FreeBSD Man Pages

jail(8)

NAME

     jail -- imprison process and its descendants

SYNOPSIS

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

DESCRIPTION

     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.

     -l 	  Run program in the clean environment.  The environment is
		  discarded 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 database for the target login are also set.

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

     -U username  The user name from jailed environment 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 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 con-
     figuration steps necessary to support either of these steps, although the
     configuration steps may be refined based on local requirements.

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

EXAMPLES

   Setting up a Jail Directory Tree
     This example shows how to set up a jail directory tree containing an
     entire FreeBSD distribution:

     D=/here/is/the/jail
     cd /usr/src
     mkdir -p $D
     make world DESTDIR=$D
     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.

     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/192.168.11.100, named for the jailed IP address.
     Substitute below as needed with your own directory, IP address, and host-
     name.

   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.  This means changing inetd(8) to only listen on the appro-
     priate IP address, and so forth.  Add the following to /etc/rc.conf in
     the host environment:

	   sendmail_enable="NO"
	   inetd_flags="-wW -a 192.168.11.23"
	   rpcbind_enable="NO"

     192.168.11.23 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 necessary to modify per-application configuration files,
     or to recompile the applications.	The following frequently deployed ser-
     vices 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
     /etc/mail/sendmail.cf.

     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
     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 /data/jail/192.168.11.100 testhostname 192.168.11.100 /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.

	   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 sys-
	       tem
	   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/192.168.11.100/var/run/log.

     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.

     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

     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.

   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 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 follow-
     ing line to /etc/sysctl.conf:

	   security.jail.set_hostname_allowed=0

   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.

     security.jail.allow_raw_sockets
	  This MIB entry determines whether or not prison root is allowed to
	  create raw sockets.  Setting this MIB to 1 allows utilities like
	  ping(8) and traceroute(8) 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 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.  As such, by default this option is
	  disabled.


     security.jail.set_hostname_allowed
	  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.

     security.jail.socket_unixiproute_only
	  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.

     security.jail.sysvipc_allowed
	  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.

     security.jail.chflags_allowed
	  This MIB entry determines how a privileged user inside a jail will
	  be treated by chflags(2).  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 constaints on
	  kern.securelevel.

     There are currently two MIB related variables that have per-jail set-
     tings.  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

     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

     The jail utility appeared in FreeBSD 4.0.

AUTHORS

     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
     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.

FreeBSD 5.4			 April 8, 2003			   FreeBSD 5.4

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