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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 should run.
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". Generally speaking making a system jail-friendly does not
require much work if you have jail.defaults.allow_listen_override set to
1. This will allow all listen sockets inside the jail to overload and
override wildcard listen sockets on the host. This methodology works
extremely well as long as you restrict the IPs you pass into the jail to
avoid any security concerns, which typically means making at least two IP
aliases for each jail that you create (an external IP and a localhost
If you do not want to allow listener socket overloading, you must modify
services you intend to leave running on the host system to listen only on
specific IPs and not all IPs, or generally only run services inside your
jails that do not conflict with services on the host system. This can be
difficult and is not recommended.
Common services include: inetd(8), sendmail(8), named(8), rpcbind(8),
mountd(8), nfsd(8), sendmail(1), and named(8).
For the purposes of our examples below, 192.168.11.1 is the host IP and
we create aliases 192.168.11.X for each jail. In addition, we leave
127.0.0.1 on the host only and create aliases 127.0.0.X for each jail.
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.
To set the jail up for real you need to ifconfig some interface aliases.
and set jail defaults. Here is an example:
ifconfig lo0 127.0.0.2 alias
ifconfig em0 192.168.11.100 netmask 255.255.255.0 alias
From within your jail you can generally run services normally. Just use
"localhost" normally. Do not inform services that localhost is
127.0.0.2. The kernel will automatically remap 'localhost' (e.g.
127.0.0.1) to the localhost IP you specify for the jail. The kernel will
also map the jail's localhost IP back to 127.0.0.1 in the accept(2),
getsockname(2), and getpeername(2) system calls.
Listen sockets can be overloaded between jails and between the host and
its jails. You can continue to use services that listen on the wildcard
*.* socket normally on both the host and its jails if you set the
allow_listen_override flag to 1, and jailed listen sockets will override
any host listen sockets listening on the wildcard address for the allowed
DragonFly also allows you to overload specific ports, but the jailed
service will not receive any connections if the host is also specifically
listening on the addr/port pair that the jail is using. If multiple
jails are listening on the same addr/port pair (as specified by the
jail's IP list), and the host is not, then only one jail will receive
connections on that pair.
In otherwords, it is still a good idea to give each jail its own local
and non-local IP address rather than have jails share.
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
configuration 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
* 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.defaults.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 em0 inet alias 192.168.11.100/32
ifconfig lo0 inet alias 127.0.0.2
mount -t procfs proc /data/jail/192.168.11.100/proc
jail /data/jail/192.168.11.100 testhostname 127.0.0.2,192.168.11.100 \
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
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. 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.
Currently 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
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
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. For each jail there
will be the same set of MIB variables as shown below but under jail.<id>
which allows control of every jail individually. The values of the
variables under jail.defaults will be copied to the per-jail MIB
variables upon creation thus serving as a kind of system-wide template.
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. Because 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, this option is disabled by
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 constraints on
This read-only MIB entry can be used to determine if a process is
running inside a jail (value is 1) or not (value is 0).
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.
This feature allows both the host and your jails to overload
services on the same ports. If enabled, the services in the jails
will override wildcarded services on the host for the jail's IP
list. As a safety mechanism, any services the host specifically
binds to an IP will not be overridden. The host has visibility to
all jail IPs but jails only have visibility to their specific IPs.
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,
permitting 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
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.
newaliases(1), ps(1), chroot(2), jail(2), procfs(5), rc.conf(5),
sysctl.conf(5), halt(8), inetd(8), jexec(8), jls(8), named(8), pw(8),
reboot(8), rpcbind(8), sendmail(8), shutdown(8), sysctl(8), syslogd(8),
The jail command appeared in FreeBSD 4.0.
Support for multiple IPs and IPv6 appeared in DragonFly 1.7.
The jail feature was originally 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.
Matthew Dillon added port overloading to make configuration easier.
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
simplest answer is to minimize services offered on the host, possibly
limiting it to services offered from inetd(8) which is easily
DragonFly 6.3-DEVELOPMENT November 8, 2022 DragonFly 6.3-DEVELOPMENT