15.4. Fine Tuning and Administration

There are several options which can be set for any jail, and various ways of combining a host FreeBSD system with jails, to produce higher level applications. This section presents:

15.4.1. System Tools for Jail Tuning in FreeBSD

Fine tuning of a jail's configuration is mostly done by setting sysctl(8) variables. A special subtree of sysctl exists as a basis for organizing all the relevant options: the security.jail.* hierarchy of FreeBSD kernel options. Here is a list of the main jail-related sysctls, complete with their default value. Names should be self-explanatory, but for more information about them, please refer to the jail(8) and sysctl(8) manual pages.

  • security.jail.set_hostname_allowed: 1

  • security.jail.socket_unixiproute_only: 1

  • security.jail.sysvipc_allowed: 0

  • security.jail.enforce_statfs: 2

  • security.jail.allow_raw_sockets: 0

  • security.jail.chflags_allowed: 0

  • security.jail.jailed: 0

These variables can be used by the system administrator of the host system to add or remove some of the limitations imposed by default on the root user. Note that there are some limitations which cannot be removed. The root user is not allowed to mount or unmount file systems from within a jail(8). The root inside a jail may not load or unload devfs(8) rulesets, set firewall rules, or do many other administrative tasks which require modifications of in-kernel data, such as setting the securelevel of the kernel.

The base system of FreeBSD contains a basic set of tools for viewing information about the active jails, and attaching to a jail to run administrative commands. The jls(8) and jexec(8) commands are part of the base FreeBSD system, and can be used to perform the following simple tasks:

  • Print a list of active jails and their corresponding jail identifier (JID), IP address, hostname and path.

  • Attach to a running jail, from its host system, and run a command inside the jail or perform administrative tasks inside the jail itself. This is especially useful when the root user wants to cleanly shut down a jail. The jexec(8) utility can also be used to start a shell in a jail to do administration in it; for example:

    # jexec 1 tcsh

15.4.2. High-Level Administrative Tools in the FreeBSD Ports Collection

Among the many third-party utilities for jail administration, one of the most complete and useful is sysutils/jailutils. It is a set of small applications that contribute to jail(8) management. Please refer to its web page for more information.

15.4.3. Keeping Jails Patched and up to Date

Jails should be kept up to date from the host operating system as attempting to patch userland from within the jail may likely fail as the default behaviour in FreeBSD is to disallow the use of chflags(1) in a jail which prevents the replacement of some files. It is possible to change this behavior but it is recommended to use freebsd-update(8) to maintain jails instead. Use -b to specify the path of the jail to be updated.

# freebsd-update -b /here/is/the/jail fetch
# freebsd-update -b /here/is/the/jail install

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