JAIL(8) FreeBSD System Manager's Manual JAIL(8)
NAME
jail - manage system jails
SYNOPSIS
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
DESCRIPTION
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 Create a new jail. The jail jid and name parameters (if
specified on the command line) must not refer to an existing
jail.
-e separator
Exhibit a list of all configured non-wildcard jails and their
parameters. No jail creation, modification or removal performed
if this option is used. The separator string is used to separate
parameters. Use jls(8) utility to list running jails.
-m Modify an existing jail. One of the jid or name parameters must
exist and refer to an existing jail. Some parameters may not be
changed on a running jail.
-r Remove the jail specified by jid or name. All jailed processes
are killed, and all jails that are children of this jail are also
removed.
-rc Restart an existing jail. The jail is first removed and then re-
created, as if "jail -r" and "jail -c" were run in succession.
-cm Create a jail if it does not exist, or modify the jail if it does
exist.
-mr Modify an existing jail. The jail may be restarted if necessary
to modify parameters than could not otherwise be changed.
-cmr Create a jail if it doesn't exist, or modify (and possibly
restart) the jail if it does exist.
Other available options are:
-d Allow making changes to a dying jail, equivalent to the
allow.dying parameter.
-f conf_file
Use configuration file conf_file instead of the default
/etc/jail.conf.
-h Resolve the host.hostname parameter (or hostname) and add all IP
addresses returned by the resolver to the list of addresses for
this jail. This is equivalent to the ip_hostname parameter.
-i Output (only) the jail identifier of the newly created jail(s).
This implies the -q option.
-J jid_file
Write a jid_file file, containing the parameters used to start
the jail.
-l Run commands in a clean environment. This is deprecated and is
equivalent to the exec.clean parameter.
-n jailname
Set the jail's name. This is deprecated and is equivalent to the
name parameter.
-p limit
Limit the number of commands from exec.* that can run
simultaneously.
-q Suppress the message printed whenever a jail is created, modified
or removed. Only error messages will be printed.
-R A variation of the -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
Set the kern.securelevel MIB entry to the specified value inside
the newly created jail. This is deprecated and is equivalent to
the securelevel parameter.
-u username
The user name from host environment as whom jailed commands
should run. This is deprecated and is equivalent to the
exec.jail_user and exec.system_jail_user parameters.
-U username
The user name from the jailed environment as whom jailed commands
should run. This is deprecated and is equivalent to the
exec.jail_user parameter.
-v Print a message on every operation, such as running commands and
mounting filesystems.
If 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).
Jail Parameters
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:
jid The jail identifier. This will be assigned automatically to a
new jail (or can be explicitly set), and can be used to identify
the jail for later modification, or for such commands as jls(8)
or jexec(8).
name The jail name. This is an arbitrary string that identifies a
jail (except it may not contain a `.'). Like the jid, it can be
passed to later 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.
path The directory which is to be the root of the jail. Any commands
run inside the jail, either by jail or from jexec(8), are run
from this directory.
ip4.addr
A list of IPv4 addresses assigned to the jail. If this is set,
the jail is restricted to using only these addresses. Any
attempts to use other addresses fail, and attempts to use
wildcard addresses silently use the jailed address instead. For
IPv4 the first address given will be used as the source address
when source address selection on unbound sockets cannot find a
better match. It is only possible to start multiple jails with
the same IP address if none of the jails has more than this
single overlapping IP address assigned to itself.
ip4.saddrsel
A boolean option to change the formerly mentioned behaviour and
disable IPv4 source address selection for the jail in favour of
the primary IPv4 address of the jail. Source address selection
is enabled by default for all jails and the ip4.nosaddrsel
setting of a parent jail is not inherited for any child jails.
ip4 Control the availability of IPv4 addresses. Possible values are
"inherit" to allow unrestricted access to all system addresses,
"new" to restrict addresses via ip4.addr, and "disable" to stop
the jail from using IPv4 entirely. Setting the ip4.addr
parameter implies a value of "new".
ip6.addr, ip6.saddrsel, ip6
A set of IPv6 options for the jail, the counterparts to ip4.addr,
ip4.saddrsel and ip4 above.
