JAIL(8) BSD System Manager’s Manual JAIL(8)


jail — imprison process and its descendants


jail [−i] [−l −u username −U username] path hostname ip-number command ...


The jail utility imprisons a process and all future descendants.

The options are as follows:


Output the jail identifier of the newly created jail.


Run program in the clean environment. The environment is discarded except for HOME, SHELL, TERM and USER. HOME and SHELL are set to the target login’s default values. USER is set to the target login. TERM is imported from your current environment. The environment variables from the login class capability database for the target login are also set.

−u username

The user name from host environment as whom the command should run.

−U username

The user name from jailed environment as whom the command should run.


Directory which is to be the root of the prison.


Hostname of the prison.


IP number assigned to the prison.


Pathname of the program which is to be executed.

Jails are typically set up using one of two philosophies: either to constrain a specific application (possibly running with privilege), or to create a ’’virtual system image’’ running a variety of daemons and services. In both cases, a fairly complete file system install of FreeBSD is required, so as to provide the necessary command line tools, daemons, libraries, application configuration files, etc. However, for a virtual server configuration, a fair amount of additional work is required so as to configure the ’’boot’’ process. This manual page documents the configuration steps necessary to support either of these steps, although the configuration steps may be refined based on local requirements.

Please see the jail(2) man page for further details.


Setting up a Jail Directory Tree
This example shows how to set up a jail directory tree containing an entire FreeBSD distribution:

cd /usr/src
mkdir -p $D
make world DESTDIR=$D
cd etc
make distribution DESTDIR=$D
mount_devfs devfs $D/dev
cd $D
ln -sf dev/null kernel

NOTE: It is important that only appropriate device nodes in devfs be exposed to a jail; access to disk devices in the jail may permit processes in the jail to bypass the jail sandboxing by modifying files outside of the jail. See devfs(8) for information on how to use devfs rules to limit access to entries in the per-jail devfs.

In many cases this example would put far more in the jail than needed. In the other extreme case a jail might contain only one file: the executable to be run in the jail.

We recommend experimentation and caution that it is a lot easier to start with a ’’fat’’ jail and remove things until it stops working, than it is to start with a ’’thin’’ jail and add things until it works.

Setting Up a Jail
Do what was described in Setting Up a Jail Directory Tree to build the jail directory tree. For the sake of this example, we will assume you built it in /data/jail/, named for the jailed IP address. Substitute below as needed with your own directory, IP address, and hostname.

Setting up the Host Environment
First, you will want to set up your real system’s environment to be ’’jail-friendly’’. For consistency, we will refer to the parent box as the ’’host environment’’, and to the jailed virtual machine as the ’’jail environment’’. Since jail is implemented using IP aliases, one of the first things to do is to disable IP services on the host system that listen on all local IP addresses for a service. If a network service is present in the host environment that binds all available IP addresses rather than specific IP addresses, it may service requests sent to jail IP addresses. This means changing inetd(8) to only listen on the appropriate IP address, and so forth. Add the following to /etc/rc.conf in the host environment:

inetd_flags="-wW -a"
rpcbind_enable="NO" is the native IP address for the host system, in this example. Daemons that run out of inetd(8) can be easily set to use only the specified host IP address. Other daemons will need to be manually configured—for some this is possible through the rc.conf(5) flags entries; for others it is necessary to modify per-application configuration files, or to recompile the applications. The following frequently deployed services must have their individual configuration files modified to limit the application to listening to a specific IP address:

To configure sshd(8), it is necessary to modify /etc/ssh/sshd_config.

To configure sendmail(8), it is necessary to modify /etc/mail/

For named(8), it is necessary to modify /etc/namedb/named.conf.

