By Mateusz Papiernik and Mark Drake
SFTP stands for SSH File Transfer Protocol. As its name suggests, it’s a secure way to transfer files between machines 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.
To follow this tutorial, you will need access to an Ubuntu 18.04 server. This server should have a non-root user with
sudo privileges, as well as a firewall enabled. For help with setting this up, follow our Initial Server Setup Guide for Ubuntu 18.04.
First, create a new user who will be granted only file transfer access to the server. Here, we’re using the username sammyfiles, but you can use any username you like.
- sudo adduser sammyfiles
You’ll be prompted to create a password for the account, followed by some information about the user. The user information is optional, so you can press
ENTER to leave those fields blank.
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.
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 Ubuntu 18.04) 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 not be writable by other users; the subdirectory
/var/sftp/uploads will be owned by sammyfiles, 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 sammyfiles.
- sudo chown sammyfiles:sammyfiles /var/sftp/uploads
Now that the directory structure is in place, we can configure the SSH server itself.
In this step, we’ll modify the SSH server configuration to disallow terminal access for sammyfiles but allow file transfer access.
Open the SSH server configuration file using
nano or your favorite text editor.
- sudo nano /etc/ssh/sshd_config
Scroll to the very bottom of the file and append the following configuration snippet:
. . . Match User sammyfiles 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 Usertells the SSH server to apply the following commands only to the user specified. Here, we specify sammyfiles.
ForceCommand internal-sftpforces the SSH server to run the SFTP server upon login, disallowing shell access.
PasswordAuthentication yesallows password authentication for this user.
ChrootDirectory /var/sftp/ensures that the user will not be allowed access to anything beyond the
AllowTcpForwarding no. and
X11Forwarding nodisables 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 the Copying your Public SSH Key section of the SSH Essentials: Working with SSH Servers, Clients, and Keys tutorial to do so. 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 sammyfiles. The last step is testing the configuration to make sure it works as intended.
Let’s ensure that our new sammyfiles user can only transfer files.
Logging in to the server as sammyfiles using normal shell access should no longer be possible. Let’s try it:
- ssh sammyfiles@localhost
You’ll see the following message before being returned to your original prompt:
Error messageThis service allows sftp connections only. Connection to localhost closed.
This means that sammyfiles can no longer can access the server shell using SSH.
Next, let’s verify if the user can successfully access SFTP for file transfer.
- sftp sammyfiles@localhost
Instead of an error message, this command will show a successful login message with an interactive prompt.
SFTP promptConnected to localhost. sftp>
You can list the directory contents using
ls in the prompt:
This will show the
uploads directory that was created in the previous step and return you to the
SFTP file list outputuploads
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.
- 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 sammyfiles user can access the server only using the SFTP protocol for file transfer and has no ability to access the full shell.
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 even 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.
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Hi, I followed all steps but I get this error when I try to connect (ssh or sftp):
packet_write_wait: Connection to xxx.xx.xxx.xxx port 22: Broken pipe
I have followed same steps and everything is fine. But showing access denied after added password. I have create new dir and under HTML and allowing that dir to user to use. But not accepting my password.
Please let me, Where I am wrong.
works like charm for me, thanks a bunch!
Regarding comment: #78000 / giacomofabbian March 16, 2019
Be aware that if by following these instructions, your problem is solved, then you are/were likely missing the sftp-server binary file.
You can check for its existence via:
Some good information on the topic and as to why you would use one over the other can be found here :
Thanks for the guide, I got this working. For some reason it’s quite slow. When I use SCP, I can get transfer speeds of at least 500kb/s, however with SFTP, I only get 80kb/s, and deleting folders is quite slow.
Any ideas if this can be addressed?
I solve your error @rabitt @marvin007dd76bbbc17604d944 @mmmrev with this:
"I was getting this error as well:
Error: Network error: Software caused connection abort Error: Could not connect to server
One entry in the sshd_config file that was omitted by the author:
Subsystem sftp /usr/lib/openssh/sftp-server ‘comment this out’ Subsystem sftp internal-sftp ‘add this line’
Once added commented out the top line and added the second line, I was able to connect without any problems."
Many thanks to @barryd solution found on tutorial for ubuntu 16.04
Please update the tutorial @mdrake @mati
I followed the tutorial on a new installation with ufw disabled and ssh default settings on but when I try to ssh or sftp into the server, I get the following response: Connection reset by xxx.xxx.xxx.x port 22.
How can I resolve this?