| JAIL(8) | MidnightBSD System Manager's Manual | JAIL(8) |
jail — manage
system jails
jail |
[-dhilqv] [-J
jid_file] [-u
username] [-U
username] [-cmr]
param=value ...
[command=command ...] |
jail |
[-dqv] [-f
conf_file] [-p
limit] [-cmr]
[jail] |
jail |
[-qv] [-f
conf_file] [-rR]
[* | jail ...] |
jail |
[-dhilqv] [-J
jid_file] [-u
username] [-U
username] [-n
jailname] [-s
securelevel] path hostname
ip[,...]
command ... |
jail |
[-f conf_file]
-e separator |
The jail utility creates new jails, or
modifies or removes existing jails. It can also print a list of configured
jails and their parameters. A jail (or “prison”) is specified
via parameters on the command line, or in the
jail.conf(5) file.
At least one of the options -c,
-e, -m or
-r must be specified. These options are used alone
or in combination to describe the operation to perform:
-c-e
separator-m-r-rcjail
-r” and
“jail -c”
were run in succession.-cm-mr-cmrOther available options are:
-d-f
conf_file-h-i-q option.-J
jid_file-l-n
jailname-p
limit-q-R-r option that removes an
existing jail without using the configuration file. No removal-related
parameters for this jail will be used — the jail will simply be
removed.-s
securelevel-u
username-U
username-vIf no arguments are given after the options, the operation (except
remove) will be performed on all jails specified in the
jail.conf(5) file. A
single argument of a jail name will operate only on the specified jail. The
-r and -R options can also
remove running jails that aren't in the
jail.conf(5) file,
specified by name or jid.
An argument of “*” is a wildcard that will operate
on all jails, regardless of whether they appear in
jail.conf(5); this is
the surest way for -r to remove all jails. If
hierarchical jails exist, a partial-matching wildcard definition may be
specified. For example, an argument of “foo.*” would apply to
jails with names like “foo.bar” and
“foo.bar.baz”.
A jail may be specified with parameters directly on the command
line. In this case, the
jail.conf(5) file will
not be used. For backward compatibility, the command line may also have four
fixed parameters, without names: path,
hostname, ip, and
command. This mode will always create a new jail, and
the -c and -m options do not
apply (and must not be present).
Parameters in the jail.conf(5) file, or on the command line, are generally of the form “name=value”. Some parameters are boolean, and do not have a value but are set by the name alone with or without a “no” prefix, e.g. persist or nopersist. They can also be given the values “true” and “false”. Other parameters may have more than one value, specified as a comma-separated list or with “+=” in the configuration file (see jail.conf(5) for details).
The jail utility recognizes two classes of
parameters. There are the true jail parameters that are passed to the kernel
when the jail is created, which can be seen with
jls(8), and can (usually) be
changed with “jail
-m”. Then there are pseudo-parameters that
are only used by jail itself.
Jails have a set of core parameters, and kernel modules can add
their own jail parameters. The current set of available parameters can be
retrieved via “sysctl
-d
security.jail.param”. Any parameters not set
will be given default values, often based on the current environment. The
core parameters are:
jail commands, or to
jls(8) or
jexec(8). If no
name is supplied, a default is assumed that is the
same as the jid. The name
parameter is implied by the
jail.conf(5) file
format, and need not be explicitly set when using the configuration
file.jail or from
jexec(8), are run from
this directory.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. A simple devfs ruleset for jails is available as ruleset #4 in /etc/defaults/devfs.rules.
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.Kernel modules may add their own parameters, which only exist when the module is loaded. These are typically headed under a parameter named after the module, with values of “inherit” to give the jail full use of the module, “new” to encapsulate the jail in some module-specific way, and “disable” to make the module unavailable to the jail. There also may be other parameters to define jail behavior within the module. Module-specific parameters include:
There are pseudo-parameters that are not passed to the kernel, but
are used by jail to set up the jail environment,
often by running specified commands when jails are created or removed. The
exec.* command parameters are
sh(1) command lines that are
run in either the system or jail environment. They may be given multiple
values, which would run the specified commands in sequence. All commands
must succeed (return a zero exit status), or the jail will not be created or
removed, as appropriate.