In addition, a number of services must be recompiled in order to run them in the host environment. This includes most applications providing services using rpc(3), such as rpcbind(8,) nfsd(8), and mountd(8). In general, applications for which it is not possible to specify which IP address to bind should not be run in the host environment unless they should also service requests sent to jail IP addresses. Attempting to serve NFS from the host environment may also cause confusion, and cannot be easily reconfigured to use only specific IPs, as some NFS services are hosted directly from the kernel. Any third-party network software running in the host environment should also be checked and configured so that it does not bind all IP addresses, which would result in those services’ also appearing to be offered by the jail environments.

Once these daemons have been disabled or fixed in the host environment, it is best to reboot so that all daemons are in a known state, to reduce the potential for confusion later (such as finding that when you send mail to a jail, and its sendmail is down, the mail is delivered to the host, etc.).

Configuring the Jail
Start any jail for the first time without configuring the network interface so that you can clean it up a little and set up accounts. As with any machine (virtual or not) you will need to set a root password, time zone, etc. Some of these steps apply only if you intend to run a full virtual server inside the jail; others apply both for constraining a particular application or for running a virtual server.

Start a shell in the jail:

jail /data/jail/ testhostname /bin/sh

Assuming no errors, you will end up with a shell prompt within the jail. You can now run /usr/sbin/sysinstall and do the post-install configuration to set various configuration options, or perform these actions manually by editing /etc/rc.conf, etc.

Create an empty /etc/fstab to quell startup warnings about missing fstab (virtual server only)

Disable the port mapper (/etc/rc.conf: rpcbind_enable="NO") (virtual server only)

Run newaliases(1) to quell sendmail(8) warnings.

Disable interface configuration to quell startup warnings about ifconfig(8) (network_interfaces="") (virtual server only)

Configure /etc/resolv.conf so that name resolution within the jail will work correctly

Set a root password, probably different from the real host system

Set the timezone

Add accounts for users in the jail environment

Install any packages the environment requires

You may also want to perform any package-specific configuration (web servers, SSH servers, etc), patch up /etc/syslog.conf so it logs as you would like, etc. If you are not using a virtual server, you may wish to modify syslogd(8) in the host environment to listen on the syslog socket in the jail environment; in this example, the syslog socket would be stored in /data/jail/

Exit from the shell, and the jail will be shut down.

Starting the Jail
You are now ready to restart the jail and bring up the environment with all of its daemons and other programs. If you are running a single application in the jail, substitute the command used to start the application for /etc/rc in the examples below. To start a virtual server environment, /etc/rc is run to launch various daemons and services. To do this, first bring up the virtual host interface, and then start the jail’s /etc/rc script from within the jail.

NOTE: If you plan to allow untrusted users to have root access inside the jail, you may wish to consider setting the security.jail.set_hostname_allowed sysctl variable to 0. Please see the management discussion later in this document as to why this may be a good idea. If you do decide to set this variable, it must be set before starting any jails, and once each boot.

ifconfig ed0 inet alias
mount -t procfs proc /data/jail/
jail /data/jail/ testhostname \

/bin/sh /etc/rc

A few warnings will be produced, because most sysctl(8) configuration variables cannot be set from within the jail, as they are global across all jails and the host environment. However, it should all work properly. You should be able to see inetd(8), syslogd(8), and other processes running within the jail using ps(1), with the ’J’ flag appearing beside jailed processes. To see an active list of jails, use the jls(8) utility. You should also be able to telnet(1) to the hostname or IP address of the jailed environment, and log in using the accounts you created previously.

Managing the Jail
Normal machine shutdown commands, such as halt(8), reboot(8), and shutdown(8), cannot be used successfully within the jail. To kill all processes in a jail, you may log into the jail and, as root, use one of the following commands, depending on what you want to accomplish:

kill -TERM -1
kill -KILL -1

This will send the SIGTERM or SIGKILL signals to all processes in the jail from within the jail. Depending on the intended use of the jail, you may also want to run /etc/rc.shutdown from within the jail. To kill processes from outside the jail, use the jexec(8) utility in conjuction with the one of the kill(1) commands above, or use the killall(1) utility with the −j option.

