218 This is fine

HTTP response status code 218 This is fine is an unofficial HTTP response that is used specifically by the Apache Web Server. It is used as a catch-all error condition that allows the passage of a message body through Apache when the ProxyErrorOverride setting is enabled.

Usage

When the 218 This is fine status code is received, the client will understand that it is communicating with an Apache Web Server with the ProxyErrorOverride setting enabled. This setting has no effect on information status codes (1XX), successful status codes (2XX), or redirect status codes (3XX). However, by default, the ProxyErrorOverride setting affects all HTTP response status codes that are client errors (4XX) or server errors (5xx).

Normally, these HTTP status codes display an error page. However, with the ProxyErrorOverride on the server-side set, the client has the opportunity to interpret the message and display a standard screen. Specific HTTP response status codes can be configured. Essentially, the setting causes Apache to automatically discard these error messages in favor of a generic HTTP response, or one that is specified by the Apache ErrorDocument directive.

Takeaway

HTTP response status code 218 This is fine is sent by Apache to indicate that there is a HTTP client error or HTTP server error, but this special status code communicating success is used to maintain a standardized look and feel by the client.