| The Microsoft DNS Server allows any character from the Unicode character set to be used in a domain name . These characters are represented in UTF-8, a particular method of encoding Unicode characters . [11] The vast majority of DNS domain names are represented with a subset of the ASCII character set: alphanumeric characters (i.e., the uppercase and lowercase letters A-Z and the digits 0-9) and the hyphen. In fact, the DNS specification has always permitted any binary value to be used in domain names, though RFC 1035one of the core RFCs that define DNSrecommends that domain names be limited to the characters just listed to avoid problems using the domain names with other protocols. For example, the Internet standards dealing with valid hostname syntax (RFCs 952 and 1123) restrict hostnames to the same ASCII alphanumeric subset. Since this hostname syntax is referenced in Internet standards for electronic mail, domain names used in email addresses must use this same limited syntax. [11] More information about the Unicode Standard is available at http://www.unicode.org. Until Windows 2000, however, networking in Microsoft operating systems was based on NetBIOS, which has more liberal hostname-syntax rules than strict alphanumerics and the hyphen. As we mentioned earlier in this chapter, limited punctuation is allowed in NetBIOS names, as well. Since DNS is the protocol used to name hosts in Windows 2000, sites that upgrade can run into trouble with hosts named according to NetBIOS syntax rules that are no longer valid according to the accepted DNS syntax rules. As a result, Microsoft extended the DNS server and the DNS console user interfaces to support a wider character seteverything in Unicode. The Microsoft DNS Server can check the syntax of domain names in zones for which it is authoritative . This behavior, called name checking , is controlled on the Advanced tab of the server properties window (see Figure 11-9). The default value is to check according to UTF-8 syntax rules, but domain names can also be restricted to alphanumerics and the hyphen or name checking can be disabled completely. A word of caution is in order here: just because the Microsoft DNS Server allows you to use all kinds of crazy characters in domain names, it doesn't mean you should. Many other protocols have certain expectations for syntax of domain names, and violating these expectations can cause applications to exhibit strange and unpredictable behavior. We recommend sticking to the accepted alphanumerics and the hyphen until standards for using additional characters are developed by the IETF (they're working on such standards at the time of this writing). If you absolutely must use other characters, do so only for hostnames used inside your organization, not those visible to the entire Internet. |
