Which option should be used with the TLSVerifyClient directive to require a valid client certificate?

The TLSVerifyClient directive is used to configure the behavior of a server regarding client certificates in a TLS (Transport Layer Security) connection. When using this directive, specifying the option "demand" indicates that a valid client certificate is not just preferred, but it is mandatory for the connection to be established.

By using "demand", the server will request a client certificate during the TLS handshake, and the connection will only succeed if the client provides a valid certificate that the server can verify. This is crucial in scenarios where security is paramount, and you need to ensure that only authenticated clients can communicate with your server.

In contrast, the other options either do not enforce this requirement or provide leniency in client certificate verification. For instance, "never" would mean that the server does not require client certificates at all, while "allow" would indicate that the client may provide a certificate but is not required to, and "try" implies that the server will attempt to verify a client certificate but will not necessitate it for a connection. Thus, "demand" is the choice that enforces the strictest security posture by ensuring that a valid client certificate is a prerequisite for establishing a connection.