Configure subdomain as "CDN" in NGINX (+ Wordpress & W3 Total Cache configuration) - http://jesus.perezpaz.es/2014/02/configure-subdomain-as-cdn-in-nginx-wordpress-w3-total-cache-configurations

cdn.conf

# -------------------------------------------------
# CDN CONFIGURATION
# -------------------------------------------------

#    Show "Not Found" 404 errors in place of other NGINX errors
  error_page 403 /404;
  error_page 404 /404;
  error_page 405 /404;
  error_page 500 501 502 503 504 /404;
  location  /404 {
    return 301 $parent_site/404/;
  }
#    Prevent access to any files starting with a dot, like .htaccess
#    or text editor temp files
  location ~ /\. { deny all; }

#    Prevent access to any files starting with a $ (usually temp files)
  location ~ ~$ { deny all; }

#    Prevent access to php files
  location ~ \.php$ { deny all; }

#    Keep images and CSS and other static files around in browser cache for as long as possible,
  location ~* .(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|css|rss|atom|js|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf|html|txt|htm)$ {
    expires max; 
    log_not_found off; 
    access_log off;
    add_header Cache-Control public;

    # don't send cookies
    fastcgi_hide_header Set-Cookie;

    # CORS config
      set $cors "true";
 
    # Determine the HTTP request method used
    if ($request_method = 'OPTIONS') {
      set $cors "${cors}options";  
    }
    if ($request_method = 'GET') {
      set $cors "${cors}get";  
    }
    if ($request_method = 'POST') {
      set $cors "${cors}post";
    }
    if ($cors = "true") {
      # Catch all incase there's a request method we're not dealing with properly
      add_header 'Access-Control-Allow-Origin' '*';
    }
     
    if ($cors = "trueget") {
      add_header 'Access-Control-Allow-Origin' '*';
      add_header 'Access-Control-Allow-Credentials' 'true';
      add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
      add_header 'Access-Control-Allow-Headers' 'DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
    }
 
    if ($cors = "trueoptions") {
      add_header 'Access-Control-Allow-Origin' '*';

      # Om nom nom cookies
      add_header 'Access-Control-Allow-Credentials' 'true';
      add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
      
      # Custom headers and headers various browsers *should* be OK with but aren't
      add_header 'Access-Control-Allow-Headers' 'DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
      
      # Tell client that this pre-flight info is valid for 20 days
      add_header 'Access-Control-Max-Age' 1728000;
      add_header 'Content-Type' 'text/plain charset=UTF-8';
      add_header 'Content-Length' 0;
      return 204;
    }
 
    if ($cors = "truepost") {
      add_header 'Access-Control-Allow-Origin' '*';
      add_header 'Access-Control-Allow-Credentials' 'true';
      add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
      add_header 'Access-Control-Allow-Headers' 'DNT,X-Mx-ReqToken,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
 
    }
  }

#    Protects the readme.html|license.txt files from being accessed 
#    (for wordpress installations)
  location ~ /(\.|readme.html|license.txt) { deny all; }

server.conf

server {
    listen   80; ## listen for ipv4
    listen   [::]:80; ## listen for ipv6
    server_name cdn.yourdomain.com;
    access_log /var/log/nginx/cdn.yourdomain.com.access.log;
    error_log  /var/log/nginx/cdn.yourdomain.com.error.log;

    root /var/www/your/web/folder/;
    
    include sites-available/cdn.conf;
}

I followed the instructions but my website returns 404 :(

when restarting nginx got

Restarting nginx: nginx: [emerg] unknown "parent_site" variable

Any idea what this could be?

@cesc1989
Old gist so I hope you already found your solution but you can set the variable
example: set $parent_site '/test'; at the top of your build and call upon this variable later on.
Cheers.