Which Samba parameter named for "valid" users prevents unauthorized access to shares?

The parameter "valid users" in Samba is specifically designed to control access to shared resources by specifying which users are allowed to connect to a particular share. When this parameter is set in the Samba configuration file (smb.conf), it takes a list of usernames or groups that are permitted access. If a user attempts to access the share but is not listed under the "valid users" parameter, they will be denied access, thus effectively preventing unauthorized users from accessing the shared resources.

This is an essential feature for maintaining security in a mixed environment, as it allows administrators to finely tune access permissions based on the needs of different users and groups. In contrast, other parameters serve different purposes: "invalid users" specifies users who are explicitly denied access, "admin users" grants elevated privileges to specified users, and "guest users" pertains to permissions for users accessing the share without a valid account. However, none of these functions directly serves as a comprehensive access control mechanism like "valid users" does.