9 Jan 2018
I know a few groups of people and associations who sometimes need a storage server for sharing big files. With a few hundreds of GB available, it's possible to offer them a secure FTP server and control how much size they can use in a chrooted directory. The end users will just have a login and a password (or a key) to connect to your server with an SFTP client like Filezilla.
Install OpenSSH, and if it does not exist, create the "sftponly" group:
sudo apt-get install openssh-server sudo addgroup sftponly
Create and add the user who will connect to your FTP server to the sftponly group, here "edmond", and remove its ability to connect through SSH:
sudo adduser edmond # adduser will create his home directory in /home/edmond sudo addgroup edmond sftponly sudo chsh -s /usr/bin/nologin edmond
Now in the SSH daemon config, usually in
/etc/ssh/sshd_config, replace the
sftp subsystem with "internal-sftp", and add restrictive rules for the sftponly
Subsystem sftp internal-sftp Match Group sftponly ChrootDirectory %h AllowTcpForwarding no X11Forwarding no ForceCommand internal-sftp
The "match" block should be placed at the end of the file or the daemon won't be
able to understand the config file. You can specify whether or not you want to
allow password and public key authentication here with the rules
PubkeyAuthentication set to
can also specify where the user will be chrooted with "%h" (her home directory)
or e.g. "/srv/jails/%u" where %u is automatically replaced with her user name.
For the chroot to work properly, the target directory needs to be owned by the
user root, without write privileges for its group and without any privileges for
others. In our example for
cd /home sudo chown root:edmond ./edmond sudo chmod 0750 ./edmond
You should now be able to connect to your server with the new account credentials and end up chrooted in your (almost) empty home directory.
To ensure users don't store more data than you want on the server, we will use a self-contained EXT4 file system stored in a file.
# Create an empty file of 10 GB (1M times 10240 bytes, set count to your needs) sudo dd if=/dev/zero of=/home/edmond.ext4 bs=1M count=10240 # Set secure file permissions sudo chown root:root /home/edmond.ext4 sudo chmod 0660 /home/edmond.ext4 # Create a file system directly in the file sudo mkfs.ext4 /home/edmond.ext4 # Finally, mount it on your home directory sudo mount -t ext4 -o loop /home/edmond.ext4 /home/edmond
Once the home directory is ready, you can create a writeable directory for your users:
# Create a writeable directory for the user cd edmond sudo mkdir SharedFiles sudo chown edmond:edmond SharedFiles sudo chmod 0770 SharedFiles # When several people might access these files, it might be a good idea to use # the setgid flag: sudo chmod g+s SharedFiles
You should now be able to read and write in this shared folder entirely through your SFTP client, without any other access to the server. The users won't be able to upload more than the size of your EXT4 file system.
This method is probably not the most efficient way to provide secure and limited remote storage but its principles are easy to grasp, so if you're lazy or in a rush to deploy a storage for a specific need, I recommend it :)