Linux user with limited permissions

Quick guide how to create a linux user with limited permissions

The idea and the purpose

In this article I want to answer this question: How can I restrict the normal user to run only limited set of commands.

So I describe step-by-step the method to create a linux user able to access to the console of a linux server but with limited permission to run commands. This method is useful when, for example, you want to give access to one of your collaborators without risking that he can launch commands as an administrator.

The idea is to use the restricted shell rbash and declare which commands the user are able to run.

The step-by-step guide

  1. create the new user

    adduser <username>

    compile all the requested fields. The most important is the password that will be used to access the server (for example in SSH).

  2. set the rbash as shell for the new user

    chsh -s /bin/rbash <username>
  3. create a bin directory under the user home directory

    mkdir /home/<username>/bin
    chmod 755 /home/<username>/bin/
  4. change the user's default PATH to the bin directory

    echo "PATH=$HOME/bin" >> /home/<username>/.bashrc
    echo "export PATH" >> /home/<username>/.bashrc
  5. create symlinks of the command(s) that the user require to bin directory under the user home directory

    ln -s /bin/ping /home/<username>/bin/
    ln -s /bin/traceroute /home/<username>/bin/
    ln -s /bin/htop /home/<username>/bin/
    ln -s /bin/top /home/<username>/bin/
    ln -s /bin/df /home/<username>/bin/
    ln -s /bin/bmon /home/<username>/bin/
    ln -s /bin/ps /home/<username>/bin/
    ln -s /bin/date /home/<username>/bin/
    ln -s /bin/netstat /home/<username>/bin/
    ln -s /bin/free /home/<username>/bin/
  6. restrict the user from modifying ~/.bashrc

    chattr +i /home/<username>/.bashrc

These are the basic steps to achieve the purpose. However, I want to add one more of non-core but stylistic step: print the list of available commands when the user login in console. For this purpose I user motd.

  1. create motd 99-available-commands

    touch /etc/update-motd.d/99-available-commands
  2. add the list of available commands

    cat << EOF >> /etc/update-motd.d/99-available-commands
    export TERM=xterm-256color
    echo "$(tput setaf 4)-------------------------------------------------------------------------------
    -- Available commands:
    -------------------------------------------------------------------------------$(tput setaf 7)
    bmon          - Bandwidth monitor and rate estimator
    date          - Display the current time or system date
    df            - Report file system disk space usage
    free          - Display amount of free and used memory in the system
    htop          - Interactive process viewer
    netstat       - Display ethernet and protocol info and statistics
    ping          - Verify IP level connectivity
    ps            - Display information about running processes
    top           - Display Linux processes
    traceroute    - Trace the path/hop from one network to another
    $(tput setaf 4)-------------------------------------------------------------------------------$(tput setaf 7)
  3. set the right permission to the file

    chmod 755 /etc/update-motd.d/99-available-commands

Now the user when logging in will see a list of enabled commands like this image.png

Thanks to

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