1.  Introduction

      The UNIX access control mechanism is designed for an environment with two types of users: those with, and without administrative privilege. Within this framework, every attempt is made to provide an open system, allowing easy sharing of files and inter-process communication. As a member of the UNIX family, FreeBSD inherits these security properties. Users of FreeBSD in non-traditional UNIX environments must balance their need for strong application support, high network performance and functionality, and low total cost of ownership with the need for alternative security models that are difficult or impossible to implement with the UNIX security mechanisms.

      One such consideration is the desire to delegate some (but not all) administrative functions to untrusted or less trusted parties, and simultaneously impose system-wide mandatory policies on process interaction and sharing. Attempting to create such an environment in the current-day FreeBSD security environment is both difficult and costly: in many cases, the burden of implementing these policies falls on user applications, which means an increase in the size and complexity of the code base, in turn translating to higher development and maintenance cost, as well as less overall flexibility.

      This abstract risk becomes more clear when applied to a practical, real-world example: many web service providers turn to the FreeBSD operating system to host customer web sites, as it provides a high-performance, network-centric server environment. However, these providers have a number of concerns on their plate, both in terms of protecting the integrity and confidentiality of their own files and services from their customers, as well as protecting the files and services of one customer from (accidental or intentional) access by any other customer. At the same time, a provider would like to provide substantial autonomy to customers, allowing them to install and maintain their own software, and to manage their own services, such as web servers and other content-related daemon programs.

      This problem space points strongly in the direction of a partitioning solution, in which customer processes and storage are isolated from those of other customers, both in terms of accidental disclosure of data or process information, but also in terms of the ability to modify files or processes outside of a compartment. Delegation of management functions within the system must be possible, but not at the cost of system-wide requirements, including integrity and privacy protection between partitions.

      However, UNIX-style access control makes it notoriously difficult to compartmentalise functionality. While mechanisms such as chroot(2) provide a modest level compartmentalisation, it is well known that these mechanisms have serious shortcomings, both in terms of the scope of their functionality, and effectiveness at what they provide [CHROOT].

      In the case of the chroot(2) call, a process's visibility of the file system name-space is limited to a single subtree. However, the compartmentalisation does not extend to the process or networking spaces and therefore both observation of and interference with processes outside their compartment is possible.

      To this end, we describe the new FreeBSD ``Jail'' facility, which provides a strong partitioning solution, leveraging existing mechanisms, such as chroot(2), to what effectively amounts to a virtual machine environment. Processes in a jail are provided full access to the files that they may manipulate, processes they may influence, and network services they can make use of, and neither access nor visibility of files, processes or network services outside their partition.

      Unlike other fine-grained security solutions, Jail does not substantially increase the policy management requirements for the system administrator, as each Jail is a virtual FreeBSD environment permitting local policy to be independently managed, with much the same properties as the main system itself, making Jail easy to use for the administrator, and far more compatible with applications.