Chroot jails made simpler

Jail Management

© Lead Image © Ints Vikmanis, 123RF.com

© Lead Image © Ints Vikmanis, 123RF.com

Article from Issue 244/2021
Author(s):

Setting up chroot jails is no simple task. Jailkit can make this job a little easier by automating setup and configuration.

Both the chroot command and a container are ways to isolate parts of a system. However, their methods are quite different. While a container is a form of virtualization with its own allocated resources, chroot is a way to limit a user account's access to the parts of the directory tree by – as the name of the command implies – changing its root directory. The result is what is known as a chroot or, sometimes, a chroot jail, which draws on the larger system's resources as needed. The result is more economical, if less trendy than containers, but it is difficult to set up. Fortunately most distros include jailkit [1], a collection of utilities that helps to automate setup and configuration.

Contrary to widespread misinformation, a chroot is not a security measure unless specifically configured as one. Although confinement in a jail can limit what an uninformed user can do, expert users could escape a jail by creating a second jail within the first. In addition, any process run with root privileges can access resources outside the chroot. Similarly, if a user has permissions for any files outside their home directory, they are not jailed. In addition, any user with root privileges can access the chroot from the main system, including those using sudo.

A chroot can be made secure with some effort. But more commonly, a chroot has other purposes, including:

  • Sandboxing: Safely testing unstable builds without risking the rest of the system
  • Creating a new environment: Usually for testing purposes
  • Dependency control: Giving an application access to only certain versions of dependencies
  • Running old software: Denying access to hardware that the software cannot handle
  • Recovery: Making the entire chroot a filesystem that can be accessed after boot from a single drive, with utilities to help in restoring the system

Most of these purposes do not require a desktop environment, although you can add one to the chroot by installing the X clients section when using jk_init, if desired.

Chroot's Wikipedia entry lists a number of common uses, including Postfix utilities, FTP servers, and package-building farms for Debian, Ubuntu, SUSE, Fedora, and Red Hat when they test for dependencies [2]. Despite being added to Version 7 Unix as long ago as 1982 – and possibly earlier – chroot remains a versatile tool today.

Whatever your purpose, setting up a chroot can be a laborious task. First, the chroot needs to be initialized. Then, depending on your purposes, you may need to configure the files, the access to devices, the shell, the user access, and daemons in the chroot. There is even a separate wrapper for using procmail within the jail. A time may come, as well, when you want to edit or update files. About half of these actions have a default configuration file in /etc/jailkit, although you will probably need to edit it for your own purposes. However, the advantage is that, should you require a clone of a chroot, it can be created quickly. You might also locate an online example you can modify to suit your purpose. Note, though, that many examples assume a Debian or Ubuntu installation and may need to be modified for other distributions. You should also check the synopsis at the start of each man page to learn whether the command can be run from outside or inside the chroot.

More to the point, these actions can be simplified by jailkit's utilities, many of which have their own man page with more examples. Generally, however, the first option in a command will be

--jail-CHROOT -j CHROOT

and the last one the command, user, or other element of the main system that will interact with the jail.

To set up a chroot, you should run the utilities in the order listed below, skipping any that are irrelevant to your purposes. Those at the end of the list can be run periodically as the chroot evolves or needs updating.

jk_init

Begin the creation of a chroot with jk_init (Figure 1). At the very least, the command must specify the directory for the chroot plus the .ini file plus the sections to install:

-usr/sbin/jk_init -j CHROOT-DIRECTORY INI-FILE --configfile =FILE (-c FILE) SECTION
Figure 1: Installing files to the chroot.

Alternatively, you can make configuration choices from the command line, which may be a more secure choice if you are not familiar with the contents of the .ini file:

jk_init -v CHROOT-DIRECTORY FILES-TO-INSTALL

The chroot's root directory, as well as its parent directories, will be made if they do not already exist, while possible sections of the .ini file for jk_init.ini can be read by using the --list option (Figure 2).

Figure 2: The --list option shows the sections in the .ini file that you can add to the chroot.

The jk_init.ini file defines the basic configuration of the chroot, as well as the behavior of the other jailkit utilities. Jailkit installs with an .ini file for a set of general purpose paths and applications (Figure 3), but often you can create a much simpler chroot. For instance, Listing 1 shows an example from the man page that creates the chroot with a limited shell so it can run the sftp command.

Listing 1

Creating a Limited Shell

[jk_lsh]
  comment = Jailkit limited shell
  paths = /usr/sbin/jk_lsh, /etc/jailkit/jk_lsh.ini
  users = root
  groups = root
  need_logsocket = 1
  includesections = uidbasics
  [sftp]
  comment = ssh secure ftp with Jailkit limited shell
  paths = /usr/lib/sftp-server
  includesections = netbasics, uidbasics
  devices = /dev/urandom, /dev/null
  emptydirs = /svr
Figure 3: Jailkit installs with an all-purpose jk_init.ini file. However, it can be edited or replaced with a simpler one for security or memory considerations.

jk_cp

Because a chroot is isolated from the rest of the system, you need to copy into the chroot any files or devices you want to run within it. This command is simply a space-separated list of the full path to files to add to the chroot. If a command is copied, its dependencies are as well – a great time-saver to manual creation using the chroot command. The copy of each file has the same permissions as the original, except that setuid and setgid permissions are removable.

jk_chrootsh

This command creates a login shell for the chroot. Since the shell has no access to the system's libraries or commands, most of those it needs must be copied into the chroot using jk_cp. Only a minimum set of commands is installed by default, such as the files in /etc/passwd needed for the user to log in. Other commands that can be executed in the chroot are defined in /etc/jailkit/jk_lsh.ini (see next).

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