You are asking how to access a LUKS encrypted disk remotely, without the server having access to the data. I will ignore that, because it's probably not what you want. You want encrypted backups, not encrypted block level access to a remote disk.
There's generally two ways I'd go about it this:
- Use an encrypting filesystem.
- Encrypt the backup in the backup program.
Encrypting file system
This is content neutral. Any content stashed onto the virtual file system will be encrypted, before being stored. The network access can be performed via e.g. Samba or NFS, sshfs, or any other technique for remotely accessing a file system.
encfs is a good encrypting virtual filesystem. You can install it with
sudo apt install encfs
To create a virtual, encrypted filesystem, run the following in a terminal
encfs /path/to/mounted/remote/storage ~/encrypted_storage
This will start a wizard which lets you configure the storage. When it's created, and mounted, anything you put inside
~/encrypted_storage will be encrypted before actually being written to disk. The encryption will happen on the machine that you ran the
To umount it run
fusermount -u ~/encrypted_storage.
To mount it again, simply run the original command used for creating it, and it will detect it as an existing FS, and ask for the passphrase.
Encrypt the backup data
In my opinion this is a better solution. Especially as duplicati supports strong encryption, allowing both AES-256 and GPG as encryption options. GPG is fairly well vetted software, security wise.
In addition this makes it trivial to move your backup to other backup providers, e.g. BackBlaze, Amazon S3 or any other service: as the backup data itself is encrypted, you don't have to worry about your provider. The server will not have access to your backed up data, only metadata, such as when last backup was performed, size and what backup system was used. This metadata will be available to the server anyway.
If you really want LUKS for remote volumes, have a look at iSCSI. It's far more work to set up, far less flexible, and probably not what you want.