Given the present non-existence of any perfect agreement on where applications should store their cached information, I propose a very simple convention that will at least allow such information to be identified effectively. Regardless of where the application decides to (or is configured to) place its cache directory, it should place within this directory a file named:

CACHEDIR.TAG

This file must be an ordinary file, not for example a symbolic link. Additionally, the first 43 octets of this file must consist of the following ASCII header string:

Signature: 8a477f597d28d172789f06886806bc55

Case is important in the header signature, there can be no whitespace or other characters in the file before the 'S', and there is exactly one space character (ASCII code 32 decimal) after the colon. The header string does not have to be followed by an LF or CR/LF in the file in order for the file to be recognized as a valid cache directory tag. The hex value in the signature happens to be the MD5 hash of the string ".IsCacheDirectory". This signature header is required to avoid the chance of an unrelated file named CACHEDIR.TAG being mistakenly interpreted as a cache directory tag by data management utilities, and (for example) causing valuable data not to be backed up.

The content of the remainder of the tag file is currently unspecified, except that it should be a plain text file in UTF-8 encoding, and any line beginning with a hash character ('#') should be treated as a comment and ignored by any software that reads the file.

We will henceforth refer to a file named as specified above, and having the required signature at the beginning of its content, as a cache directory tag.

For the benefit of anyone who happens to find and look at a cache directory tag directly, it is recommended that applications include in the file a comment referring back to this specification. For example:

Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by (application name).
# For information about cache directory tags, see:
#   http://www.brynosaurus.com/cachedir/