\" $NetBSD: magic.5,v 1.25 2023/08/18 19:00:10 christos Exp $

\" $NetBSD: magic.5,v 1.25 2023/08/18 19:00:10 christos Exp $
\"
\" $File: magic.man,v 1.103 2023/07/20 14:32:07 christos Exp $
Dd Arpil 18, 2023
Dt MAGIC 5
Os
\" install as magic.4 on USG, magic.5 on V7, Berkeley and Linux systems.
Sh NAME
Nm magic
Nd file command's magic pattern file
Sh DESCRIPTION
This manual page documents the format of magic files as
used by the
Xr file 1
command, version 5.45.
The
Xr file 1
command identifies the type of a file using,
among other tests,
a test for whether the file contains certain
Dq "magic patterns" .
The database of these
Dq "magic patterns"
is usually located in a binary file in
Pa /usr/share/misc/magic.mgc
or a directory of source text magic pattern fragment files in
Pa /usr/share/misc/magic .
The database specifies what patterns are to be tested for, what message or
MIME type to print if a particular pattern is found,
and additional information to extract from the file.
Pp
The format of the source fragment files that are used to build this database
is as follows:
Each line of a fragment file specifies a test to be performed.
A test compares the data starting at a particular offset
in the file with a byte value, a string or a numeric value.
If the test succeeds, a message is printed.
The line consists of the following fields:
Bl -tag -width ".Dv message"
It Dv offset
A number specifying the offset (in bytes) into the file of the data
which is to be tested.
This offset can be a negative number if it is:
Bl -bullet -compact
It
The first direct offset of the magic entry (at continuation level 0),
in which case it is interpreted an offset from end end of the file
going backwards.
This works only when a file descriptor to the file is available and it
is a regular file.
It
A continuation offset relative to the end of the last up-level field
Dv ( \*[Am] ) .
El
It Dv type
The type of the data to be tested.
The possible values are:
Bl -tag -width ".Dv lestring16"
It Dv byte
A one-byte value.
It Dv short
A two-byte value in this machine's native byte order.
It Dv long
A four-byte value in this machine's native byte order.
It Dv quad
An eight-byte value in this machine's native byte order.
It Dv float
A 32-bit single precision IEEE floating point number in this machine's native byte order.
It Dv double
A 64-bit double precision IEEE floating point number in this machine's native byte order.
It Dv string
A string of bytes.
The string type specification can be optionally followed by a /<width>
option and optionally followed by a set of flags /[bCcftTtWw]*.
The width limits the number of characters to be copied.
Zero means all characters.
The following flags are supported:
Bl -tag -width B -compact -offset XXXX
It b
Force binary file test.
It C
Use upper case insensitive matching: upper case
characters in the magic match both lower and upper case characters in the
target, whereas lower case characters in the magic only match upper case
characters in the target.
It c
Use lower case insensitive matching: lower case
characters in the magic match both lower and upper case characters in the
target, whereas upper case characters in the magic only match upper case
characters in the target.
To do a complete case insensitive match, specify both
Dq c
and
Dq C .
It f
Require that the matched string is a full word, not a partial word match.
It T
Trim the string, i.e. leading and trailing whitespace
It t
Force text file test.
It W
Compact whitespace in the target, which must
contain at least one whitespace character.
If the magic has
Dv n
consecutive blanks, the target needs at least
Dv n
consecutive blanks to match.
It w
Treat every blank in the magic as an optional blank.
is deleted before the string is printed.
El
It Dv pstring
A Pascal-style string where the first byte/short/int is interpreted as the
unsigned length.
The length defaults to byte and can be specified as a modifier.
The following modifiers are supported:
Bl -tag -width B -compact -offset XXXX
It B
A byte length (default).
It H
A 2 byte big endian length.
It h
A 2 byte little endian length.
It L
A 4 byte big endian length.
It l
A 4 byte little endian length.
It J
The length includes itself in its count.
El
The string is not NUL terminated.
