Chapter 14. Jails

Contributed by Matteo Riondato.

14.1. Synopsis

Since system administration is a difficult task, many tools have been developed to make life easier for the administrator. These tools often enhance the way systems are installed, configured, and maintained. One of the tools which can be used to enhance the security of a FreeBSD system is jails. Jails have been available since FreeBSD 4.X and continue to be enhanced in their usefulness, performance, reliability, and security.

Jails build upon the chroot(2) concept, which is used to change the root directory of a set of processes. This creates a safe environment, separate from the rest of the system. Processes created in the chrooted environment can not access files or resources outside of it. For that reason, compromising a service running in a chrooted environment should not allow the attacker to compromise the entire system. However, a chroot has several limitations. It is suited to easy tasks which do not require much flexibility or complex, advanced features. Over time, many ways have been found to escape from a chrooted environment, making it a less than ideal solution for securing services.

Jails improve on the concept of the traditional chroot environment in several ways. In a traditional chroot environment, processes are only limited in the part of the file system they can access. The rest of the system resources, system users, running processes, and the networking subsystem are shared by the chrooted processes and the processes of the host system. Jails expand this model by virtualizing access to the file system, the set of users, and the networking subsystem. More fine-grained controls are available for tuning the access of a jailed environment. Jails can be considered as a type of operating system-level virtualization.

A jail is characterized by four elements:

  • A directory subtree: the starting point from which a jail is entered. Once inside the jail, a process is not permitted to escape outside of this subtree.

  • A hostname: which will be used by the jail.

  • An IP address: which is assigned to the jail. The IP address of a jail is often an alias address for an existing network interface.

  • A command: the path name of an executable to run inside the jail. The path is relative to the root directory of the jail environment.

Jails have their own set of users and their own root account which are limited to the jail environment. The root account of a jail is not allowed to perform operations to the system outside of the associated jail environment.

This chapter provides an overview of the terminology and commands for managing FreeBSD jails. Jails are a powerful tool for both system administrators, and advanced users.

After reading this chapter, you will know:

  • What a jail is and what purpose it may serve in FreeBSD installations.

  • How to build, start, and stop a jail.

  • The basics of jail administration, both from inside and outside the jail.


Jails are a powerful tool, but they are not a security panacea. While it is not possible for a jailed process to break out on its own, there are several ways in which an unprivileged user outside the jail can cooperate with a privileged user inside the jail to obtain elevated privileges in the host environment.

Most of these attacks can be mitigated by ensuring that the jail root is not accessible to unprivileged users in the host environment. As a general rule, untrusted users with privileged access to a jail should not be given access to the host environment.