Ubuntu Manpage: jailkit - utilities for jailing a user or process

NAME

       jailkit - utilities for jailing a user or process

DESCRIPTION

Jailkit is a set of utilities that can limit user accounts to a specific directory tree
and to specific commands. Setting up a jail is much easier using the jailkit utilities
that doing so 'by hand'. A jail is a directory tree that you create within your file
system; the user cannot see any directories or files that are outside the jail directory.
The user is jailed in that directory and it subdirectories. The chroot(2) system call is
used by jailkit to put the user inside the jail.

If you want the user to be able to do just one thing, you can set up the jail so that the
user is able to do exactly and only that one thing. For example, if you want the user to
be able to run scp, you install a copy of scp in the jail along with just enough support
to execute it (e.g., using a limited shell). As you can understand, the fewer executables
you have in a jail (and the more their capabilities are limited such as using strict
configurations), the more work a hacker needs to break out of it. It is important to note
that a chroot jail can be easily escaped if the user is able to elevate to the root level,
so it's very important to prevent the user from doing so.

In this summary, the top-level directory of the jail is referred to as JAIL. You can
configure the JAIL to be any suitable directory (e.g., your JAIL may be
/usr/local/chrootjail or /home/chroot). The JAIL directory should obviously be chosen so
as not collide or interfere with other standard directories (e.g., it's probably a bad
idea to use /home/chroot as the JAIL and also create a user named 'chroot'). A reference
to JAIL/etc means "the etc/ subdirectory in your top-level jail directory". From the
jailed user's point of view, the top-level jail directory is "/".

SECURITY CONSIDERATIONS

       A badly configured jail is a security risk!

       If  a  jailed  user or a jailed process can modify files in (for example) the JAIL/lib/ or
       JAIL/etc/ directory (i.e., those within the jail directory), the user can bypass  security
       checks and gain root privileges.

       No  directory  inside  the  jail  except  for  the  user's home directory or tmp should be
       writable by the user. Especially the root of the jail should not be writable by the  user.
       Jailkit utilities can be used to perform some basic checks to verify that a jail is secure
       and abort if a jail is not secure. Check your logfiles if things don't work as expected.

       The super user (root), or any process running with root privileges, can always  break  out
       of  a  jail. It is therefore important that the processes inside the jail do not have root
       privileges, nor have the means to receive those privileges. Avoid setuid (+s)  executables
       inside  the jail. If the jail is on a separate filesystem, the jail filesystem can mounted
       with the nosuid flag.

This section gives summary sketches of the various programs that comprise jailkit. For
details on how a program operates and is configured, read the reference pages of each
program.

jk_init can be used to quickly create a jail with several files or directories needed for
a specific task or profile. Creating the same jail over and over again is easily automated
with jk_init. There are many tasks in /etc/jailkit/jk_init.ini predefined that work on
Debian or Ubuntu systems. For other platforms you might need to update the predefined
configuration. For example, you can use jk_init to quickly set up a limited shell, a jail
to run apache, or a jail for just sftp and scp. It will copy the binaries, the required
libraries (and related symlinks) as well as other files such as /etc/passwd. These are all
copied into the jail directory so that a jailed process can run them.

jk_cp can be used to copy a file or device into a jail. Each file or device is copied with
the same permissions with the exception that any setuid or setgid permissions are removed.
If the file is a binary executable, the libraries required to execute it (as reported by
ldd) are copied as well.

jk_chrootsh is a shell that jails a user in a specific directory, called the JAIL. It does
this using the chroot(2) (change root) system call. This makes the filesystem 'above' the
JAIL directory inaccessible to the user. Because the user can no longer access directories
such as /usr/bin that are accessible to a 'non-jailed' user, the JAIL directory must
recreate enough of a file system to allow the jailed user to execute programs. For
example, the JAIL directory typically contains a lib/ directory to contain shared
libraries, and it normally has a etc/ directory to contain a minimal set of files such as
etc/passwd (though this passwd file contains only a few key entries, not all those of the
'real' /etc/passwd). The jk_chrootsh program is normally installed as the user's shell
(replacing /bin/bash) in the 'real' /etc/passwd file. When the user logs in, jk_chrootsh
is executed as the user's shell. The jk_chrootsh enacts the chroot into the JAIL
directory. It then reads the passwd file found within the JAIL (i.e., JAIL/etc/passwd ),
obtains the program to be run as the user's shell (typically jk_lsh, the limited shell),
and executes it within the jail. This combination limits the user's file system access to
the JAIL directory (implemented by jk_chrootsh) and limits which programs the user is
allowed to execute (implemented by jk_lsh).

