Mastering Development System & Network

What do these sporadic floods of 301 GET log entries every mean?

I have a new CentOS7 web server running virtualmin. It hosts a new WordPress site that just went into production.

Starting mid-day today, on the quarter hour, to the second, I am troubleshooting spikes in server load. At various time, but mostly during these spikes, the access_log file fills with hundreds of entries like this: - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-" - - [25/Mar/2020:22:00:09 -0700] "GET / HTTP/1.1" 301 - "-" "-"

I can’t tell if these entries are the cause or the result of spikes in server load, or something else entirely. The lack of user agent data is also puzzling.

I have eliminated wp-cron.php as the culprit (disabled cron and spike still happens, ran manually, doesn’t spike).

I’ve removed all custom 301 redirects in my .htaccess file, disabled a 301 redirect plugin, still get the entries. The only redirect in place right now are via WordPress General Settings where I have as the WordPress and Site URLs.

What do these entries mean and how do they relate to spikes in server load?

Edit: addendum – if I search these IP addresses in the rest of the access log, they only have these repeated 301 entries (all the ones I have checked are every few seconds or minutes for over 48 hours) and no requests for any other web pages. I’m almost thinking this is an attack? Would that make sense? There are two other sites on this server (same IP, different SNI) that have normal-looking logs without the 301 floods. I spot-checked IP addresses to see if they are on any blacklists and they came back clean.

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