1 .\" Copyright (c) 2000, 2003 Robert N. M. Watson
2 .\" Copyright (c) 2008-2012 James Gritton
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33 .Nd "manage system jails"
41 .Ar param Ns = Ns Ar value ...
42 .Op Cm command Ns = Ns Ar command ...
53 .Op Cm * | Ar jail ...
60 .Op Fl s Ar securelevel
61 .Op Ar path hostname [ Ar ip Ns [ Ns Ar ,... Ns ]] Ar command ...
65 utility creates new jails, or modifies or removes existing jails.
68 is specified via parameters on the command line, or in the
72 At least one of the options
78 These options are used alone or in combination to describe the operation to
80 .Bl -tag -width indent
87 parameters (if specified on the command line)
88 must not refer to an existing jail.
90 Modify an existing jail.
95 parameters must exist and refer to an existing jail.
96 Some parameters may not be changed on a running jail.
100 specified by jid or name.
101 All jailed processes are killed, and all jails that are
102 children of this jail are also
105 Restart an existing jail.
106 The jail is first removed and then re-created, as if
110 were run in succession.
112 Create a jail if it does not exist, or modify the jail if it does exist.
114 Modify an existing jail.
115 The jail may be restarted if necessary to modify parameters than could
116 not otherwise be changed.
118 Create a jail if it doesn't exist, or modify (and possibly restart) the
119 jail if it does exist.
122 Other available options are:
123 .Bl -tag -width indent
125 Allow making changes to a dying jail, equivalent to the
128 .It Fl f Ar conf_file
129 Use configuration file
131 instead of the default
138 and add all IP addresses returned by the resolver
139 to the list of addresses for this jail.
140 This is equivalent to the
144 Output (only) the jail identifier of the newly created jail(s).
151 file, containing the parameters used to start the jail.
153 Run commands in a clean environment.
154 This is deprecated and is equivalent to the exec.clean parameter.
157 This is deprecated and is equivalent to the
161 Limit the number of commands from
163 that can run simultaneously.
165 Suppress the message printed whenever a jail is created, modified or removed.
166 Only error messages will be printed.
170 option that removes an existing jail without using the configuration file.
171 No removal-related parameters for this jail will be used \(em the jail will
173 .It Fl s Ar securelevel
176 MIB entry to the specified value inside the newly created jail.
177 This is deprecated and is equivalent to the
181 The user name from host environment as whom jailed commands should run.
182 This is deprecated and is equivalent to the
185 .Va exec.system_jail_user
188 The user name from the jailed environment as whom jailed commands should run.
189 This is deprecated and is equivalent to the
193 Print a message on every operation, such as running commands and
194 mounting filesystems.
197 If no arguments are given after the options, the operation (except
198 remove) will be performed on all jails specified in the
201 A single argument of a jail name will operate only on the specified jail.
206 options can also remove running jails that aren't in the
208 file, specified by name or jid.
212 is a wildcard that will operate on all jails, regardless of whether
215 this is the surest way for
218 If hierarchical jails exist, a partial-matching wildcard definition may
220 For example, an argument of
222 would apply to jails with names like
227 A jail may be specified with parameters directly on the command line.
230 file will not be used.
231 For backward compatibility, the command line may also have four fixed
232 parameters, without names:
238 This mode will always create a new jail, and the
242 options do not apply (and must not be present).
246 file, or on the command line, are generally of the form
248 Some parameters are boolean, and do not have a value but are set by the
249 name alone with or without a
255 They can also be given the values
259 Other parameters may have more than one value, specified as a
260 comma-separated list or with
262 in the configuration file (see
268 utility recognizes two classes of parameters.
269 There are the true jail
270 parameters that are passed to the kernel when the jail is created,
271 which can be seen with
273 and can (usually) be changed with
275 Then there are pseudo-parameters that are only used by
279 Jails have a set of core parameters, and kernel modules can add their own
281 The current set of available parameters can be retrieved via
282 .Dq Nm sysctl Fl d Va security.jail.param .
