Allowing SFTP Access While Chrooting The User And Denying Shell Access

Published Published June 28th, 2011

In my opinion File Transfer Protocol (FTP) is a dated protocol which no administrator in their right mind should ever allow to run on any server they are in charge of administrating. FTP was never a protocol designed to be secure and just as Telnet to access remote servers was replaced long ago so should FTP. I will not go into the many weaknesses of FTP security wise when used over the Internet but I am sure if you are reading this you already understand there is a huge risk allowing such a protocol to run on any of your systems.

There are a few solutions to FTP’s shortcomings when it comes to security such as FTPS and SFTP. In this post I will cover SSH File Transfer Protocol or SFTP. SFTP allows for secure file access, transfer and management over SSH which most Linux servers generally have installed to allow for remote administration.

By default any account with SSH access already has SFTP access as well. The problem with just adding a user and letting them have at it is that by default along with SFTP access users which you only wish to grant file management access also have shell access and may be able to install and run processes you may not wish them to run on your server. The following is a command by command walk through on how to allow users only file management access via SFTP while not allowing them to gain shell access on your server.

These command by command instructions are Ubuntu/Debian centric but only with the in the case of apt-get and process restart commands and should be no problem to adapt to any other distro. The first thing we will want to do is install SSH which generally is installed by default but just in case…

sudo apt-get install openssh-server

Once we know SSH is installed we can then begin to add users we wish to allow only SFTP access only to. We will start by adding a new user group to our system. I will use “sftponly” for the name of this group but the name of the group is up to you. Just be sure that if you use a different name to modify any proceeding referances to the “sftponly” group name with the one you used.

sudo groupadd sftponly

Now open the file /etc/ssh/sshd_config in your favorite text edit. I prefer nano for such trivial edits.

sudo nano /etc/ssh/sshd_config

Look for the line near the bottom of this file that looks like this.

Subsystem sftp /usr/lib/openssh/sftp-server

And change it to this.

Subsystem sftp internal-sftp

At the very bottom of this file you will need to add the following lines which restrict the “sftponly” user groups access when logging in via SSH.

# Rules for sftponly group
Match group sftponly
ChrootDirectory %h
X11Forwarding no
AllowTcpForwarding no
ForceCommand internal-sftp

Once you have added these lines save the file and restart the SSH process.

sudo /etc/init.d/ssh restart

Next we will create the home directory for the user we are about to add where they will be allowed file access to as well as where they will be chrooted or jailed. This directory can be located wherever but for the sake of this tutorial I will use the following directory.

sudo mkdir /var/www/vhost/

Now we will need to add a user as we normally would under Linux. At this time we will also specify the users home directory which we just created using the “-d” flag. Remember to replace “sally” with the login name for the user you wish to add.

sudo useradd -d /var/www/vhosts/ sally

Then we will need to change the group which the user we just added will belong to.

sudo usermod -g sftponly sally

Next we will set our new user’s shell to /bin/false which will not allowing our new user shell login.

sudo usermod -s /bin/false sally

Set a password this user will use.

sudo passwd sally

We will now need to give ownership to any files and folders within the new users home directory which may exist such as folders brought over from a skel or created by an administrator setting up the users home directory manually.

sudo chown sally:sftponly -R /var/www/vhosts/

Now comes an important part the newly created users home directory MUST be owned by root. If this directory is not owned by root then the newly created user may not be chooted or jailed within their home directory possibly allowing them access to other directories under theirs.

sudo chown root:root /var/www/vhosts/

And that is it you now have set up a user allowed only SFTP access over SSH with no shell access and chroot them within their directory. You are now much safer then you were when running FTP.