Create an SSH user who only has permission to access specific folders

Question

I installed SSH, but I found if I use my original account to login to Ubuntu, it has too many permissions.

I want to constrain the user to only have permissions for specific folders in Ubuntu. How can I configure such a user?

Answer 1

This is simple. Just create a new user with its home directory set to the one you need him to have access to (this command must be run under sudo or in root shell):

adduser --home /restricted/directory restricted_user

This will create a user restricted_user, the directory /restricted/directory and then permissions on the directory will be set so the user can write to it. It won't have an ability to write to any other directory by default.

If you have the directory already, you can run adduser command with a --no-create-home option appended and set permissions manually (also with root privileges), like:

chown restricted_user:restricted_user /restricted/directory
chmod 755 /restricted/directory

If you need to make even world-writable directories unaccessible for this user, there are two variants.

1) If you want to provide an interactive shell session to the user, then consider following this manual on creating a chroot jail (in your /restricted/directory).

After that, add the following to your sshd_config:

Match user restricted_user
  ChrootDirectory /restricted/directory

2) If you only need him to copy files between his endpoint of connection and your host, everything is much easier. Add these lines at the end of your sshd_config:

Match user restricted_user
  ForceCommand internal-sftp
  ChrootDirectory /restricted/directory

Subsystem       sftp    internal-sftp

Then comment out the Subsystem sftp /usr/lib/openssh/sftp-server by placing a hash (#) sign at the start.

After restarting your SSH server (it does not kill interactive sessions on restart, so it is safe even if you misconfigured something; also, does not close your running session before you have checked that you are still able to log in), everything should work as intended.

Answer 2

The easiest way to create restricted user that cannot wander off the given directory (e.g., to the upper directory etc), and have a limited/picked set of command to use, is to use a Restricted Shell. Ref:

http://man.he.net/man1/rbash

First, create a symlink called rbash (run as root user).

ln -s /bin/bash /bin/rbash

Then just create a normal user with this Restricted Shell, and set it's home dir to the desired folder:

useradd -s /bin/rbash -d /home/restricted_folder username

Even without the Restricted Shell, if you explicitly do not add this user to sudoer's list, or any special groups, then it will be limited by default.

With the Restricted Shell, the following are disallowed or not performed:

• changing directories with cd

• setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

• specifying command names containing /

• specifying a file name containing a / as an argument to the . builtin command

• Specifying a filename containing a slash as an argument to the -p option to the hash builtin command

• importing function definitions from the shell environment at startup

• parsing the value of SHELLOPTS from the shell environment at startup

• redirecting output using the >, >|, <>, >&, &>, and >> redirect- ion operators

• using the exec builtin command to replace the shell with another command

• adding or deleting builtin commands with the -f and -d options to the enable builtin command

• Using the enable builtin command to enable disabled shell builtins

• specifying the -p option to the command builtin command

• turning off restricted mode with set +r or set +o restricted.

These restrictions are enforced after any startup files

Moreover/Optionally, to restrict the user to a limited/picked set of command to use, you can create a .bash_profile read-only to that user, with

PATH=$HOME/bin

and symlink whatever commands you allows into the ~/bin folder to that user:

ln -s /bin/ls /home/restricted_folder/bin/ls
ln -s /bin/mkdir /home/restricted_folder/bin/mkdir
ln -s /bin/rm /home/restricted_folder/bin/rm

etc.

HTH