Jail

Jails sind eine einfache Lösung zur Virtualisierung von Computersystemen im FreeBSD Betriebssystem. Im Gegensatz zu den meisten gängigen Virtualisierungslösungen wird eine virtuelle Maschine nur sehr rudimentär implementiert. So findet keine tatsächliche Emulation eines vollständigen eigenen Systems samt entsprechender virtueller Hardware statt. Vielmehr läuft ein Jail auf der selben Kernel-Instanz wie das Hostsystem und wird darunter nach einem chroot in ein eigenes Unterverzeichnis in einer eigenen Prozessgruppe gestartet. Der Kernel kapselt dabei die Prozesstabellen ab, so dass ein Jail stets nur seine eigenen Prozesse sehen kann. Ausschließlich das Hostsystem hat Zugriff auf die vollständige Prozesstabelle.

Genaugenommen handelt es sich bei einem Jail mehr oder weniger um ein aufgebohrtes chroot. Dadurch muss der Kernel zwingend ein gleichartiges Betriebssystem im Jail starten.

Einschränkungen

Bis auf ein paar Einschränkungen ist ein Jail genauso funktional wie das native Hostsystem. Im Detail funktionieren nicht:

• IPv6
• Mounten und unmounten von Dateisystemen,
• Abhängig von der Policy, Dienste die Raw Sockets benötigen: Ping, Traceroute, ...
• Shared Memory
• Das Anlegen von Paketfilter-Regeln

Hier ist die Entwicklung aber nicht zum Stehen gekommen. Einer Überarbeitung des Jail-Konzepts wird in FreeBSD Jail2 einfließen. Einen Überblick über das Projekt bietet die FreeVPS Projektseite.

Sysctl MIB Entries

(Quelle: FreeBSD man pages jail(8))

Certain aspects of the jail containments environment may be modified from
the host environment using sysctl(8) MIB variables.  Currently, these
variables affect all jails on the system, although in the future this
functionality may be finer grained.

security.jail.allow_raw_sockets:
	  This MIB entry determines whether or not prison root is allowed to
	  create raw sockets.  Setting this MIB to 1 allows utilities like
	  ping(8) and traceroute(8) to operate inside the prison.  If this MIB
	  is set, the source IP addresses are enforced to comply with the IP
	  address bound to the jail, regardless of whether or not the
	  IP_HDRINCL flag has been set on the socket.  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.  As such, by default this option is
	  disabled.

security.jail.enforce_statfs:
	  This MIB entry 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 direc-
	  tory 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.

security.jail.set_hostname_allowed:
	  This MIB entry determines whether or not processes within a jail are
	  allowed to change their hostname via hostname(1) or sethostname(3).
	  In the current jail implementation, the ability to set the hostname
	  from within the jail can impact management tools relying on the
	  accuracy of jail information in /proc.  As such, this should be dis-
	  abled in environments where privileged access to jails is given out
	  to untrusted parties.

security.jail.socket_unixiproute_only:
	  The jail functionality binds an IPv4 address to each jail, and lim-
	  its access to other network addresses in the IPv4 space that may be
	  available in the host environment.  However, jail is not currently
	  able to limit access to other network protocol stacks that have not
	  had jail functionality added to them.  As such, by default, pro-
	  cesses within jails may only access protocols in the following
	  domains: PF_LOCAL, PF_INET, and PF_ROUTE, permitting them access to
	  UNIX domain sockets, IPv4 addresses, and routing sockets.  To enable
	  access to other domains, this MIB variable may be set to 0.

security.jail.sysvipc_allowed:
	  This MIB entry determines whether or not processes within a jail
	  have access to System V IPC primitives.  In the current jail imple-
	  mentation, System V primitives share a single namespace across the
	  host and jail environments, meaning that processes within a jail
	  would be able to communicate with (and potentially interfere with)
	  processes outside of the jail, and in other jails.  As such, this
	  functionality is disabled by default, but can be enabled by setting
	  this MIB entry to 1.

security.jail.chflags_allowed:
	  This MIB entry determines how a privileged user inside a jail will
	  be treated by chflags(2).  If zero, such users are treated as
	  unprivileged, and are unable to set or clear system file flags; if
	  non-zero, such users are treated as privileged, and may manipulate
	  system file flags subject to the usual constraints on
	  kern.securelevel.

There are currently two MIB related variables that have per-jail set-
tings.  Changes to these variables by a jailed process do not effect the
host environment, only the jail environment.  The variables are
kern.securelevel and kern.hostname.

Siehe auch

• Top-Jailkonzept zur Umsetzung bei der UUGRN.
• Paper zu FreeBSD Jails auf Real Virtuality:

• Vortragsfolien zu FreeBSD Jails als ODP (OASIS OpenDocument Präsentation / OpenOffice)
• Vortragsfolien zu FreeBSD Jails als PDF