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echo "$HOSTNAME" > /srv/jail/$JID/hostname echo "$HOSTNAME" > /srv/jail/$JID/etc/hostname


A complete Debian GNU/kFreeBSD system should work within a jail, on a GNU/kFreeBSD or regular FreeBSD host system, with a few limitations.

Jails work a lot like Linux OpenVZ. On the host you can see all processes running in all jails. Within a jail, you can only see the processes running in that jail.


Be aware that some files in /proc or /sys, such as /proc/mounts, are not partitioned per jail, and this may leak some (read-only) information about the host, or other guests' mountpoints.

Some features such as sysvipc are not namespaced for individual jails, so for security reasons they are disabled by default. fakeroot requires this to be enabled. Running more than one postgresql-server instance in a shared sysvipc namespace would clash, and not normally work.

The raw_sockets feature is normally disabled, to prevent IP spoofing from inside the jail. The ping tool will not work properly as a result.

Creating a new jailed system


debootstrap \
 --exclude=devd,dmidecode,isc-dhcp-client,isc-dhcp-common,kldutils,pf,vidcontrol \
 wheezy /srv/jail/$JID "$MIRROR"


echo "$HOSTNAME" > /srv/jail/$JID/etc/hostname
echo "$IP $HOSTNAME" >> /srv/jail/$JID/etc/hosts

The --exclude to debootrap lists some packages that are probably not useful in a jailed system. The devd package will typically not work in a jail.

Ensure the jailed system's /etc/resolv.conf is suitable. If a DNS resolver runs on the host system, you should reference it by unicast (public or private) IP address such as "nameserver", instead of e.g. "nameserver".

The jailed system will not have a loopback interface unless you create one.

Starting or stopping a jail

Assuming a debootstrap'd installation already exists in /srv/jail/$JID/, here is an example of how to start it up inside a jail:


# Linux-like /proc and /sys filesystems
mount -t linprocfs linprocfs /srv/jail/$JID/proc
mount -t linsysfs linsysfs /srv/jail/$JID/sys
# Ramdisk required for /run
mount -t tmpfs tmpfs /srv/jail/$JID/run
# A restricted, read-only /dev filesystem
mount -t devfs devfs /srv/jail/$JID/dev
# Compatibility symlink from /dev/shm to /run/shm
ln -s /run/shm /srv/jail/$JID/dev/
# Optionally enable networking
# :XXX: this IP address *must* be assigned to one of the host's interfaces before the guest can use it

mkdir -p /var/run/jail
jail -J /var/run/jail/$JID.jid -c jid=$JID \
  name=jail$JID \
  path=/srv/jail/$JID \
  host.hostname=$HOSTNAME \
  ip4.addr=$IP \
  command=/bin/sh -- -c "/etc/init.d/rc S && /etc/init.d/rc 2"

You may want to install and configure locales on the first boot:

apt-get install locales
dpkg-reconfigure locales

If openssh-server is installed within the jail, you should be able to SSH into it like a virtual private server.

jls (to list running jails) is not available yet. 709225

jexec is not available yet, but you can probably get by with:

# jail -m jid=$JID command=/bin/bash
# cd

A jail stops 'running' when all processes within it exit. (Within the jail, /etc/init.d/rc 0 ; exec kill -1 might be a way to force a shutdown?)


The libjail package was not distributed with Squeeze. The kernel functionality has existed since FreeBSD 4.x however, so it may work if you can build the necessary userland tools.