Description of Valid Labels for Domain Name System

This article describes what constitutes a valid label for Domain Name System (DNS) resource records, including the associated restrictions for DNS labels.

Request For Comment (RFC) 1035 and 1123 defined the legal syntax of an Internet host name as consisting of the following:

• The letters a-z and A-Z.
• The numbers 0-9.
• The hyphen (-) character.

Host names may begin with a letter or a number and cannot be longer than 63 characters.

However, RFC 2181 (which updates RFC 1035 and 1123), states that any binary string may be used in the label of any resource record. The only restriction is the length of the label (63 characters for the label, and 255 characters for the full domain name). It also states that the DNS protocols must not place any restrictions (other than length) on the labels used for resource records. A DNS server must not refuse to service a zone because of labels that may be considered questionable or invalid.

Client programs that use the data in the DNS may impose whatever label restrictions are deemed necessary or appropriate. However, the client program (not the DNS server program) is responsible for ensuring that the data is valid. For example, older resolution tools and e-mail protocols may require strict adherence to RFC 1123. To ensure compatibility with these programs, Microsoft recommends that the DNS administrator use labels that conform to RFC 1123 unless a client program requires something different.