Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
JAIL(8)                 FreeBSD System Manager's Manual                JAIL(8)

     jail -- imprison process and its descendants

     jail [-u username] path hostname ip-number command ...

     The jail command imprisons a process and all future descendants.

     The options are as follows:

     -u username  The user name as whom the command should run.

     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.

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

   Setting up a Jail Directory Tree
     This example shows how to setup 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 -DNO_MAKEDEV_RUN
     cd $D/dev
     sh MAKEDEV jail
     cd $D
     ln -sf dev/null kernel

     In many cases this example would put far more stuff in the jail than is
     needed.  In the other extreme case a jail might contain only one single
     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 host-

     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''.  Because 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.  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"
           portmap_enable="NO" 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 not possible without munging the per-application config-
     uration files, or even recompiling.  For those applications that cannot
     specify the IP they run on, it is better to disable them, if possible.

     A number of daemons ship with the base system that may have problems when
     run from outside of a jail in a jail-centric environment.  This includes
     sendmail(8), named(8), and portmap(8).  While sendmail(8) and named(8)
     can be configured to listen only on a specific IP using their configura-
     tion files, in most cases it is easier to simply run the daemons in jails
     only, and not in the host environment.  Attempting to serve NFS from the
     host environment may also cause confusion, and cannot be easily reconfig-
     ured 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 appear-
     ing 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.)

     Start any jails 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.  Before beginning, you may want to copy sysinstall(8) into the
     tree so that you can use it to set things up easily.  Do this using:

           mkdir /data/jail/
           cp /stand/sysinstall /data/jail/

     Now start the jail:

           jail /data/jail/ testhostname /bin/sh

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

           +o   Create an empty /etc/fstab to quell startup warnings about
               missing fstab
           +o   Disable the port mapper (/etc/rc.conf: portmap_enable="NO")
           +o   Run newaliases(1) to quell sendmail(8) warnings.
           +o   Disable interface configuration to quell startup warnings about
               ifconfig(8) (network_interfaces="")
           +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 that you think 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.

     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.  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 jail.set_hostname_allowed to
     0.  Please see the management reasons why this is 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 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.  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.  Cur-
     rently there is no way to insert new processes into a jail, so you must
     first log into the jail before performing these actions.

     To kill processes from outside the jail, you must individually identify
     the PID of each process to be killed.  The /proc/pid/status file con-
     tains, 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.  How-
     ever, 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
     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:


     In a future version of FreeBSD, the mechanisms for managing jails will be
     more refined.

   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 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.

     newaliases(1), ps(1), chroot(2), jail(2), procfs(5), rc.conf(5),
     sysctl.conf(5), halt(8), inetd(8), named(8), portmap(8), reboot(8),
     sendmail(8), shutdown(8), sysctl(8), syslogd(8)

     The jail command 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 strong management functionality, such as the ability
     to deliver signals to all processes in a jail, and 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 sim-
     plist answer is to minimize services offered on the host, possibly limit-
     ing it to services offered from inetd(8) which is easily configurable.

FreeBSD 4.10                   December 12, 2001                  FreeBSD 4.10


Want to link to this manual page? Use this URL:

home | help