The /proc/pid/status file contains, as its last field, the hostname of the jail in which the process runs, or ’’-’’ to indicate that the process is not running within a jail. The ps(1) command also shows a ’J’ flag for processes in a jail. However, the hostname for a jail may be, by default, modified from within the jail, so the /proc status entry is unreliable by default. To disable the setting of the hostname from within a jail, set the security.jail.set_hostname_allowed sysctl variable in the host environment to 0, which will affect all jails. You can have this sysctl set on each boot using sysctl.conf(5). Just add the following line to /etc/sysctl.conf:


Sysctl MIB Entries
Certain aspects of the jail containments environment may be modified from the host environment using sysctl(8) MIB variables. Currently, these variables affect all jails on the system, although in the future this functionality may be finer grained.


This MIB entry determines whether or not prison root is allowed to create raw sockets. Setting this MIB to 1 allows utilities like ping(8) and traceroute(8) to operate inside the prison. If this MIB is set, the source IP addresses are enforced to comply with the IP address bound to the jail, regardless of whether or not the IP_HDRINCL flag has been set on the socket. Since raw sockets can be used to configure and interact with various network subsystems, extra caution should be used where privileged access to jails is given out to untrusted parties. As such, by default this option is disabled.


This MIB entry determines whether or not processes within a jail are able to see data for all mountpoints. When set to 1 (default), the getfsstat(2) system call returns only (when called by jailed processes) the data for the file system on which the jail’s root vnode is located. Note: this also has the effect of hiding other mounts inside a jail, such as /dev, /tmp, and /proc, but errs on the side of leaking less information.


This MIB entry determines whether or not processes within a jail are allowed to change their hostname via hostname(1) or sethostname(3). In the current jail implementation, the ability to set the hostname from within the jail can impact management tools relying on the accuracy of jail information in /proc. As such, this should be disabled in environments where privileged access to jails is given out to untrusted parties.


The jail functionality binds an IPv4 address to each jail, and limits access to other network addresses in the IPv4 space that may be available in the host environment. However, jail is not currently able to limit access to other network protocol stacks that have not had jail functionality added to them. As such, by default, processes within jails may only access protocols in the following domains: PF_LOCAL, PF_INET, and PF_ROUTE, permitting them access to UNIX domain sockets, IPv4 addresses, and routing sockets. To enable access to other domains, this MIB variable may be set to 0.


This MIB entry determines whether or not processes within a jail have access to System V IPC primitives. In the current jail implementation, System V primitives share a single namespace across the host and jail environments, meaning that processes within a jail would be able to communicate with (and potentially interfere with) processes outside of the jail, and in other jails. As such, this functionality is disabled by default, but can be enabled by setting this MIB entry to 1.

There are currently two MIB related variables that have per-jail settings. Changes to these variables by a jailed process do not effect the host environment, only the jail environment. The variables are kern.securelevel and kern.hostname.


killall(1), newaliases(1), ps(1), chroot(2), jail(2), jail_attach(2), procfs(5), rc.conf(5), sysctl.conf(5), devfs(8), halt(8), inetd(8), jexec(8), jls(8), mount_devfs(8), named(8), reboot(8), rpcbind(8), sendmail(8), shutdown(8), sysctl(8), syslogd(8)


The jail utility appeared in FreeBSD 4.0.


The jail feature was written by Poul-Henning Kamp for R&D Associates who contributed it to FreeBSD.

Robert Watson wrote the extended documentation, found a few bugs, added a few new features, and cleaned up the userland jail environment.


Jail currently lacks the ability to allow access to specific jail information via ps(1) as opposed to procfs(5). Similarly, it might be a good idea to add an address alias flag such that daemons listening on all IPs (INADDR_ANY) will not bind on that address, which would facilitate building a safe host environment such that host daemons do not impose on services offered from within jails. Currently, the simplest answer is to minimize services offered on the host, possibly limiting it to services offered from inetd(8) which is easily configurable.

BSD April 8, 2003 BSD