This is an attempt at having a central post about using QNAP as a backend / destination for Duplicati backups.
For now this is mostly placeholders and links to other instructions - but this post is a “wiki” so if you have experience using QNAP as your backend, feel free to use the 
button at the bottom right to add / change any content that might help others.
QNAP as a destination
Why would I want to use a QNAP NAS for a destination?
QNAP devices support containers (including Docker containers) which opens up a lot of possibilities for destination technologies including Minio (Amazon S3 compatible).
Types of backends supported
SSH / SFTP (built in)
SSH / SFTP is a great (secure) alternative to FTP. You can set up the built in SSH as described here and here.
Minio (Amazon S3 compatible) Docker container
You can install the Minio Docker on your QNAP device then set it up like any other S3 destination.
Note that it has been unofficially reported that sometimes Minio changes target directory.
FTP (built in)
You can use the built in FTP services to set up a standard FTP server as described here. Note that FTP is considered MUCH LESS SECURE than other options such as SSH / SFTP.
Windows / Mac / Linux drive share (built in)
You can use the built in Windows / Mac / NFS (Linux) Network Services to set up a “drive share” as described here. Note that this type of destination works great for local networks but is NOT suggested for internet use.
Securing your QNAP device
While securing your QNAP device is not really in the scope of this post, it should be noted that if exposing your NAS to the internet the safest way to do it is to use specific port forwarding on your router to allow ONLY the necessary ports through to your QNAP device.
The actual method for doing this will vary by router and the ports needed will vary by backend type (see above) and individual settings.
General notes
The model of your QNAP can make a big difference in what backends are supported (for example, some models may not support Docker containers).