vnet Create the jail with its own virtual network stack, with its own
network interfaces, addresses, routing table, etc. The kernel
must have been compiled with the VIMAGE option for this to be
available. Possible values are "inherit" to use the system
network stack, possibly with restricted IP addresses, and "new"
to create a new network stack.
host.hostname
The hostname of the jail. Other similar parameters are
host.domainname, host.hostuuid and host.hostid.
host Set the origin of hostname and related information. Possible
values are "inherit" to use the system information and "new" for
the jail to use the information from the above fields. Setting
any of the above fields implies a value of "new".
securelevel
The value of the jail's kern.securelevel sysctl. A jail never
has a lower securelevel than its parent system, but by setting
this parameter it may have a higher one. If the system
securelevel is changed, any jail securelevels will be at least as
secure.
devfs_ruleset
The number of the devfs ruleset that is enforced for mounting
devfs in this jail. A value of zero (default) means no ruleset
is enforced. Descendant jails inherit the parent jail's devfs
ruleset enforcement. Mounting devfs inside a jail is possible
only if the allow.mount and allow.mount.devfs permissions are
effective and enforce_statfs is set to a value lower than 2.
Devfs rules and rulesets cannot be viewed or modified from inside
a jail.
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.
children.max
The number of child jails allowed to be created by this jail (or
by other jails under this jail). This limit is zero by default,
indicating the jail is not allowed to create child jails. See
the Hierarchical Jails section for more information.
children.cur
The number of descendants of this jail, including its own child
jails and any jails created under them.
enforce_statfs
This determines what information processes in a jail are able to
get about mount points. It affects the behaviour of the
following syscalls: statfs(2), fstatfs(2), getfsstat(2), and
fhstatfs(2) (as well as similar compatibility syscalls). When
set to 0, all mount points are available without any
restrictions. When set to 1, only mount points below the jail's
chroot directory are visible. In addition to that, the path to
the jail's chroot directory is removed from the front of their
pathnames. When set to 2 (default), above syscalls can operate
only on a mount-point where the jail's chroot directory is
located.
persist
Setting this boolean parameter allows a jail to exist without any
processes. Normally, a command is run as part of jail creation,
and then the jail is destroyed as its last process exits. A new
jail must have either the persist parameter or exec.start or
command pseudo-parameter set.
cpuset.id
The ID of the cpuset associated with this jail (read-only).
dying This is true if the jail is in the process of shutting down
(read-only).
parent The jid of the parent of this jail, or zero if this is a top-
level jail (read-only).
osrelease
The string for the jail's kern.osrelease sysctl and uname -r.
osreldate
The number for the jail's kern.osreldate and uname -K.
allow.*
Some restrictions of the jail environment may be set on a per-
jail basis. With the exception of allow.set_hostname and
allow.reserved_ports, these boolean parameters are off by
default.
allow.set_hostname
The jail's hostname may be changed via hostname(1) or
sethostname(3).
allow.sysvipc
A process within the jail has access to System V IPC
primitives. This is deprecated in favor of the per-
module parameters (see below). When this parameter is
set, it is equivalent to setting sysvmsg, sysvsem, and
sysvshm all to "inherit".
allow.raw_sockets
The jail root is allowed to create raw sockets. Setting
this parameter allows utilities like ping(8) and
traceroute(8) to operate inside the jail. If this 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.
allow.chflags
Normally, privileged users inside a jail are treated as
unprivileged by chflags(2). When this parameter is set,
such users are treated as privileged, and may manipulate
system file flags subject to the usual constraints on
kern.securelevel.
allow.mount
privileged users inside the jail will be able to mount
and unmount file system types marked as jail-friendly.
