Description of Valid Labels for Domain Name System

ID: Q241980

This article discusses a Beta release of a Microsoft product. The information in this article is provided as-is and is subject to change without notice.

No formal product support is available from Microsoft for this Beta product. For information about obtaining support for a Beta release, please see the documentation included with the Beta product files, or check the Web location from which you downloaded the release.

The information in this article applies to:

Microsoft Windows NT Server version 4.0
Microsoft Windows NT Server, Enterprise Edition version 4.0
Microsoft Windows 2000 Server
Microsoft Windows 2000 Advanced Server

SUMMARY

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

MORE INFORMATION

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.

REFERENCES

For more information about name syntax for DNS, please see RFC 2181. RFCs may be obtained via the Internet as follows: Paper copies of all RFCs are available from the NIC, either individually or on a subscription basis (for more information, contact NIC@NIC.DDN.MIL). Online copies are available via FTP or Kermit from NIC.DDN.MIL as rfc/rfc2181.txt or rfc/rfc2181.PS (2181 is the RFC number without leading zeros). Additionally, RFCs may be requested through electronic mail from the automated NIC mail server by sending a message to SERVICE@NIC.DDN.MIL with a subject line of "rfc 2181" for text versions, or a subject line of "rfc 2181.PS" for PostScript versions. To obtain the RFC index, the subject line of your message should read "rfc index".

Additional query words:

Keywords : kbnetwork kbtool
Version : WINDOWS:2000; winnt:4.0
Platform : WINDOWS winnt
Issue type : kbinfo