Name Formatting

Hostname sizes and character sets can also cause problems. Originally, hostnames were defined as 64 characters consisting of letters, numbers, and a few symbols. Some registration systems have attempted to extend or redefine the hostname format:

• Although fully qualified hostnames, with domain names, could be up to 64 characters, many operating systems in the 1980s and 1990s required hostname elements to be no longer than 8 characters. This included SunOS and HP-UX. Although recent versions of these operating systems do not have this limitation, hostnames are still recommended to be no longer than eight characters for backwards compatibility.
• Some international providers have opted to extend the character set to include nonprintable ASCII or multibyte characters.
• Many developers have chosen to support hostname sizes that are either smaller than 64 characters or much larger.

Applications that expect fixed hostname lengths or specific character sets may be vulnerable to memory overflows or parsing issues. For example, in April 2005, the security vendor iDefense announced a vulnerability in the Internet Explorer (IE) Web browser. IE would generate a buffer overflow if the URL's hostname was longer than 256 characters.