usr.sbin/jail/jail.8

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.Dd February 9, 2012
.Dt JAIL 8
.Os
.Sh NAME
.Nm jail
.Nd "create or modify a system jail"
.Sh SYNOPSIS
.Nm
.Op Fl dhi
.Op Fl J Ar jid_file
.Op Fl l u Ar username | Fl U Ar username
.Op Fl c | m
.Op Ar parameter=value ...
.Nm
.Op Fl hi
.Op Fl n Ar jailname
.Op Fl J Ar jid_file
.Op Fl s Ar securelevel
.Op Fl l u Ar username | Fl U Ar username
.Op Ar path hostname [ip[,..]] command ...
.Nm
.Op Fl r Ar jail
.Sh DESCRIPTION
The
.Nm
utility creates a new jail or modifies an existing jail, optionally
imprisoning the current process (and future descendants) inside it.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl d
Allow making changes to a dying jail.
.It Fl h
Resolve the
.Va host.hostname
parameter (or
.Va hostname )
and add all IP addresses returned by the resolver
to the list of
.Va ip
addresses for this prison.
This may affect default address selection for outgoing IPv4 connections
of prisons.
The address first returned by the resolver for each address family
will be used as primary address.
See the
.Va ip4.addr
and
.Va ip6.addr
parameters further down for details.
.It Fl i
Output the jail identifier of the newly created jail.
.It Fl n Ar jailname
Set the jail's name.
This is deprecated and is equivalent to setting the
.Va name
parameter.
.It Fl J Ar jid_file
Write a
.Ar jid_file
file, containing jail identifier, path, hostname, IP and
command used to start the jail.
.It Fl l
Run program in the clean environment.
The environment is discarded except for
.Ev HOME , SHELL , TERM
and
.Ev USER .
.Ev HOME
and
.Ev SHELL
are set to the target login's default values.
.Ev USER
is set to the target login.
.Ev TERM
is imported from the current environment.
The environment variables from the login class capability database for the
target login are also set.
.It Fl s Ar securelevel
Set the
.Va kern.securelevel
MIB entry to the specified value inside the newly created jail.
This is deprecated and is equivalent to setting the
.Va securelevel
parameter.
.It Fl u Ar username
The user name from host environment as whom the
.Ar command
should run.
.It Fl U Ar username
The user name from jailed environment as whom the
.Ar command
should run.
.It Fl c
Create a new jail.
The
.Va jid
and
.Va name
parameters (if specified) must not refer to an existing jail.
.It Fl m
Modify an existing jail.
One of the
.Va jid
or
.Va name
parameters must exist and refer to an existing jail.
.It Fl cm
Create a jail if it does not exist, or modify a jail if it does exist.
.It Fl r
Remove the
.Ar jail
specified by jid or name.
All jailed processes are killed, and all children of this jail are also
removed.
.El
.Pp
At least one of the
.Fl c ,
.Fl m
or
.Fl r
options must be specified.
.Pp
.Ar Parameters
are listed in
.Dq name=value
form, following the options.
Some parameters are boolean, and do not have a value but are set by the
name alone with or without a
.Dq no
prefix, e.g.
.Va persist
or
.Va nopersist .
Any parameters not set will be given default values, often based on the
current environment.
.Pp
The pseudo-parameter
.Va command
specifies that the current process should enter the new (or modified) jail,
and run the specified command.
It must be the last parameter specified, because it includes not only
the value following the
.Sq =
sign, but also passes the rest of the arguments to the command.
.Pp
Instead of supplying named
.Ar parameters ,
four fixed parameters may be supplied in order on the command line:
.Ar path ,
.Ar hostname ,
.Ar ip ,
and
.Ar command .
As the
.Va jid
and
.Va name
parameters aren't in this list, this mode will always create a new jail, and
the
.Fl c
and
.Fl m
options don't apply (and must not exist).
.Pp
Jails have a set a core parameters, and modules can add their own jail
parameters.
The current set of available parameters can be retrieved via
.Dq Nm sysctl Fl d Va security.jail.param .
The core parameters are:
.Bl -tag -width indent
.It Va jid
The jail identifier.
This will be assigned automatically to a new jail (or can be explicitly
set), and can be used to identify the jail for later modification, or
for such commands as
.Xr jls 8
or
.Xr jexec 8 .
.It Va name
The jail name.
