FreeBSD Manual Pages
JAIL(8) System Manager's Manual JAIL(8) NAME jail -- imprison process and its descendants SYNOPSIS jail [-l -u username | -U username] path hostname ip-number command ... DESCRIPTION The jail command imprisons a process and all future descendants. The options are as follows: -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 your 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. Please see the jail(2) man page for further details. EXAMPLES Setting up a Jail Directory Tree This example shows how to setup a jail directory tree containing an en- tire FreeBSD distribution: D=/here/is/the/jail 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/192.168.11.100, named for the jailed IP ad- dress. Substitute below as needed with your own directory, IP address, and hostname. 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. 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: sendmail_enable="NO" inetd_flags="-wW -a 192.168.11.23" portmap_enable="NO" 192.168.11.23 is the native IP address for the host system, in this ex- ample. 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 en- tries, for others it is not possible without munging the per-applica- tion configuration 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 configuration files, in most cases it is easier to simply run the dae- mons in jails only, and not in the host environment. Attempting to serve NFS from the host environment may also cause confusion, and can- not be easily reconfigured to use only specific IPs, as some NFS ser- vices 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 re- sult in those services also appearing to be offered by the jail envi- ronments. 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 re- duce 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 in- terface 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/192.168.11.100/stand cp /stand/sysinstall /data/jail/192.168.11.100/stand Now start the jail: jail /data/jail/192.168.11.100 testhostname 192.168.11.100 /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 con- figuration to set various configuration options, or perform these ac- tions manually by editing /etc/rc.conf, etc. • Create an empty /etc/fstab to quell startup warnings about missing fstab • Disable the port mapper (/etc/rc.conf: portmap_enable="NO") • Run newaliases(1) to quell sendmail(8) warnings. • Disable interface configuration to quell startup warnings about ifconfig(8) (network_interfaces="") • Configure /etc/resolv.conf so that name resolution within the jail will work correctly • Set a root password, probably different from the real host system • Set the timezone • Add accounts for users in the jail environment • 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 192.168.11.100/32 mount -t procfs proc /data/jail/192.168.11.100/proc jail /data/jail/192.168.11.100 testhostname 192.168.11.100 \ /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 processes running within the jail using ps(1), with the `J' flag ap- pearing beside jailed processes. You should also be able to telnet(1) to the hostname or IP address of the jailed environment, and log in us- ing 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 dis- able 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: jail.set_hostname_allowed=0 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. 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 disabled in environments where privileged ac- cess to jails is given out to untrusted parties. jail.socket_unixiproute_only 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. 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 im- plementation, 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. SEE ALSO 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) HISTORY The jail command 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 few new features, and cleaned up the userland jail environment. BUGS Jail currently lacks strong management functionality, such as the abil- ity 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). Simi- larly, 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 ad- dress, 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. GNU December 12, 2001 JAIL(8)
NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | HISTORY | AUTHORS | BUGS
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=jail&manpath=FreeBSD+4.11-RELEASE>