Dq J
is used rather than the more
valuable
Dq I
because this type of length is a feature of the JPEG
format.
It Dv date
A four-byte value interpreted as a UNIX date.
It Dv qdate
An eight-byte value interpreted as a UNIX date.
It Dv ldate
A four-byte value interpreted as a UNIX-style date, but interpreted as
local time rather than UTC.
It Dv qldate
An eight-byte value interpreted as a UNIX-style date, but interpreted as
local time rather than UTC.
It Dv qwdate
An eight-byte value interpreted as a Windows-style date.
It Dv beid3
A 32-bit ID3 length in big-endian byte order.
It Dv beshort
A two-byte value in big-endian byte order.
It Dv belong
A four-byte value in big-endian byte order.
It Dv bequad
An eight-byte value in big-endian byte order.
It Dv befloat
A 32-bit single precision IEEE floating point number in big-endian byte order.
It Dv bedouble
A 64-bit double precision IEEE floating point number in big-endian byte order.
It Dv bedate
A four-byte value in big-endian byte order,
interpreted as a Unix date.
It Dv beqdate
An eight-byte value in big-endian byte order,
interpreted as a Unix date.
It Dv beldate
A four-byte value in big-endian byte order,
interpreted as a UNIX-style date, but interpreted as local time rather
than UTC.
It Dv beqldate
An eight-byte value in big-endian byte order,
interpreted as a UNIX-style date, but interpreted as local time rather
than UTC.
It Dv beqwdate
An eight-byte value in big-endian byte order,
interpreted as a Windows-style date.
It Dv bestring16
A two-byte unicode (UCS16) string in big-endian byte order.
It Dv leid3
A 32-bit ID3 length in little-endian byte order.
It Dv leshort
A two-byte value in little-endian byte order.
It Dv lelong
A four-byte value in little-endian byte order.
It Dv lequad
An eight-byte value in little-endian byte order.
It Dv lefloat
A 32-bit single precision IEEE floating point number in little-endian byte order.
It Dv ledouble
A 64-bit double precision IEEE floating point number in little-endian byte order.
It Dv ledate
A four-byte value in little-endian byte order,
interpreted as a UNIX date.
It Dv leqdate
An eight-byte value in little-endian byte order,
interpreted as a UNIX date.
It Dv leldate
A four-byte value in little-endian byte order,
interpreted as a UNIX-style date, but interpreted as local time rather
than UTC.
It Dv leqldate
An eight-byte value in little-endian byte order,
interpreted as a UNIX-style date, but interpreted as local time rather
than UTC.
It Dv leqwdate
An eight-byte value in little-endian byte order,
interpreted as a Windows-style date.
It Dv lestring16
A two-byte unicode (UCS16) string in little-endian byte order.
It Dv melong
A four-byte value in middle-endian (PDP-11) byte order.
It Dv medate
A four-byte value in middle-endian (PDP-11) byte order,
interpreted as a UNIX date.
It Dv meldate
A four-byte value in middle-endian (PDP-11) byte order,
interpreted as a UNIX-style date, but interpreted as local time rather
than UTC.
It Dv indirect
Starting at the given offset, consult the magic database again.
The offset of the
Dv indirect
magic is by default absolute in the file, but one can specify
Dv /r
to indicate that the offset is relative from the beginning of the entry.
It Dv name
Define a
Dq named
magic instance that can be called from another
Dv use
magic entry, like a subroutine call.
Named instance direct magic offsets are relative to the offset of the
previous matched entry, but indirect offsets are relative to the beginning
of the file as usual.
Named magic entries always match.
It Dv use
Recursively call the named magic starting from the current offset.
If the name of the referenced begins with a
Dv ^
then the endianness of the magic is switched; if the magic mentioned
Dv leshort
for example,
it is treated as
Dv beshort
and vice versa.
This is useful to avoid duplicating the rules for different endianness.
It Dv regex
A regular expression match in extended POSIX regular expression syntax
(like egrep).
Regular expressions can take exponential time to process, and their
performance is hard to predict, so their use is discouraged.
