func (r *Resolver) goLookupPTR(ctx context.Context, addr string, order hostLookupOrder, conf *dnsConfig) ([]string, error) {
	if order == hostLookupFiles || order == hostLookupFilesDNS {
		names := lookupStaticAddr(addr)
		if len(names) > 0 {
			return names, nil
		}

		if order == hostLookupFiles {
			return nil, newDNSError(errNoSuchHost, addr, "")
		}
	}

	arpa, err := reverseaddr(addr)