283 Any parameters not set will be given default values, often based on the
285 The core parameters are:
286 .Bl -tag -width indent
289 This will be assigned automatically to a new jail (or can be explicitly
290 set), and can be used to identify the jail for later modification, or
297 This is an arbitrary string that identifies a jail (except it may not
302 it can be passed to later
310 is supplied, a default is assumed that is the same as the
314 parameter is implied by the
316 file format, and need not be explicitly set when using the configuration
319 The directory which is to be the root of the jail.
320 Any commands run inside the jail, either by
324 are run from this directory.
326 A list of IPv4 addresses assigned to the jail.
327 If this is set, the jail is restricted to using only these addresses.
328 Any attempts to use other addresses fail, and attempts to use wildcard
329 addresses silently use the jailed address instead.
330 For IPv4 the first address given will be used as the source address
331 when source address selection on unbound sockets cannot find a better
333 It is only possible to start multiple jails with the same IP address
334 if none of the jails has more than this single overlapping IP address
337 A boolean option to change the formerly mentioned behaviour and disable
338 IPv4 source address selection for the jail in favour of the primary
339 IPv4 address of the jail.
340 Source address selection is enabled by default for all jails and the
342 setting of a parent jail is not inherited for any child jails.
344 Control the availability of IPv4 addresses.
347 to allow unrestricted access to all system addresses,
349 to restrict addresses via
353 to stop the jail from using IPv4 entirely.
356 parameter implies a value of
358 .It Va ip6.addr , Va ip6.saddrsel , Va ip6
359 A set of IPv6 options for the jail, the counterparts to
366 Create the jail with its own virtual network stack,
367 with its own network interfaces, addresses, routing table, etc.
368 The kernel must have been compiled with the
370 for this to be available.
373 to use the system network stack, possibly with restricted IP addresses,
376 to create a new network stack.
378 The hostname of the jail.
379 Other similar parameters are
380 .Va host.domainname ,
385 Set the origin of hostname and related information.
388 to use the system information and
390 for the jail to use the information from the above fields.
391 Setting any of the above fields implies a value of
394 The value of the jail's
397 A jail never has a lower securelevel than its parent system, but by
398 setting this parameter it may have a higher one.
399 If the system securelevel is changed, any jail securelevels will be at
402 The number of the devfs ruleset that is enforced for mounting devfs in
404 A value of zero (default) means no ruleset is enforced.
405 Descendant jails inherit the parent jail's devfs ruleset enforcement.
406 Mounting devfs inside a jail is possible only if the
409 .Va allow.mount.devfs
410 permissions are effective and
412 is set to a value lower than 2.
413 Devfs rules and rulesets cannot be viewed or modified from inside a jail.
415 NOTE: It is important that only appropriate device nodes in devfs be
416 exposed to a jail; access to disk devices in the jail may permit processes
417 in the jail to bypass the jail sandboxing by modifying files outside of
421 for information on how to use devfs rules to limit access to entries
422 in the per-jail devfs.
423 A simple devfs ruleset for jails is available as ruleset #4 in
424 .Pa /etc/defaults/devfs.rules .
426 The number of child jails allowed to be created by this jail (or by
427 other jails under this jail).
428 This limit is zero by default, indicating the jail is not allowed to
431 .Sx "Hierarchical Jails"
432 section for more information.
434 The number of descendants of this jail, including its own child jails
435 and any jails created under them.
436 .It Va enforce_statfs
437 This determines what information processes in a jail are able to get
439 It affects the behaviour of the following syscalls:
445 (as well as similar compatibility syscalls).
446 When set to 0, all mount points are available without any restrictions.
447 When set to 1, only mount points below the jail's chroot directory are
449 In addition to that, the path to the jail's chroot directory is removed
450 from the front of their pathnames.
451 When set to 2 (default), above syscalls can operate only on a mount-point
452 where the jail's chroot directory is located.
454 Setting this boolean parameter allows a jail to exist without any
456 Normally, a command is run as part of jail creation, and then the jail
457 is destroyed as its last process exits.
458 A new jail must have either the
464 pseudo-parameter set.
