Section: Linux-PAM Manual (5)
The pam_limits.so module applies ulimit limits, nice priority and number of simultaneous login sessions limit to user login sessions. This description of the configuration file syntax applies to the /etc/security/limits.conf file and *.conf files in the /etc/security/limits.d directory.
The syntax of the lines is as follows:
The fields listed above should be filled as follows:
- • a username
- • a groupname, with @group syntax. This should not be confused with netgroups.
- • the wildcard *, for default entry.
- • the wildcard %, for maxlogins limit only, can also be used with %group syntax. If the % wildcard is used alone it is identical to using * with maxsyslogins limit. With a group specified after % it limits the total number of logins of all users that are member of the group.
- • an uid range specified as <min_uid>
- <max_uid>. If min_uid is omitted, the match is exact for the max_uid. If max_uid is omitted, all uids greater than or equal min_uid match.:
- • a gid range specified as @<min_gid>
- <max_gid>. If min_gid is omitted, the match is exact for the max_gid. If max_gid is omitted, all gids greater than or equal min_gid match. For the exact match all groups including the user's supplementary groups are examined. For the range matches only the user's primary group is examined.:
- • a gid specified as %
- <gid> applicable to maxlogins limit only. It limits the total number of logins of all users that are member of the group with the specified gid.:
NOTE: group and wildcard limits are not applied to the root user. To set a limit for the root user, this field must contain the literal username root.
- for enforcing hard resource limits. These limits are set by the superuser and enforced by the Kernel. The user cannot raise his requirement of system resources above such values.
- for enforcing soft resource limits. These limits are ones that the user can move up or down within the permitted range by any pre-existing hard limits. The values specified with this token can be thought of as default values, for normal system usage.
:for enforcing both soft and hard resource limits together.
Note, if you specify a type of '-' but neglect to supply the item and value fields then the module will never enforce any limits on the specified user/group etc. .
- limits the core file size (KB)
- maximum data size (KB)
- maximum filesize (KB)
- maximum locked-in-memory address space (KB)
- maximum number of open files
- maximum resident set size (KB) (Ignored in Linux 2.4.30 and higher)
- maximum stack size (KB)
- maximum CPU time (minutes)
- maximum number of processes
- address space limit (KB)
- maximum number of logins for this user except for this with uid=0
- maximum number of all logins on system
- the priority to run user process with (negative values boost process priority)
- maximum locked files (Linux 2.4 and higher)
- maximum number of pending signals (Linux 2.6 and higher)
- maximum memory used by POSIX message queues (bytes) (Linux 2.6 and higher)
- maximum nice priority allowed to raise to (Linux 2.6.12 and higher) values
- maximum realtime priority allowed for non-privileged processes (Linux 2.6.12 and higher)
- the directory to chroot the user to
All items support the values -1, unlimited or infinity indicating no limit, except for priority and nice.
If a hard limit or soft limit of a resource is set to a valid value, but outside of the supported range of the local system, the system may reject the new limit or unexpected behavior may occur. If the control value required is used, the module will reject the login if a limit could not be set.
In general, individual limits have priority over group limits, so if you impose no limits for admin group, but one of the members in this group have a limits line, the user will have its limits set according to this line.
Also, please note that all limit settings are set per login. They are not global, nor are they permanent; existing only for the duration of the session. One exception is the maxlogin option, this one is system wide. But there is a race, concurrent logins at the same time will not always be detect as such but only counted as one.
In the limits configuration file, the '#' character introduces a comment - after which the rest of the line is ignored.
These are some example lines which might be specified in /etc/security/limits.conf.
* soft core 0 root hard core 100000 * hard nofile 512 @student hard nproc 20 @faculty soft nproc 20 @faculty hard nproc 50 ftp hard nproc 0 @student - maxlogins 4 :123 hard cpu 5000 @500: soft cpu 10000 600:700 hard locks 10
pam_limits was initially written by Cristian Gafton <[email protected]>