A site I host ended up getting this a couple of months ago. I was at first adding the IP addresses the attack was coming from to the iptables rules on the server but this ended up getting a little tedious. I ended up upping my Linode plan until I could figure out what was going on. That day, I wrote a .htaccess file which solved the issue for me.
RewriteEngine OnRewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
RewriteRule ^xmlrpc\.php$ "http\:\/\/0\.0\.0\.0\/" [R=301,L]
This site is rather popular and after adding the .htaccess file into the site-owners wordpress folder I saw the issue was resolved. They were hitting the webserver for a wee while and just getting 403's for everything.