466 The ID of the cpuset associated with this jail (read-only).
468 This is true if the jail is in the process of shutting down (read-only).
472 of the parent of this jail, or zero if this is a top-level jail
475 The string for the jail's
479 The number for the jail's
483 Some restrictions of the jail environment may be set on a per-jail
485 With the exception of
486 .Va allow.set_hostname ,
487 these boolean parameters are off by default.
488 .Bl -tag -width indent
489 .It Va allow.set_hostname
490 The jail's hostname may be changed via
495 A process within the jail has access to System V IPC primitives.
496 This is deprecated in favor of the per-module parameters (see below).
497 When this parameter is set, it is equivalent to setting
504 .It Va allow.raw_sockets
505 The jail root is allowed to create raw sockets.
506 Setting this parameter allows utilities like
510 to operate inside the jail.
511 If this is set, the source IP addresses are enforced to comply
512 with the IP address bound to the jail, regardless of whether or not
515 flag has been set on the socket.
516 Since raw sockets can be used to configure and interact with various
517 network subsystems, extra caution should be used where privileged access
518 to jails is given out to untrusted parties.
520 Normally, privileged users inside a jail are treated as unprivileged by
522 When this parameter is set, such users are treated as privileged, and
523 may manipulate system file flags subject to the usual constraints on
524 .Va kern.securelevel .
526 privileged users inside the jail will be able to mount and unmount file
527 system types marked as jail-friendly.
530 command can be used to find file system types available for mount from
532 This permission is effective only if
534 is set to a value lower than 2.
535 .It Va allow.mount.devfs
536 privileged users inside the jail will be able to mount and unmount the
538 This permission is effective only together with
542 is set to a value lower than 2.
543 The devfs ruleset should be restricted from the default by using the
546 .It Va allow.mount.fdescfs
547 privileged users inside the jail will be able to mount and unmount the
549 This permission is effective only together with
553 is set to a value lower than 2.
554 .It Va allow.mount.nullfs
555 privileged users inside the jail will be able to mount and unmount the
557 This permission is effective only together with
561 is set to a value lower than 2.
562 .It Va allow.mount.procfs
563 privileged users inside the jail will be able to mount and unmount the
565 This permission is effective only together with
569 is set to a value lower than 2.
570 .It Va allow.mount.linprocfs
571 privileged users inside the jail will be able to mount and unmount the
572 linprocfs file system.
573 This permission is effective only together with
577 is set to a value lower than 2.
578 .It Va allow.mount.linsysfs
579 privileged users inside the jail will be able to mount and unmount the
580 linsysfs file system.
581 This permission is effective only together with
585 is set to a value lower than 2.
586 .It Va allow.mount.tmpfs
587 privileged users inside the jail will be able to mount and unmount the
589 This permission is effective only together with
593 is set to a value lower than 2.
594 .It Va allow.mount.zfs
595 privileged users inside the jail will be able to mount and unmount the
597 This permission is effective only together with
601 is set to a value lower than 2.
604 for information on how to configure the ZFS filesystem to operate from
607 The jail root may administer quotas on the jail's filesystem(s).
608 This includes filesystems that the jail may share with other jails or
609 with non-jailed parts of the system.
610 .It Va allow.read_msgbuf
611 Jailed users may read the kernel message buffer.
613 .Va security.bsd.unprivileged_read_msgbuf
614 MIB entry is zero, this will be restricted to the root user.
615 .It Va allow.socket_af
616 Sockets within a jail are normally restricted to IPv4, IPv6, local
617 (UNIX), and route. This allows access to other protocol stacks that
618 have not had jail functionality added to them.
622 Kernel modules may add their own parameters, which only exist when the
624 These are typically headed under a parameter named after the module,
627 to give the jail full use of the module,
629 to encapsulate the jail in some module-specific way,
632 to make the module unavailable to the jail.
633 There also may be other parameters to define jail behavior within the module.
634 Module-specific parameters include:
635 .Bl -tag -width indent
637 Determine how a jail's Linux emulation environment appears.
640 will keep the same environment, and
642 will give the jail it's own environment (still originally inherited when
643 the jail is created).
644 .It Va linux.osname , linux.osrelease , linux.oss_version
645 The Linux OS name, OS release, and OSS version associated with this jail.
647 Allow access to SYSV IPC message primitives.
650 all IPC objects on the system are visible to this jail, whether they
651 were created by the jail itself, the base system, or other jails.
