[nginx] limit requests from searchengine crawlers/bots to 1r/m (prevent DDOS)

gistfile1.txt

 http {

     map $http_user_agent $limit_bots {
         default '';
         ~*(bing|yandex|msnbot) $binary_remote_addr;
     }

     limit_req_zone $limit_bots zone=bots:10m rate=1r/m;

     server {
         location / {
             limit_req zone=bots burst=5 nodelay;
         }
     }
 }

Clever use of mapping plus the $binary_remote_addr variable to apply the limit to matching user agents, but let everyone else through.

What would happen if I used a fixed string instead of $binary_remote_addr?

My understanding is that I would put all "bots" into one key/bucket and disallow them all.
Each bot could have many IP adresses and could rotate them so $binary_remote_addr will not with that.

map $http_user_agent $limit_bots {
    default '';
    ~UptimeRobot ''; ## allow
    ~*\(.*bot.*\) 'mybotmarker';
}