code: func getMainDomain(domain string) string {
  	domain = dns01.UnFqdn(domain)
  
  	split := dns.Split(domain)
  	if strings.HasSuffix(strings.ToLower(domain), "duckdns.org") {
  		if len(split) < 3 {
  			return ""
  		}
  
  		firstSubDomainIndex := split[len(split)-3]
  		return domain[firstSubDomainIndex:]
  	}
  
  	return domain[split[len(split)-1]:]
  }
docs: // DuckDNS only lets you write to your subdomain
  // so it must be in format subdomain.duckdns.org
  // not in format subsubdomain.subdomain.duckdns.org
  // so strip off everything that is not top 3 levels