WordPress at a glance
function is not described

allowed_http_request_hosts() WP 3.6.0

Whitelists allowed redirect hosts for safe HTTP requests as well.

Attached to the 'http_request_host_is_external' filter.

No Hooks.

Return

true/false.

Usage

allowed_http_request_hosts( $is_external, $host );
$is_external(true/false) (required)
$host(string) (required)

Code of allowed http request hosts: wp-includes/http.php WP 5.2.1

<?php
function allowed_http_request_hosts( $is_external, $host ) {
	if ( ! $is_external && wp_validate_redirect( 'http://' . $host ) ) {
		$is_external = true;
	}
	return $is_external;
}