code: func NewDNSProviderConfig(config *Config) (*DNSProvider, error) {
  	if config.SecretKey == "" || config.AccessKey == "" || config.DoServiceCode == "" {
  		return nil, fmt.Errorf("iij: credentials missing")
  	}
  
  	return &DNSProvider{
  		api:    doapi.NewAPI(config.AccessKey, config.SecretKey),
  		config: config,
  	}, nil
  }
docs: // NewDNSProviderConfig takes a given config
  // and returns a custom configured DNSProvider instance