When someone logs in, the file access.conf is scanned for the first entry that matches the (user/group, host) or (user/group, network/netmask) combination, or, in case of non-networked logins, the first entry that matches the (user/group, tty) combination, or in the case of non-networked logins without a tty, the first entry that matches the (user/group, X-$DISPLAY-value) or (user/group, pam-service-name/) combination. The permissions field of that table entry determines whether the login will be accepted or refused.
Each line of the login access control table has three fields separated by a ":" character (colon):
The first field, the permission field, can be either a "+" character (plus) for access granted or a "-" character (minus) for access denied.
The second field, the users/group field, should be a list of one or more login names, group names, or ALL (which always matches). To differentiate user entries from group entries, group entries should be written with brackets, e.g. (group).
The third field, the origins field, should be a list of one or more tty names (for non-networked logins), X $DISPLAY values or PAM service names (for non-networked logins without a tty), host names, domain names (begin with "."), host addresses, internet network numbers (end with "."), internet network addresses with network mask (where network mask can be a decimal number or an internet address also), ALL (which always matches) or LOCAL. The LOCAL keyword matches if and only if pam_get_item(3), when called with an item_type of PAM_RHOST, returns NULL or an empty string (and therefore the origins field is compared against the return value of pam_get_item(3) called with an item_type of PAM_TTY or, absent that, PAM_SERVICE).
If supported by the system you can use @netgroupname in host or user patterns. The @@netgroupname syntax is supported in the user pattern only and it makes the local system hostname to be passed to the netgroup match call in addition to the user name. This might not work correctly on some libc implementations causing the match to always fail.
The EXCEPT operator makes it possible to write very compact rules.
If the nodefgroup is not set, the group file is searched when a name does not match that of the logged-in user. Only groups are matched in which users are explicitly listed. However the PAM module does not look at the primary group id of a user.
The "#" character at start of line (no space at front) can be used to mark this line as a comment line.
User root should be allowed to get access via cron, X11 terminal :0, tty1, ..., tty5, tty6.
+:root:crond :0 tty1 tty2 tty3 tty4 tty5 tty6
User root should be allowed to get access from hosts which own the IPv4 addresses. This does not mean that the connection have to be a IPv4 one, a IPv6 connection from a host with one of this IPv4 addresses does work, too.
+:root:192.168.200.1 192.168.200.4 192.168.200.9
User root should get access from network 192.168.201. where the term will be evaluated by string matching. But it might be better to use network/netmask instead. The same meaning of 192.168.201. is 192.168.201.0/24 or 192.168.201.0/255.255.255.0.
User root should be able to have access from hosts foo1.bar.org and foo2.bar.org (uses string matching also).
User root should be able to have access from domain foo.bar.org (uses string matching also).
User root should be denied to get access from all other sources.
User foo and members of netgroup admins should be allowed to get access from all sources. This will only work if netgroup service is available.
User john and foo should get access from IPv6 host address.
User john should get access from IPv6 net/mask.
Members of group wheel should be allowed to get access from all sources.
Disallow console logins to all but the shutdown, sync and all other accounts, which are a member of the wheel group.
-:ALL EXCEPT (wheel) shutdown sync:LOCAL
All other users should be denied to get access from all sources.
Network address / netmask description and example text was introduced by Mike Becher <email@example.com>.