(no user agent)

Its full user-agent string, exactly as it arrives at the gate:

There is no operator to verify against. This name is what software calls itself when nobody gave it one.

(no user agent) does not read robots.txt, so a polite sign is wasted on it. Refuse it at your web server or firewall instead. User agents are easy to fake, so pair this with rate limiting.

# robots.txt will not stop (no user agent). Block it at the server.
# nginx
if ($http_user_agent ~* "^$") {
    return 403;
}

The same thing on Apache:

# Apache (.htaccess)
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} ^$ [NC]
RewriteRule .* - [F,L]

Scripts and scanners run from wherever their owners rent a server. These are the networks behind the visits on file:

Networks and countries come from the visitor's IP address, looked up in a local copy of the DB-IP database. The addresses themselves are never stored.