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Note: FreeBSD introduced container technology twenty years ago, long before the industry standardized on the term "container". Internally, FreeBSD refers to these containers as "jails".

jail.conf

In this section we'll look at the default config for a new container. The defaults are sane for most applications, but if you want to tweak the settings here they are.

A jail.conf template is used each time a new container is created. This template looks like this:

{name} {
  devfs_ruleset = 4;
  enforce_statfs = 2;
  exec.clean;
  exec.consolelog = /usr/local/bastille/logs/{name}_console.log;
  exec.start = '/bin/sh /etc/rc';
  exec.stop = '/bin/sh /etc/rc.shutdown';
  host.hostname = {name};
  interface = {interface};
  mount.devfs;
  mount.fstab = /usr/local/bastille/jails/{name}/fstab;
  path = /usr/local/bastille/jails/{name}/root;
  securelevel = 2;

  ip4.addr = x.x.x.x;
  ip6 = disable;
}

devfs_ruleset

devfs_ruleset
  The number of the devfs ruleset that is enforced for mounting
  devfs in this jail.  A value of zero (default) means no ruleset
  is enforced.  Descendant jails inherit the parent jail's devfs
  ruleset enforcement.  Mounting devfs inside a jail is possible
  only if the allow.mount and allow.mount.devfs permissions are
  effective and enforce_statfs is set to a value lower than 2.
  Devfs rules and rulesets cannot be viewed or modified from inside
  a jail.

  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.

enforce_statfs

enforce_statfs
  This determines what 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.

exec.clean

exec.clean
  Run commands in a 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.

exec.consolelog

exec.consolelog
  A file to direct command output (stdout and stderr) to.

exec.start

exec.start
  Command(s) to run in the jail environment when a jail is created.
  A typical command to run is "sh /etc/rc".

exec.stop

exec.stop
  Command(s) to run in the jail environment before a jail is
  removed, and after any exec.prestop commands have completed.  A
  typical command to run is "sh /etc/rc.shutdown".

host.hostname

host.hostname
  The hostname of the jail.  Other similar parameters are
  host.domainname, host.hostuuid and host.hostid.

interface

interface
  A network interface to add the jail's IP addresses (ip4.addr and
  ip6.addr) to.  An alias for each address will be added to the
  interface before the jail is created, and will be removed from
  the interface after the jail is removed.

mount.devfs

mount.devfs
  Mount a devfs(5) filesystem on the chrooted /dev directory, and
  apply the ruleset in the devfs_ruleset parameter (or a default of
  ruleset 4: devfsrules_jail) to restrict the devices visible
  inside the jail.

mount.fstab

mount.fstab
  An fstab(5) format file containing filesystems to mount before
  creating a jail.

path

path
  The directory which is to be the root of the jail.  Any commands
  run inside the jail, either by jail or from jexec(8), are run
  from this directory.

securelevel

By default, Bastille containers run at securelevel = 2;. See below for the implications of kernel security levels and when they might be altered.

Note: Bastille does not currently have any mechanism to automagically change securelevel settings. My recommendation is this only be altered manually on a case-by-case basis and that "Highly secure mode" is a sane default for most use cases.

The kernel runs with five different security levels.  Any super-user
process can raise the level, but no process can lower it.  The security
levels are:

-1    Permanently insecure mode - always run the system in insecure mode.
      This is the default initial value.

0     Insecure mode - immutable and append-only flags may be turned off.
      All devices may be read or written subject to their permissions.

1     Secure mode - the system immutable and system append-only flags may
      not be turned off; disks for mounted file systems, /dev/mem and
      /dev/kmem may not be opened for writing; /dev/io (if your platform
      has it) may not be opened at all; kernel modules (see kld(4)) may
      not be loaded or unloaded.  The kernel debugger may not be entered
      using the debug.kdb.enter sysctl.  A panic or trap cannot be forced
      using the debug.kdb.panic and other sysctl's.

2     Highly secure mode - same as secure mode, plus disks may not be
      opened for writing (except by mount(2)) whether mounted or not.
      This level precludes tampering with file systems by unmounting
      them, but also inhibits running newfs(8) while the system is multi-
      user.

      In addition, kernel time changes are restricted to less than or
      equal to one second.  Attempts to change the time by more than this
      will log the message "Time adjustment clamped to +1 second".

3     Network secure mode - same as highly secure mode, plus IP packet
      filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be
      changed and dummynet(4) or pf(4) configuration cannot be adjusted.
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