The pseudo-parameters are:
jail command line as its own arguments.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.SIGTERM signal (which happens after
the exec.stop commands have completed). After this
many seconds have passed, the jail will be removed, which will kill any
remaining processes. If this is set to zero, no
SIGTERM is sent and the jail is immediately
removed. The default is 10 seconds.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. However, for a virtual server configuration, a fair amount of additional work is required so as to replace the “boot” process. This manual page documents the configuration steps necessary to support either of these steps, although the configuration steps may need to be refined based on local requirements.
To set up a jail directory tree containing an entire FreeBSD distribution, the following sh(1) command script can be used:
D=/here/is/the/jail cd /usr/src mkdir -p $D make world DESTDIR=$D make distribution DESTDIR=$D
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 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.
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/testjail, for a jail named “testjail”. Substitute below as needed with your own directory, IP address, and hostname.
First, set up the 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 jails are 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 if the jail did not bind the port. 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 192.0.2.23" rpcbind_enable="NO"
192.0.2.23 is the native IP address for
the host system, in this example. Daemons that run out of
inetd(8) can be easily
configured to use only the specified host IP address. Other daemons will
need to be manually configured — for some this is possible through
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 /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 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.).
Start any jail 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 both for constraining a particular application or for running a virtual server.
Start a shell in the jail:
jail -c path=/data/jail/testjail mount.devfs \ host.hostname=testhostname ip4.addr=192.0.2.100 \ command=/bin/sh
Assuming no errors, you will end up with a shell prompt within the jail. You can now run bsdconfig(8) and do the post-install configuration to set various configuration options, or perform these actions manually by editing /etc/rc.conf, etc.
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/testjail/var/run/log.
Exit from the shell, and the jail will be shut down.
You are now ready to restart the jail and bring up the environment with all of its daemons and other programs. Create an entry for the jail in /etc/jail.conf:
testjail {
path = /tmp/jail/testjail;
mount.devfs;
host.hostname = testhostname;
ip4.addr = 192.0.2.100;
interface = ed0;
exec.start = "/bin/sh /etc/rc";
exec.stop = "/bin/sh /etc/rc.shutdown jail";
}
To start a virtual server environment,
/etc/rc is run to launch various daemons and
services, and /etc/rc.shutdown is run to shut them
down when the jail is removed. If you are running a single application in
the jail, substitute the command used to start the application for
“/bin/sh /etc/rc”; there may be some script available to
cleanly shut down the application, or it may be sufficient to go without a
stop command, and have jail send
SIGTERM to the application.
Start the jail by running:
jail -c testjail
A few warnings may be produced; 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
jls(8). If
sshd(8) is enabled in the
jail environment, you should be able to
ssh(1) to the hostname or IP
address of the jailed environment, and log in using the accounts you created
previously.
It is possible to have jails started at boot time. Please refer to the “jail_*” variables in rc.conf(5) for more information.
Normal machine shutdown commands, such as halt(8), reboot(8), and shutdown(8), cannot be used successfully within the jail. To kill all processes from within a jail, you may 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
— be careful not to run this from the host environment! Once all of
the jail's processes have died, unless the jail was created with the
persist parameter, the jail will be removed. Depending
on the intended use of the jail, you may also want to run
/etc/rc.shutdown from within the jail.
To shut down the jail from the outside, simply remove it with:
jail -r
which will run any commands specified by
exec.stop, and then send
SIGTERM and eventually
SIGKILL to any remaining jailed processes.
The
/proc/pid/status
file contains, as its last field, the name 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.
