) =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");

        listenOptions.Use((context, next) =>
        {
            var tlsFeature = context.Features.Get<ITlsHandshakeFeature>()!;

            if (tlsFeature.CipherAlgorithm == CipherAlgorithmType.Null)
            {
                throw new NotSupportedException(
                    $"Prohibited cipher: {tlsFeature.CipherAlgorithm}");
            }

            return next();
        });
    });
});

Set the HTTP protocol from configuration

By default, Kestrel configuration is loaded from the Kestrel section. The following appsettings.json example establishes HTTP/1.1 as the default connection protocol for all endpoints:

{
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

The following appsettings.json example establishes the HTTP/1.1 connection protocol for a specific endpoint:

{
  "Kestrel": {
    "Endpoints": {
      "HttpsDefaultCert": {
        "Url": "https://localhost:5001",
        "Protocols": "Http1"
      }
    }
  }
}

Protocols specified in code override values set by configuration.

URL prefixes

When using UseUrls, --urls command-line argument, urls host configuration key, or ASPNETCORE_URLS environment variable, the URL prefixes can be in any of the following formats.

Only HTTP URL prefixes are valid. Kestrel doesn't support HTTPS when configuring URL bindings using UseUrls.

• IPv4 address with port number

  http://65.55.39.10:80/
  

  0.0.0.0 is a special case that binds to all IPv4 addresses.

• IPv6 address with port number

  http://[0:0:0:0:0:ffff:4137:270a]:80/
  

  [::] is the IPv6 equivalent of IPv4 0.0.0.0.

• Host name with port number

  http://contoso.com:80/
  http://*:80/
  

  Host names, *, and +, aren't special. Anything not recognized as a valid IP address or localhost binds to all IPv4 and IPv6 IPs. To bind different host names to different ASP.NET Core apps on the same port, use HTTP.sys or a reverse proxy server. Reverse proxy server examples include IIS, Nginx, or Apache.

  Warning

  Hosting in a reverse proxy configuration requires host filtering.

• Host localhost name with port number or loopback IP with port number

  http://localhost:5000/
  http://127.0.0.1:5000/
  http://[::1]:5000/
  

  When localhost is specified, Kestrel attempts to bind to both IPv4 and IPv6 loopback interfaces. If the requested port is in use by another service on either loopback interface, Kestrel fails to start. If either loopback interface is unavailable for any other reason (most commonly because IPv6 isn't supported), Kestrel logs a warning.

By default, ASP.NET Core binds to:

• http://localhost:5000
• https://localhost:5001 (when a local development certificate is present)

Specify URLs using the:

• ASPNETCORE_URLS environment variable.
• --urls command-line argument.
• urls host configuration key.
• UseUrls extension method.

The value provided using these approaches can be one or more HTTP and HTTPS endpoints (HTTPS if a default cert is available). Configure the value as a semicolon-separated list (for example, "Urls": "http://localhost:8000;http://localhost:8001").

For more information on these approaches, see Server URLs and Override configuration.

A development certificate is created:

• When the .NET SDK is installed.
• The dev-certs tool is used to create a certificate.

Some browsers require granting explicit permission to trust the local development certificate.

Project templates configure apps to run on HTTPS by default and include HTTPS redirection and HSTS support.

Call Listen or ListenUnixSocket methods on KestrelServerOptions to configure URL prefixes and ports for Kestrel.

UseUrls, the --urls command-line argument, urls host configuration key, and the ASPNETCORE_URLS environment variable also work but have the limitations noted later in this section (a default certificate must be available for HTTPS endpoint configuration).

KestrelServerOptions configuration:

ConfigureEndpointDefaults(Action<ListenOptions>)

Specifies a configuration Action to run for each specified endpoint. Calling ConfigureEndpointDefaults multiple times replaces prior Actions with the last Action specified.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // Configure endpoint defaults
    });
});

Note

Endpoints created by calling Listen before calling ConfigureEndpointDefaults won't have the defaults applied.

