Creating SFTP server with isolated directory access

First make sure your repository is up to date.

sudo apt-get update

Now install OpenSSH server software.

sudo apt-get install openssh-server

We then create a user group for SFTP access, I will be calling it sftponly. For security I think it is best practice not to allow accounts with SFTP access additional admission to the server using secure shell (SSH) remote log in.

sudo groupadd sftponly

Run the following to display your new group. It will be listed as the last entry.

cat /etc/group

Cat allows you to quickly display a text file while /etc/group is the file that defines the groups on the server. You should see something like this.

Output of cat /etc/group.

Each line is an individual group, you can see the name, the password which is set to x which means none, the numeric group id and users who are associated with the group. For sftponly there are currently no assigned users.

Take note of the group id, in this screenshot it is the value 1001.

We now add a new user that we will use exclusively for SFTP access.

sudo useradd [user name] -d / -g [sftponly group id] -M -N -o -u [sftponly group id]
sudo passwd [user name]

The arguments we used.

  • -d is the user home directory which needs to be set to / (root).
  • -g is the user group id to assign which in our example needs to be assigned to sftponly.
  • -M stops the useradd command creating a home directory.
  • -N useradd by default creates a group with the same name as the new user, this disables that behaviour.
  • -u is the user id, which in our case needs to be the same id value as sftponly.
  • -o allows duplicate, non-unique user ids.
  • The passwd command sets an encrypted user password.

Add a user of your choice, I will use ben_example. To display your users.

cat /etc/passwd

Output cat /etc/passwd.

We now backup and edit the SSH Daemon configuration file.

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
sudo nano +76 /etc/ssh/sshd_config

The line

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

Needs to be replaced with

Subsystem sftp internal-sftp

Now go to the end of the document, the key combination Alt / should take you there or you could simply use the Page Down key. After UsePAM Yes add the following lines to configure our sftponly group permissions and settings. The ChrootDirectory setting will confine all sftponly users to this directory. Otherwise sftponly will have access to your server root which you do not want. /var/www is often the default Debian/Ubuntu location for web servers to place their assets such as HTML, CSS files and images. Though you can use a different directory for ChrootDirectory such as /var/sftp.

Match group sftponly
ChrootDirectory /var/www
X11Forwarding no
AllowTcpForwarding no
ForceCommand internal-sftp

Once finished use the key combination Ctrl O to save and then Ctrl X exit.

Output of sudo nano /etc/ssh/sshd_config.

Now make sure the directory you assigned to ChrootDirectory actually exists and if it does not then create it. Also the directory group and owner need to be root which it should by default if you use the command.

sudo mkdir /var/www

Output of SFTP root directory.

Now for later testing create the first of 3 directories within your ChrootDirectory.

cd /var/www
sudo mkdir test_readonly
sudo chmod 755 test_readonly

If you do not understand the chmod command shown we will quickly go through it but it is beyond the scope of this article. Change mode (chmod) parameter 755 is a permission code in octal notation. You break it up into 3 parts, 7/5/5.

  1. 1st part is root permission for this directory.
  2. 2nd is group permission.
  3. 3rd is everyone else.

The value of the individual part grants permissions for its user. A value of 0 grants no permission, a value of 1 grants execute permissions, 2 grants write and 4 grants read. These values can be summed to create multiple permissions. So 1 (execute+ 2 (write) + 4 (read) equals 7 which grants executewrite and read aka total access. chmod 755 test_readonly means the root has total access. While users associated with the directory’s group and everyone else only have execute and read access, 4 (read) + 1 (execute) = 5. More can be read about Linux permissions here http://en.wikipedia.org/wiki/File_system_permissions#Symbolic_notation.

sudo mkdir test_readwrite
sudo chown root:sftponly test_readwrite
sudo chmod 775 test_readwrite

The above commands creates a test_readwrite directory whose owner is root and group is sftponly. Both root and sftponly members have full access permissions within test_readwrite, which allows the creation and deletion of files and sub-directories.

To remove browsing access to a directory you remove the read permission as shown below. 1 (execute) + 2 (write) = 3, execute and write access but critically no read which leaves the other two permissions redundant.

sudo mkdir test_noaccess
sudo chmod 733 test_noaccess

Restart the SSH server.

sudo /etc/init.d/ssh restart

If you don’t know your server IP address.

ip -o -f inet addr

Discover your server’s IP address.

lo is your local host, eth0 is your Ethernet cable connected address, wlan0 is probably wireless.

Connect to your SSH server using an SFTP client such as FileZilla. Make sure you use the correct IP address and port number which by default is 22.

Filezilla site manager connecting to my example SFTP server.

You should be connected to the directory assigned to ChRootDirectory (var/www) in your SSHD configuration and that doubles as the SFTP client root folder. Also listed should be the test_readonly,test_readwrite and test_noaccess directories that we created earlier. Play or navigate around, hopefully you can upload files and create/delete directories within test_readwrite. While test_noaccess should be displayed but limited to browsing or download access.

Browsing your SFTP root in Filezilla.

In the screen capture below we have Filezilla’s message log with 3 sections highlighted. The orange section shows my failed attempt at accessing test_noaccess. The purple is the successful attempt at accessing test_readonly, but a failure in creating a New directory sub-folder within. While the green section shows access into test_readwrite as well as being able to create a New directory sub-folder and its subsequent removal.

Colour-coded message log from browsing our SFTP service in FileZilla.

Congratulations you now have a working example of a SFTP service running on your server. It should be mentioned that a number of these instructions were originally learnt from the blog post SFTP on Ubuntu and Debian in 9 easy steps and the reader comments.

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One thought on “Creating SFTP server with isolated directory access

  1. Pingback: Creating Secure File Upload/Download sites using UBUNTU Server | Andrew's blog of things

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