This adds the ask sub-directive to tls that defines the URL of a backend HTTP service to be queried during the TLS handshake to determine if an on-demand TLS certificate should be acquired for incoming hostnames. When the ask sub-directive is defined, Caddy will query the URL for permission to acquire a cert by making a HTTP GET request to the URL including the requested domain in the query string. If the backend service returns a 2xx response Caddy will acquire a cert. Any other response code (including 3xx redirects) are be considered a rejection and the certificate will not be acquired.
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