How to provide Secure Isolated (Restricted) SFTP Access for file transfer

Often we need to provide secure but restricted sftp access to our clients to upload or download large files.

Regular sftp accounts allow the users to view login names of other users/clients and in many default installations even allow them to browse through the directories and files. In addition, they can also view your system files and any unprotected files and directories including much of your system settings under etc.

When you provide sftp access, you also provide ssh access (sftp is a subsystem of ssh) and as such users can login.

We address these issues by enclosing the user’s home directory in chroot jail, which makes it impossible for the user to break out of his home directory and view other’s files and directories.

Follow the steps below to configure your system.

As root (or sudo) modify /etc/ssh/sshd_config:Change Subsystem sftp line to:

Subsystem sftp internal-sftp

Add this to the end of the file:

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

Save the changes and restart OpenSSH:

sudo service ssh restart

Create a system group for users with restricted sftp access:

addgroup --system sftp

Now you can add users with addsftpuser and delete with delsftpuser from GitHub project restricted-sftp.

Clone the repository with:

git clone https://github.com/angsuman/restricted-sftp.git

Copy the files addsftpuser and delsftpuser to your ~/bin or any other directory in your PATH like /usr/bin.

Run it with by specifying the login name of the sftp user:

addsftpuser new-login

The only downside is that the user cannot upload/download files and directories under his home directory directly (as it is owned by root) but can do under Files sub-directory. There you have full access and can upload/download/delete/rename files and directories. To create additional directories under home, edit the addftpuser script and create them following the same procedure as Files.

OVH: How to add Additional IP Addresses (IP Alias) on Ubuntu 16.04 / 18.04

Adding additional IP addresses in OVH is rather unconventional and the guide they point to in their mail is non-existent. The process, fortunately, is simple:

  1. ssh to the server as root
  2. cd /etc/systemd/network/
  3. vi *-default.network
  4. Add the new IP address after the original IPv4 Address
    DHCP=no
    Address=Main_IP/24
    Address=Failover_IP/32
  5. Save and close the file
  6. Reboot the server

You should now be able to ssh to the server using the new IP Address (which OVH likes to call Failover IP Address) in addition to the old one.
Repeat the process for any additional IP addresses.
Note: I have tested it on two different OVH Servers (in France & Canada) for Ubuntu 16.04 and Ubuntu 18.04.

Ubuntu: How to Change Host name and Propagate to DHCP Server

How to change Hostname on Ubuntu

First find your current host name by typing:
hostname
Open /etc/hostname and change the host name specified there to the new host name.
sudo vi /etc/hostname
Now open /etc/hosts and locate the old host name in the file and change it to the new host name.
sudo vi /etc/hosts
Now restart hostname with:
sudo service hostname restart

How to propagate Hostname Change to DHCP Server and other machines

The chosen host name should be propagated to the DHCP server so that any other computer can refer to this computer by your new chosen hostname. To do that run the following commands:
sudo dhcclient -r ; sudo dhclient
Now you should be able to access this machine by its newly chosen host name from any other machine in the same network and served the same DHCP server.
An exception would be if a machine is configured statically or not configured to use the name server associated with this dhcp server.
Most routers today comes with DHCP server and an associated Name Server which can be configured as shown above.