16.4. Creating and Controlling Jails

Some administrators divide jails into the following two types: complete jails, which resemble a real FreeBSD system, and service jails, dedicated to one application or service, possibly running with privileges. This is only a conceptual division and the process of building a jail is not affected by it. The jail(8) manual page is quite clear about the procedure for building a jail:

# setenv D /here/is/the/jail # mkdir -p $D 1 # cd /usr/src # make buildworld 2 # make installworld DESTDIR=$D 3 # make distribution DESTDIR=$D 4 # mount -t devfs devfs $D/dev 5


Selecting a location for a jail is the best starting point. This is where the jail will physically reside within the file system of the jail's host. A good choice can be /usr/jail/jailname, where jailname is the hostname identifying the jail. The /usr/ file system usually has enough space for the jail file system, which for complete jails is, essentially, a replication of every file present in a default installation of the FreeBSD base system.


If you have already rebuilt your userland using make world or make buildworld, you can skip this step and install your existing userland into the new jail.


This command will populate the directory subtree chosen as jail's physical location on the file system with the necessary binaries, libraries, manual pages and so on.


The distribution target for make installs every needed configuration file. In simple words, it installs every installable file of /usr/src/etc/ to the /etc directory of the jail environment: $D/etc/.


Mounting the devfs(8) file system inside a jail is not required. On the other hand, any, or almost any application requires access to at least one device, depending on the purpose of the given application. It is very important to control access to devices from inside a jail, as improper settings could permit an attacker to do nasty things in the jail. Control over devfs(8) is managed through rulesets which are described in the devfs(8) and devfs.conf(5) manual pages.

Once a jail is installed, it can be started by using the jail(8) utility. The jail(8) utility takes four mandatory arguments which are described in the Section 16.3.1, “What is a Jail”. Other arguments may be specified too, e.g., to run the jailed process with the credentials of a specific user. The command argument depends on the type of the jail; for a virtual system, /etc/rc is a good choice, since it will replicate the startup sequence of a real FreeBSD system. For a service jail, it depends on the service or application that will run within the jail.

Jails are often started at boot time and the FreeBSD rc mechanism provides an easy way to do this.

  1. A list of the jails which are enabled to start at boot time should be added to the rc.conf(5) file:

    jail_enable="YES" # Set to NO to disable starting of any jails jail_list="www" # Space separated list of names of jails


    Jail names in jail_list should contain alphanumeric characters only.

  2. For each jail listed in jail_list, a group of rc.conf(5) settings, which describe the particular jail, should be added:

    jail_www_rootdir="/usr/jail/www" # jail's root directory jail_www_hostname="www.example.org" # jail's hostname jail_www_ip="" # jail's IP address jail_www_devfs_enable="YES" # mount devfs in the jail jail_www_devfs_ruleset="www_ruleset" # devfs ruleset to apply to jail

    The default startup of jails configured in rc.conf(5), will run the /etc/rc script of the jail, which assumes the jail is a complete virtual system. For service jails, the default startup command of the jail should be changed, by setting the jail_jailname_exec_start option appropriately.


    For a full list of available options, please see the rc.conf(5) manual page.

service(8) can be used to start or stop a jail by hand, if an entry for it exists in rc.conf:

# service jail start www # service jail stop www

A clean way to shut down a jail(8) is not available at the moment. This is because commands normally used to accomplish a clean system shutdown cannot be used inside a jail. The best way to shut down a jail is to run the following command from within the jail itself or using the jexec(8) utility from outside the jail:

# sh /etc/rc.shutdown

More information about this can be found in the jail(8) manual page.

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>.