code: func NewDNSProvider() (*DNSProvider, error) {
  	values, err := env.Get("RFC2136_NAMESERVER")
  	if err != nil {
  		return nil, fmt.Errorf("rfc2136: %v", err)
  	}
  
  	config := NewDefaultConfig()
  	config.Nameserver = values["RFC2136_NAMESERVER"]
  	config.TSIGKey = env.GetOrFile("RFC2136_TSIG_KEY")
  	config.TSIGSecret = env.GetOrFile("RFC2136_TSIG_SECRET")
  
  	return NewDNSProviderConfig(config)
  }
docs: // NewDNSProvider returns a DNSProvider instance configured for rfc2136
  // dynamic update. Configured with environment variables:
  // RFC2136_NAMESERVER: Network address in the form "host" or "host:port".
  // RFC2136_TSIG_ALGORITHM: Defaults to hmac-md5.sig-alg.reg.int. (HMAC-MD5).
  // See https://github.com/miekg/dns/blob/master/tsig.go for supported values.
  // RFC2136_TSIG_KEY: Name of the secret key as defined in DNS server configuration.
  // RFC2136_TSIG_SECRET: Secret key payload.
  // RFC2136_PROPAGATION_TIMEOUT: DNS propagation timeout in time.ParseDuration format. (60s)
  // To disable TSIG authentication, leave the RFC2136_TSIG* variables unset.