Restic

A high-performance HTTP server that implements the REST backend protocol the restic backup client speaks, so any machine can push encrypted snapshots to a repository over plain HTTP or HTTPS. It authenticates users through an htpasswd file, can give each person a private subdirectory, supports TLS, offers an append-only mode that stops existing backups from being deleted, and can expose Prometheus metrics.

It works well on small or low-powered devices, behind firewalls that only permit HTTP, and as the remote half of a setup where machines back up to a central box.

Running it yourself means the backup endpoint lives on your network, costs nothing per client, and can be snapshotted and restored as ordinary server data.