This is an arbitrary string that identifies a jail (except it may not
contain a
.Sq \&. ) .
Like the
.Va jid ,
it can be passed to later
.Nm
commands, or to
.Xr jls 8
or
.Xr jexec 8 .
If no
.Va name
is supplied, a default is assumed that is the same as the
.Va jid .
.It Va path
Directory which is to be the root of the prison.
The
.Va command
(if any) is run from this directory, as are commands from
.Xr jexec 8 .
.It Va ip4.addr
A comma-separated list of IPv4 addresses assigned to the prison.
If this is set, the jail is restricted to using only these addresses.
Any attempts to use other addresses fail, and attempts to use wildcard
addresses silently use the jailed address instead.
For IPv4 the first address given will be kept used as the source address
in case source address selection on unbound sockets cannot find a better
match.
It is only possible to start multiple jails with the same IP address,
if none of the jails has more than this single overlapping IP address
assigned to itself.
.It Va ip4.saddrsel
A boolean option to change the formerly mentioned behaviour and disable
IPv4 source address selection for the prison in favour of the primary
IPv4 address of the jail.
Source address selection is enabled by default for all jails and a
.Va ip4.nosaddrsel
setting of a parent jail is not inherited for any child jails.
.It Va ip4
Control the availability of IPv4 addresses.
Possible values are
.Dq inherit
to allow unrestricted access to all system addresses,
.Dq new
to restrict addresses via
.Va ip4.addr
above, and
.Dq disable
to stop the jail from using IPv4 entirely.
Setting the
.Va ip4.addr
parameter implies a value of
.Dq new .
.It Va ip6.addr , Va ip6.saddrsel , Va ip6
A set of IPv6 options for the prison, the counterparts to
.Va ip4.addr ,
.Va ip4.saddrsel
and
.Va ip4
above.
.It Va host.hostname
Hostname of the prison.
Other similar parameters are
.Va host.domainname ,
.Va host.hostuuid
and
.Va host.hostid .
.It Va host
Set the origin of hostname and related information.
Possible values are
.Dq inherit
to use the system information and
.Dq new
for the jail to use the information from the above fields.
Setting any of the above fields implies a value of
.Dq new .
.It Va securelevel
The value of the jail's
.Va kern.securelevel
sysctl.
A jail never has a lower securelevel than the default system, but by
setting this parameter it may have a higher one.
If the system securelevel is changed, any jail securelevels will be at
least as secure.
.It Va devfs_ruleset
The number of the devfs ruleset that is enforced for mounting devfs in
this jail and its descendants. A value of zero means no ruleset is enforced
or if set inside a jail for a descendant jail, the parent jails's devfs
ruleset enforcement is inherited. A value of -1 (default) means mounting a
devfs filesystem is not allowed. Mounting devfs inside a jail is possible
only if the
.Va allow.mount
permission is effective and
.Va enforce_statfs
is set to a value lower than 2.
.It Va children.max
The number of child jails allowed to be created by this jail (or by
other jails under this jail).
This limit is zero by default, indicating the jail is not allowed to
create child jails.
See the
.Va "Hierarchical Jails"
section for more information.
.It Va children.cur
The number of descendents of this jail, including its own child jails
and any jails created under them.
.It Va enforce_statfs
This determines which information processes in a jail are able to get
about mount points.
It affects the behaviour of the following syscalls:
.Xr statfs 2 ,
.Xr fstatfs 2 ,
.Xr getfsstat 2
and
.Xr fhstatfs 2
(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
visible.
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.
.It Va persist
Setting this boolean parameter allows a jail to exist without any
processes.
Normally, a jail is destroyed as its last process exits.
A new jail must have either the
.Va persist
parameter or
.Va command
pseudo-parameter set.
.It Va cpuset.id
The ID of the cpuset associated with this jail (read-only).
.It Va dying
This is true if the jail is in the process of shutting down (read-only).
.It Va parent
The
.Va jid
of the parent of this jail, or zero if this is a top-level jail
(read-only).
.It Va allow.*
Some restrictions of the jail environment may be set on a per-jail
basis.
With the exception of
.Va allow.set_hostname ,
these boolean parameters are off by default.
.Bl -tag -width indent
.It Va allow.set_hostname
The jail's hostname may be changed via
.Xr hostname 1
or
.Xr sethostname 3 .
.It Va allow.sysvipc
A process within the jail has 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.
