Cross-platform file export through means like public FTP directories has long
been an important mechanism of Internet information sharing. More recently,
the growth of the Internet has been driven by technologies such as the
World Wide Web, which give it (among other things) the aspect of a single
huge distributed file store.
In such an environment, it is very desirable that files should
generally present themselves as self-describing objects from which an
application launcher or navigation tool can readily deduce both their
uses and at least some of the semantics of their contents. An
effective set of such conventions can enable tools such as Web
browsers to inform users according to such deductions, and to dispatch
to appropriate sub-interpreters and user agents on the file object's
The Internet's major operating systems have traditionally shared two
means of ad-hoc semantic typing for files. The better known is the
quasi-systematic use of file name "extensions" to carry semantic
information (most Internet habitues, for example would feel
justifiably surprised if a file with extension ".gif" were not to
contain picture data in the well-known Graphic Interchange Format).
Standardization of such file name extensions would be a desirable goal,
but is beyond the scope of this RFC.
The other, prevalent especially under Unix, is the quasi-systematic
use of "magic numbers" in binary-format files. Many Unix binary
formats have an initial segment of fixed pattern intended to uniquely
identify the file type for the purposes of loaders, linkers, graphics
viewers, and other tools.
The purpose of this RFC is threefold:
(1) To enunciate guidelines and design principles appropriate when choosing
magic numbers for a new file type.
(2) To advertise a central Internet registry of "magic numbers" (initially
based on Unix prior art but intended to be platform-independent), with
procedures for checking candidate segment patterns against the registry
and for claiming new patterns.
(3) To define a new standards-compliance category of MAGIC programs (MAGIC =
"Magic Against Galloping Internet Complexity") which are required to
be aware, in certain well-defined ways, of registered magic numbers.
The goal of encouraging MAGIC compliance is to formalize existing practice in
a way which creates a uniform semantic file type system for the Internet.
HOW TO PICK MAGIC NUMBERS
Magic numbers used to identify the type of data stream or file. Any given
file has only one primary magic block.
Magic numbers used to identify characteristics of the data stream
of file such as version, sub-type, etc. A file may have more than
one secondary magic block.
The first and third are required for compatibility with both
traditional and POSIX versions of file(1). To be eligible for
inclusion in the Registry, a new file format MUST have these properties:
1. The primary and all secondary magic blocks must be located at predictable
constant offsets from the beginning of the data stream. (The purpose of
this requirement is to permit tests for magic to be expressed in the
simple, rapidly-interpretable notation of file(1)).
2. The primary and all secondary magic blocks must be located within 512
bytes of the beginning of the file or stream. (The purpose of this
requirement is to limit the amount of data which MAGIC-compliant programs
must read to determine a file's type.)
3. Both primary and secondary magic must consist of single, contiguous
octet-aligned blocks of octets.
1. Both primary and secondary magic blocks should be limited to 8 bytes each.
2. Primary magic blocks should not contain NULs.
3. Magic should be not composed solely of characters in ASCII, ISO Latin-1,
or other standard character sets unless the file type is a text format
with characters limited to that set.
4. For binary files, magic should be a random mix of text and non-text
characters, preferably including characters in each of these ranges:
ASCII, Latin-1 extensions to ASCII, and everything else.
5. The primary magic should be constant for a particular application or data
type. Only secondary magic should change depending on the version of
an application or data type.
Each registry entry includes the following attributes:
This is a pattern, expressed in a recipe notation compatible with
that of Unix file(1), describing the primary and/or secondary magic
numbers and their offsets.
A Web page or email contact for the person(s) responsible for the
One of: Experimental, Production, or Obsolete.
A code of at most 8 ASCII characters intended for use as a type
representation icon in text-only navigation tools.
One or more 32x32 PNG icons intended for use as a type representation
icon in graphic navigation tools.
One of a set of semantic-class codes described below.
Text description of the format (in the style of Unix file(1)).
Zero or more quadruples consisting of
(a) A name
(b) A resource type
Resource types may include Documentation, Viewer, Library,
Browser, Toolkit, Editor.
(c) An URL
Date of last update.
(9) Entered By
Email address of person responsible for entry.
Registry maintainer's comments.
The purpose of MAGIC-compliance is to enable programs such as Web
browsers, Macintosh-style program launchers, etc. to benefit from an
Internet-standard file semantic type system.
A program may be MAGIC-compliant in one of two ways. Either the
program itself contains information equivalent to (a subset of) the
magic-number registry, or it is able to reference an external resource
(such as the Unix /usr/lib/magic file or the master Registry itself)
which is presumed to be a snapshot of appropriate portions of the
In the former case, a program may describe itself as being conformant
with the particular release version of the registry (i.e "MAGIC 1.00
compliant") abstracted in its code. In the latter case, it may simply
describe itself as MAGIC-compliant.
Certain requirements are placed on MAGIC-compliant programs:
1. A MAGIC-compliant program MUST be able to recognize (automatically
or on user query) every file type marked Production in its registry
version. It SHOULD be able to recognize every Experimental type.
It MAY recognize every Obsolete type.
2. A MAGIC-compliant program MUST allow users to see or request
the Description field of a file's type, once it has been recognized.
3. A MAGIC-compliant program's interface SHOULD present for each recognized
file at least one of the following:
(a) The type's Code field (recommended for text-only interfaces)
(b) Text or icon representing the Class field
(c) Icon from the Icons list (recommended for graphical interfaces).
A documented C library implementing an API for querying the /usr/lib/magic
file will be made available at the Registry site. This code is freely
reusable, and developers are encouraged to incorporate it as a way of
making their programs minimally MAGIC-compliant.
Freely redistributable C sources of the MAGIC-compliant Unix utility file(1)
will be made available along with the library.
-- <a href="http://www.ccil.org/~esr/home.html">Eric S. Raymond</a>