654 the jail will have its own key namespace, and can only see the objects
656 the system (or parent jail) has access to the jail's objects, but not to
660 the jail cannot perform any sysvmsg-related system calls.
661 .It Va sysvsem, sysvshm
662 Allow access to SYSV IPC semaphore and shared memory primitives, in the
667 There are pseudo-parameters that are not passed to the kernel, but are
670 to set up the jail environment, often by running specified commands
671 when jails are created or removed.
674 command parameters are
676 command lines that are run in either the system or jail environment.
677 They may be given multiple values, which would run the specified
678 commands in sequence.
679 All commands must succeed (return a zero exit status), or the jail will
680 not be created or removed, as appropriate.
682 The pseudo-parameters are:
683 .Bl -tag -width indent
685 Command(s) to run in the system environment before a jail is created.
687 Command(s) to run in the jail environment when a jail is created.
688 A typical command to run is
693 for use when specifying a jail directly on the command line.
694 Unlike other parameters whose value is a single string,
696 uses the remainder of the
698 command line as its own arguments.
699 .It Va exec.poststart
700 Command(s) to run in the system environment after a jail is created,
703 commands have completed.
705 Command(s) to run in the system environment before a jail is removed.
707 Command(s) to run in the jail environment before a jail is removed,
710 commands have completed.
711 A typical command to run is
712 .Dq sh /etc/rc.shutdown .
714 Command(s) to run in the system environment after a jail is removed.
716 Run commands in a clean environment.
717 The environment is discarded except for
718 .Ev HOME , SHELL , TERM
724 are set to the target login's default values.
726 is set to the target login.
728 is imported from the current environment.
729 The environment variables from the login class capability database for the
730 target login are also set.
731 .It Va exec.jail_user
732 The user to run commands as, when running in the jail environment.
733 The default is to run the commands as the current user.
734 .It Va exec.system_jail_user
735 This boolean option looks for the
739 file, instead of in the jail's file.
740 .It Va exec.system_user
741 The user to run commands as, when running in the system environment.
742 The default is to run the commands as the current user.
744 The maximum amount of time to wait for a command to complete, in
746 If a command is still running after this timeout has passed,
747 the jail will not be created or removed, as appropriate.
748 .It Va exec.consolelog
749 A file to direct command output (stdout and stderr) to.
751 The FIB (routing table) to set when running commands inside the jail.
753 The maximum amount of time to wait for a jail's processes to exit
756 signal (which happens after the
758 commands have completed).
759 After this many seconds have passed, the jail will be removed, which
760 will kill any remaining processes.
761 If this is set to zero, no
763 is sent and the jail is immediately removed.
764 The default is 10 seconds.
766 A network interface to add the jail's IP addresses
771 An alias for each address will be added to the interface before the
772 jail is created, and will be removed from the interface after the
775 In addition to the IP addresses that are passed to the kernel, an
776 interface, netmask and additional parameters (as supported by
778 may also be specified, in the form
779 .Dq Ar interface Ns | Ns Ar ip-address Ns / Ns Ar netmask param ... .
780 If an interface is given before the IP address, an alias for the address
781 will be added to that interface, as it is with the
784 If a netmask in either dotted-quad or CIDR form is given
785 after an IP address, it will be used when adding the IP alias.
786 If additional parameters are specified then they will also be used when
789 In addition to the IP addresses that are passed to the kernel,
790 an interface, prefix and additional parameters (as supported by
792 may also be specified, in the form
793 .Dq Ar interface Ns | Ns Ar ip-address Ns / Ns Ar prefix param ... .
794 .It Va vnet.interface
795 A network interface to give to a vnet-enabled jail after is it created.
796 The interface will automatically be released when the jail is removed.
800 parameter and add all IP addresses returned by the resolver
801 to the list of addresses
806 This may affect default address selection for outgoing IPv4 connections
808 The address first returned by the resolver for each address family
809 will be used as the primary address.
811 A filesystem to mount before creating the jail (and to unmount after
812 removing it), given as a single
818 format file containing filesystems to mount before creating a jail.