jk_lsh is a limited shell that allows only those commands to be executed as specified in
its configuration file. /etc/jailkit/jk_lsh.ini. It is typically started in one of two
ways, by specifying it as the user's shell or by using the jk_chrootsh program. The first
way is implemented by specifying jk_lsh as the shell in the user's entry in the 'real'
/etc/passwd file. In this case, it executes in the normal file system and reads its
configuration from /etc/jailkit/jk_lsh.ini. In the second way, jk_lsh is started from
within jk_chrootsh by specifying it as the shell in the passwd file located inside the
JAIL directory: JAIL/etc/passwd, in which case it reads its configuration from within the
JAIL: JAIL/etc/jailkit/jk_lsh.ini. The latter is the recommended approach for highest
security. Use this program if you want to deny regular shell access (e.g. logins) but you
want to allow execution of only one or a few commands such sftp, scp, rsync, or cvs.

jk_uchroot is a utility to give regular users access to the chroot(2) (change root) system
call in a safe way. Which users are allowed in which jails is controlled from
/etc/jailkit/jk_uchroot.ini Use this utility for users that can run processes both inside
a jail and outside a jail.

jk_socketd is a daemon that allows logging safely to syslog from within a jail. It limits
the logging rate based on parameters set in its configuration file:
/etc/jailkit/jk_socketd.ini

jk_chrootlaunch is a utility to start a daemon that cannot do a chroot(2) call itself in a
jail. It can change the user and group id after jailing the process, and before executing
the daemon.

jk_jailuser is a tool to move an existing user account into a jail. This moves the user's
entire home directory (and subdirectories) into the appropriate place in the JAIL
directory (e.g., JAIL/home/someuser).

jk_check is a jail integrity checker. It checks a jail for some of the potential security
problems. (Obviously it does not check all possible weaknesses.) It reports any setuid and
setgid programs, checks for any modified programs, checks for world writable directories,
and more. It is configured by /etc/jailkit/jk_check.ini

jk_list lists all jailed processes on a system, showing the PID, UID, and the jail
directory.

jk_procmailwrapper is a wrapper for procmail. For regular users, it runs procmail and
allows access to their normal .procmailrc file. For jailed users, it runs procmail and
allows access only to the jailed .procmailrc (e.g., JAIL/home/someuser/.procmailrc). In
the latter case, procmail must be available inside the jail.

jk_update is a tool to update files inside a jail according to updates on the real system.

EXAMPLE

       Suppose  you  wish  to create an account 'test' that is permitted to execute only sftp and
       scp. Assume you also want it contained in a jail called /home/sftproot, in which it has  a
       home  directory  /home/test (as seen by processes run by user 'test'; the actual directory
       will be JAIL/home/test).

       # Initialise the jail
       mkdir /home/sftproot
       chown root:root /home/sftproot
       chmod 0755 /home/sftproot
       jk_init -j /home/sftproot jk_lsh
       jk_init -j /home/sftproot sftp
       jk_init -j /home/sftproot scp
       # Move the account into the jail
       jk_jailuser -j /home/sftproot -s /usr/bin/jk_lsh -m test
       # Edit the jk_lsh configfile in the jail; see man jk_lsh.
       # You can use every editor you want; I choose 'joe'
       joe /home/sftproot/etc/jailkit/jk_lsh.ini
       # Restart jk_socketd so that log messages are transferred
       killall jk_socketd
       jk_socketd
       # Test the account
       sftp test@localhost
       # Check the logs to see if everything is correct
       tail /var/log/daemon.log /var/log/auth.log
       # or if you use systemd
       journalctl --since=-1h

FILES

       The jailkit configuration files are located in /etc/jailkit/ Note that in some  cases  the
       configuration  files  must  be  replicated  into the JAIL/etc/jailkit directory and edited
       appropriately. A jk program that is run within the jail directory  is  able  to  read  its
       configuration from only the jailed etc/jailkit directory.

COPYRIGHT

       Copyright (C) 2003, 2004, 2005, 2006, 2007, 2009 Olivier Sessink

       Copying  and distribution of this file, with or without modification, are permitted in any
       medium without royalty provided the copyright notice and this notice are preserved.