Which Samba directive must be set for security authentication as a user?

The directive that must be set for security authentication as a user in Samba is "security = user." This setting means that Samba will require a valid username and password for clients to access shared resources. When this directive is used, Samba checks the credentials provided by the client against its own user database or an external authentication method, such as a Windows domain.

This mode is commonly used because it allows for greater control over access to shared resources, ensuring that only authorized users can access certain files or directories. It is particularly important in environments where sensitive data is handled, as it helps in maintaining a secure access policy.

In contrast, other security modes like "share," "domain," or "guest" do not enforce the same level of user-based authentication. For example, "security = share" allows access based on shared resource control rather than individual user credentials, while "security = domain" engages in authenticating against a domain controller. "Security = guest" permits access without any valid user credentials, posing potential security risks for sensitive information.