SFTP server configuration in Debian Linux

SFTP stands for SSH file transfer protocol, It is not the actual FTP server, rather the extension of SSH, which uses SSH tunnel to transfer the files between server and client.

SFTP is more than SCP, as it supports more features than SCP with an integration of remote SCP client. If you want to learn more about SFTP check out this wiki.

OK, to configure SFTP we don't need to install extra packages, as I have already told you , it's just an extension of ssh, you might be guessing,  you need to make some tweaks to your sshd_config file, you are right.
OK,
Open your sshd_config file, which resides under /etc/ directory, feel free to use any text editor you want. I love VIM.

vim /etc/ssh/sshd_config

Inside sshd_config file on line no 77 change,  #Subsystem sftp /usr/lib/openssh/sftp-server to

Subsystem sftp internal-sftp

and uncomment this line. Next, we need to define a policy,  based on user or a group. At the last of a sshd_config file, put the following code given below.

 Match Group Sftp_Group
  ChrootDirectory %h
  ForceCommand internal-sftp
  AllowTcpForwarding no
  PermitTunnel no
  X11Forwarding no

The above policy tells sftp server to authenticate all the users which belong to Sftp_Group and Chroot them only into their home directory so that they can't browse and access the parent directories and other user directories, which they don't have permission to do.  And one important thing to keep in mind while configuring sftp server is, the parent directory must be  always owned by a root user. Let's say, We want to export the home directory of a user test and we want her to confine into her home directory only. To do that first we need to create a test user with a home directory test(of course, you can create a home directory with any name) and make her a member of Sftp_Group, such that the parent directory of a test user must own by the root user. Let's create a test user.

 
 useradd -m -d /home/test -s /usr/sbin/nologin -G Sftp_Group -g Sftp_Group

The above command adds a user test with a home directory test and the secondary group Sftp_Group. As we know parent directory should be owned by the root user
ONLY BY THE ROOT USER. Now, the user test has been created. Let's set the password for user test

passwd test

The above command will let you set a password for user test. Using password command we have also set a password for user test. Now it's time to create a public_html dir under the /test directory, such that it is owned by the user test and the parent directory of this public_html is owned by a user root.

mkdir /home/test/public_html
chown test:test /home/test/public_html
chown root:root /home/test

With the above series of commands, we can now access the sftp server via sftp client.
To test using SFTP, You can use any SFTP client, in a market there are many sftp clients some of them are advance with a feature of a window explorer integrations. For testing purpose. Filezilla would be perfect for us.

One final thing, Before connecting to our SFTP server, we need to restart our SSH server,

systemctl restart ssh

To connect to the SFTP server, we need to prefix the server name with a a sftp prefix as shown below.