822 filesystem on the chrooted
824 directory, and apply the ruleset in the
826 parameter (or a default of ruleset 4: devfsrules_jail)
827 to restrict the devices visible inside the jail.
831 filesystem on the chrooted
837 filesystem on the chrooted
841 Allow making changes to a
845 Specify a jail (or jails) that this jail depends on.
846 When this jail is to be created, any jail(s) it depends on must already exist.
847 If not, they will be created automatically, up to the completion of the last
849 command, before any action will taken to create this jail.
850 When jails are removed the opposite is true:
851 this jail will be removed, up to the last
853 command, before any jail(s) it depends on are stopped.
856 Jails are typically set up using one of two philosophies: either to
857 constrain a specific application (possibly running with privilege), or
859 .Dq "virtual system image"
860 running a variety of daemons and services.
861 In both cases, a fairly complete file system install of
864 required, so as to provide the necessary command line tools, daemons,
865 libraries, application configuration files, etc.
866 However, for a virtual server configuration, a fair amount of
867 additional work is required so as to replace the
870 This manual page documents the configuration steps necessary to support
871 either of these steps, although the configuration steps may need to be
872 refined based on local requirements.
873 .Ss "Setting up a Jail Directory Tree"
874 To set up a jail directory tree containing an entire
876 distribution, the following
878 command script can be used:
883 make world DESTDIR=$D
884 make distribution DESTDIR=$D
887 In many cases this example would put far more in the jail than needed.
888 In the other extreme case a jail might contain only one file:
889 the executable to be run in the jail.
891 We recommend experimentation, and caution that it is a lot easier to
894 jail and remove things until it stops working,
895 than it is to start with a
897 jail and add things until it works.
898 .Ss "Setting Up a Jail"
899 Do what was described in
900 .Sx "Setting Up a Jail Directory Tree"
901 to build the jail directory tree.
902 For the sake of this example, we will
903 assume you built it in
904 .Pa /data/jail/testjail ,
907 Substitute below as needed with your
908 own directory, IP address, and hostname.
909 .Ss "Setting up the Host Environment"
910 First, set up the real system's environment to be
912 For consistency, we will refer to the parent box as the
913 .Dq "host environment" ,
914 and to the jailed virtual machine as the
915 .Dq "jail environment" .
916 Since jails are implemented using IP aliases, one of the first things to do
917 is to disable IP services on the host system that listen on all local
918 IP addresses for a service.
919 If a network service is present in the host environment that binds all
920 available IP addresses rather than specific IP addresses, it may service
921 requests sent to jail IP addresses if the jail did not bind the port.
924 to only listen on the
925 appropriate IP address, and so forth.
928 in the host environment:
929 .Bd -literal -offset indent
931 inetd_flags="-wW -a 192.0.2.23"
936 is the native IP address for the host system, in this example.
937 Daemons that run out of
939 can be easily configured to use only the specified host IP address.
941 will need to be manually configured \(em for some this is possible through
943 flags entries; for others it is necessary to modify per-application
944 configuration files, or to recompile the application.
945 The following frequently deployed services must have their individual
946 configuration files modified to limit the application to listening
947 to a specific IP address:
951 it is necessary to modify
952 .Pa /etc/ssh/sshd_config .
956 it is necessary to modify
957 .Pa /etc/mail/sendmail.cf .
961 it is necessary to modify
962 .Pa /etc/namedb/named.conf .
964 In addition, a number of services must be recompiled in order to run
965 them in the host environment.
966 This includes most applications providing services using
973 In general, applications for which it is not possible to specify which
974 IP address to bind should not be run in the host environment unless they
975 should also service requests sent to jail IP addresses.
977 NFS from the host environment may also cause confusion, and cannot be
978 easily reconfigured to use only specific IPs, as some NFS services are
979 hosted directly from the kernel.
980 Any third-party network software running
981 in the host environment should also be checked and configured so that it
982 does not bind all IP addresses, which would result in those services also
983 appearing to be offered by the jail environments.
