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

     jail — create or modify a system jail

     jail [-dhi] [-J jid_file] [-l -u username | -U username] [-c | -m]
	  [parameter=value ...]
     jail [-hi] [-n jailname] [-J jid_file] [-s securelevel]
	  [-l -u username | -U username] [path hostname [ip[,..]] command ...]
     jail [-r jail]

     The jail utility creates a new jail or modifies an existing jail, option‐
     ally imprisoning the current process (and future descendants) inside it.

     The options are as follows:

     -d	     Allow making changes to a dying jail.

     -h	     Resolve the host.hostname parameter (or hostname) and add all IP
	     addresses returned by the resolver to the list of 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 ip4.addr and ip6.addr parameters further down
	     for details.

     -i	     Output the jail identifier of the newly created jail.

     -n jailname
	     Set the jail's name.  This is deprecated and is equivalent to
	     setting the name parameter.

     -J jid_file
	     Write a jid_file file, containing jail identifier, path, host‐
	     name, IP and command used to start the jail.

     -l	     Run program in the clean environment.  The environment is dis‐
	     carded 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 data‐
	     base for the target login are also set.

     -s securelevel
	     Set the kern.securelevel MIB entry to the specified value inside
	     the newly created jail.  This is deprecated and is equivalent to
	     setting the securelevel parameter.

     -u username
	     The user name from host environment as whom the command should

     -U username
	     The user name from jailed environment as whom the command should

     -c	     Create a new jail.	 The jid and name parameters (if specified)
	     must not refer to an existing jail.

     -m	     Modify an existing jail.  One of the jid or name parameters must
	     exist and refer to an existing jail.

     -cm     Create a jail if it does not exist, or modify a jail if it does

     -r	     Remove the jail specified by jid or name.	All jailed processes
	     are killed, and all children of this jail are also removed.

     At least one of the -c, -m or -r options must be specified.

     Parameters are listed in “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 “no” prefix, e.g.	persist or nopersist.  Any
     parameters not set will be given default values, often based on the cur‐
     rent environment.

     The pseudo-parameter 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 ‘=’ sign, but also passes the rest of the arguments to the

     Instead of supplying named parameters, four fixed parameters may be sup‐
     plied in order on the command line: path, hostname, ip, and command.  As
     the jid and name parameters aren't in this list, this mode will always
     create a new jail, and the -c and -m options don't apply (and must not

     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
     “sysctl -d security.jail.param”.  The core parameters are:

     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 jls(8)
	     or jexec(8).

     name    The jail name.  This is an arbitrary string that identifies a
	     jail (except it may not contain a ‘.’).  Like the jid, it can be
	     passed to later jail commands, or to jls(8) or jexec(8).  If no
	     name is supplied, a default is assumed that is the same as the

     path    Directory which is to be the root of the prison.  The command (if
	     any) is run from this directory, as are commands from jexec(8).

	     A comma-separated list of IPv4 addresses assigned to the prison.
	     If this is set, the jail is restricted to using only these
	     address.  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.

	     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 ip4.nosaddrsel setting
	     of a parent jail is not inherited for any child jails.

     ip4     Control the availablity of IPv4 addresses.	 Possible values are
	     “inherit” to allow unrestricted access to all system addresses,
	     “new” to restrict addresses via ip4.addr above, and “disable” to
	     stop the jail from using IPv4 entirely.  Setting the ip4.addr
	     parameter implies a value of “new”.

     ip6.addr, ip6.saddrsel, ip6
	     A set of IPv6 options for the prison, the counterparts to
	     ip4.addr, ip4.saddrsel and ip4 above.

	     Hostname of the prison.  Other similar parameters are
	     host.domainname, host.hostuuid and host.hostid.

     host    Set the origin of hostname and related information.  Possible
	     values are “inherit” to use the system information and “new” for
	     the jail to use the information from the above fields.  Setting
	     any of the above fields implies a value of “new”.

	     The value of the jail's 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

	     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 Hierarchical Jails section for more information.

	     The number of descendents of this jail, including its own child
	     jails and any jails created under them.

	     This determines which information processes in a jail are able to
	     get about mount points.  It affects the behaviour of the follow‐
	     ing syscalls: statfs(2), fstatfs(2), getfsstat(2) and 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 visi‐
	     ble.  In addition to that, the path to the jail's chroot direc‐
	     tory 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.

	     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 persist parameter or
	     command pseudo-parameter set.

	     The ID of the cpuset associated with this jail (read-only).

     dying   This is true if the jail is in the process of shutting down

     parent  The jid of the parent of this jail, or zero if this is a top-
	     level jail (read-only).

	     Some restrictions of the jail environment may be set on a per-
	     jail basis.  With the exception of allow.set_hostname, these
	     boolean parameters are off by default.

		     The jail's hostname may be changed via hostname(1) or

		     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 inter‐
		     fere with) processes outside of the jail, and in other

		     The prison root is allowed to create raw sockets.	Set‐
		     ting this parameter allows utilities like ping(8) and
		     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 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.