When used in production environments, their performance
should be carefully checked.
The size of the string to search should also be limited by specifying
Dv /<length> ,
to avoid performance issues scanning long files.
The type specification can also be optionally followed by
Dv /[c][s][l] .
The
Dq c
flag makes the match case insensitive, while the
Dq s
flag update the offset to the start offset of the match, rather than the end.
The
Dq l
modifier, changes the limit of length to mean number of lines instead of a
byte count.
Lines are delimited by the platforms native line delimiter.
When a line count is specified, an implicit byte count also computed assuming
each line is 80 characters long.
If neither a byte or line count is specified, the search is limited automatically
to 8KiB.
Dv ^
and
Dv $
match the beginning and end of individual lines, respectively,
not beginning and end of file.
It Dv search
A literal string search starting at the given offset.
The same modifier flags can be used as for string patterns.
The search expression must contain the range in the form
Dv /number,
that is the number of positions at which the match will be
attempted, starting from the start offset.
This is suitable for
searching larger binary expressions with variable offsets, using
Dv \e
escapes for special characters.
The order of modifier and number is not relevant.
It Dv default
This is intended to be used with the test
Em x
(which is always true) and it has no type.
It matches when no other test at that continuation level has matched before.
Clearing that matched tests for a continuation level, can be done using the
Dv clear
test.
It Dv clear
This test is always true and clears the match flag for that continuation level.
It is intended to be used with the
Dv default
test.
It Dv der
Parse the file as a DER Certificate file.
The test field is used as a der type that needs to be matched.
The DER types are:
Dv eoc ,
Dv bool ,
Dv int ,
Dv bit_str ,
Dv octet_str ,
Dv null ,
Dv obj_id ,
Dv obj_desc ,
Dv ext ,
Dv real ,
Dv enum ,
Dv embed ,
Dv utf8_str ,
Dv rel_oid ,
Dv time ,
Dv res2 ,
Dv seq ,
Dv set ,
Dv num_str ,
Dv prt_str ,
Dv t61_str ,
Dv vid_str ,
Dv ia5_str ,
Dv utc_time ,
Dv gen_time ,
Dv gr_str ,
Dv vis_str ,
Dv gen_str ,
Dv univ_str ,
Dv char_str ,
Dv bmp_str ,
Dv date ,
Dv tod ,
Dv datetime ,
Dv duration ,
Dv oid-iri ,
Dv rel-oid-iri .
These types can be followed by an optional numeric size, which indicates
the field width in bytes.
It Dv guid
A Globally Unique Identifier, parsed and printed as
XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX.
It's format is a string.
It Dv offset
This is a quad value indicating the current offset of the file.
It can be used to determine the size of the file or the magic buffer.
For example the magic entries:
Bd -literal -offset indent
-0 offset x this file is %lld bytes
-0 offset <=100 must be more than 100 \e
bytes and is only %lld
Ed
It Dv octal
A string representing an octal number.
El
El
Pp
For compatibility with the Single
Ux
Standard, the type specifiers
Dv dC
and
Dv d1
are equivalent to
Dv byte ,
the type specifiers
Dv uC
and
Dv u1
are equivalent to
Dv ubyte ,
the type specifiers
Dv dS
and
Dv d2
are equivalent to
Dv short ,
the type specifiers
Dv uS
and
Dv u2
are equivalent to
Dv ushort ,
the type specifiers
Dv dI ,
Dv dL ,
and
Dv d4
are equivalent to
Dv long ,
the type specifiers
Dv uI ,
Dv uL ,
and
Dv u4
are equivalent to
Dv ulong ,
the type specifier
Dv d8
is equivalent to
Dv quad ,
the type specifier
Dv u8
is equivalent to
Dv uquad ,
and the type specifier
Dv s
is equivalent to
Dv string .
In addition, the type specifier
Dv dQ
is equivalent to
Dv quad
and the type specifier
Dv uQ
is equivalent to
Dv uquad .