Configure(IConfiguration)

Enables Kestrel to load endpoints from an IConfiguration. The configuration must be scoped to the configuration section for Kestrel.

The Configure(IConfiguration, bool) overload can be used to enable reloading endpoints when the configuration source changes.

IHostBuilder.ConfigureWebHostDefaults calls Configure(context.Configuration.GetSection("Kestrel"), reloadOnChange: true) by default to load Kestrel configuration and enable reloading.

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "Https": {
        "Url": "https://localhost:5001"
      }
    }
  }
}

If reloading configuration is enabled and a change is signaled then the following steps are taken:

• The new configuration is compared to the old one, any endpoint without configuration changes are not modified.
• Removed or modified endpoints are given 5 seconds to complete processing requests and shut down.
• New or modified endpoints are started.

Clients connecting to a modified endpoint may be disconnected or refused while the endpoint is restarted.

ConfigureHttpsDefaults(Action<HttpsConnectionAdapterOptions>)

Specifies a configuration Action to run for each HTTPS endpoint. Calling ConfigureHttpsDefaults multiple times replaces prior Actions with the last Action specified.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        // certificate is an X509Certificate2
        listenOptions.ServerCertificate = certificate;
    });
});

Note

Endpoints created by calling Listen before calling ConfigureHttpsDefaults won't have the defaults applied.

ListenOptions.UseHttps

Configure Kestrel to use HTTPS.

ListenOptions.UseHttps extensions:

• UseHttps: Configure Kestrel to use HTTPS with the default certificate. Throws an exception if no default certificate is configured.
• UseHttps(string fileName)
• UseHttps(string fileName, string password)
• UseHttps(string fileName, string password, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(StoreName storeName, string subject)
• UseHttps(StoreName storeName, string subject, bool allowInvalid)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location)
• UseHttps(StoreName storeName, string subject, bool allowInvalid, StoreLocation location, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(X509Certificate2 serverCertificate)
• UseHttps(X509Certificate2 serverCertificate, Action<HttpsConnectionAdapterOptions> configureOptions)
• UseHttps(Action<HttpsConnectionAdapterOptions> configureOptions)

ListenOptions.UseHttps parameters:

• filename is the path and file name of a certificate file, relative to the directory that contains the app's content files.
• password is the password required to access the X.509 certificate data.
• configureOptions is an Action to configure the HttpsConnectionAdapterOptions. Returns the ListenOptions.
• storeName is the certificate store from which to load the certificate.
• subject is the subject name for the certificate.
• allowInvalid indicates if invalid certificates should be considered, such as self-signed certificates.
• location is the store location to load the certificate from.
• serverCertificate is the X.509 certificate.

In production, HTTPS must be explicitly configured. At a minimum, a default certificate must be provided.

Supported configurations described next:

• No configuration
• Replace the default certificate from configuration
• Change the defaults in code

No configuration

Kestrel listens on http://localhost:5000 and https://localhost:5001 (if a default cert is available).

Replace the default certificate from configuration

A default HTTPS app settings configuration schema is available for Kestrel. Configure multiple endpoints, including the URLs and the certificates to use, either from a file on disk or from a certificate store.

In the following appsettings.json example:

• Set AllowInvalid to true to permit the use of invalid certificates (for example, self-signed certificates).
• Any HTTPS endpoint that doesn't specify a certificate (HttpsDefaultCert in the example that follows) falls back to the cert defined under Certificates:Default or the development certificate.

{
  "Kestrel": {
    "Endpoints": {
      "Http": {
        "Url": "http://localhost:5000"
      },
      "HttpsInlineCertFile": {
        "Url": "https://localhost:5001",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertAndKeyFile": {
        "Url": "https://localhost:5002",
        "Certificate": {
          "Path": "<path to .pem/.crt file>",
          "KeyPath": "<path to .key file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      },
      "HttpsInlineCertStore": {
        "Url": "https://localhost:5003",
        "Certificate": {
          "Subject": "<subject; required>",
          "Store": "<certificate store; required>",
          "Location": "<location; defaults to CurrentUser>",
          "AllowInvalid": "<true or false; defaults to false>"
        }
      },
      "HttpsDefaultCert": {
        "Url": "https://localhost:5004"
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

