[Solved] Synology Hyper Backup Tailscale target is offline

I have two Synology DS923+ NAS devices, one running at home and the other remotely. I backup specific folders of each NAS to its remote location with Hyper Backup and Tailscale, running DSM 7.2.2¹. Tailscale puts both machines in the same network, removing the need to expose my networks publicly and letting Hyper Backup find the remote machine by its Tailscale IP address. This workflow works great, but yesterday I noticed Hyper Backup couldn't find the remote machine, marking it as Off-line.

Before you test my solution below, try restarting both machines from the DSM interface.

If that doesn't work, here's how I fixed the issue and restored my backups.

I uninstalled and reinstalled Tailscale on both Synology DSMs and restarting the machines. Note that this changes the Tailscale IP address of the machine and requires reauthentication, which isn't straightforward for a remote machine because you'll lose the ability to connect via SSH (the machine isn't in the Tailscale network anymore). I had to run scheduled tasks with logging on to (a) enable Tailscale, (b) update it, (c) start it, and then (d) see the authentication URL for me to log into the browser.

These are the commands I triggered manually, and the first two are scheduled to run on boot and daily, respectively.

# Start Tailscale's service.
# Scheduled to run on boot.
/var/packages/Tailscale/target/bin/tailscale configure-host; synosystemctl restart pkgctl-Tailscale.service

# Update to Tailscale's latest version for Linux.
# Scheduled to run daily.
tailscale update --yes

# Start Tailscale, which requires authenticating via browser.
tailscale up

The last command will output a URL to your task's logs (so you need to set up a logging file first) which you need to open in your browser to authenticate the machine to your Tailscale network.

Voila, you can now go to your Tailscale admin page to see your machines' new IP addresses and set that as your Hyper Backup task's target.

One machine did not find the other because they pinged the other machine's Tailscale IP before the NAS was on the network, which mapped the IP to a non-existing source. Restarting, it seems, flushes the DNS cache, letting the machine resolve the other machine's address correctly.

If this helped you in any way, I'd love to know. You can message me here.