Install ProFTPd

The ProFTPd software is in Ubuntu’s default repositories. We can install it by typing:

sudo apt-get update && sudo apt-get install proftpd

Choose “stand alone” when prompted during installation.

After it has been installed, we need to edit some basic configuration variables. Open the ProFTPd configuration file with root privileges with your text editor:

sudo nano /etc/proftpd/proftpd.conf

Change the ServerName parameter to match your domain name or IP address.

ServerName      "yourDomainOrIPAddress"

Remove the # from in front of the DefaultRoot parameter to uncomment it:

DefaultRoot     ~

Save and close the file.

Configure SFTP Access with ProFTPd

Now, we need to configure the service to use SFTP.

The default file looks in the conf.d subdirectory for additional configuration. We will create a file there to enable the use of SFTP:

sudo nano /etc/proftpd/conf.d/sftp.conf

ProFTPd can take configuration with the same formatting as Apache. If you are familiar with Apache, this should look familiar. If you are not familiar, it’s easy to figure out.

Copy and paste the following into the file:

<IfModule mod_sftp.c>

        SFTPEngine on
        Port 2222
        SFTPLog /var/log/proftpd/sftp.log

        # Configure both the RSA and DSA host keys, using the same host key
        # files that OpenSSH uses.
        SFTPHostKey /etc/ssh/ssh_host_rsa_key
        SFTPHostKey /etc/ssh/ssh_host_dsa_key

        SFTPAuthMethods publickey

        SFTPAuthorizedUserKeys file:/etc/proftpd/authorized_keys/%u

        # Enable compression
        SFTPCompression delayed

</IfModule>

Deconstructing the SFTP Configuration

Let’s break the file down into its component pieces so that we can understand it better.

The entire section is wrapped in IfModule tags to make sure that the configuration options are only applied if the SFTP module is available (which it is).

• SFTPEngine on: Enables the SFTP ability for the server
• Port 2222: Specifies the port where the SFTP connections will be accepted. Since SSH already is looking for connections on port 22, we want a different port.
• SFTPLog: Configures the location of the log file that will be created.
• SFTPHostKey: These two lines point to the SSH host keys. This is how the server identifies itself to clients. For the most part, the lines we used should be correct.
• SFTPAuthMethods: This line configures the server to only accept connections with SSH keys.
• SFTPAuthorizedUserKeys: This parameter names the location of the SFTP keys that can be used to authenticate someone. The %u portion will substitute the authenticating user’s name.
• SFTPCompression delayed: This sets the compression mechanism that will be utilized during file transfers.

Configure Key Based Authentication

The ProFTPd can use SSH keys to authenticate users, but the keys must be converted to use the RFC4716 format. Luckily, the SSH suite has the ability to convert these files natively.

Begin by creating a directory to house these files:

sudo mkdir /etc/proftpd/authorized_keys

Now, we need to convert the public keys that are currently used to log into the server. If you only have one user, you can use this command:

sudo ssh-keygen -e -f ~username/.ssh/authorized_keys | sudo tee /etc/proftpd/authorized_keys/username

If you have multiple users and you need to separate their log in credentials, you will have to use the actual public key instead of the authorized_keys file, like this:

sudo ssh-keygen -e -f /path/to/id_rsa.pub | sudo tee /etc/proftpd/authorized_keys/username_who_owns_key

You can add as many keys as you would like.

When you are finished, restart the ProFTPd server:

sudo service proftpd restart

Disable SFTP Access on the SSH Port

Now that we have enabled SFTP through ProFTPd, we can disable it on the normal SSH port. This will allow us to configure user access and lock down what each user can see and manipulate through ProFTPd, without worrying about people being able to leave their home directories.

Open the SSHD configuration file:

sudo nano /etc/ssh/sshd_config

Towards the bottom of the file, you should see a line that looks like this:

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

Put a hash (#) in front of it to comment out the line:

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

Save and close the file.

Now, restart the SSH server so to enable your changes:

sudo service ssh restart