The lsvfs(1) command can be used to find file system
types available for mount from within a jail. This
permission is effective only if enforce_statfs is set to
a value lower than 2.
allow.mount.devfs
privileged users inside the jail will be able to mount
and unmount the devfs file system. This permission is
effective only together with allow.mount and only when
enforce_statfs is set to a value lower than 2. The devfs
ruleset should be restricted from the default by using
the devfs_ruleset option.
allow.quotas
The jail root may administer quotas on the jail's
filesystem(s). This includes filesystems that the jail
may share with other jails or with non-jailed parts of
the system.
allow.read_msgbuf
Jailed users may read the kernel message buffer. If the
security.bsd.unprivileged_read_msgbuf MIB entry is zero,
this will be restricted to the root user.
allow.socket_af
Sockets within a jail are normally restricted to IPv4,
IPv6, local (UNIX), and route. This allows access to
other protocol stacks that have not had jail
functionality added to them.
allow.mlock
Locking or unlocking physical pages in memory are
normally not available within a jail. When this
parameter is set, users may mlock(2) or munlock(2) memory
subject to security.bsd.unprivileged_mlock and resource
limits.
allow.reserved_ports
The jail root may bind to ports lower than 1024.
allow.unprivileged_proc_debug
Unprivileged processes in the jail may use debugging
facilities.
allow.suser
The value of the jail's security.bsd.suser_enabled
sysctl. The super-user will be disabled automatically if
its parent system has it disabled. The super-user is
enabled by default.
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:
allow.mount.fdescfs
privileged users inside the jail will be able to mount and
unmount the fdescfs file system. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.fusefs
privileged users inside the jail will be able to mount and
unmount fuse-based file systems. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.nullfs
privileged users inside the jail will be able to mount and
unmount the nullfs file system. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.procfs
privileged users inside the jail will be able to mount and
unmount the procfs file system. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.linprocfs
privileged users inside the jail will be able to mount and
unmount the linprocfs file system. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.linsysfs
privileged users inside the jail will be able to mount and
unmount the linsysfs file system. This permission is effective
only together with allow.mount and only when enforce_statfs is
set to a value lower than 2.
allow.mount.tmpfs
privileged users inside the jail will be able to mount and
unmount the tmpfs file system. This permission is effective only
together with allow.mount and only when enforce_statfs is set to
a value lower than 2.
allow.mount.zfs
privileged users inside the jail will be able to mount and
unmount the ZFS file system. This permission is effective only
together with allow.mount and only when enforce_statfs is set to
a value lower than 2. See zfs(8) for information on how to
configure the ZFS filesystem to operate from within a jail.
allow.vmm
The jail may access vmm(4). This flag is only available when the
vmm(4) kernel module is loaded.
linux Determine how a jail's Linux emulation environment appears. A
value of "inherit" will keep the same environment, and "new" will
give the jail its own environment (still originally inherited
when the jail is created).
linux.osname, linux.osrelease, linux.oss_version
The Linux OS name, OS release, and OSS version associated with
this jail.
sysvmsg
Allow access to SYSV IPC message primitives. If set to
"inherit", all IPC objects on the system are visible to this
jail, whether they were created by the jail itself, the base
system, or other jails. If set to "new", the jail will have its
own key namespace, and can only see the objects that it has
created; the system (or parent jail) has access to the jail's
objects, but not to its keys. If set to "disable", the jail
cannot perform any sysvmsg-related system calls.
sysvsem, sysvshm
Allow access to SYSV IPC semaphore and shared memory primitives,
in the same manner as sysvmsg.
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:
exec.prepare
Command(s) to run in the system environment to prepare a jail for
creation. These commands are executed before assigning IP
addresses and mounting filesystems, so they may be used to create
a new jail filesystem if it does not already exist.
exec.prestart
Command(s) to run in the system environment before a jail is
created.
exec.created
Command(s) to run in the system environment right after a jail
has been created, but before commands (or services) get executed
in the jail.
exec.start
Command(s) to run in the jail environment when a jail is created.
A typical command to run is "sh /etc/rc".
command
A synonym for exec.start for use when specifying a jail directly
on the command line. Unlike other parameters whose value is a
single string, command uses the remainder of the jail command
line as its own arguments.
exec.poststart
Command(s) to run in the system environment after a jail is
created, and after any exec.start commands have completed.
exec.prestop
Command(s) to run in the system environment before a jail is
removed.
exec.stop
Command(s) to run in the jail environment before a jail is
removed, and after any exec.prestop commands have completed. A
typical command to run is "sh /etc/rc.shutdown jail".
exec.poststop
Command(s) to run in the system environment after a jail is
removed.
exec.release
Command(s) to run in the system environment after all other
actions are done. These commands are executed after unmounting
filesystems and removing IP addresses, so they may be used to
remove a jail filesystem if it is no longer needed.
