3.  Access control

      The printer system maintains protected spooling areas so that users cannot circumvent printer accounting or remove files other than their own. The strategy used to maintain protected spooling areas is as follows:

• The spooling area is writable only by a daemon user and daemon group.
• The lpr program runs set-user-id to root and set-group-id to group daemon. The root access permits reading any file required. Accessibility is verified with an access(2) call. The group ID is used in setting up proper ownership of files in the spooling area for lprm.
• Control files in a spooling area are made with daemon ownership and group ownership daemon. Their mode is 0660. This insures control files are not modified by a user and that no user can remove files except through lprm.
• The spooling programs, lpd, lpq, and lprm run set-user-id to root and set-group-id to group daemon to access spool files and printers.
• The printer server, lpd, uses the same verification procedures as rshd(8C) in authenticating remote clients. The host on which a client resides must be present in the file /etc/hosts.equiv or /etc/hosts.lpd and the request message must come from a reserved port number.

      In practice, none of lpd, lpq, or lprm would have to run as user root if remote spooling were not supported. In previous incarnations of the printer system lpd ran set-user-id to daemon, set-group-id to group spooling, and lpq and lprm ran set-group-id to group spooling.