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I recommend turning off password authentication in SSH if at all possible. In my opinion, this is the single most important thing you can do to secure ANY UNIX-like system on the Internet. Automated password-guessing attacks on SSH servers have been going on for years, and this stops them cold.
To turn off password authentication, go into /etc/ssh/sshd_config, find the lines that say
# Change to no to disable tunnelled clear text passwords
You MUST have already generated and set up RSA authentication for every login you intend to access with SSH. The RSA public key(s) go(es) into the file ~/.ssh/authorized_keys. Test this before you disable password authentication.
I use Zeroconf to find local devices and services. That's a set of IETF standard protocols that originated as Apple's "Bonjour"; they're especially popular for discovering local printers. The KiwiSDR already runs 'avahi', which is the main Zeroconf daemon for Linux. (mDNS is just part of Zeroconf; that's how it responds to "kiwisdr.local".) If you create files under /etc/avahi/services, then it will also advertise those services so they can be found by a Zeroconf client like avahi-browse on Linux, the Safari web browser with the Zeroconf extension, or the third-party Discovery application on Mac OSX. I've attached the files /etc/avahi/ssh.service and /etc/avahi/http.service that I added to my KiwiSDRs. To get them past the forum's file name filter I had to append ".txt" to each one; remove before you place them in /etc/avahi.
One advantage of doing this, rather than running only mDNS, is that you can still discover a KiwiSDR whose name has been changed from "kiwisdr.local". I have had to do that when there's more than one on my network.
I concur with the suggestion to try another power supply. They can vary enormously in noise generation.
I've been switching to Cat 7 ethernet cables where possible. Each pair is shielded, and the entire cable has an end-to-end shield that connects the body of the connectors. This usually helps, sometimes a lot, but connecting the two device grounds can sometimes complete a ground loop that actually makes problems worse.
I've also switched to fiber for all inter-switch connections. This gets rid of the longer copper runs and can be a major win. Unfortunately fiber is rarely if ever supported by end devices even though it's getting fairly cheap.
I'm listening to your SDR now but I don't hear it, probably because it's too late (0108 UTC). In fact you have a remarkably quiet QTH compared to mine (San Diego CA DM12ju). But your description sounds remarkably like one source of QRM I'm working on here: clearly power line related, very unstable lines from a switching power supply, present after dark until the middle of the night, worst on the 49m and 40m bands (receiving with a G5RV).
The switching frequency appears around 20 kHz, i.e., the lines blanket the spectrum every 20 kHz. My hunch is a compact fluorescent bulb, as their switch mode ballasts produce 20 kHz AC where CFLs are most efficient.
As for the noise coming down your power line, yes, this is exactly what I'd expect. Electronic noise below about mid HF is generally conducted out through the power line since the device itself is too small to be an efficient radiator.
Have you offered to buy your neighbor a new, clean light bulb? That's what I plan to do once I find out who to offer it to.