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.