Pp
Each top-level magic pattern (see below for an explanation of levels)
is classified as text or binary according to the types used.
Types
Dq regex
and
Dq search
are classified as text tests, unless non-printable characters are used
in the pattern.
All other tests are classified as binary.
A top-level
pattern is considered to be a test text when all its patterns are text
patterns; otherwise, it is considered to be a binary pattern.
When
matching a file, binary patterns are tried first; if no match is
found, and the file looks like text, then its encoding is determined
and the text patterns are tried.
Pp
The numeric types may optionally be followed by
Dv \*[Am]
and a numeric value,
to specify that the value is to be AND'ed with the
numeric value before any comparisons are done.
Prepending a
Dv u
to the type indicates that ordered comparisons should be unsigned.
It Dv test
The value to be compared with the value from the file.
If the type is
numeric, this value
is specified in C form; if it is a string, it is specified as a C string
with the usual escapes permitted (e.g. \en for new-line).
Pp
Numeric values
may be preceded by a character indicating the operation to be performed.
It may be
Dv = ,
to specify that the value from the file must equal the specified value,
Dv \*[Lt] ,
to specify that the value from the file must be less than the specified
value,
Dv \*[Gt] ,
to specify that the value from the file must be greater than the specified
value,
Dv \*[Am] ,
to specify that the value from the file must have set all of the bits
that are set in the specified value,
Dv ^ ,
to specify that the value from the file must have clear any of the bits
that are set in the specified value, or
Dv ~ ,
the value specified after is negated before tested.
Dv x ,
to specify that any value will match.
If the character is omitted, it is assumed to be
Dv = .
Operators
Dv \*[Am] ,
Dv ^ ,
and
Dv ~
don't work with floats and doubles.
The operator
Dv !\&
specifies that the line matches if the test does
Em not
succeed.
Pp
Numeric values are specified in C form; e.g.
Dv 13
is decimal,
Dv 013
is octal, and
Dv 0x13
is hexadecimal.
Pp
Numeric operations are not performed on date types, instead the numeric
value is interpreted as an offset.
Pp
For string values, the string from the
file must match the specified string.
The operators
Dv = ,
Dv \*[Lt]
and
Dv \*[Gt]
(but not
Dv \*[Am] )
can be applied to strings.
The length used for matching is that of the string argument
in the magic file.
This means that a line can match any non-empty string (usually used to
then print the string), with
Em \*[Gt]\e0
(because all non-empty strings are greater than the empty string).
Pp
Dates are treated as numerical values in the respective internal
representation.
Pp
The special test
Em x
always evaluates to true.
It Dv message
The message to be printed if the comparison succeeds.
If the string contains a
Xr printf 3
format specification, the value from the file (with any specified masking
performed) is printed using the message as the format string.
If the string begins with
Dq \eb ,
the message printed is the remainder of the string with no whitespace
added before it: multiple matches are normally separated by a single
space.
El
Pp
An APPLE 4+4 character APPLE creator and type can be specified as:
Bd -literal -offset indent
!:apple CREATYPE
Ed
Pp
A slash-separated list of commonly found filename extensions can be specified
as:
Bd -literal -offset indent
!:ext ext[/ext...]
Ed
Pp
i.e. the literal string
Dq !:ext
followed by a slash-separated list of commonly found extensions; for example
for JPEG images:
Bd -literal -offset indent
!:ext jpeg/jpg/jpe/jfif
Ed
Pp
A MIME type is given on a separate line, which must be the next
non-blank or comment line after the magic line that identifies the
file type, and has the following format:
Bd -literal -offset indent
!:mime MIMETYPE
Ed
Pp
i.e. the literal string
Dq !:mime
followed by the MIME type.
Pp
An optional strength can be supplied on a separate line which refers to
the current magic description using the following format:
Bd -literal -offset indent
!:strength OP VALUE
Ed
Pp
The operand
Dv OP
can be:
Dv + ,
Dv - ,
Dv * ,
or
Dv /
and
Dv VALUE
is a constant between 0 and 255.
