Introduction

Duplicati is a free, open-source backup solution that encrypts, compresses, and stores files in the cloud. Duplicati is mainly written in C# and supports various storage protocols like SFTP and S3-compatible storage solutions like Vultr Object Storage.

You can use the Duplicate intuitive web interface to set up and schedule regular backups for your Cloud Compute instances to remote storage. Duplicate backups are fully automated and secured with solid data encryption algorithms like AES-256 and TLS/SSL protocols.

This guide walks you through setting up automatic backups with Duplicati and Vultr Object Storage on Ubuntu 22.04.

You’ll set up:

• Server A – The source VPS, where Duplicati will back up data from. IP_PUB_A (IP LAN: 10.22.1.24, user: green1)

• Server B – The destination VPS, where Duplicati will store the backup (via LAN). (IP LAN: 10.22.1.25, user: green)

• Goal: Web UI access: http://PUB_A:8200 (management from the Internet)Data transmission: LAN_A <-> LAN_B (SFTP over a private LAN)

Prerequisites

Before you begin:

• Two VPS running Ubuntu 22.04.

• Both servers are connected to the same LAN (or private network).

• SSH access to both.

• Sudo privileges.

Step 1: Prepare VPS B (storage target) — set directory + user, + SSH key

On VPS B (LAN_B): user green

1. Create a dedicated user and folder:

#Create a backup folder.
mkdir -p /home/green/duplicati_backups/ServerA
# Guarantee users' green rights.
chown -R green:green /home/green/duplicati_backups
chmod 700 /home/green/duplicati_backups

2. Ensure OpenSSH server is installed and running:

apt update
apt install openssh-server -y
systemctl enable --now ssh

3. Create an SSH key on VPS A (or on your admin machine) and copy it to VPS B:

mkdir -p ~/.ssh
ssh-keygen -t ed25519 -f ~/.ssh/duplicati_key -C "duplicati@ServerA" -N ""
# file private: ~/.ssh/duplicati_key
# file public:  ~/.ssh/duplicati_key.pub

Copy public key sang Server B (user green):

On VPS A, display the key contents.

cat ~/.ssh/duplicati_key.pub

Copy the entire line (starting with ssh-rsa or ssh-ed25519).
On VPS B (backup machine), log in as the user green and run:

mkdir -p ~/.ssh
chmod 700 ~/.ssh
echo "PASTE_PUBLIC_KEY_Here" >> ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

Check the login from VPS A: 

ssh -i ~/.ssh/duplicati_key [email protected] 'ls -la /home/green/duplicati_backups'

👉 If you can log in without a password, you’re successful.

If you encounter a fingerprint confirmation error, you can disregard it by following the provided instructions.

 Step 1: Clean up old fingerprints (if any)

on Server A:

mkdir -p ~/.ssh
rm -f ~/.ssh/known_hosts

Step 2: Create a new known_hosts file.

To avoid SSH getting stuck when manually confirming fingerprints, you can automatically accept fingerprints using this command:

ssh-keyscan 10.22.1.25 >> ~/.ssh/known_hosts
chmod 600 ~/.ssh/known_hosts

🔸 Explain: ssh-keyscan takes Server B’s fingerprint and saves it to known_hosts, helping SSH to no longer ask, “Are you sure you want to continue connecting (yes/no)?

Step 2: Install and Configure Duplicati on A VPS

Duplicati maintains a central repository where you can download the latest installation files, tailored to your operating system. Follow the steps below to install Duplicati.

Install Duplicati

Install Duplicati using Ubuntu apt command:

1. Navigate to the /tmp directory.

   $ cd /tmp

2. Visit the Official Duplicati Download page and get a link to download the installation file for Debian/Ubuntu.
3. Install the Mono dependency package.

   $ sudo apt install mono-complete -y

4. Use the following Linux wget command to download the Duplicati installation package file.

   $ wget https://updates.duplicati.com/stable/duplicati-2.1.0.5_stable_2025-03-04-linux-x64-gui.deb

5. Install the Duplicati package.

   $ sudo apt install ./duplicati-2.1.0.5_stable_2025-03-04-linux-x64-gui.deb -y
   

Configure Duplicati

Make the following configuration changes before you start using the Duplicati service:

1. Open the default Duplicati configuration file using a text editor like Nano.

   $ sudo nano /etc/default/duplicati

2. Locate the DAEMON_OPTS directive.

   ...
   # Additional options that are passed to the Daemon.
   DAEMON_OPTS=""

3. Change the default value "" to the following content. This setting allows you to access the Duplicati web interface through your server’s private IP address on port 8200.

   DAEMON_OPTS="--webservice-interface=0.0.0.0 --webservice-port=8200 --webservice-password=YourStrongPassword"    #Replace YourStrongPassword with the password you want to use.

   Save and close the file.

4. Allow port 8200 through Uncomplicated Firewall (UFW) and reload the new firewall changes.

   $ sudo ufw allow 8200
   $ sudo ufw reload

5. Start Duplicati and enable the service to start at boot.

   $ sudo systemctl start duplicati 
   $ sudo systemctl enable duplicati

   Duplicati runs a web interface on port 8200 by default.

   Open it in your browser:

   http://<SERVER-IP>:8200

Step 3: Configure Backup on Server A (Duplicati Web UI)

Open the browser to:http://IP_PUB_A:8200

 You will be asked for a password before logging in. The password was configured in step 2.

Step-by-step in the UI:

Select Add backup → Configure a new backup → Next

General:

• • Name: Backup_ServerA_to_ServerB

  • Encryption: No Encryption

    

Destination:

• Storage Type: SFTP (SSH)
• Server: 10.22.1.25
• Port: 22
• Path on server: /home/green/duplicati_backups/ServerA
• Username: green
• Password: Password of the user green
• Authentication: Private SSH key file

If Duplicati runs under green1 (local GUI): select the file key from the client browser (upload) and then select the ~/.ssh/duplicati_key that you created.

Click Test connection → if “Connection Successful” is displayed.

Source data: select the folder you want to back up, e.g. /home/green1/data or /etc depending on your needs

Schedule & Options: set up a calendar (daily, weekly) and a retention policy

Save and click Run now to try it immediately.

After the job runs, log on to Server B

 You’ll see .dblock.zip, .dindex, .dlist, or duplicati-* backup files, indicating the backup is successful.

ssh [email protected]
ls -la /home/green/duplicati_backups/ServerA

You’ll see .dblock.zip, .dindex, .dlist, or duplicati-* backup files, indicating the backup is successful.

Troubleshooting

Conclusions

Duplicati is a multi-platform, feature-rich, easy-to-use backup solution for personal computers. It supports a wide variety of backup repositories, making it a very versatile tool that can adapt to most personal needs.