How to add captcha to the site from frequent requests from IP?

Question

Computer networks

Good time.
it became necessary to install protection against spam bots on the site, but the protection should not respond to comments / registration / orders, etc., but to frequent requests to pages (essentially, the essence is like antispam in search engines, Yandex, etc.) - if from one IP many requests are received per second, the user must see the captcha - and until he enters it, he does not get to the site.
the meaning is that on the site (portal) the supporters of the attack, which overload the server, have become more frequent.
maybe someone came across and can advise ways to solve or direction where to dig.
20 minutes of googling gave only information about protection against spat attacks in comments, registration and the like.