Source

File: wp-includes/http.php

function wp_http_validate_url( $url ) {
	$original_url = $url;
	$url          = wp_kses_bad_protocol( $url, array( 'http', 'https' ) );
	if ( ! $url || strtolower( $url ) !== strtolower( $original_url ) ) {
		return false;
	}

	$parsed_url = parse_url( $url );
	if ( ! $parsed_url || empty( $parsed_url['host'] ) ) {
		return false;
	}

	if ( isset( $parsed_url['user'] ) || isset( $parsed_url['pass'] ) ) {
		return false;
	}

	if ( false !== strpbrk( $parsed_url['host'], ':#?[]' ) ) {
		return false;
	}

	$parsed_home = parse_url( get_option( 'home' ) );

	if ( isset( $parsed_home['host'] ) ) {
		$same_host = strtolower( $parsed_home['host'] ) === strtolower( $parsed_url['host'] );
	} else {
		$same_host = false;
	}

	if ( ! $same_host ) {
		$host = trim( $parsed_url['host'], '.' );
		if ( preg_match( '#^(([1-9]?\d|1\d\d|25[0-5]|2[0-4]\d)\.){3}([1-9]?\d|1\d\d|25[0-5]|2[0-4]\d)$#', $host ) ) {
			$ip = $host;
		} else {
			$ip = gethostbyname( $host );
			if ( $ip === $host ) { // Error condition for gethostbyname().
				return false;
			}
		}
		if ( $ip ) {
			$parts = array_map( 'intval', explode( '.', $ip ) );
			if ( 127 === $parts[0] || 10 === $parts[0] || 0 === $parts[0]
				|| ( 172 === $parts[0] && 16 <= $parts[1] && 31 >= $parts[1] )
				|| ( 192 === $parts[0] && 168 === $parts[1] )
			) {
				// If host appears local, reject unless specifically allowed.
				/**
				 * Check if HTTP request is external or not.
				 *
				 * Allows to change and allow external requests for the HTTP request.
				 *
				 * @since 3.6.0
				 *
				 * @param bool   $external Whether HTTP request is external or not.
				 * @param string $host     Host name of the requested URL.
				 * @param string $url      Requested URL.
				 */
				if ( ! apply_filters( 'http_request_host_is_external', false, $host, $url ) ) {
					return false;
				}
			}
		}
	}

	if ( empty( $parsed_url['port'] ) ) {
		return $url;
	}

	$port = $parsed_url['port'];
	if ( 80 === $port || 443 === $port || 8080 === $port ) {
		return $url;
	}

	if ( $parsed_home && $same_host && isset( $parsed_home['port'] ) && $parsed_home['port'] === $port ) {
		return $url;
	}

	return false;
}