For years, FreeBSD has used a resource limits database controlled through a flat file, /etc/login.conf. While it has been discussed previously and is still supported, it is not the most optimal method of controlling resources. The flat file requires users to be divided into various group labels known as classes, which require changes not only to this flat file but also the password database. Potentially a single, more constrained user would require an additional label to be added, the resource database rebuilt using cap_mkdb, and edits made to /etc/master.passwd. In addition, the password database must be rebuilt using pwd_mkdb. This multi-step process could be very time consuming depending on how many users must be singled out.

A new command in FreeBSD, rctl(8), allows for a more fine grained method of controlling resources limits for users. This command will support much more than users, it will also set resource constraints on processes, jails, and the original login class. These advanced features provide administrators and users with methods to control resources through the command line and set rules on system initialization using a configuration file.

To enable this feature, add these lines to GENERIC, or the custom kernel configuration file, and rebuild.:

The entire system will need rebuilt. See Chapter 9, Configuring the FreeBSD Kernel, which will provide instructions for the process. Once this is complete, rctl may be used to set rules for the system.

Rule syntax is simple, controlled through the use of a subject, a subject-id, resource, and action. Take the following example rule:

This rule shows a basic premise of a rule, here the subject is user and the subject-id is trhodes. The maxproc is, of course, max number of processes, which is considered the resource. The action here is set to deny, which blocks any new processes from being created. In the previous example, the user, trhodes will be constrained to 10 (ten) processes and no greater. Other actions are available and could be log to the console, pass a notification to devd(8), or send a sigterm to the process.

Some care must be taken while adding rules. The one above will unfortunately block my user from doing the most simple tasks after I have logged in and executed a screen session. When a resource limit has been hit, an error will be printed, as in this example:

For another example, rctl(8) can be used to prevent a jail from exceeding a memory limit. This rule could be written as:

Rules may also persist across reboots if they have been added to /etc/rctl.conf. The format is a rule, without the preceding command. For example, the previous rule could be added like the following:

To remove a rule, just ask rctl to remove it from the list:

The manual page shows a method for removing all rules; however, if removing all rules for a single user is required, this command may be issued:

Many other resources exist which can be used to exert additional control over various subjects. See rctl(8) to learn about them.