Schema notes:

• Endpoints names are case-insensitive. For example, HTTPS and Https are equivalent.
• The Url parameter is required for each endpoint. The format for this parameter is the same as the top-level Urls configuration parameter except that it's limited to a single value.
• These endpoints replace those defined in the top-level Urls configuration rather than adding to them. Endpoints defined in code via Listen are cumulative with the endpoints defined in the configuration section.
• The Certificate section is optional. If the Certificate section isn't specified, the defaults defined in Certificates:Default are used. If no defaults are available, the development certificate is used. If there are no defaults and the development certificate isn't present, the server throws an exception and fails to start.
• The Certificate section supports multiple certificate sources.
• Any number of endpoints may be defined in Configuration as long as they don't cause port conflicts.

Certificate sources

Certificate nodes can be configured to load certificates from a number of sources:

• Path and Password to load .pfx files.
• Path, KeyPath and Password to load .pem/.crt and .key files.
• Subject and Store to load from the certificate store.

For example, the Certificates:Default certificate can be specified as:

"Default": {
  "Subject": "<subject; required>",
  "Store": "<cert store; required>",
  "Location": "<location; defaults to CurrentUser>",
  "AllowInvalid": "<true or false; defaults to false>"
}

ConfigurationLoader

options.Configure(context.Configuration.GetSection("{SECTION}")) returns a KestrelConfigurationLoader with an .Endpoint(string name, listenOptions => { }) method that can be used to supplement a configured endpoint's settings:

webBuilder.UseKestrel((context, serverOptions) =>
{
    serverOptions.Configure(context.Configuration.GetSection("Kestrel"))
        .Endpoint("HTTPS", listenOptions =>
        {
            listenOptions.HttpsOptions.SslProtocols = SslProtocols.Tls12;
        });
});

KestrelServerOptions.ConfigurationLoader can be directly accessed to continue iterating on the existing loader, such as the one provided by CreateDefaultBuilder.

• The configuration section for each endpoint is available on the options in the Endpoint method so that custom settings may be read.
• Multiple configurations may be loaded by calling options.Configure(context.Configuration.GetSection("{SECTION}")) again with another section. Only the last configuration is used, unless Load is explicitly called on prior instances. The metapackage doesn't call Load so that its default configuration section may be replaced.
• KestrelConfigurationLoader mirrors the Listen family of APIs from KestrelServerOptions as Endpoint overloads, so code and config endpoints may be configured in the same place. These overloads don't use names and only consume default settings from configuration.

Change the defaults in code

ConfigureEndpointDefaults and ConfigureHttpsDefaults can be used to change default settings for ListenOptions and HttpsConnectionAdapterOptions, including overriding the default certificate specified in the prior scenario. ConfigureEndpointDefaults and ConfigureHttpsDefaults should be called before any endpoints are configured.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureEndpointDefaults(listenOptions =>
    {
        // Configure endpoint defaults
    });

    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.SslProtocols = SslProtocols.Tls12;
    });
});

Configure endpoints using Server Name Indication

Server Name Indication (SNI) can be used to host multiple domains on the same IP address and port. For SNI to function, the client sends the host name for the secure session to the server during the TLS handshake so that the server can provide the correct certificate. The client uses the furnished certificate for encrypted communication with the server during the secure session that follows the TLS handshake.

SNI can be configured in two ways:

• Create an endpoint in code and select a certificate using the host name with the ServerCertificateSelector callback.
• Configure a mapping between host names and HTTPS options in Configuration. For example, JSON in the appsettings.json file.

