utility imprisons a process and all future descendants.
The options are as follows:
Output the jail identifier of the newly created jail.
file, containing jail identifier, path, hostname, IP and
command used to start the jail.
Run program in the clean environment.
The environment is discarded except for
HOME , SHELL , TERM
are set to the target login's default values.
is set to the target login.
is imported from the current environment.
The environment variables from the login class capability database for the
target login are also set.
sysctl variable to the specified value inside the newly created jail.
The user name from host environment as whom the
The user name from jailed environment as whom the
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
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
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
man page for further details.
Setting up a Jail Directory Tree
To set up a jail directory tree containing an entire
Fx distribution, the following
command script can be used:
mkdir -p $D
make world DESTDIR=$D
make distribution DESTDIR=$D
mount -t devfs devfs $D/dev
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
for information on how to use devfs rules to limit access to entries
in the per-jail devfs.
A simple devfs ruleset for jails is available as ruleset #4 in
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
jail and remove things until it stops working,
than it is to start with a
jail and add things until it works.
Setting Up a Jail
Do what was described in
Sx 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
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
For consistency, we will refer to the parent box as the
and to the jailed virtual machine as the
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
to only listen on the
appropriate IP address, and so forth.
Add the following to
in the host environment:
inetd_flags="-wW -a 192.0.2.23"
is the native IP address for the host system, in this example.
Daemons that run out of
can be easily set to use only the specified host IP address.
will need to be manually configured---for some this is possible through
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:
it is necessary to modify
it is necessary to modify
it is necessary to modify
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
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.
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,
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.
with any machine (virtual or not) you will need to set a root password, time
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.
Assuming no errors, you will end up with a shell prompt within the jail.
You can now run
and do the post-install configuration to set various configuration options,
or perform these actions manually by editing
Create an empty
to quell startup warnings about missing fstab (virtual server only)
Disable the port mapper
(virtual server only)
so that name resolution within the jail will work correctly
Disable interface configuration to quell startup warnings about
(virtual server only)
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
so it logs as you would like, etc.
If you are not using a virtual server, you may wish to modify
in the host environment to listen on the syslog socket in the jail
environment; in this example, the syslog socket would be stored in
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
in the examples below.
To start a virtual server environment,
is run to launch various daemons and services.
To do this, first bring up the
virtual host interface, and then start the jail's
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
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 192.0.2.100/32
mount -t procfs proc /data/jail/192.0.2.100/proc
jail /data/jail/192.0.2.100 testhostname 192.0.2.100 \
A few warnings will be produced, because most
configuration variables cannot be set from within the jail, as they are
global across all jails and the host environment.
However, it should all
You should be able to see
and other processes running within the jail using
flag appearing beside jailed processes.
To see an active list of jails, use the
You should also be able to
to the hostname or IP address of the jailed environment, and log
in using the accounts you created previously.
It is possible to have jails started at boot time.
Please refer to the
for more information.
jail script provides a flexible system to start/stop jails:
Normal machine shutdown commands, such as
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
signals to all processes in the jail from within the jail.
the intended use of the jail, you may also want to run
from within the jail.
To kill processes from outside the jail, use the
utility in conjunction with the one of 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.
command also shows a
flag for processes in a jail.
However, the hostname for a jail may be, by
default, modified from within the jail, so the
status entry is unreliable by default.
To disable the setting of the hostname
from within a jail, set the
sysctl variable in the host environment to 0, which will affect all jails.
You can have this sysctl set on each boot using
Just add the following line to
You can also list/kill processes based on their jail ID.
To show processes and their jail ID, use the following command:
"ps ax -o pid,jid,args"
To show and then kill processes in jail number 3 use the following commands:
pgrep -lfj 3
pkill -j 3
"killall -j 3"
Sysctl MIB Entries
Certain aspects of the jail containments environment may be modified from
the host environment using
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
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
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.
by default this option is disabled.
This MIB entry determines which information processes in a jail are
able to get about mount-points.
It affects the behaviour of the following syscalls:
(as well as similar compatibility syscalls).
When set to 0, all mount-points are available without any restrictions.
When set to 1, only mount-points below the jail's chroot directory are
In addition to that, the path to the jail's chroot directory is removed
from the front of their pathnames.
When set to 2 (default), above syscalls can operate only on a mount-point
where the jail's chroot directory is located.
This MIB entry determines whether or not processes within a jail are
allowed to change their hostname via
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
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
permitting them access to
IPv4 addresses, and routing sockets.
To enable access to other domains, this MIB variable may be set to
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.
This MIB entry determines how a privileged user inside a jail will be
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
This MIB entry determines if a privileged user inside a jail will be
able to mount and unmount file system types marked as jail-friendly.
command can be used to find file system types available for mount from within
This functionality is disabled by default, but can be enabled by setting this
MIB entry to 1.
The read-only sysctl variable
can be used to determine if a process is running inside a jail (value
is one) or not (value is zero).
MIB entry is read-only and it returns an array of
Vt struct xprison
In sys/jail.h .
It is recommended to use the
utility to see current active list of jails.
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
An 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
as opposed to
Similarly, it might be a good idea to add an
address alias flag such that daemons listening on all IPs
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
which is easily configurable.