Block request that is not coming through the load balancer (Fixed) - nginx

Question

I have a two servers that serve the same static content behind an Nginx load balancer
`
upstream backend {
server xxx.xxx.xxx.xx;
server xxx.xxx.xxx.xx;
}
server {
server_name example.com www.example.com;
location / {
proxy_pass http://backend;
}
listen 443 ssl; # managed by Certbot
ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot
include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}
server {
if ($host = www.example.com) {
return 301 https://$host$request_uri;
} # managed by Certbot
if ($host = example.com) {
return 301 https://$host$request_uri;
} # managed by Certbot
if ($host !~* ^(www\.)?example.com$) {
return 444;
}
listen 80;
server_name example.com www.example.com;
return 404; # managed by Certbot
}
`
So far I have blocked access to the load balancer from the IP and I want to do the same for the servers I want only the requests that goes through the load balancer to be accpted by both backend servers.
servers have same configuration here is how it looks:
`
server {
listen 80 default_server;
listen [::]:80 default_server;
root /var/www/html;
server_name _;
location / {
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
try_files $uri $uri/ =404;
}
access_log /var/log/nginx/access.log;
}
`
FIXED:
I created a special header that the load balancer adds to the request and if it is not included the server will close the connection.