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NAME    [Toc]    [Back]

     jail -- imprison process and its descendants

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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.

     -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.

     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 FreeBSD
     is required, so as to provide the necessary command line tools, daemons,
     libraries, application configuration files, etc are available.  However,
     for a virtual server configuration, a fair amount of additional work is
     required so as to configure the "boot" process.  This man page documents
     the configuration steps necessary to support either of these steps,
     althoguh the configuration steps may be refined based on local requirements.

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

EXAMPLES    [Toc]    [Back]

   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
     mount_devfs devfs $D/dev
     cd $D
     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 processes
 in the jail to bypass the jail sandboxing by modifying files outside
 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 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    [Toc]    [Back]
     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 hostname.

   Setting up the Host Environment    [Toc]    [Back]
     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 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 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
     specified host IP address.  Other daemons will need to be manually configured--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 application.  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 services
 using rpc(3), such as rpcbind(8,) nfsd(8), and 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.

     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    [Toc]    [Back]
     Start any jails 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 for both constraining a particular
 application or for a virtual server.

     Start a shell in 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 /usr/sbin/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 (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 system

	   +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/

     Exit from the shell, and the jail will be shut down.

   Starting the Jail    [Toc]    [Back]
     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 /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 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 properly.
  You should be able to see inetd(8), syslogd(8), and other processes
 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 created

   Managing the Jail    [Toc]    [Back]
     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 following
 line to /etc/sysctl.conf:


   Sysctl MIB Entries    [Toc]    [Back]
     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 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, 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 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.

     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
     kern.securelevel and kern.hostname.

SEE ALSO    [Toc]    [Back]

     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)

HISTORY    [Toc]    [Back]

     The jail utility appeared in FreeBSD 4.0.

AUTHORS    [Toc]    [Back]

     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.

BUGS    [Toc]    [Back]

     Jail currently lacks the ability 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 simplist answer is to
     minimize services offered on the host, possibly limiting it to services
     offered from inetd(8) which is easily configurable.

FreeBSD 5.2.1			 April 8, 2003			 FreeBSD 5.2.1
[ Back ]
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