SNI with ServerCertificateSelector

Kestrel supports SNI via the ServerCertificateSelector callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate. The following callback code can be used in the ConfigureWebHostDefaults method call of a project's Program.cs file:

// using System.Security.Cryptography.X509Certificates;
// using Microsoft.AspNetCore.Server.Kestrel.Https;

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var subExampleCert = CertificateLoader.LoadFromStoreCert(
                "sub.example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var certs = new Dictionary<string, X509Certificate2>(StringComparer.OrdinalIgnoreCase)
            {
                { "localhost", localhostCert },
                { "example.com", exampleCert },
                { "sub.example.com", subExampleCert },
            };            

            httpsOptions.ServerCertificateSelector = (connectionContext, name) =>
            {
                if (name != null && certs.TryGetValue(name, out var cert))
                {
                    return cert;
                }

                return exampleCert;
            };
        });
    });
});

SNI with ServerOptionsSelectionCallback

Kestrel supports additional dynamic TLS configuration via the ServerOptionsSelectionCallback callback. The callback is invoked once per connection to allow the app to inspect the host name and select the appropriate certificate and TLS configuration. Default certificates and ConfigureHttpsDefaults are not used with this callback.

// using System.Security.Cryptography.X509Certificates;
// using Microsoft.AspNetCore.Server.Kestrel.Https;

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenAnyIP(5005, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            var localhostCert = CertificateLoader.LoadFromStoreCert(
                "localhost", "My", StoreLocation.CurrentUser,
                allowInvalid: true);
            var exampleCert = CertificateLoader.LoadFromStoreCert(
                "example.com", "My", StoreLocation.CurrentUser,
                allowInvalid: true);

            listenOptions.UseHttps((stream, clientHelloInfo, state, cancellationToken) =>
            {
                if (string.Equals(clientHelloInfo.ServerName, "localhost", StringComparison.OrdinalIgnoreCase))
                {
                    return new ValueTask<SslServerAuthenticationOptions>(new SslServerAuthenticationOptions
                    {
                        ServerCertificate = localhostCert,
                        // Different TLS requirements for this host
                        ClientCertificateRequired = true,
                    });
                }

                return new ValueTask<SslServerAuthenticationOptions>(new SslServerAuthenticationOptions
                {
                    ServerCertificate = exampleCert,
                });
            }, state: null);
        });
    });
});

SNI in configuration

Kestrel supports SNI defined in configuration. An endpoint can be configured with an Sni object that contains a mapping between host names and HTTPS options. The connection host name is matched to the options and they are used for that connection.

The following configuration adds an endpoint named MySniEndpoint that uses SNI to select HTTPS options based on the host name:

{
  "Kestrel": {
    "Endpoints": {
      "MySniEndpoint": {
        "Url": "https://*",
        "SslProtocols": ["Tls11", "Tls12"],
        "Sni": {
          "a.example.org": {
            "Protocols": "Http1AndHttp2",
            "SslProtocols": ["Tls11", "Tls12", "Tls13"],
            "Certificate": {
              "Subject": "<subject; required>",
              "Store": "<certificate store; required>",
            },
            "ClientCertificateMode" : "NoCertificate"
          },
          "*.example.org": {
            "Certificate": {
              "Path": "<path to .pfx file>",
              "Password": "$CREDENTIAL_PLACEHOLDER$"
            }
          },
          "*": {
            // At least one subproperty needs to exist per SNI section or it
            // cannot be discovered via IConfiguration
            "Protocols": "Http1",
          }
        }
      }
    },
    "Certificates": {
      "Default": {
        "Path": "<path to .pfx file>",
        "Password": "$CREDENTIAL_PLACEHOLDER$"
      }
    }
  }
}

Warning

In the preceding example, certificate passwords are stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for each certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

HTTPS options that can be overridden by SNI:

• Certificate configures the certificate source.
• Protocols configures the allowed HTTP protocols.
• SslProtocols configures the allowed SSL protocols.
• ClientCertificateMode configures the client certificate requirements.

The host name supports wildcard matching:

• Exact match. For example, a.example.org matches a.example.org.
• Wildcard prefix. If there are multiple wildcard matches then the longest pattern is chosen. For example, *.example.org matches b.example.org and c.example.org.
• Full wildcard. * matches everything else, including clients that aren't using SNI and don't send a host name.

