public function prohibitUrlByConfig($url, IOInterface $io = null)
    {
        // Return right away if the URL is malformed or custom (see issue #5173)
        if (false === filter_var($url, FILTER_VALIDATE_URL)) {
            return;
        }

        // Extract scheme and throw exception on known insecure protocols
        $scheme = parse_url($url, PHP_URL_SCHEME);
        if (in_array($scheme, array('http', 'git', 'ftp', 'svn'))) {
            if ($this->get('secure-http')) {
                throw new TransportException("Your configuration does not allow connections to $url. See https://getcomposer.org/doc/06-config.md#secure-http for details.");
            } elseif ($io) {
                $host = parse_url($url, PHP_URL_HOST);
                if (!isset($this->warnedHosts[$host])) {
                    $io->writeError("<warning>Warning: Accessing $host over $scheme which is an insecure protocol.</warning>");
                }
                $this->warnedHosts[$host] = true;
            }
        }
    }