986 these daemons have been disabled or fixed in the host environment, it is
987 best to reboot so that all daemons are in a known state, to reduce the
988 potential for confusion later (such as finding that when you send mail
989 to a jail, and its sendmail is down, the mail is delivered to the host,
991 .Ss "Configuring the Jail"
992 Start any jail for the first time without configuring the network
993 interface so that you can clean it up a little and set up accounts.
995 with any machine (virtual or not), you will need to set a root password, time
997 Some of these steps apply only if you intend to run a full virtual server
998 inside the jail; others apply both for constraining a particular application
999 or for running a virtual server.
1001 Start a shell in the jail:
1002 .Bd -literal -offset indent
1003 jail -c path=/data/jail/testjail mount.devfs \\
1004 host.hostname=testhostname ip4.addr=192.0.2.100 \\
1008 Assuming no errors, you will end up with a shell prompt within the jail.
1010 .Pa /usr/sbin/sysinstall
1011 and do the post-install configuration to set various configuration options,
1012 or perform these actions manually by editing
1016 .Bl -bullet -offset indent -compact
1019 .Pa /etc/resolv.conf
1020 so that name resolution within the jail will work correctly.
1028 Set a root password, probably different from the real host system.
1032 Add accounts for users in the jail environment.
1034 Install any packages the environment requires.
1037 You may also want to perform any package-specific configuration (web servers,
1038 SSH servers, etc), patch up
1039 .Pa /etc/syslog.conf
1040 so it logs as you would like, etc.
1041 If you are not using a virtual server, you may wish to modify
1043 in the host environment to listen on the syslog socket in the jail
1044 environment; in this example, the syslog socket would be stored in
1045 .Pa /data/jail/testjail/var/run/log .
1047 Exit from the shell, and the jail will be shut down.
1048 .Ss "Starting the Jail"
1049 You are now ready to restart the jail and bring up the environment with
1050 all of its daemons and other programs.
1051 Create an entry for the jail in
1052 .Pa /etc/jail.conf :
1053 .Bd -literal -offset indent
1055 path = /tmp/jail/testjail;
1057 host.hostname = testhostname;
1058 ip4.addr = 192.0.2.100;
1060 exec.start = "/bin/sh /etc/rc";
1061 exec.stop = "/bin/sh /etc/rc.shutdown";
1065 To start a virtual server environment,
1067 is run to launch various daemons and services, and
1068 .Pa /etc/rc.shutdown
1069 is run to shut them down when the jail is removed.
1070 If you are running a single application in the jail,
1071 substitute the command used to start the application for
1072 .Dq /bin/sh /etc/rc ;
1073 there may be some script available to cleanly shut down the application,
1074 or it may be sufficient to go without a stop command, and have
1080 Start the jail by running:
1081 .Bd -literal -offset indent
1085 A few warnings may be produced; however, it should all work properly.
1086 You should be able to see
1089 and other processes running within the jail using
1093 flag appearing beside jailed processes.
1094 To see an active list of jails, use
1098 is enabled in the jail environment, you should be able to
1100 to the hostname or IP address of the jailed environment, and log
1101 in using the accounts you created previously.
1103 It is possible to have jails started at boot time.
1108 for more information.
1109 .Ss "Managing the Jail"
1110 Normal machine shutdown commands, such as
1115 cannot be used successfully within the jail.
1116 To kill all processes from within a jail, you may use one of the
1117 following commands, depending on what you want to accomplish:
1118 .Bd -literal -offset indent
1127 signals to all processes in the jail \(em be careful not to run this from
1128 the host environment!
1129 Once all of the jail's processes have died, unless the jail was created
1132 parameter, the jail will be removed.
1134 the intended use of the jail, you may also want to run
1135 .Pa /etc/rc.shutdown
1136 from within the jail.
1138 To shut down the jail from the outside, simply remove it with
1141 which will run any commands specified by
1147 to any remaining jailed processes.
1150 .Pa /proc/ Ns Ar pid Ns Pa /status
1151 file contains, as its last field, the name of the jail in which the
1154 to indicate that the process is not running within a jail.
1157 command also shows a
1159 flag for processes in a jail.
