lshell man page

lshell — Limited Shell

Synopsis

lshell [Options]

Description

lshell provides a limited shell configured per user. The configuration is done quite simply using a configuration file. Coupled with ssh's authorized_keys or with /etc/shells and /etc/passwd , it becomes very easy to restrict user's access to a limited set of command.

Options

--config <FILE>
Specify config file
--log <DIR>
Specify the log directory
--<param> <value>
where <param> is *any* config file parameter
-h, --help
Show help message
--version
Show version

Configuration

You can configure lshell through its configuration file:

On Linux -> /etc/lshell.conf
On *BSD  -> /usr/{pkg,local}/etc/lshell.conf

The configuration is dynamically reloaded. Which means that you can edit the configuration, and all the connected users will automatically load it. In case you are using multiple configuration files (see include_dir), you will need to refresh the main configuration's timestamp, in order to reload the configuration:

touch /path/to/lshell.conf

lshell configuration has 4 types of sections:

[global]   -> lshell system configuration (only 1)
[default]  -> lshell default user configuration (only 1)
[foo]      -> UNIX username "foo" specific configuration
[grp:bar]  -> UNIX groupname "bar" specific configuration

Order of priority when loading preferences is the following:

1- User configuration
2- Group configuration
3- Default configuration

[global]

logpath
config path (default is /var/log/lshell/)
loglevel
0, 1, 2, 3 or 4 (0: no logs -> 4: logs everything)
logfilename

- set to syslog in order to log to syslog

- set log file name, e.g. %u-%y%m%d (i.e foo-20091009.log): %u -> username

%d -> day [1..31]

%m -> month [1..12]

%y -> year [00..99]

%h -> time [00:00..23:59]

syslogname
in case you are using syslog, set your logname (default: lshell)
include_dir
include a directory containing multiple configuration files. These files can only contain default/user/group configuration. The global configuration will only be loaded from the default configuration file. This variable will be expanded (e.g. /path/*.conf).
path_noexec
set path to sudo noexec library. This path is usually autodetected, only set this variable to use alternate path. If set and the shared object is not found, lshell will exit immediately. Otherwise, please check your logs to verify that a standard path is detected.

[default] and/or [username] and/or [grp:groupname]

aliases
command aliases list (similar to bash's alias directive)
allowed

a list of the allowed commands or set to 'all' to allow all commands in user's PATH

if sudo(8) is installed and sudo_noexec.so is available, it will be loaded before running every command, preventing it from running further commands itself. If not available, beware of commands like vim/find/more/etc. that will allow users to execute code (e.g. /bin/sh) from within the application, thus easily escaping lshell. See variable 'path_noexec' to use an alternative path to library.

allowed_shell_escape

a list of the allowed commands that are permitted to execute other programs (e.g. shell scripts with exec(3)). Setting this variable to 'all' is NOT allowed. Warning: do not put here any command that can execute arbitrary commands (e.g. find, vim, xargs).

important: commands defined in 'allowed_shell_escape' override their definition in the ´allowed´ variable.

allowed_cmd_path
a list of path; all executable files inside these path will be allowed
env_path
update the environment variable $PATH of the user (optional)
env_vars
set environment variables (optional)
forbidden
a list of forbidden characters or commands
history_file

set the history filename. A wildcard can be used:

%u -> username (e.g. '/home/%u/.lhistory')

history_size
set the maximum size (in lines) of the history file
home_path (deprecated)

set the home folder of your user. If not specified, the home directory is set to the $HOME environment variable. This variable will be removed in the next version of lshell, please use your system's tools to set a user's home directory. A wildcard can be used:

%u -> username (e.g. '/home/%u')

intro
set the introduction to print at login
login_script
define the script to run at user login
passwd
password of specific user (default is empty)
path
list of path to restrict the user geographically. It is possible to use wildcards (e.g. '/var/log/ap*').
prompt

set the user's prompt format (default: username)

%u -> username

%h -> hostname

prompt_short

set prompt style for current directory - 0, 1 or 2. Default is 0.