exec.clean
Run commands in a 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.
exec.jail_user
The user to run commands as, when running in the jail
environment. The default is to run the commands as the current
user.
exec.system_jail_user
This boolean option looks for the exec.jail_user in the system
passwd(5) file, instead of in the jail's file.
exec.system_user
The user to run commands as, when running in the system
environment. The default is to run the commands as the current
user.
exec.timeout
The maximum amount of time to wait for a command to complete, in
seconds. If a command is still running after this timeout has
passed, the jail will not be created or removed, as appropriate.
exec.consolelog
A file to direct command output (stdout and stderr) to.
exec.fib
The FIB (routing table) to set when running commands inside the
jail.
stop.timeout
The maximum amount of time to wait for a jail's processes to exit
after sending them a 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.
interface
A network interface to add the jail's IP addresses (ip4.addr and
ip6.addr) to. An alias for each address will be added to the
interface before the jail is created, and will be removed from
the interface after the jail is removed.
ip4.addr
In addition to the IP addresses that are passed to the kernel, an
interface, netmask and additional parameters (as supported by
ifconfig(8)) may also be specified, in the form
"interface|ip-address/netmask param ...". If an interface is
given before the IP address, an alias for the address will be
added to that interface, as it is with the interface parameter.
If a netmask in either dotted-quad or CIDR form is given after an
IP address, it will be used when adding the IP alias. If
additional parameters are specified then they will also be used
when adding the IP alias.
ip6.addr
In addition to the IP addresses that are passed to the kernel, an
interface, prefix and additional parameters (as supported by
ifconfig(8)) may also be specified, in the form
"interface|ip-address/prefix param ...".
vnet.interface
A network interface to give to a vnet-enabled jail after is it
created. The interface will automatically be released when the
jail is removed.
ip_hostname
Resolve the host.hostname parameter and add all IP addresses
returned by the resolver to the list of addresses (ip4.addr or
ip6.addr) for this jail. This may affect default address
selection for outgoing IPv4 connections from jails. The address
first returned by the resolver for each address family will be
used as the primary address.
mount A filesystem to mount before creating the jail (and to unmount
after removing it), given as a single fstab(5) line.
mount.fstab
An fstab(5) format file containing filesystems to mount before
creating a jail.
mount.devfs
Mount a devfs(5) filesystem on the chrooted /dev directory, and
apply the ruleset in the devfs_ruleset parameter (or a default of
ruleset 4: devfsrules_jail) to restrict the devices visible
inside the jail.
mount.fdescfs
Mount a fdescfs(5) filesystem on the chrooted /dev/fd directory.
mount.procfs
Mount a procfs(5) filesystem on the chrooted /proc directory.
allow.dying
Allow making changes to a dying jail.
depend Specify a jail (or jails) that this jail depends on. When this
jail is to be created, any jail(s) it depends on must already
exist. If not, they will be created automatically, up to the
completion of the last exec.poststart command, before any action
will taken to create this jail. When jails are removed the
opposite is true: this jail will be removed, up to the last
exec.poststop command, before any jail(s) it depends on are
stopped.
EXAMPLES
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.
Setting up a Jail Directory Tree
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.
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/testjail, for a jail named "testjail". Substitute
below as needed with your own directory, IP address, and hostname.
Setting up the Host Environment
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.).
Configuring the Jail
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.
• Configure /etc/resolv.conf so that name resolution within the
jail will work correctly.
• Run newaliases(1) to quell sendmail(8) warnings.
• 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 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/testjail/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. 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 = em0;
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.
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 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,args
To show and then kill processes in jail number 3 use the following
commands:
pgrep -lfj 3
pkill -j 3
or:
killall -j 3
Jails and File Systems
It 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.
Sysctl MIB Entries
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,
security.bsd.suser_enabled, kern.hostname, kern.domainname, kern.hostid,
and kern.hostuuid.
Hierarchical Jails
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.
SEE ALSO
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)
HISTORY
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.
AUTHORS
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.
BUGS
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.
NOTES
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.
FreeBSD 13.1-RELEASE-p6 April 4, 2021 FreeBSD 13.1-RELEASE-p6
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