limit_req per subnet?

[c0nw0nk]

proxy_cache / fastcgi_cache the pages output will help. Flood all you want
Nginx handles flooding and lots of connections fine your back end is your
weakness / bottleneck that is allowing them to be successful in effecting
your service.

You could also use the secure_link module to help on your index.php or .html
what ever it is you have going on that generates the link they are
attacking, You can generate a unique hash that expires for that IP only.
There are allot of solutions.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,271483,271537#msg-271537