In the past two weeks I set up a new VPS, and I run a small experiment. I share the results for those who are curious.
Consider that this is a backup server only, meaning that there is no outgoing traffic unless a backup is actually to be recovered, or as we will see, because of sshd.
I initially left the standard "port 22 open to the world" for 4-5 days, I then moved sshd to a different port (still open to the whole world), and finally I closed everything and turned on tailscale. You find a visualization of the resulting egress traffic in the image. Different colors are different areas of the world. Ignore the orange spikes which were my own ssh connections to set up stuff.
Main points:
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there were about 10 Mb of egress per day due just to sshd answering to scanners. Not to mention the cluttering of access logs.
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moving to a non standard port is reasonably sufficient to avoid traffic and log cluttering even without IP restrictions
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Tailscale causes a bit of traffic, negligible of course, but continuous.
There is quite a significant difference. An ssh server - even when running on a non-default port - is easily detectable by scanning for it. With a properly configured Wireguard setup this is not the case. As someone scanning from the outside, it is impossible to tell if there is Wireguard listening or not, since it simply won't send any reply to you if you don't have the correct key. Since it uses UDP it isn't even possible to tell if there is any service running on a given UDP port.