Restrict shell user to run only limited set of commands

Issue

Need to restrict the normal users to run only limited set of commands assigned to him/her and all other commands for which normal user have permission to execute by-default, shall not be executed.

E.g: user XYZ can run only gzip and gunzip commands.

Resolution

The normal user has been given permission to execute some commands which are available in /bin/ and /usr/local/bin/, So to remove those permissions and to restrict the user to run only particular set of commands, following steps shall be useful.

1. Create the restricted shell.

# cp /bin/bash /bin/rbash

2. Modify the target user for the shell as restricted shell

While creating user:

# useradd -s /bin/rbash localuser

For existing user:

# usermod -s /bin/rbash localuser

Then the user localuser is chrooted and can’t access the links outside his home directory /home/localuser

3. Create a directory under /home/localuser/, e.g. programs

# mkdir /home/localuser/programs

4. Now if you check, the user localuser can access all commands which he/she has allowed to execute. These commands are taken from the environmental PATH variable which is set in /home/localuser/.bash_profile. Modify it as follows.

# cat /home/localuser/.bash_profile  
# .bash_profile  

# Get the aliases and functions  
if [ -f ~/.bashrc ]; then  
. ~/.bashrc  
fi  
# User specific environment and startup programs  
PATH=$HOME/programs  
export PATH

Here the PATH variable is set to ~/programs directory, as /usr/local/bin is binded to /home/username/bin and /bin is binded to /home/username/bin so replacing that.

5. Now after logging with the username localuser, user cant run a simple command too. The output will be like this,

[[email protected] ~]$ ls  
-rbash: ls: command not found  
[[email protected] ~]$ less file1  
-rbash: less: command not found  
[[email protected] ~]$ clear  
-rbash: clear: command not found  
[[email protected] ~]$ date  
-rbash: date: command not found  
[[email protected] ~]$ ping redhat.com  
-rbash: ping: command not found

6. Now create the softlinks of commands which are required for user localuser to execute in the directory /home/localuser/programs

# ln -s /bin/date /home/localuser/programs/  
# ln -s /bin/ls /home/localuser/programs/  
# ll /home/localuser/programs/  
total 8  
lrwxrwxrwx 1 root root 9 Oct 17 15:53 date -> /bin/date  
lrwxrwxrwx 1 root root 7 Oct 17 15:43 ls -> /bin/ls

Here examples of date and ls commands has been taken

7. Again login with user localuser and try to execute the commands.

[[email protected] ~]$ date  
Mon Oct 17 15:55:45 IST 2011  
[[email protected] ~]$ ls  
file1 file10 file2 file3 file4 file5 file6 file7 file8 file9 programs  
[[email protected] ~]$ clear  
-rbash: clear: command not found

8. One more step can be added to restrict the user for making any modifications in their .bash_profile , as users can change it.

Run the following command to make the user localuser‘s .bash_profile file as immutable so that root/localuser can’t modify it until root removes immutable permission from it.

# chattr +i /home/localuser/.bash_profile

To remove immutable tag,

# chattr -i /home/localuser/.bash_profile

Make file .bash_profile as immutable so that user localuser can’t change the environmental paths.

source: redhat.com

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