I perma-ban any /16 that hits fail2ban 100+ times. That cuts down dramatically on the attacks from the usual suspects.
Manual reviewing like this also helped me find a bunch of organisations that just probe the entire IPv4 range on a regular basis, trying to map it for 'security' purposes. Fuck them, blocked!
P.S. I wholeheartedly support your choice of blocking for your reasons.
Yep, #1 source of junk traffic, in my experience. I set those prefixes go right into nullroute on every server I set up:
https://raw.githubusercontent.com/UninvitedActivity/Uninvite...
#2 are IP ranges of Azure, DO, OVH, vultr, etc... A bit harder to block those outright.
On the plus side, it does not waste CPU cycles used to block unwanted IPv4 traffic.
But not that much, unfortunately. Those same "cYbeRseCUrITy" orgs also ingest SSL transparency logs, resolve A and AAAA for all the names in the cert, then turn around and start scanning those addresses.
In my experience, it only takes a few hours from getting an SSL certificate to junk traffic to start rolling in, even for IPv6-only servers.
Small percentage of that could be attributed directly, based on "BitSightBot", "CMS-Checker", "Netcraft Web Server Survey", "Cortex-Xpans" and similar keywords in user-agent and referer headers. And purely based on timing, there's a lot more of that stuff where scanners try and blend in.
Yes. Fucking censys and internet-measurement and the predatory "opt-out" of scans. What about opting-in to scan my website? Fuck you, i'm blocking you forever
Nowadays, wireguard would probably be a better choice.
(both of above of course assume one is to do a sensible thing and add "perma-bans" a bit lower in firewall rules, below "established" and "port-knock")