mTLS (mutual TLS) requires a separate setup from the server side. mTLS is valid, when clientCert is negotiated on the connection establishment, meaning when ASP.NET code calls context.Connection.ClientCertificate: it should be already available and there must not be any need to invoke context.Connection.GetClientCertificateAsync()

in case of Kestrel setup is extremely easy - just set a specific enum value:

options.ClientCertificateMode = ClientCertificateMode.RequireCertificate;
options.ClientCertificateValidation = AllowAnyCertificateValidationWithLogging;

however in case of HTTP.SYS the only option is to execute a netsh command netsh http add sslcert.

I have added a code in ASP.NET application, which on start binds the test certificate using netsh command, and then using an applicationLifetime ApplicationStopping callback it deletes the cert binding from the machine. It can be seem from the logs (below).

Basically the order is the following:

1. bind the test cert to http.sys with clientcertnegotiation=enable to enable client cert negotiation
   optionally) netsh http show sslcert can show the ssl cert bindings on a machine. Important property is "Negotiate Client Certificate : Enabled"
2. execute the benchmark (in other words process the traffic). If verbose logging of cert validation is enabled, you should see message "client certificate (...) already exists on the connection", meaning that client cert negotiation happened on the connection establishment
3. in the end of the benchmark run during a graceful shutdown of an app a rollback of http.sys configuration will happen - we will execute netsh http delete sslcert

args: --urls https://10.0.0.110:5000 --mTLS true --certValidationConsoleEnabled true --statsEnabled false --tlsRenegotiation true --httpSysLogs true

Setting up mTLS for http.sys
Loaded testCert.pfx from local file system
Added testCert.pfx to localMachine cert store
Executing command: netsh http add sslcert ipport=10.0.0.110:5000 certstorename=MY certhash=6A283FAAA7FFFC129DF24CC0E7521186C7926219 appid={75e4f5b6-776f-448f-8a36-64e964bf2147} clientcertnegotiation=enable
Configured http.sys settings for mTLS
Registered client cert validation middleware
Executing command: netsh http show sslcert

SSL Certificate bindings:

IP:port                      : 10.0.0.110:5000
Certificate Hash             : 6a283faaa7fffc129df24cc0e7521186c7926219
Application ID               : {75e4f5b6-776f-448f-8a36-64e964bf2147}
Certificate Store Name       : MY
Verify Client Certificate Revocation : Enabled
Verify Revocation Using Cached Client Certificate Only : Disabled
Usage Check                  : Enabled
Revocation Freshness Time    : 0
URL Retrieval Timeout        : 0
Ctl Identifier               : (null)
Ctl Store Name               : (null)
DS Mapper Usage              : Disabled
**Negotiate Client Certificate : Enabled**
Reject Connections           : Disabled
Disable HTTP2                : Not Set
Disable QUIC                 : Not Set
Disable TLS1.2               : Not Set
Disable TLS1.3               : Not Set
Disable OCSP Stapling        : Not Set
Enable Token Binding         : Not Set
Log Extended Events          : Not Set
Disable Legacy TLS Versions  : Not Set
Enable Session Ticket        : Not Set

Application Info:

mTLS is enabled (client cert is required)
enabled logging certificate validation events to console
listening endpoints: https://10.0.0.110:5000

Application started.
client certificate (6A283FAAA7FFFC129DF24CC0E7521186C7926219) already exists on the connection -648518319766231673
client certificate (6A283FAAA7FFFC129DF24CC0E7521186C7926219) already exists on the connection -216172777013495157
client certificate (6A283FAAA7FFFC129DF24CC0E7521186C7926219) already exists on the connection -144115161500731759
client certificate (6A283FAAA7FFFC129DF24CC0E7521186C7926219) already exists on the connection -576460747203134805
client certificate (6A283FAAA7FFFC129DF24CC0E7521186C7926219) already exists on the connection -432345559127283053
...
...
...
Disabling mTLS for http.sys
Executing command: netsh http delete sslcert ipport=10.0.0.110:5000
Disabled http.sys settings for mTLS