The matched SNI configuration is applied to the endpoint for the connection, overriding values on the endpoint. If a connection doesn't match a configured SNI host name then the connection is refused.

SNI requirements

• Running on target framework netcoreapp2.1 or later. On net461 or later, the callback is invoked but the name is always null. The name is also null if the client doesn't provide the host name parameter in the TLS handshake.
• All websites run on the same Kestrel instance. Kestrel doesn't support sharing an IP address and port across multiple instances without a reverse proxy.

SSL/TLS Protocols

SSL Protocols are protocols used for encrypting and decrypting traffic between two peers, traditionally a client and a server.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.SslProtocols = SslProtocols.Tls13;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "SslProtocols": ["Tls12", "Tls13"],
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

The default value, SslProtocols.None, causes Kestrel to use the operating system defaults to choose the best protocol. Unless you have a specific reason to select a protocol, use the default.

Client Certificates

ClientCertificateMode configures the client certificate requirements.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.ClientCertificateMode = ClientCertificateMode.AllowCertificate;
    });
});

{
  "Kestrel": {
    "Endpoints": {
      "MyHttpsEndpoint": {
        "Url": "https://localhost:5001",
        "ClientCertificateMode": "AllowCertificate",
        "Certificate": {
          "Path": "<path to .pfx file>",
          "Password": "$CREDENTIAL_PLACEHOLDER$"
        }
      }
    }
  }
}

Warning

In the preceding example, the certificate password is stored in plain-text in appsettings.json. The $CREDENTIAL_PLACEHOLDER$ token is used as a placeholder for the certificate's password. To store certificate passwords securely in development environments, see Protect secrets in development. To store certificate passwords securely in production environments, see Azure Key Vault configuration provider. Development secrets shouldn't be used for production or test.

The default value is ClientCertificateMode.NoCertificate where Kestrel will not request or require a certificate from the client.

For more information, see Configure certificate authentication in ASP.NET Core.

Connection logging

Call UseConnectionLogging to emit Debug level logs for byte-level communication on a connection. Connection logging is helpful for troubleshooting problems in low-level communication, such as during TLS encryption and behind proxies. If UseConnectionLogging is placed before UseHttps, encrypted traffic is logged. If UseConnectionLogging is placed after UseHttps, decrypted traffic is logged. This is built-in Connection Middleware.

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseConnectionLogging();
    });
});

Bind to a TCP socket

The Listen method binds to a TCP socket, and an options lambda permits X.509 certificate configuration:

public static void Main(string[] args)
{
    CreateHostBuilder(args).Build().Run();
}

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.ConfigureKestrel(serverOptions =>
            {
                serverOptions.Listen(IPAddress.Loopback, 5000);
                serverOptions.Listen(IPAddress.Loopback, 5001, 
                    listenOptions =>
                    {
                        listenOptions.UseHttps("testCert.pfx", 
                            "testPassword");
                    });
            })
            .UseStartup<Startup>();
        });

The example configures HTTPS for an endpoint with ListenOptions. Use the same API to configure other Kestrel settings for specific endpoints.

On Windows, self-signed certificates can be created using the New-SelfSignedCertificate PowerShell cmdlet. For an unsupported example, see UpdateIISExpressSSLForChrome.ps1.

On macOS, Linux, and Windows, certificates can be created using OpenSSL.

Bind to a Unix socket

Listen on a Unix socket with ListenUnixSocket for improved performance with Nginx, as shown in this example:

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock");
    serverOptions.ListenUnixSocket("/tmp/kestrel-test.sock", 
        listenOptions =>
        {
            listenOptions.UseHttps("testCert.pfx", 
                "testpassword");
        });
})

• In the Nginx configuration file, set the server > location > proxy_pass entry to http://unix:/tmp/{KESTREL SOCKET}:/;. {KESTREL SOCKET} is the name of the socket provided to ListenUnixSocket (for example, kestrel-test.sock in the preceding example).
• Ensure that the socket is writeable by Nginx (for example, chmod go+w /tmp/kestrel-test.sock).