.It Va allow.raw_sockets
The prison root is allowed to create raw sockets.
Setting this parameter allows utilities like
.Xr ping 8
and
.Xr traceroute 8
to operate inside the prison.
If this is set, the source IP addresses are enforced to comply
with the IP address bound to the jail, regardless of whether or not
the
.Dv 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.
.It Va allow.chflags
Normally, privileged users inside a jail are treated as unprivileged by
.Xr chflags 2 .
When this parameter is set, such users are treated as privileged, and
may manipulate system file flags subject to the usual constraints on
.Va kern.securelevel .
.It Va allow.mount
privileged users inside the jail will be able to mount and unmount file
system types marked as jail-friendly.
The
.Xr lsvfs 1
command can be used to find file system types available for mount from
within a jail.
This permission is effective only if
.Va enforce_statfs
is set to a value lower than 2.
.It Va allow.quotas
The prison root may administer quotas on the jail's filesystem(s).
This includes filesystems that the jail may share with other jails or
with non-jailed parts of the system.
.It Va allow.socket_af
Sockets within a jail are normally restricted to IPv4, IPv6, local
(UNIX), and route.  This allows access to other protocol stacks that
have not had jail functionality added to them.
.El
.El
.Pp
Jails are typically set up using one of two philosophies: either to
constrain a specific application (possibly running with privilege), or
to create a
.Dq "virtual system image"
running a variety of daemons and services.
In both cases, a fairly complete file system install of
.Fx
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
.Dq 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.
.Sh EXAMPLES
.Ss "Setting up a Jail Directory Tree"
To set up a jail directory tree containing an entire
.Fx
distribution, the following
.Xr sh 1
command script can be used:
.Bd -literal
D=/here/is/the/jail
cd /usr/src
mkdir -p $D
make world DESTDIR=$D
make distribution DESTDIR=$D
mount -t devfs devfs $D/dev
.Ed
.Pp
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
.Xr devfs 8
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
.Pa /etc/defaults/devfs.rules .
.Pp
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.
.Pp
We recommend experimentation and caution that it is a lot easier to
start with a
.Dq fat
jail and remove things until it stops working,
than it is to start with a
.Dq thin
jail and add things until it works.
.Ss "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
.Pa /data/jail/192.0.2.100 ,
named for the jailed IP address.
Substitute below as needed with your
own directory, IP address, and hostname.
.Ss "Setting up the Host Environment"
First, you will want to set up your real system's environment to be
.Dq jail-friendly .
For consistency, we will refer to the parent box as the
.Dq "host environment" ,
and to the jailed virtual machine as the
.Dq "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 if the jail did not bind the port.
This means changing
.Xr inetd 8
to only listen on the
appropriate IP address, and so forth.
Add the following to
.Pa /etc/rc.conf
in the host environment:
.Bd -literal -offset indent
sendmail_enable="NO"
inetd_flags="-wW -a 192.0.2.23"
rpcbind_enable="NO"
.Ed
.Pp
.Li 192.0.2.23
is the native IP address for the host system, in this example.
Daemons that run out of
.Xr inetd 8
can be easily set to use only the specified host IP address.
Other daemons
will need to be manually configured\(emfor some this is possible through
the
.Xr 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:
.Pp
To configure
.Xr sshd 8 ,
it is necessary to modify
.Pa /etc/ssh/sshd_config .
.Pp
To configure
.Xr sendmail 8 ,
it is necessary to modify
.Pa /etc/mail/sendmail.cf .
.Pp
For
.Xr named 8 ,
it is necessary to modify
.Pa /etc/namedb/named.conf .
.Pp
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
.Xr rpc 3 ,
such as
.Xr rpcbind 8 ,
.Xr nfsd 8 ,
and
.Xr 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.
.Pp
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.).
.Ss "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.
.Pp
Start a shell in the jail:
.Bd -literal -offset indent
jail -c path=/data/jail/192.0.2.100 host.hostname=testhostname \\
	ip4.addr=192.0.2.100 command=/bin/sh
.Ed
.Pp
Assuming no errors, you will end up with a shell prompt within the jail.
You can now run
.Pa /usr/sbin/sysinstall
and do the post-install configuration to set various configuration options,
or perform these actions manually by editing
.Pa /etc/rc.conf ,
etc.