0 -> will show the current directory as compared to home directory ~/current/dir

1 -> will only show the current directory name

2 -> will show the complete path to the current directory

overssh
list of command allowed to execute over ssh (e.g. rsync, rdiff-backup, scp, etc.)
scp
allow or forbid the use of scp connection - set to 1 or 0
scpforce
force files sent through scp to a specific directory
scp_download
set to 0 to forbid scp downloads (default is 1)
scp_upload
set to 0 to forbid scp uploads (default is 1)
sftp

allow or forbid the use of sftp connection - set to 1 or 0.

WARNING: This option will not work if you are using OpenSSH's internal-sftp service (e.g. when configured in chroot)

sudo_commands

a list of the allowed commands that can be used with sudo(8). If set to ´all', all the 'allowed' commands will be accessible through sudo(8).

It is possible to use the -u sudo flag in order to run a command as a different user than the default root.

timer
a value in seconds for the session timer
strict
logging strictness. If set to 1, any unknown command is considered as forbidden, and user's warning counter is decreased. If set to 0, command is considered as unknown, and user is only warned (i.e. *** unknown synthax)
warning_counter
number of warnings when user enters a forbidden value before getting exited from lshell. Set to -1 to disable the counter, and just warn the user.
winscp

enable support for WinSCP with scp mode (NOT sftp)

When enabled, the following parameters will be overridden:

scp_upload: 1 (uses scp(1) from within session)

scp_download: 1 (uses scp(1) from within session)

scpforce: ignored (uses scp(1) from within session)

forbidden: -[';']

allowed: +['scp', 'env', 'pwd', 'groups', 'unset', 'unalias']

Shell Builtin Commands

Here is the set of commands that are always available with lshell:

clear
clears the terminal
export
name of exported shell variable. Disabled by default, enable it by adding it to allowed commands.
help, ?
print the list of allowed commands
history
print the commands history
lpath
lists all allowed and forbidden path
lsudo
lists all sudo allowed commands

Examples

$ lshell
Tries to run lshell using default ${PREFIX}/etc/lshell.conf as configuration file. If it fails a warning is printed and lshell is interrupted. lshell options are loaded from the configuration file
$ lshell --config /path/to/myconf.file --log /path/to/mylog.log
This will override the default options specified for configuration and/or log file

Use Case

The primary goal of lshell, was to be able to create shell accounts with ssh access and restrict their environment to a couple a needed commands. In this example, User 'foo' and user 'bar' both belong to the 'users' UNIX group:

User foo:
- must be able to access /usr and /var but not /usr/local
- user all command in his PATH but 'su'
- has a warning counter set to 5
- has his home path set to '/home/users'
User bar:

- must be able to access /etc and /usr but not /usr/local
- is allowed default commands plus 'ping' minus 'ls'
- strictness is set to 1 (meaning he is not allowed to type an unknown command)

In this case, my configuration file will look something like this:

# CONFIURATION START
[global]
logpath         : /var/log/lshell/
loglevel        : 2

[default]
allowed         : ['ls','pwd']
forbidden       : [';', '&', '|'] 
warning_counter : 2
timer           : 0
path            : ['/etc', '/usr']
env_path        : ':/sbin:/usr/bin/'
scp             : 1 # or 0
sftp            : 1 # or 0
overssh         : ['rsync','ls']
aliases         : {'ls':'ls --color=auto','ll':'ls -l'}

[grp:users]
warning_counter : 5
overssh         : - ['ls']

[foo]
allowed         : 'all' - ['su']
path            : ['/var', '/usr'] - ['/usr/local']
home_path       : '/home/users'

[bar]
allowed         : + ['ping'] - ['ls'] 
path            : - ['/usr/local']
strict          : 1
scpforce        : '/home/bar/uploads/'
# CONFIURATION END

Notes

In order to log a user's warnings into the logging directory (default /var/log/lshell/) , you must firt create the folder (if it doesn't exist yet) and chown it to lshell group:
# addgroup --system lshell
# mkdir /var/log/lshell
# chown :lshell /var/log/lshell
# chmod 770 /var/log/lshell

then add the user to the lshell group:

# usermod -aG lshell user_name

In order to set lshell as default shell for a user:

On Linux:
# chsh -s /usr/bin/lshell user_name

On *BSD:
# chsh -s /usr/{pkg,local}/bin/lshell user_name

Author

Currently maintained by Ignace Mouzannar (ghantoos)

Email

Feel free to send me your recommendations at <ghantoos@ghantoos.org>

Info

July, 2015 v0.9.18