What value should TLSVerifyClient be set to if a valid client certificate is required?

Setting TLSVerifyClient to "demand" is the correct choice when a valid client certificate is required during the TLS handshake process. This configuration mandates that the client present a valid certificate for authentication to successfully establish a secure connection with the server.

When TLSVerifyClient is set to "demand," the server will strictly require the client to authenticate itself using a valid certificate. If the client does not provide a certificate or provides an invalid one, the handshake will fail, and the server will deny the connection. This setting is particularly important in environments where security is paramount, such as in financial services or sensitive data transactions where ensuring the identity of communicating parties is critical.

The other settings do not provide the same level of requirement for client authentication. "Never" means that the server will not request or require a client certificate at all. "Allow" indicates that the server will accept connections from clients with or without a valid certificate, and "try" means that the server will attempt to authenticate but will not enforce it, allowing connections to proceed even if no certificate is provided or if an invalid one is presented. Thus, "demand" is the ideal configuration for enforcing strict client authentication via certificates.