.Pp
.Bl -bullet -offset indent -compact
.It
Create an empty
.Pa /etc/fstab
to quell startup warnings about missing fstab (virtual server only)
.It
Disable the port mapper
.Pa ( /etc/rc.conf :
.Li rpcbind_enable="NO" )
(virtual server only)
.It
Configure
.Pa /etc/resolv.conf
so that name resolution within the jail will work correctly
.It
Run
.Xr newaliases 1
to quell
.Xr sendmail 8
warnings.
.It
Disable interface configuration to quell startup warnings about
.Xr ifconfig 8
.Pq Li network_interfaces=""
(virtual server only)
.It
Set a root password, probably different from the real host system
.It
Set the timezone
.It
Add accounts for users in the jail environment
.It
Install any packages the environment requires
.El
.Pp
You may also want to perform any package-specific configuration (web servers,
SSH servers, etc), patch up
.Pa /etc/syslog.conf
so it logs as you would like, etc.
If you are not using a virtual server, you may wish to modify
.Xr 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
.Pa /data/jail/192.0.2.100/var/run/log .
.Pp
Exit from the shell, and the jail will be shut down.
.Ss "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
.Pa /etc/rc
in the examples below.
To start a virtual server environment,
.Pa /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
.Pa /etc/rc
script from within the jail.
.Bd -literal -offset indent
ifconfig ed0 inet alias 192.0.2.100/32
mount -t procfs proc /data/jail/192.0.2.100/proc
jail -c path=/data/jail/192.0.2.100 host.hostname=testhostname \\
	ip4.addr=192.0.2.100 command=/bin/sh /etc/rc
.Ed
.Pp
A few warnings will be produced, because most
.Xr 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
.Xr inetd 8 ,
.Xr syslogd 8 ,
and other processes running within the jail using
.Xr ps 1 ,
with the
.Ql J
flag appearing beside jailed processes.
To see an active list of jails, use the
.Xr jls 8
utility.
You should also be able to
.Xr telnet 1
to the hostname or IP address of the jailed environment, and log
in using the accounts you created previously.
.Pp
It is possible to have jails started at boot time.
Please refer to the
.Dq jail_*
variables in
.Xr rc.conf 5
for more information.
The
.Xr rc 8
jail script provides a flexible system to start/stop jails:
.Bd -literal
/etc/rc.d/jail start
/etc/rc.d/jail stop
/etc/rc.d/jail start myjail
/etc/rc.d/jail stop myjail
.Ed
.Ss "Managing the Jail"
Normal machine shutdown commands, such as
.Xr halt 8 ,
.Xr reboot 8 ,
and
.Xr 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:
.Bd -literal -offset indent
kill -TERM -1
kill -KILL -1
.Ed
.Pp
This will send the
.Dv SIGTERM
or
.Dv 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
.Pa /etc/rc.shutdown
from within the jail.
To kill processes from outside the jail, use the
.Xr jexec 8
utility in conjunction with the one of the
.Xr kill 1
commands above.
You may also remove the jail with
.Nm
.Ar -r ,
which will killall the jail's processes with
.Dv SIGKILL .
.Pp
The
.Pa /proc/ Ns Ar pid Ns Pa /status
file contains, as its last field, the name of the jail in which the
process runs, or
.Dq Li -
to indicate that the process is not running within a jail.
The
.Xr ps 1
command also shows a
.Ql J
flag for processes in a jail.
.Pp
You can also list/kill processes based on their jail ID.
To show processes and their jail ID, use the following command:
.Pp
.Dl "ps ax -o pid,jid,args"
.Pp
To show and then kill processes in jail number 3 use the following commands:
.Bd -literal -offset indent
pgrep -lfj 3
pkill -j 3
.Ed
or:
.Pp
.Dl "killall -j 3"
.Ss "Jails and File Systems"
It is not possible to
.Xr mount 8
or
.Xr umount 8
any file system inside a jail unless the file system is marked
jail-friendly, the jail's
.Va allow.mount
parameter is set and the jail's
.Va enforce_statfs
parameter is lower than 2.
.Pp
Multiple jails sharing the same file system can influence each other.
For example a user in one jail can fill the file system also
leaving no space for processes in the other jail.
Trying to use
.Xr quota 1
to prevent this will not work either as the file system quotas
are not aware of jails but only look at the user and group IDs.
This means the same user ID in two jails share the same file
system quota.
One would need to use one file system per jail to make this work.
