Limiting Access with SFTP Jails on Debian and Ubuntu

As the system administrator for your Linode, you may want to give your users the ability to securely upload files to your server. The most common way to do this is to allow file transfers via SFTP, which uses SSH to provide encryption. This means you need to give your users SSH logins. But, by default, SSH users are able to view your Linode’s entire filesystem, which may not be desirable.

Limiting Access with SFTP Jails on Debian and Ubuntu

This guide will help you configure OpenSSH to restrict users to their home directories, and to SFTP access only. Please note that these instructions are not intended to support shell logins; any user accounts modified in accordance with this guide will have the ability to transfer files, but not the ability to log into a remote shell session.

These instructions will work for Ubuntu 9.04, Debian 5, and later. Unfortunately, the version of SSH packaged with Ubuntu 8.04 is too old to support this configuration.

Configure OpenSSH

First, you need to configure OpenSSH.

1. Edit your /etc/ssh/sshd_config file with your favorite text editor:

vim /etc/ssh/sshd_config

2. Add or modify the Subsystem sftp line to look like the following: /etc/ssh/sshd_config

Subsystem sftp internal-sftp

3. Add this block of settings to the end of the file: /etc/ssh/sshd_config

Match Group filetransfer
ChrootDirectory %h
X11Forwarding no
AllowTcpForwarding no
ForceCommand internal-sftp

Save the changes to your file.

4. Restart OpenSSH:

service ssh restart

OpenSSH has been successfully modified.

Modify User Accounts

In this section, we’ll set up the correct new groups, ownership, and permissions for your user accounts.

1. Create a system group for users whom you want to restrict to SFTP access:

addgroup filetransfer

2. Modify the user accounts that you wish to restrict to SFTP. Issue the following commands for each account, substituting the appropriate username. Please keep in mind that this will prevent these users from being able to log into a remote shell session.

usermod -G filetransfer username
sudo chown root /home/username
sudo chmod go-w /home/username

These users will now be unable to create files in their home directories, since these directories are owned by the root user.

3. Next, you need to create new directories for each user, to which they will have full access. Issue the following commands for each user, changing the directories created to suit your needs:

sudo mkdir /home/username/writable
sudo chown username:filetransfer /home/username/writable
sudo chmod ug+rwX /home/username/writable

Your users should now be able to log into their accounts via SFTP and transfer files to and from their assigned subdirectories, but they shouldn’t be able to see the rest of your Linode’s filesystem.

Reference :
1. https://www.linode.com/docs/tools-reference/tools/limiting-access-with-sftp-jails-on-debian-and-ubuntu

2. https://askubuntu.com/questions/134425/how-can-i-chroot-sftp-only-ssh-users-into-their-homes

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