1161 You can also list/kill processes based on their jail ID.
1162 To show processes and their jail ID, use the following command:
1164 .Dl "ps ax -o pid,jid,args"
1166 To show and then kill processes in jail number 3 use the following commands:
1167 .Bd -literal -offset indent
1174 .Ss "Jails and File Systems"
1175 It is not possible to
1179 any file system inside a jail unless the file system is marked
1180 jail-friendly, the jail's
1182 parameter is set, and the jail's
1184 parameter is lower than 2.
1186 Multiple jails sharing the same file system can influence each other.
1187 For example, a user in one jail can fill the file system,
1188 leaving no space for processes in the other jail.
1191 to prevent this will not work either, as the file system quotas
1192 are not aware of jails but only look at the user and group IDs.
1193 This means the same user ID in two jails share a single file
1195 One would need to use one file system per jail to make this work.
1196 .Ss "Sysctl MIB Entries"
1198 .Va security.jail.jailed
1199 can be used to determine if a process is running inside a jail (value
1200 is one) or not (value is zero).
1203 .Va security.jail.max_af_ips
1204 determines how may address per address family a jail may have.
1207 Some MIB variables have per-jail settings.
1208 Changes to these variables by a jailed process do not affect the host
1209 environment, only the jail environment.
1211 .Va kern.securelevel ,
1213 .Va kern.domainname ,
1217 .Ss "Hierarchical Jails"
1220 parameter, processes within a jail may be able to create jails of their own.
1221 These child jails are kept in a hierarchy, with jails only able to see and/or
1222 modify the jails they created (or those jails' children).
1223 Each jail has a read-only
1225 parameter, containing the
1227 of the jail that created it; a
1229 of 0 indicates the jail is a child of the current jail (or is a top-level
1230 jail if the current process isn't jailed).
1232 Jailed processes are not allowed to confer greater permissions than they
1233 themselves are given, e.g., if a jail is created with
1235 it is not able to create a jail with
1238 Similarly, such restrictions as
1242 may not be bypassed in child jails.
1244 A child jail may in turn create its own child jails if its own
1246 parameter is set (remember it is zero by default).
1247 These jails are visible to and can be modified by their parent and all
1250 Jail names reflect this hierarchy, with a full name being an MIB-type string
1252 For example, if a base system process creates a jail
1254 and a process under that jail creates another jail
1256 then the second jail will be seen as
1258 in the base system (though it is only seen as
1260 to any processes inside jail
1262 Jids on the other hand exist in a single space, and each jail must have a
1265 Like the names, a child jail's
1267 appears relative to its creator's own
1269 This is by virtue of the child jail being created in the chrooted
1270 environment of the first jail.
1309 Hierarchical/extensible jails were introduced in
1311 The configuration file was introduced in
1315 The jail feature was written by
1316 .An Poul-Henning Kamp
1318 who contributed it to
1322 wrote the extended documentation, found a few bugs, added
1323 a few new features, and cleaned up the userland jail environment.
1326 added multi-IP jail support for IPv4 and IPv6 based on a patch
1328 .An Pawel Jakub Dawidek
1332 added the extensible jail parameters, hierarchical jails,
1333 and the configuration file.
1335 It might be a good idea to add an
1336 address alias flag such that daemons listening on all IPs
1338 will not bind on that address, which would facilitate building a safe
1339 host environment such that host daemons do not impose on services offered
1341 Currently, the simplest answer is to minimize services
1342 offered on the host, possibly limiting it to services offered from
1344 which is easily configurable.
1346 Great care should be taken when managing directories visible within the jail.
1347 For example, if a jailed process has its current working directory set to a
1348 directory that is moved out of the jail's chroot, then the process may gain
1349 access to the file space outside of the jail.
1350 It is recommended that directories always be copied, rather than moved, out
1353 In addition, there are several ways in which an unprivileged user
1354 outside the jail can cooperate with a privileged user inside the jail
1355 and thereby obtain elevated privileges in the host environment.
1356 Most of these attacks can be mitigated by ensuring that the jail root
1357 is not accessible to unprivileged users in the host environment.
1358 Regardless, as a general rule, untrusted users with privileged access
1359 to a jail should not be given access to the host environment.
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