6.14. pam_limits - limit resources
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6.14. pam_limits - limit resources

pam_limits.so [ conf=/path/to/limits.conf ] [ debug ] [ set_all ] [ utmp_early ] [ noaudit ]

6.14.1. DESCRIPTION

The pam_limits PAM module sets limits on the system resources that can be obtained in a user-session. Users of uid=0 are affected by this limits, too.

By default limits are taken from the /etc/security/limits.conf config file. Then individual *.conf files from the /etc/security/limits.d/ directory are read. The files are parsed one after another in the order of "C" locale. The effect of the individual files is the same as if all the files were concatenated together in the order of parsing. If a config file is explicitly specified with a module option then the files in the above directory are not parsed.

The module must not be called by a multithreaded application.

If Linux PAM is compiled with audit support the module will report when it denies access based on limit of maximum number of concurrent login sessions.

6.14.2. DESCRIPTION

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:

<domain> <type> <item> <value>

The fields listed above should be filled as follows:

<domain>

  • 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.

<type>
hard

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.

soft

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. .

<item>
core

limits the core file size (KB)

data

maximum data size (KB)

fsize

maximum filesize (KB)

memlock

maximum locked-in-memory address space (KB)

nofile

maximum number of open file descriptors

rss

maximum resident set size (KB) (Ignored in Linux 2.4.30 and higher)

stack

maximum stack size (KB)

cpu

maximum CPU time (minutes)

nproc

maximum number of processes

as

address space limit (KB)

maxlogins

maximum number of logins for this user (this limit does not apply to user with uid=0)

maxsyslogins

maximum number of all logins on system; user is not allowed to log-in if total number of all user logins is greater than specified number (this limit does not apply to user with uid=0)

nonewprivs

value of 0 or 1; if set to 1 disables acquiring new privileges by invoking prctl(PR_SET_NO_NEW_PRIVS)

priority

the priority to run user process with (negative values boost process priority)

locks

maximum locked files (Linux 2.4 and higher)

sigpending

maximum number of pending signals (Linux 2.6 and higher)

msgqueue

maximum memory used by POSIX message queues (bytes) (Linux 2.6 and higher)

nice

maximum nice priority allowed to raise to (Linux 2.6.12 and higher) values: [-20,19]

rtprio

maximum realtime priority allowed for non-privileged processes (Linux 2.6.12 and higher)

All items support the values -1, unlimited or infinity indicating no limit, except for priority, nice, and nonewprivs. If nofile is to be set to one of these values, it will be set to the contents of /proc/sys/fs/nr_open instead (see setrlimit(3)).

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.

The pam_limits module does report configuration problems found in its configuration file and errors via syslog(3).

6.14.3. OPTIONS

conf=/path/to/limits.conf

Indicate an alternative limits.conf style configuration file to override the default.

debug

Print debug information.

set_all

Set the limits for which no value is specified in the configuration file to the one from the process with the PID 1. Please note that if the init process is systemd these limits will not be the kernel default limits and this option should not be used.

utmp_early

Some broken applications actually allocate a utmp entry for the user before the user is admitted to the system. If some of the services you are configuring PAM for do this, you can selectively use this module argument to compensate for this behavior and at the same time maintain system-wide consistency with a single limits.conf file.

noaudit

Do not report exceeded maximum logins count to the audit subsystem.

6.14.4. MODULE TYPES PROVIDED

Only the session module type is provided.

6.14.5. RETURN VALUES

PAM_ABORT

Cannot get current limits.

PAM_IGNORE

No limits found for this user.

PAM_PERM_DENIED

New limits could not be set.

PAM_SERVICE_ERR

Cannot read config file.

PAM_SESSION_ERR

Error recovering account name.

PAM_SUCCESS

Limits were changed.

PAM_USER_UNKNOWN

The user is not known to the system.

6.14.6. FILES

/etc/security/limits.conf

Default configuration file

6.14.7. EXAMPLES

These are some example lines which might be specified in /etc/security/limits.conf. 

*               soft    core            0
*               hard    nofile          512
@student        hard    nproc           20
@faculty        soft    nproc           20
@faculty        hard    nproc           50
ftp             hard    nproc           0
@student        -       maxlogins       4
@student        -       nonewprivs      1
:123            hard    cpu             5000
@500:           soft    cpu             10000
600:700         hard    locks           10

6.14.8. AUTHORS

pam_limits was initially written by Cristian Gafton <gafton@redhat.com>

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