Port 0

When the port number 0 is specified, Kestrel dynamically binds to an available port. The following example shows how to determine which port Kestrel bound at runtime:

public void Configure(IApplicationBuilder app)
{
    var serverAddressesFeature =
        app.ServerFeatures.Get<IServerAddressesFeature>();

    app.UseStaticFiles();

    app.Run(async (context) =>
    {
        context.Response.ContentType = "text/html";
        await context.Response
            .WriteAsync("<!DOCTYPE html><html lang=\"en\"><head>" +
                "<title></title></head><body><p>Hosted by Kestrel</p>");

        if (serverAddressesFeature != null)
        {
            await context.Response
                .WriteAsync("<p>Listening on the following addresses: " +
                    string.Join(", ", serverAddressesFeature.Addresses) +
                    "</p>");
        }

        await context.Response.WriteAsync("<p>Request URL: " +
            $"{context.Request.GetDisplayUrl()}<p>");
    });
}

When the app is run, the console window output indicates the dynamic port where the app can be reached:

Listening on the following addresses: http://127.0.0.1:48508

Limitations

Configure endpoints with the following approaches:

• UseUrls
• --urls command-line argument
• urls host configuration key
• ASPNETCORE_URLS environment variable

These methods are useful for making code work with servers other than Kestrel. However, be aware of the following limitations:

• HTTPS can't be used with these approaches unless a default certificate is provided in the HTTPS endpoint configuration (for example, using KestrelServerOptions configuration or a configuration file as shown earlier in this article).
• When both the Listen and UseUrls approaches are used simultaneously, the Listen endpoints override the UseUrls endpoints.

IIS endpoint configuration

When using IIS, the URL bindings for IIS override bindings are set by either Listen or UseUrls. For more information, see ASP.NET Core Module.

ListenOptions.Protocols

The Protocols property establishes the HTTP protocols (HttpProtocols) enabled on a connection endpoint or for the server. Assign a value to the Protocols property from the HttpProtocols enum.

HttpProtocols enum value	Connection protocol permitted
Http1	HTTP/1.1 only. Can be used with or without TLS.
Http2	HTTP/2 only. May be used without TLS only if the client supports a Prior Knowledge mode.
Http1AndHttp2	HTTP/1.1 and HTTP/2. HTTP/2 requires the client to select HTTP/2 in the TLS Application-Layer Protocol Negotiation (ALPN) handshake; otherwise, the connection defaults to HTTP/1.1.

The default ListenOptions.Protocols value for any endpoint is HttpProtocols.Http1AndHttp2.

TLS restrictions for HTTP/2:

• TLS version 1.2 or later
• Renegotiation disabled
• Compression disabled
• Minimum ephemeral key exchange sizes:
  • Elliptic curve Diffie-Hellman (ECDHE) [RFC4492]: 224 bits minimum
  • Finite field Diffie-Hellman (DHE) [TLS12]: 2048 bits minimum
• Cipher suite not prohibited.

TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186] is supported by default.

The following example permits HTTP/1.1 and HTTP/2 connections on port 8000. Connections are secured by TLS with a supplied certificate:

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
    });
});

On Linux, CipherSuitesPolicy can be used to filter TLS handshakes on a per-connection basis:

// using System.Net.Security;
// using Microsoft.AspNetCore.Hosting;
// using Microsoft.AspNetCore.Server.Kestrel.Core;
// using Microsoft.Extensions.DependencyInjection;
// using Microsoft.Extensions.Hosting;

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.ConfigureHttpsDefaults(listenOptions =>
    {
        listenOptions.OnAuthenticate = (context, sslOptions) =>
        {
            sslOptions.CipherSuitesPolicy = new CipherSuitesPolicy(
                new[]
                {
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                    TlsCipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
                    // ...
                });
        };
    });
});

Connection Middleware

Custom connection middleware can filter TLS handshakes on a per-connection basis for specific ciphers if necessary.

