code: func (p *passthrough) addFqdnToList(ctx context.Context, r string) error {
  	rgx, err := regexp.Compile(r)
  	if err == nil {
  		p.mutex.Lock()
  		p.proxypassthrough = append(p.proxypassthrough, rgx)
  		p.mutex.Unlock()
  	}
  	return err
  }
docs: // addFqdnToList add all the passthrough fqdn in a list