This constant is applied using the specified operand
to the currently computed default magic strength.
Pp
Some file formats contain additional information which is to be printed
along with the file type or need additional tests to determine the true
file type.
These additional tests are introduced by one or more
Em \*[Gt]
characters preceding the offset.
The number of
Em \*[Gt]
on the line indicates the level of the test; a line with no
Em \*[Gt]
at the beginning is considered to be at level 0.
Tests are arranged in a tree-like hierarchy:
if the test on a line at level
Em n
succeeds, all following tests at level
Em n+1
are performed, and the messages printed if the tests succeed, until a line
with level
Em n
(or less) appears.
For more complex files, one can use empty messages to get just the
"if/then" effect, in the following way:
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Lt]0x40 MS-DOS executable
\*[Gt]0x18 leshort \*[Gt]0x3f extended PC executable (e.g., MS Windows)
Ed
Pp
Offsets do not need to be constant, but can also be read from the file
being examined.
If the first character following the last
Em \*[Gt]
is a
Em \&(
then the string after the parenthesis is interpreted as an indirect offset.
That means that the number after the parenthesis is used as an offset in
the file.
The value at that offset is read, and is used again as an offset
in the file.
Indirect offsets are of the form:
Em (( x [[.,][bBcCeEfFgGhHiIlmsSqQ]][+\-][ y ]) .
The value of
Em x
is used as an offset in the file.
A byte, id3 length, short or long is read at that offset depending on the
Em [bBcCeEfFgGhHiIlmsSqQ]
type specifier.
The value is treated as signed if
Dq ,
is specified or unsigned if
Dq .
is specified.
The capitalized types interpret the number as a big endian
value, whereas the small letter versions interpret the number as a little
endian value;
the
Em m
type interprets the number as a middle endian (PDP-11) value.
To that number the value of
Em y
is added and the result is used as an offset in the file.
The default type if one is not specified is long.
The following types are recognized:
Bl -column -offset indent "Type" "Half/Short" "Little" "Size"
It Sy Type Sy Mnemonic Sy Endian Sy Size
It bcBc Byte/Char N/A 1
It efg Double Little 8
It EFG Double Big 8
It hs Half/Short Little 2
It HS Half/Short Big 2
It i ID3 Little 4
It I ID3 Big 4
It m Middle Middle 4
It o Octal Textual Variable
It q Quad Little 8
It Q Quad Big 8
El
Pp
That way variable length structures can be examined:
Bd -literal -offset indent
# MS Windows executables are also valid MS-DOS executables
0 string MZ
\*[Gt]0x18 leshort \*[Lt]0x40 MZ executable (MS-DOS)
# skip the whole block below if it is not an extended executable
\*[Gt]0x18 leshort \*[Gt]0x3f
\*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
\*[Gt]\*[Gt](0x3c.l) string LX\e0\e0 LX executable (OS/2)
Ed
Pp
This strategy of examining has a drawback: you must make sure that you
eventually print something, or users may get empty output (such as when
there is neither PE\e0\e0 nor LE\e0\e0 in the above example).
Pp
If this indirect offset cannot be used directly, simple calculations are
possible: appending
Em [+-*/%\*[Am]|^]number
inside parentheses allows one to modify
the value read from the file before it is used as an offset:
Bd -literal -offset indent
# MS Windows executables are also valid MS-DOS executables
0 string MZ
# sometimes, the value at 0x18 is less that 0x40 but there's still an
# extended executable, simply appended to the file
\*[Gt]0x18 leshort \*[Lt]0x40
\*[Gt]\*[Gt](4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
\*[Gt]\*[Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
Ed
Pp
Sometimes you do not know the exact offset as this depends on the length or
position (when indirection was used before) of preceding fields.
