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

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.