14.4. Limiting Users

FreeBSD provides several methods for an administrator to limit the amount of system resources an individual may use. These limits are discussed in two sections: disk quotas and other resource limits.

Disk quotas limit the amount of disk space available to users and provide a way to quickly check that usage without calculating it every time. Quotas are discussed in Section 19.13, “File System Quotas”.

The other resource limits include ways to limit the amount of CPU, memory, and other resources a user may consume. These are defined using login classes and are discussed here.

Login classes are defined in /etc/login.conf and are described in detail in login.conf(5). Each user account is assigned to a login class, default by default, and each login class has a set of login capabilities associated with it. A login capability is a name=value pair, where name is a well-known identifier and value is an arbitrary string which is processed accordingly depending on the name. Setting up login classes and capabilities is rather straightforward and is also described in login.conf(5).


FreeBSD does not normally read the configuration in /etc/login.conf directly, but instead reads the /etc/login.conf.db database which provides faster lookups. Whenever /etc/login.conf is edited, the /etc/login.conf.db must be updated by executing the following command:

# cap_mkdb /etc/login.conf

Resource limits differ from the default login capabilities in two ways. First, for every limit, there is a soft (current) and hard limit. A soft limit may be adjusted by the user or application, but may not be set higher than the hard limit. The hard limit may be lowered by the user, but can only be raised by the superuser. Second, most resource limits apply per process to a specific user, not to the user as a whole. These differences are mandated by the specific handling of the limits, not by the implementation of the login capability framework.

Below are the most commonly used resource limits. The rest of the limits, along with all the other login capabilities, can be found in login.conf(5).


The limit on the size of a core file generated by a program is subordinate to other limits on disk usage, such as filesize, or disk quotas. This limit is often used as a less-severe method of controlling disk space consumption. Since users do not generate core files themselves, and often do not delete them, setting this may save them from running out of disk space should a large program crash.


The maximum amount of CPU time a user's process may consume. Offending processes will be killed by the kernel.


This is a limit on CPU time consumed, not percentage of the CPU as displayed in some fields by top(1) and ps(1).


The maximum size of a file the user may own. Unlike disk quotas, this limit is enforced on individual files, not the set of all files a user owns.


The maximum number of processes a user can run. This includes foreground and background processes. This limit may not be larger than the system limit specified by the kern.maxproc sysctl(8). Setting this limit too small may hinder a user's productivity as it is often useful to be logged in multiple times or to execute pipelines. Some tasks, such as compiling a large program, spawn multiple processes and other intermediate preprocessors.


The maximum amount of memory a process may request to be locked into main memory using mlock(2). Some system-critical programs, such as amd(8), lock into main memory so that if the system begins to swap, they do not contribute to disk thrashing.


The maximum amount of memory a process may consume at any given time. It includes both core memory and swap usage. This is not a catch-all limit for restricting memory consumption, but is a good start.


The maximum number of files a process may have open. In FreeBSD, files are used to represent sockets and IPC channels, so be careful not to set this too low. The system-wide limit for this is defined by the kern.maxfiles sysctl(8).


The limit on the amount of network memory, and thus mbufs, a user may consume in order to limit network communications.


The maximum size of a process stack. This alone is not sufficient to limit the amount of memory a program may use so it should be used in conjunction with other limits.

There are a few other things to remember when setting resource limits. Following are some general tips, suggestions, and miscellaneous comments.

For further information on resource limits and login classes and capabilities in general, refer to cap_mkdb(1), getrlimit(2), and login.conf(5).

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.

Send questions about this document to <freebsd-doc@FreeBSD.org>.