What does HTTP status 444 (Bo Response) mean?

HTTP_STATUS_BO_RESPONSE

This is an NGINX-specific behavior, not a registered HTTP response. It is often configured deliberately to drop unwanted traffic, bots, malformed requests, or requests matching a security rule.

From the client side it may look like an empty response or connection reset. On the server, inspect NGINX rewrite, map, and access-control rules that can return 444.

NGINX uses 444 internally to terminate the client connection without a response status line, so a remote HTTP client usually observes a closed connection rather than an actual 444 response. The number is primarily useful in NGINX access logs for rules that deliberately drop traffic without returning an HTTP message.

NGINX return directive