The following example throws NotSupportedException for any cipher algorithm that the app doesn't support. Alternatively, define and compare ITlsHandshakeFeature.CipherAlgorithm to a list of acceptable cipher suites.

No encryption is used with a CipherAlgorithmType.Null cipher algorithm.

// using System.Net;
// using Microsoft.AspNetCore.Connections;

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
        listenOptions.UseTlsFilter();
    });
});

using System;
using System.Security.Authentication;
using Microsoft.AspNetCore.Connections.Features;

namespace Microsoft.AspNetCore.Connections
{
    public static class TlsFilterConnectionMiddlewareExtensions
    {
        public static IConnectionBuilder UseTlsFilter(
            this IConnectionBuilder builder)
        {
            return builder.Use((connection, next) =>
            {
                var tlsFeature = connection.Features.Get<ITlsHandshakeFeature>();

                if (tlsFeature.CipherAlgorithm == CipherAlgorithmType.Null)
                {
                    throw new NotSupportedException("Prohibited cipher: " +
                        tlsFeature.CipherAlgorithm);
                }

                return next();
            });
        }
    }
}

Connection filtering can also be configured via an IConnectionBuilder lambda:

// using System;
// using System.Net;
// using System.Security.Authentication;
// using Microsoft.AspNetCore.Connections;
// using Microsoft.AspNetCore.Connections.Features;

webBuilder.ConfigureKestrel(serverOptions =>
{
    serverOptions.Listen(IPAddress.Any, 8000, listenOptions =>
    {
        listenOptions.UseHttps("testCert.pfx", "testPassword");
        listenOptions.Use((context, next) =>
        {
            var tlsFeature = context.Features.Get<ITlsHandshakeFeature>();

            if (tlsFeature.CipherAlgorithm == CipherAlgorithmType.Null)
            {
                throw new NotSupportedException(
                    $"Prohibited cipher: {tlsFeature.CipherAlgorithm}");
            }

            return next();
        });
    });
});

Set the HTTP protocol from configuration

CreateDefaultBuilder calls serverOptions.Configure(context.Configuration.GetSection("Kestrel")) by default to load Kestrel configuration.

The following appsettings.json example establishes HTTP/1.1 as the default connection protocol for all endpoints:

{
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

The following appsettings.json example establishes the HTTP/1.1 connection protocol for a specific endpoint:

{
  "Kestrel": {
    "Endpoints": {
      "HttpsDefaultCert": {
        "Url": "https://localhost:5001",
        "Protocols": "Http1"
      }
    }
  }
}

Protocols specified in code override values set by configuration.

URL prefixes

When using UseUrls, --urls command-line argument, urls host configuration key, or ASPNETCORE_URLS environment variable, the URL prefixes can be in any of the following formats.

Only HTTP URL prefixes are valid. Kestrel doesn't support HTTPS when configuring URL bindings using UseUrls.

• IPv4 address with port number

  http://65.55.39.10:80/
  

  0.0.0.0 is a special case that binds to all IPv4 addresses.

• IPv6 address with port number

  http://[0:0:0:0:0:ffff:4137:270a]:80/
  

  [::] is the IPv6 equivalent of IPv4 0.0.0.0.

• Host name with port number

  http://contoso.com:80/
  http://*:80/
  

  Host names, *, and +, aren't special. Anything not recognized as a valid IP address or localhost binds to all IPv4 and IPv6 IPs. To bind different host names to different ASP.NET Core apps on the same port, use HTTP.sys or a reverse proxy server. Reverse proxy server examples include IIS, Nginx, or Apache.

  Warning

  Hosting in a reverse proxy configuration requires host filtering.

• Host localhost name with port number or loopback IP with port number

  http://localhost:5000/
  http://127.0.0.1:5000/
  http://[::1]:5000/
  

  When localhost is specified, Kestrel attempts to bind to both IPv4 and IPv6 loopback interfaces. If the requested port is in use by another service on either loopback interface, Kestrel fails to start. If either loopback interface is unavailable for any other reason (most commonly because IPv6 isn't supported), Kestrel logs a warning.