NAME¶

jail — create or modify a system jail

SYNOPSIS¶

jail	[-dhi] [-J jid_file] [-l -u username | -U username] [-c | -m] [parameter=value ...]

jail	[-hi] [-n jailname] [-J jid_file] [-s securelevel] [-l -u username | -U username] [path hostname [ip[,..]] command ...]

jail

[-r jail]

DESCRIPTION¶

The jail utility creates a new jail or modifies an existing jail, optionally imprisoning the current process (and future descendants) inside it. The options are as follows:

-d

Allow making changes to a dying jail.

-h

Resolve the host.hostname parameter (or hostname) and add all IP addresses returned by the resolver to the list of ip addresses for this prison. This may affect default address selection for outgoing IPv4 connections of prisons. The address first returned by the resolver for each address family will be used as primary address. See the ip4.addr and ip6.addr parameters further down for details.

-i

Output the jail identifier of the newly created jail.

-n jailname

Set the jail's name. This is deprecated and is equivalent to setting the name parameter.

-J jid_file

Write a jid_file file, containing jail identifier, path, hostname, IP and command used to start the 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.

-s securelevel

Set the kern.securelevel MIB entry to the specified value inside the newly created jail. This is deprecated and is equivalent to setting the securelevel parameter.

-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.

-c

Create a new jail. The jid and name parameters (if specified) must not refer to an existing jail.

-m

Modify an existing jail. One of the jid or name parameters must exist and refer to an existing jail.

-cm

Create a jail if it does not exist, or modify a jail if it does exist.

-r

Remove the jail specified by jid or name. All jailed processes are killed, and all children of this jail are also removed.

At least one of the -c, -m or -r options must be specified. Parameters are listed in “name=value” form, following the options. 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. Any parameters not set will be given default values, often based on the current environment. The pseudo-parameter command specifies that the current process should enter the new (or modified) jail, and run the specified command. It must be the last parameter specified, because it includes not only the value following the ‘=’ sign, but also passes the rest of the arguments to the command. Instead of supplying named parameters, four fixed parameters may be supplied in order on the command line: path, hostname, ip, and command. As the jid and name parameters aren't in this list, this mode will always create a new jail, and the -c and -m options don't apply (and must not exist). Jails have a set a core parameters, and modules can add their own jail parameters. The current set of available parameters can be retrieved via “sysctl -d security.jail.param”. 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.

path

Directory which is to be the root of the prison. The command (if any) is run from this directory, as are commands from jexec(8).

ip4.addr

A comma-separated list of IPv4 addresses assigned to the prison. 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 kept used as the source address in case 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 prison in favour of the primary IPv4 address of the jail. Source address selection is enabled by default for all jails and a 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 above, 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 prison, the counterparts to ip4.addr, ip4.saddrsel and ip4 above.

host.hostname

Hostname of the prison. 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 the default 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.

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 descendents of this jail, including its own child jails and any jails created under them.

enforce_statfs

This determines which 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 jail is destroyed as its last process exits. A new jail must have either the persist parameter 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).

allow.*

Some restrictions of the jail environment may be set on a per-jail basis. With the exception of allow.set_hostname, 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. 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.

allow.raw_sockets

The prison root is allowed to create raw sockets. Setting this parameter allows utilities like ping(8) and traceroute(8) to operate inside the prison. 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.quotas

The prison 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.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.

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 configure the “boot” process. This manual page documents the configuration steps necessary to support either of these steps, although the configuration steps may be refined based on local requirements.

EXAMPLES¶

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: 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. 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/192.0.2.100, named for the jailed IP address. Substitute below as needed with your own directory, IP address, and hostname.

Setting up the Host Environment¶

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”. Since 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. 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: 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 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 entries; for others it is necessary to modify per-application configuration files, or to recompile the applications. 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: Assuming no errors, you will end up with a shell prompt within the jail. You can now run /usr/sbin/sysinstall and do the post-install configuration to set various configuration options, or perform these actions manually by editing /etc/rc.conf, etc.

• Create an empty /etc/fstab to quell startup warnings about missing fstab (virtual server only)
• Disable the port mapper (/etc/rc.conf: rpcbind_enable="NO") (virtual server only)
• Configure /etc/resolv.conf so that name resolution within the jail will work correctly
• Run newaliases(1) to quell sendmail(8) warnings.
• Disable interface configuration to quell startup warnings about ifconfig(8) (network_interfaces="") (virtual server only)
• 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/192.0.2.100/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. If you are running a single application in the jail, substitute the command used to start the application for /etc/rc in the examples below. To start a virtual server environment, /etc/rc is run to launch various daemons and services. To do this, first bring up the virtual host interface, and then start the jail's /etc/rc script from within the jail. 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. To see an active list of jails, use the jls(8) utility. 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. It is possible to have jails started at boot time. Please refer to the “jail_*” variables in rc.conf(5) for more information. The rc(8) jail script provides a flexible system to start/stop jails:

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: 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. To kill processes from outside the jail, use the jexec(8) utility in conjunction with the one of the kill(1) commands above. You may also remove the jail with jail -r, which will killall the jail's processes with SIGKILL. 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: To show and then kill processes in jail number 3 use the following commands: or:

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 also 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 the same 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 prison may have. The default is 255. Some MIB variables have per-jail settings. Changes to these variables by a jailed process do not effect the host environment, only the jail environment. These variables are kern.securelevel, 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 is 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), chroot(2), jail_set(2), jail_attach(2), procfs(5), rc.conf(5), sysctl.conf(5), devfs(8), halt(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.

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. 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 and hierarchical jails.

BUGS¶

Jail currently lacks the ability 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 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.