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JAIL(8) DragonFly System Manager's Manual JAIL(8)
jail -- imprison process and its descendants
jail [-i] [-l -u username | -U username] path hostname ip-list command
The jail command 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 as whom the command should run.
-U username The user name from jailed environment as whom the command
path Directory which is to be the root of the prison.
hostname Hostname of the prison.
ip-list Comma separated IP list 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 DragonFly distribution, provided that you built world before
mkdir -p $D
make installworld DESTDIR=$D
make distribution DESTDIR=$D
ln -sf dev/null boot/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 external jail IP
address. Substitute below as needed with your own directory, IP
addresses, 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 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 192.168.11.23"
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 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 rpcbind(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
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
Now start the jail:
jail /data/jail/192.168.11.100 testhostname
You will end up with a shell prompt, assuming no errors, within the jail.
You can now do the post-install configuration to set various configura-
tion options by editing /etc/rc.conf, etc.
* Disable the port mapper (/etc/rc.conf: rpcbind_enable="NO")
* Run newaliases(1) to quell sendmail(8) warnings.
* Disable interface configuration to quell startup warnings about
* Configure /etc/resolv.conf so that name resolution within the
jail will work correctly
* Set a root password, probably different from the real host sys-
* Set the timezone with tzsetup(8)
* 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 127.0.0.1,192.168.11.100 \
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 DragonFly, the mechanisms for managing jails may
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 IPv4 and IPv6 addresses to each jail,
and limits access to other network addresses in the IPv4 and IPv6
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, PF_INET6, and PF_ROUTE, per-
mitting them access to UNIX domain sockets, IPv4 and IPv6 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), pw(8), reboot(8),
rpcbind(8), sendmail(8), shutdown(8), sysctl(8), syslogd(8), tzsetup(8)
The jail command appeared in FreeBSD 4.0.
Support for multiple IPs and IPv6 appeared in DragonFly 1.7.
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.
Victor Balada Diaz wrote the support for multiple IPs and IPv6. Multiple
IPs support is based on work done by Pawel Jakub Dawidek.
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.
DragonFly 5.3 August 6, 2009 DragonFly 5.3