jk_chrootsh(8) - Linux man page

Name

jk_chrootsh - a shell that will put the user inside a changed root

Synopsis

jk_chrootsh

Description

jk_chrootsh can be used as a shell for a user. That user will be put into a changed root. The directory where to put the user in is read from the users home directory, the last occuring /./ sequence is used to mark the location of the changed root. An example

line in /etc/passwd would look like

test:x:10000:10000::/home/testchroot/./home/test:/usr/sbin/jk_chrootsh

In this example the user will be chroot-ed into /home/testchroot

Inside the chroot-ed directory, it will look for /etc/passwd and it will execute the shell for the user from that file. For the above example the /etc/passwd

file inside the jail should have an entry like

test:x:10000:10000::/home/test:/usr/sbin/jk_lsh

Notice that the home directory and the shell are local inside the chroot

jk_chrootsh needs root access to make the chroot(2) system call. Therefore it is setuid root. It will drop its root priveleges immediately after making the chroot() system call.

By default jk_chrootsh does not copy any environment variables. For some functionality, however, environment variables need to be copied (e.g. the TERM variable for a functional terminal emulation, or the DISPLAY variable for X forwarding). In /etc/jailkit/jk_chrootsh.ini the required environment variables can be listed. An example config file is shown below. In the example, user bill will get the DISPLAY variable, and all users in group jail will get the TERM and PATH variables.

By default jk_chrootsh requires a home directory that has the same group as the primary group from the user, and requires the home directory to be non-writable for group and others. You can relax these requirements in the configfile as shown below.

[DEFAULT]
relax_home_group=1
[bill]
env= DISPLAY
relax_home_group_permissions=1
relax_home_other_permissions=1
[group jail]
env = TERM, PATH

If user bill is in group jail, however, he will not get the TERM variable in the above example. Neither will any user in group jail get relaxed requirements for the group of the home directory. First the user is checked, and only if no user section is found the group section is looked for, and if no group section is found, the DEFAULT section is used.

jk_chrootsh can be configured not to read the final shell from the /etc/passwd file in the jail. An example configfile is shown below.

[group jail2]
skip_injail_passwd_check=1
injail_shell=/bin/bash

Files

/etc/passwd /etc/jailkit/jk_chrootsh.ini

Diagnostics

jk_chrootsh logs everything to syslog, please check the log files

commonly made mistakes are:

forgetting to add the user to JAIL/etc/passwd or the group to JAIL/etc/group

forgetting to have the correct permissions on all files inside the jail, or forgetting files inside the jail (the shell itself, or any libraries used by the shell)

referring to a file outside the chroot

Copyright

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.

Referenced By

jk_addjailuser(8), jk_jailuser(8)