code: function () {
      return function (req, res, next) {
        var apphost = config.url.host,
            outputHost = undefsafe(config, 'security.preview'),
            host = req.header('Host', ''),
            offset = host.indexOf(apphost);
  
        if (host === outputHost) {
          offset = host.indexOf(outputHost);
        }
  
        if (offset > 0) {
          // Slice the host from the subdomain and subtract 1 for
          // trailing . on the subdomain.
          req.subdomain = host.slice(0, offset - 1);
        }
        next();
      };
    }
docs: Checks for a subdomain in the current url, if found it sets the req.subdomain property. This supports existing behaviour that allows subdomains to load custom config files.