code: static URL getStorageConfigURL() throws ConfigurationException
      {
          String configUrl = System.getProperty("cassandra.config");
          if (configUrl == null)
              configUrl = DEFAULT_CONFIGURATION;
  
          URL url;
          try
          {
              url = new URL(configUrl);
              url.openStream().close(); // catches well-formed but bogus URLs
          }
          catch (Exception e)
          {
              ClassLoader loader = DatabaseDescriptor.class.getClassLoader();
              url = loader.getResource(configUrl);
              if (url == null)
              {
                  String required = "file:" + File.separator + File.separator;
                  if (!configUrl.startsWith(required))
                      throw new ConfigurationException("Expecting URI in variable: [cassandra.config].  Please prefix the file with " + required + File.separator +
                              " for local files or " + required + "<server>" + File.separator + " for remote files. Aborting. If you are executing this from an external tool, it needs to set Config.setClientMode(true) to avoid loading configuration.");
                  throw new ConfigurationException("Cannot locate " + configUrl + ".  If this is a local file, please confirm you've provided " + required + File.separator + " as a URI prefix.");
              }
          }
  
          return url;
      }
docs: Inspect the classpath to find storage configuration file