Another observation. While looking at one particular group of IP addresses accessing us via HTTP/1.0 (very few in number), they were hitting us in bursts of six or seven accesses per second, and each one was three different URLs. Two of the URLs were the home pages of the site (
www.xyz.com and forums.xxx.com) while the third was for a different thread each time.
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I don't know if any of the server-side solutions like Anubis or any of the XF add-ons would rate limit accesses (or at least flag them as suspicious), but it's pattern I see with others--the hits can be for a few different pages in the forum (threads, home page, etc.) all during a very short period of time, like a matter of seconds.
Cloudflare does have very basic rate limiting (on the free account level) but it won't catch this kind of logic, such as, accessing different resources on a site within a very short burst of time.
I think my rules may have caught these specific IPs with a managed challenge, but it's another pattern to watch for with other bots. No human can visit three different page views on a forum within one second's time.