PAM, through the /lib/security/pam_listfile.so module, allows great flexibility in denying specific accounts. This allows the administrator to point the module at a list of users who are not allowed to log in. Below is an example of how the module is used for the vsftpd FTP server in the /etc/pam.d/vsftpd PAM configuration file (the \ character at the end of the first line in the following example is not necessary if the directive is on one line):

         auth required /lib/security/pam_listfile.so item=user \ sense=deny file=/etc/vsftpd.ftpusers onerr=succeed 
      

This tells PAM to consult the file /etc/vsftpd.ftpusers and deny access to the service for any user listed. The administrator is free to change the name of this file, and can keep separate lists for each service or use one central list to deny access to multiple services.

If the administrator wants to deny access to multiple services, a similar line can be added to the PAM configuration services, such as /etc/pam.d/pop and /etc/pam.d/imap for mail clients or /etc/pam.d/ssh for SSH clients.

For more information about PAM, refer to the chapter titled Pluggable Authentication Modules (PAM) in the Red Hat Enterprise Linux Reference Guide.