.Ss "Sysctl MIB Entries"
The read-only entry
.Va security.jail.jailed
can be used to determine if a process is running inside a jail (value
is one) or not (value is zero).
.Pp
The variable
.Va security.jail.max_af_ips
determines how may address per address family a prison may have.
The default is 255.
.Pp
Some MIB variables have per-jail settings.
Changes to these variables by a jailed process do not effect the host
environment, only the jail environment.
These variables are
.Va kern.securelevel ,
.Va kern.hostname ,
.Va kern.domainname ,
.Va kern.hostid ,
and
.Va kern.hostuuid .
.Ss "Hierarchical Jails"
By setting a jail's
.Va children.max
parameter, processes within a jail may be able to create jails of their own.
These child jails are kept in a hierarchy, with jails only able to see and/or
modify the jails they created (or those jails' children).
Each jail has a read-only
.Va parent
parameter, containing the
.Va jid
of the jail that created it; a
.Va jid
of 0 indicates the jail is a child of the current jail (or is a top-level
jail if the current process isn't jailed).
.Pp
Jailed processes are not allowed to confer greater permissions than they
themselves are given, e.g. if a jail is created with
.Va allow.nomount ,
it is not able to create a jail with
.Va allow.mount
set.
Similarly, such restrictions as
.Va ip4.addr
and
.Va securelevel
may not be bypassed in child jails.
.Pp
A child jail may in turn create its own child jails if its own
.Va children.max
parameter is set (remember it is zero by default).
These jails are visible to and can be modified by their parent and all
ancestors.
.Pp
Jail names reflect this hierarchy, with a full name being an MIB-type string
separated by dots.
For example, if a base system process creates a jail
.Dq foo ,
and a process under that jail creates another jail
.Dq bar ,
then the second jail will be seen as
.Dq foo.bar
in the base system (though it is only seen as
.Dq bar
to any processes inside jail
.Dq foo ) .
Jids on the other hand exist in a single space, and each jail must have a
unique jid.
.Pp
Like the names, a child jail's
.Va path
is relative to its creator's own
.Va path .
This is by virtue of the child jail being created in the chrooted
environment of the first jail.
.Sh SEE ALSO
.Xr killall 1 ,
.Xr lsvfs 1 ,
.Xr newaliases 1 ,
.Xr pgrep 1 ,
.Xr pkill 1 ,
.Xr ps 1 ,
.Xr quota 1 ,
.Xr chroot 2 ,
.Xr jail_set 2 ,
.Xr jail_attach 2 ,
.Xr procfs 5 ,
.Xr rc.conf 5 ,
.Xr sysctl.conf 5 ,
.Xr devfs 8 ,
.Xr halt 8 ,
.Xr inetd 8 ,
.Xr jexec 8 ,
.Xr jls 8 ,
.Xr mount 8 ,
.Xr named 8 ,
.Xr reboot 8 ,
.Xr rpcbind 8 ,
.Xr sendmail 8 ,
.Xr shutdown 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8 ,
.Xr umount 8
.Sh HISTORY
The
.Nm
utility appeared in
.Fx 4.0 .
Hierarchical/extensible jails were introduced in
.Fx 8.0 .
.Sh AUTHORS
.An -nosplit
The jail feature was written by
.An Poul-Henning Kamp
for R&D Associates
.Pa http://www.rndassociates.com/
who contributed it to
.Fx .
.Pp
.An Robert Watson
wrote the extended documentation, found a few bugs, added
a few new features, and cleaned up the userland jail environment.
.Pp
.An Bjoern A. Zeeb
added multi-IP jail support for IPv4 and IPv6 based on a patch
originally done by
.An Pawel Jakub Dawidek
for IPv4.
.Pp
.An James Gritton
added the extensible jail parameters and hierarchical jails.
.Sh BUGS
Jail currently lacks the ability to allow access to
specific jail information via
.Xr ps 1
as opposed to
.Xr procfs 5 .
Similarly, it might be a good idea to add an
address alias flag such that daemons listening on all IPs
.Pq Dv 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
.Xr inetd 8
which is easily configurable.
.Sh NOTES
Great care should be taken when managing directories visible within the jail.
For example, if a jailed process has its current working directory set to a
directory that is moved out of the jail's chroot, then the process may gain
access to the file space outside of the jail.
It is recommended that directories always be copied, rather than moved, out
of a jail.