Setting up an SFTP Server on Ubuntu

Secure File Transfer Protocol (SFTP) is a secure protocol for transferring files between local and remote servers. Unlike standard FTP, it uses SSH (Secure Shell) to encrypt all data transfers, protecting against common threats like data interception and packet sniffing. In this guide, we will detail the steps to set up an SFTP server on an Ubuntu machine.

Prerequisites

Before starting, ensure you have:

• A machine running Ubuntu (version 18.04 or later is recommended for the best compatibility)
• SSH installed on your Ubuntu machine. This is usually pre-installed on most Ubuntu distributions.
• Sudo privileges or root access to the Ubuntu machine

Step 1: Create a New User

First, we’ll create a new user for the SFTP connection. This is important for security reasons – it’s best practice not to use the root user for SFTP.

Open your terminal and type the following command to add a new user. Replace ‘sftpuser’ with the username you want to use.

sudo adduser sftpuser 

You’ll be prompted to enter a new UNIX password and to confirm it. Remember this password as you will need it later.

Step 2: Install SSH (if not already installed)

If SSH is not already installed on your server, you can install it by running the following command:

sudo apt-get update 
sudo apt-get install openssh-server 

Check if the SSH service is running by using:

sudo systemctl status ssh 

If it’s not running, start it with:

sudo systemctl start ssh 

Step 3: Configuring SSH for SFTP

Now, we need to edit the SSH configuration file to specify the SFTP settings. We will use nano editor for this, but you can use vim or any other text editor you are comfortable with.

sudo nano /etc/ssh/sshd_config 

Scroll to the very bottom of the file and add the following lines:

Match User sftpuser
    ForceCommand internal-sftp
    PasswordAuthentication yes
    ChrootDirectory /home/sftpuser
    PermitTunnel no
    AllowAgentForwarding no
    AllowTcpForwarding no
    X11Forwarding no

In these settings:

• ‘Match User’ specifies that the following lines only apply to the ‘sftpuser’ user.
• ‘ForceCommand internal-sftp’ restricts the user to SFTP only and disallows SSH.
• ‘PasswordAuthentication yes’ allows password authentication for this user.
• ‘ChrootDirectory /home/sftpuser’ sets the user’s home directory as their root directory, preventing them from accessing other parts of the server’s file system.
• The last four lines disable various SSH features to improve security.

Once you’ve made these changes, save and exit the editor. If you’re using nano, you can do this by pressing ‘Ctrl + X’, then ‘Y’, then ‘Enter’.

Step 4: Restart the SSH Service

After making the changes, restart the SSH service for them to take effect.

sudo systemctl restart ssh 

Step 5: Testing the SFTP Connection

Now it’s time to test the SFTP connection. From your local machine, attempt to connect to your server using the ‘sftp’ command, replacing ‘your_server_ip’ with your server’s IP address.

sftp sftpuser@your_server_ip 

You’ll be prompted to enter your password. If everything is configured correctly, you should now be connected to your SFTP server.

Step 6: Permissions and Ownership

After setting up the SFTP server, it’s important to check and manage the ownership and permissions of the user’s directory. The ChrootDirectory (in our case, /home/sftpuser) should be owned by root and should not be writable by any other user or group. This is a requirement of the SFTP setup.

First, change the ownership of the directory to root:

sudo chown root:root /home/sftpuser 

Next, set the permissions for this directory. This command removes write permissions for group and other users:

sudo chmod 755 /home/sftpuser 

Please note that the sftpuser will not be able to write in the root of their home directory because it is owned by root. To allow the sftpuser to upload files, we need to create a directory inside the home directory that sftpuser owns.

sudo mkdir /home/sftpuser/files 
sudo chown sftpuser:sftpuser /home/sftpuser/files 

Now, sftpuser can upload files to the /files directory.

Step 7: Final Testing

Let’s perform a final test to make sure everything is working as expected. Try to connect again from your local machine:

sftp sftpuser@your_server_ip 

Once you’re logged in, navigate to the files directory and try to create a new file:

cd files 
touch test.txt 

If the file is created without errors, this means that you have successfully set up your SFTP server and the user permissions are correct. Don’t forget to exit the SFTP shell:

exit 

Conclusion

In this tutorial, we’ve walked you through the process of setting up an SFTP server on Ubuntu. We’ve created a new user, installed and configured SSH for SFTP, and set the correct permissions and ownership for our user’s directory. Now you can securely transfer files to and from your Ubuntu server using SFTP. It’s important to remember that while SFTP is secure, you should always follow best practices for managing your server, like regularly updating your software and using strong, unique passwords for all your users.

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