		     Normally, privileged users inside a jail are treated as
		     unprivileged by 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

		     privileged users inside the jail will be able to mount
		     and unmount file system types marked as jail-friendly.
		     The lsvfs(1) command can be used to find file system
		     types available for mount from within a jail.

		     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.

		     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 functional‐
		     ity added to them.

     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 configu‐
     ration steps necessary to support either of these steps, although the
     configuration steps may be refined based on local requirements.

   Setting up a Jail Directory Tree
     To set up a jail directory tree containing an entire FreeBSD distribu‐
     tion, the following sh(1) command script can be used:

     cd /usr/src
     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 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.	A simple devfs ruleset
     for jails is available as ruleset #4 in /etc/defaults/devfs.rules.

     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/, 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 if the jail did not bind the port.  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 speci‐
     fied host IP address.  Other daemons will need to be manually configured—
     for some this is possible through the rc.conf(5) flags entries; for oth‐
     ers 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

     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
     host, etc.).

   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 -c path=/data/jail/ host.hostname=testhostname \
		   ip4.addr= command=/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.

	   ·   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)
	   ·   Configure /etc/resolv.conf so that name resolution within the
	       jail will work correctly
	   ·   Run newaliases(1) to quell sendmail(8) warnings.
	   ·   Disable interface configuration to quell startup warnings about
	       ifconfig(8) (network_interfaces="") (virtual server only)
	   ·   Set a root password, probably different from the real host sys‐
	   ·   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 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.

	   ifconfig ed0 inet alias
	   mount -t procfs proc /data/jail/
	   jail -c path=/data/jail/ host.hostname=testhostname \
		   ip4.addr= command=/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 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‐
     ated previously.

     It is possible to have jails started at boot time.	 Please refer to the
     “jail_*” variables in rc.conf(5) for more information.  The rc(8) jail
     script provides a flexible system to start/stop jails:

     /etc/rc.d/jail start
     /etc/rc.d/jail stop
     /etc/rc.d/jail start myjail
     /etc/rc.d/jail stop myjail

   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 conjunction
     with the one of the kill(1) commands above.  You may also remove the jail
     with jail -r, which will killall the jail's processes with SIGKILL.

     The /proc/pid/status file contains, as its last field, the name 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.

     You can also list/kill processes based on their jail ID.  To show pro‐
     cesses 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 com‐

	   pgrep -lfj 3
	   pkill -j 3

	   killall -j 3

   Jails and File Systems
     It is not possible to mount(8) or umount(8) any file system inside a jail
     unless the file system is marked jail-friendly and the jail's allow.mount
     parameter is set.

     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 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.

   Sysctl MIB Entries
     The read-only entry security.jail.jailed can be used to determine if a
     process is running inside a jail (value is one) or not (value is zero).

     The variable security.jail.max_af_ips determines how may address per
     address family a prison may have.	The default is 255.

     Some MIB variables have per-jail settings.	 Changes to these variables by
     a jailed process do not effect the host environment, only the jail envi‐
     ronment.  These variables are kern.securelevel, kern.hostname,
     kern.domainname, kern.hostid, and kern.hostuuid.

   Hierarchical Jails
     By setting a jail's 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 cre‐
     ated (or those jails' children).  Each jail has a read-only parent param‐
     eter, containing the jid of the jail that created it; a jid of 0 indi‐
     cates the jail is a child of the current jail (or is a top-level jail if
     the current process isn't jailed).

     Jailed processes are not allowed to confer greater permissions than they
     themselves are given, e.g. if a jail is created with allow.nomount, it is
     not able to create a jail with allow.mount set.  Similarly, such restric‐
     tions as ip4.addr and securelevel may not be bypassed in child jails.

     A child jail may in turn create its own child jails if its own
     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 ances‐

     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 “foo”, and a process under that jail creates another jail “bar”,
     then the second jail will be seen as “foo.bar” in the base system (though
     it is only seen as “bar” to any processes inside jail “foo”).  Jids on
     the other hand exist in a single space, and each jail must have a unique

     Like the names, a child jail's path is relative to its creator's own
     path.  This is by virtue of the child jail being created in the chrooted
     environment of the first jail.

     killall(1), lsvfs(1), newaliases(1), pgrep(1), pkill(1), ps(1), quota(1),
     chroot(2), jail_set(2), jail_attach(2), procfs(5), rc.conf(5),
     sysctl.conf(5), devfs(8), halt(8), inetd(8), jexec(8), jls(8), mount(8),
     named(8), reboot(8), rpcbind(8), sendmail(8), shutdown(8), sysctl(8),
     syslogd(8), umount(8)

     The jail utility appeared in FreeBSD 4.0.	Hierarchical/extensible jails
     were introduced in FreeBSD 8.0.

     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.

     Bjoern A. Zeeb added multi-IP jail support for IPv4 and IPv6 based on a
     patch originally done by Pawel Jakub Dawidek for IPv4.

     James Gritton added the extensible jail parameters and hierchical jails.

     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			       January 17, 2010				   BSD

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