How To Limit User’s Access To The Linux System

In this brief tutorial, we are going to learn what is Restricted shell and how to limit user's access to the Linux system using Restricted shell.

Introduction

Picture this scenario.

You want to allow an user to do only certain tasks and execute certain commands. The user shouldn't change the environment variables/paths.

The user can't visit to the other directories except her/his home directory and can't switch to other users etc. The user can only be able to execute a few commands assigned by the system administrator. Is that possible? Yes! This is where Restricted Shell comes in help.

Restricted Shell is used to limit user's access to the Linux system. Once you put the users in restricted shell mode, they are allowed to execute only limited set of commands.

I tested this guide on CentOS 7 minimal server. However, It will work on most Unix-like distributions.

What is Restricted Shell?

First, let me clarify what exactly Restricted Shell is. It is not a separate shell like Bash, Korn Shell etc. If you start any existing shell using "rbash", "--restricted", "-r" options, then it will become Restricted shell. For instance, the Bourne shell can be started as a restricted shell with the command bsh -r, and the Korn shell with the command ksh -r.

The Restricted Shell will limit the users from executing most commands and from changing the current working directory. The Restricted Shell will impose the following restrictions to the users:

• It will not allow you to execute cd command. So you can't go anywhere. You can simply stay in the current working directory.
• It will not allow you to modify the values of $PATH, $SHELL, $BASH_ENV or $ENV environmental variables.
• It will not allow you to execute a program that contains a /(slash) character. For example, you can't run /usr/bin/uname or ./uname command. You can however execute uname command. In other words, you are allowed to run the commands in the current path only.
• You can't redirect the output using ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’ redirection operators.
• It will not allow you to get out of the restricted shell mode within scripts.
• It will not allow you to turn off restricted shell mode with ‘set +r’ or ‘set +o restricted’.

This can be very useful when a large number of users are using a shared system. So, If you want to allow the users to execute only specific commands, Restricted Shell is one way to do this.

Limit User's Access To The Linux System Using Restricted Shell

First, create a symlink called rbash from Bash as shown below. The following commands should be run as root user.

# ln -s /bin/bash /bin/rbash

Next, create an user called "ostechnix" with rbash as his/her default login shell.

# useradd ostechnix -s /bin/rbash

Set password to the new user.

# passwd ostechnix

Create a bin directory inside the home folder of the the new user.

# mkdir /home/ostechnix/bin

Now, we need to specify which commands the user can run.

Here, I am going to let the user to run only "ls", "mkdir", and "ping" commands. You can assign any commands of your choice.

To do so, run the following commands:

# ln -s /bin/ls /home/ostechnix/bin/ls

# ln -s /bin/mkdir /home/ostechnix/bin/mkdir

# ln -s /bin/ping /home/ostechnix/bin/ping

Now, you understand why we created the "bin" directory in the earlier step. The users can't run any commands except the above three commands.

Next, prevent the user from modifying .bash_profile.

# chown root. /home/ostechnix/.bash_profile

# chmod 755 /home/ostechnix/.bash_profile

Edit /home/ostechnix/.bash_profile file:

# vi /home/ostechnix/.bash_profile

Modify the PATH variable like below.

[...]
PATH=$HOME/bin
[...]

Modify PATH variable

Press ESC key and type :wq to save and close the file.

Now when the user logs in, the restricted shell(rbash) will run as the default login shell and read the .bash_profile, which will set PATH to $HOME/bin so that the user will only be able to run the ls, mkdir and ping commands. The restricted shell will not allow the user to change PATH, and the permissions on .bash_profile will not allow the user to alter the environment to bypass the restrictions during the next login session.

Verifying Rbash

Now, log out from root user and log back in with the newly created user i.e ostechnix in our case.

Then, run some commands to check whether it works or not. For example, I want to clear the Terminal.

To do so, I ran:

$ clear

Sample output:

-rbash: clear: command not found

You can't use cd command to change to the different directory.

$ cd /root

Sample output:

-rbash: cd: restricted

You can't redirect the output using > operator either.

$ cat > file.txt

Sample output:

-rbash: file.txt: restricted: cannot redirect output

Limit User's Access To The Linux System Using Restricted Shell

The user "ostechnix" is allowed to use only the commands assigned by you (the system admin, of course). In our case, the user can execute ls, mkdir and ping commands.

$ ls

$ mkdir ostechnix

$ ping -c 3 google.com

Restricted Shell in CentOS

Apart from these three commands, the user can't perform anything. S/He is completely under your control.

Suggested read:

• How To Monitor User Activity In Linux

Allow new commands to users

If you want to assign more commands to a user, log out out from the current user and back in to the root user again and assign the commands as shown below.

For example, to allow the user (i.e. ostechnix) to execute rm command, run the following command as root user.

# ln -s /bin/rm /home/ostechnix/bin/rm

Now the user can able to use "rm" command.

For more details, refer the man pages in the link given below.

• Rbash man pages