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Introduction

SFTP stands for SSH File Transfer Protocol. As its name suggests, it’s a secure way of transferring files to a server using an encrypted SSH connection. Despite the name, it’s a completely different protocol than FTP (File Transfer Protocol), though it’s widely supported by modern FTP clients.

SFTP is available by default with no additional configuration on all servers that have SSH access enabled. It’s secure and easy to use, but comes with a disadvantage: in a standard configuration, the SSH server grants file transfer access and terminal shell access to all users with an account on the system.

In some cases, you might want only certain users to be allowed file transfers and no SSH access. In this tutorial, we’ll set up the SSH daemon to limit SFTP access to one directory with no SSH access allowed on per user basis.

Prerequisites

To follow this tutorial, you will need:

• One CentOS 7 server set up with this server setup tutorial, including a sudo non-root user.
• Optionally, the nano text editor installed with the command: yum install nano. CentOS comes with the vi text editor by default, but nano can be more user friendly.

Step 1 — Creating a New User

First, create a new user who will be granted only file transfer access to the server. Here, we’re using the username non_root_username, but you can use any username you like.

sudo adduser non_root_username

Next, assign a password to the new user:

sudo passwd non_root_username

Enter a strong password, and repeat it again to verify it.

You have now created a new user that will be granted access to the restricted directory. In the next step we will create the directory for file transfers and set up the necessary permissions.

Step 2 — Creating a Directory for File Transfers

In order to restrict SFTP access to one directory, we first have to make sure the directory complies with the SSH server’s permissions requirements, which are very particular.

Specifically, the directory itself and all directories above it in the filesystem tree must be owned by root and not writable by anyone else. Consequently, it’s not possible to simply give restricted access to a user’s home directory because home directories are owned by the user, not root.

Note: Some versions of OpenSSH do not have such strict requirements for the directory structure and ownership, but most modern Linux distributions (including CentOS 7) do.

There are a number of ways to work around this ownership issue. In this tutorial, we’ll create and use /var/sftp/uploads as the target upload directory. /var/sftp will be owned by root and will be unwritable by other users; the subdirectory /var/sftp/uploads will be owned by non_root_username, so that user will be able to upload files to it.

First, create the directories.

sudo mkdir -p /var/sftp/uploads

Set the owner of /var/sftp to root.

sudo chown root:root /var/sftp

Give root write permissions to the same directory, and give other users only read and execute rights.

sudo chmod 755 /var/sftp

Change the ownership on the uploads directory to non_root_username.

sudo chown non_root_username:non_root_username /var/sftp/uploads

Now that the directory structure is in place, we can configure the SSH server itself.

Step 3 — Restricting Access to One Directory

In this step, we’ll modify the SSH server configuration to disallow terminal access for non_root_username but allow file transfer access.

Let’s open the SSH server configuration file with vi or your favourite text editor.

sudo vi /etc/ssh/sshd_config

Scroll to the very bottom of the file and append the following configuration snippet:

 . . .

Match User non_root_username
 ForceCommand internal-sftp
 PasswordAuthentication yes
 ChrootDirectory /var/sftp
 PermitTunnel no
 AllowAgentForwarding no
 AllowTcpForwarding no
 X11Forwarding no

Then save and close the file.

Here’s what each of those directives do:

• Match User tells the SSH server to apply the following commands only to the user specified. Here, we specify non_root_username.
• ForceCommand internal-sftp forces the SSH server to run the SFTP server upon login, disallowing shell access.
• PasswordAuthentication yes allows password authentication for this user.
• ChrootDirectory /var/sftp/ ensures that the user will not be allowed access to anything beyond the /var/sftp directory. You can learn more about chroot in this Ubuntu-based chroot tutorial.
• AllowAgentForwarding no, AllowTcpForwarding no. and X11Forwarding no disables port forwarding, tunneling and X11 forwarding for this user.

This set of commands, starting with Match User, can be copied and repeated for different users too. Make sure to modify the username in the Match User line accordingly.

Note: You can omit the PasswordAuthentication yes line and instead set up SSH key access for increased security. Follow this guide on how to work with SSH. Make sure to do this before you disable shell access for the user.

In the next step, we’ll test the configuration by SSHing locally with password access, but if you set up SSH keys, you’ll instead need access to a computer with the user’s keypair.

To apply the configuration changes, restart the service.

sudo systemctl restart sshd

You have now configured the SSH server to restrict access to file transfer only for non_root_username. The last step is testing the configuration to make sure it works as intended.

Step 4 — Verifying the Configuration

Let’s ensure that our new non_root_username user can only transfer files.

Logging in to the server as non_root_username using normal shell access should no longer be possible. Let’s try it:

ssh non_root_username@localhost

You’ll see the following message before being returned to your original prompt:

Error message:
 This service allows sftp connections only.
 Connection to localhost closed.
 This means that non_root_usernamecan no longer can access the server shell using SSH.

Next, let’s verify if the user can successfully access SFTP for file transfer.

sftp non_root_username@localhost

Instead of an error message, this command will show a successful login message with an interactive prompt.

SFTP prompt:
 Connected to localhost.
 sftp>

You can list the directory contents using ls in the prompt:

ls

This will show the uploads directory that was created in the previous step and return you to the sftp> prompt.

SFTP file list output:
 uploads

To verify that the user is indeed restricted to this directory and cannot access any directory above it, you can try changing the directory to the one above it.

sftp> cd ..

This command will not give an error, but listing the directory contents as before will show no change, proving that the user was not able to switch to the parent directory.

You have now verified that the restricted configuration works as intended. The newly created non_root_usernameuser can access the server only using he SFTP protocol for file transfer and has no ability to access the full shell.

Conclusion

You’ve restricted a user to SFTP-only access to a single directory on a server without full shell access. While this tutorial uses only one directory and one user for brevity, you can extend this example to multiple users and multiple directories.

The SSH server allows more complex configuration schemes, including limiting access to groups or multiple users at once or limited access to certain IP addresses. You can find examples of additional configuration options and explanation of possible directives in the OpenSSH Cookbook. If you run into any issues with SSH, you can debug and fix them with this troubleshooting SSH series.

Original Article by Mateusz Papiernik, edited by the author of this post according to the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.