You can specify an offset relative to the end of the last up-level
field using
Sq \*[Am]
as a prefix to the offset:
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Gt]0x3f
\*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
# immediately following the PE signature is the CPU type
\*[Gt]\*[Gt]\*[Gt]\*[Am]0 leshort 0x14c for Intel 80386
\*[Gt]\*[Gt]\*[Gt]\*[Am]0 leshort 0x184 for DEC Alpha
Ed
Pp
Indirect and relative offsets can be combined:
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Lt]0x40
\*[Gt]\*[Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
# if it's not COFF, go back 512 bytes and add the offset taken
# from byte 2/3, which is yet another way of finding the start
# of the extended executable
\*[Gt]\*[Gt]\*[Gt]\*[Am](2.s-514) string LE LE executable (MS Windows VxD driver)
Ed
Pp
Or the other way around:
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Gt]0x3f
\*[Gt]\*[Gt](0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
# at offset 0x80 (-4, since relative offsets start at the end
# of the up-level match) inside the LE header, we find the absolute
# offset to the code area, where we look for a specific signature
\*[Gt]\*[Gt]\*[Gt](\*[Am]0x7c.l+0x26) string UPX \eb, UPX compressed
Ed
Pp
Or even both!
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Gt]0x3f
\*[Gt]\*[Gt](0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
# at offset 0x58 inside the LE header, we find the relative offset
# to a data area where we look for a specific signature
\*[Gt]\*[Gt]\*[Gt]\*[Am](\*[Am]0x54.l-3) string UNACE \eb, ACE self-extracting archive
Ed
Pp
If you have to deal with offset/length pairs in your file, even the
second value in a parenthesized expression can be taken from the file itself,
using another set of parentheses.
Note that this additional indirect offset is always relative to the
start of the main indirect offset.
Bd -literal -offset indent
0 string MZ
\*[Gt]0x18 leshort \*[Gt]0x3f
\*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
# search for the PE section called ".idata"...
\*[Gt]\*[Gt]\*[Gt]\*[Am]0xf4 search/0x140 .idata
# ...and go to the end of it, calculated from start+length;
# these are located 14 and 10 bytes after the section name
\*[Gt]\*[Gt]\*[Gt]\*[Gt](\*[Am]0xe.l+(-4)) string PK\e3\e4 \eb, ZIP self-extracting archive
Ed
Pp
If you have a list of known values at a particular continuation level,
and you want to provide a switch-like default case:
Bd -literal -offset indent
# clear that continuation level match
\*[Gt]18 clear
\*[Gt]18 lelong 1 one
\*[Gt]18 lelong 2 two
\*[Gt]18 default x
# print default match
\*[Gt]\*[Gt]18 lelong x unmatched 0x%x
Ed
Sh SEE ALSO
Xr file 1
\- the command that reads this file.
Sh BUGS
The formats
Dv long ,
Dv belong ,
Dv lelong ,
Dv melong ,
Dv short ,
Dv beshort ,
and
Dv leshort
do not depend on the length of the C data types
Dv short
and
Dv long
on the platform, even though the Single
Ux
Specification implies that they do.
However, as OS X Mountain Lion has passed the Single
Ux
Specification validation suite, and supplies a version of
Xr file 1
in which they do not depend on the sizes of the C data types and that is
built for a 64-bit environment in which
Dv long
is 8 bytes rather than 4 bytes, presumably the validation suite does not
test whether, for example
Dv long
refers to an item with the same size as the C data type
Dv long .
There should probably be
Dv type
names
Dv int8 ,
Dv uint8 ,
Dv int16 ,
Dv uint16 ,
Dv int32 ,
Dv uint32 ,
Dv int64 ,
and
Dv uint64 ,
and specified-byte-order variants of them,
to make it clearer that those types have specified widths.
\"
\" From: [email protected] (Guy Harris)
\" Newsgroups: net.bugs.usg
\" Subject: /etc/magic's format isn't well documented
\" Message-ID: <[email protected]>
\" Date: 3 Sep 85 08:19:07 GMT
\" Organization: Sun Microsystems, Inc.
\" Lines: 136
\"
\" Here's a manual page for the format accepted by the "file" made by adding
\" the changes I posted to the S5R2 version.
\"
\" Modified for Ian Darwin's version of the file command.