Nginx Reverse Proxying Multiple Domains Using map Module

map_hash_bucket_size 128;
map $http_host $backend_servers {
hostnames;
    default                         www.example.com;
    frontend.example2.com           backend.example2.com
    frontend.example3.com           backend.example3.com
    www.example.org                 backend.example.org
}
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
server {
    location / {
        proxy_pass  http://$backend_servers
    }
}