You can also list/kill processes based on their jail ID. To show processes and their jail ID, use the following command:
ps ax -o pid,jid,argsTo show and then kill processes in jail number 3 use the following commands:
pgrep -lfj 3 pkill -j 3
killall -j 3It is not possible to mount(8) or umount(8) any file system inside a jail unless the file system is marked jail-friendly, the jail's allow.mount parameter is set, and the jail's enforce_statfs parameter is lower than 2.
Multiple jails sharing the same file system can influence each other. For example, a user in one jail can fill the file system, leaving no space for processes in the other jail. Trying to use quota(1) to prevent this will not work either, as the file system quotas are not aware of jails but only look at the user and group IDs. This means the same user ID in two jails share a single file system quota. One would need to use one file system per jail to make this work.
The read-only entry security.jail.jailed can be used to determine if a process is running inside a jail (value is one) or not (value is zero).
The variable security.jail.max_af_ips determines how may address per address family a jail may have. The default is 255.
Some MIB variables have per-jail settings. Changes to these variables by a jailed process do not affect the host environment, only the jail environment. These variables are kern.securelevel, kern.hostname, kern.domainname, kern.hostid, and kern.hostuuid.
By setting a jail's children.max parameter, processes within a jail may be able to create jails of their own. These child jails are kept in a hierarchy, with jails only able to see and/or modify the jails they created (or those jails' children). Each jail has a read-only parent parameter, containing the jid of the jail that created it; a jid of 0 indicates the jail is a child of the current jail (or is a top-level jail if the current process isn't jailed).
Jailed processes are not allowed to confer greater permissions than they themselves are given, e.g., if a jail is created with allow.nomount, it is not able to create a jail with allow.mount set. Similarly, such restrictions as ip4.addr and securelevel may not be bypassed in child jails.
A child jail may in turn create its own child jails if its own children.max parameter is set (remember it is zero by default). These jails are visible to and can be modified by their parent and all ancestors.
Jail names reflect this hierarchy, with a full name being an MIB-type string separated by dots. For example, if a base system process creates a jail “foo”, and a process under that jail creates another jail “bar”, then the second jail will be seen as “foo.bar” in the base system (though it is only seen as “bar” to any processes inside jail “foo”). Jids on the other hand exist in a single space, and each jail must have a unique jid.
Like the names, a child jail's path appears relative to its creator's own path. This is by virtue of the child jail being created in the chrooted environment of the first jail.
killall(1), lsvfs(1), newaliases(1), pgrep(1), pkill(1), ps(1), quota(1), jail_set(2), vmm(4), devfs(5), fdescfs(5), jail.conf(5), linprocfs(5), linsysfs(5), procfs(5), rc.conf(5), sysctl.conf(5), bsdconfig(8), chroot(8), devfs(8), halt(8), ifconfig(8), inetd(8), jexec(8), jls(8), mount(8), named(8), reboot(8), rpcbind(8), sendmail(8), shutdown(8), sysctl(8), syslogd(8), umount(8)
The jail utility appeared in
FreeBSD 4.0. Hierarchical/extensible jails were
introduced in FreeBSD 8.0. The configuration file
was introduced in FreeBSD 9.1.
The jail feature was written by Poul-Henning Kamp for R&D Associates 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.
Bjoern A. Zeeb added multi-IP jail support for IPv4 and IPv6 based on a patch originally done by Pawel Jakub Dawidek for IPv4.
James Gritton added the extensible jail parameters, hierarchical jails, and the configuration file.
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
configurable.
Great care should be taken when managing directories visible within the jail. For example, if a jailed process has its current working directory set to a directory that is moved out of the jail's chroot, then the process may gain access to the file space outside of the jail. It is recommended that directories always be copied, rather than moved, out of a jail.
In addition, there are several ways in which an unprivileged user outside the jail can cooperate with a privileged user inside the jail and thereby obtain elevated privileges in the host environment. Most of these attacks can be mitigated by ensuring that the jail root is not accessible to unprivileged users in the host environment. Regardless, as a general rule, untrusted users with privileged access to a jail should not be given access to the host environment.
| July 18, 2022 | midnightbsd-3.1 |