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:
-i Output the jail identifier of the newly created jail.
-l 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 the 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
-U username The user name from jailed environment as whom the command
path Directory which is to be the root of the prison.
hostname Hostname of the prison.
ip-number IP number assigned to the prison.
command Pathname of the program which is to be executed.
Jails are typically set up using one of two philosophies: either to con-
strain a specific application (possibly running with privilege), or to
create a ``virtual system image'' running a variety of daemons and ser-
vices. 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 con-
figuration 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:
mkdir -p $D
make world DESTDIR=$D
make distribution DESTDIR=$D
mount_devfs devfs $D/dev
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 pro-
cesses in the jail to bypass the jail sandboxing by modifying files out-
side 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 exe-
cutable 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/192.168.11.100, named for the jailed IP address.
Substitute below as needed with your own directory, IP address, and host-
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 lis-
ten 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 appro-
priate IP address, and so forth. Add the following to /etc/rc.conf in
the host environment:
inetd_flags="-wW -a 192.168.11.23"
192.168.11.23 is the native IP address for the host system, in this exam-
ple. 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 con-
figured--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 ser-
vices 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
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 ser-
vices using rpc(3), such as rpcbind(8,) nfsd(8), and mountd(8). In gen-
eral, 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 run-
ning in the host environment should also be checked and configured so
that it does not bind all IP addresses, which would result in those ser-
vices' 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
Configuring the Jail
Start any jail for the first time without configuring the network inter-
face 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 par-
ticular application or for running a virtual server.
Start a shell in the jail:
jail /data/jail/192.168.11.100 testhostname 192.168.11.100 /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 configura-
tion to set various configuration options, or perform these actions manu-
ally by editing /etc/rc.conf, etc.
o Create an empty /etc/fstab to quell startup warnings about
missing fstab (virtual server only)
o Disable the port mapper (/etc/rc.conf: rpcbind_enable="NO")
(virtual server only)
o Run newaliases(1) to quell sendmail(8) warnings.
o Disable interface configuration to quell startup warnings about
ifconfig(8) (network_interfaces="") (virtual server only)
o Configure /etc/resolv.conf so that name resolution within the
jail will work correctly
o Set a root password, probably different from the real host sys-
o Set the timezone
o Add accounts for users in the jail environment
o 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/192.168.11.100/var/run/log.
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 appli-
cation 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 192.168.11.100/32
mount -t procfs proc /data/jail/192.168.11.100/proc
jail /data/jail/192.168.11.100 testhostname 192.168.11.100 \
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 prop-
erly. You should be able to see inetd(8), syslogd(8), and other pro-
cesses 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 cre-
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 follow-
ing 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
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 pro-
cesses) 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 dis-
abled in environments where privileged access to jails is given out
to untrusted parties.
The jail functionality binds an IPv4 address to each jail, and lim-
its 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, pro-
cesses 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 imple-
mentation, 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.
This MIB entry determines how a privileged user inside a jail will
be treated by chflags(2). If zero, such users are treated as
unprivileged, and are unable to set or clear system file flags; if
non-zero, such users are treated as privileged, and may manipulate
system file flags subject to the usual constaints on
There are currently two MIB related variables that have per-jail set-
tings. 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
http://www.rndassociates.com/ 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 infor-
mation 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 build-
ing a safe host environment such that host daemons do not impose on ser-
vices 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