MAGIC(5) OpenBSD Programmer's Manual MAGIC(5)NAMEmagic - file command's magic pattern file
DESCRIPTION
This manual page documents the format of the magic file as used by the
file(1) command, version 4.24. The file(1) command identifies the type
of a file using, among other tests, a test for whether the file contains
certain ``magic patterns''. The file /etc/magic specifies what magic
numbers are to be tested for, what message to print if a particular magic
number is found, and additional information to extract from the file.
Each line of the 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:
offset A number specifying the offset, in bytes, into the file of
the data which is to be tested.
type The type of the data to be tested. The possible values are:
byte A one-byte value.
short A two-byte value in this machine's native
byte order.
long A four-byte value in this machine's native
byte order.
quad An eight-byte value in this machine's native
byte order.
float A 32-bit single precision IEEE floating
point number in this machine's native byte
order.
double A 64-bit double precision IEEE floating
point number in this machine's native byte
order.
string A string of bytes. The string type
specification can be optionally followed by
/[Bbc]*. The ``B'' flag compacts whitespace
in the target, which must contain at least
one whitespace character. If the magic has
n consecutive blanks, the target needs at
least n consecutive blanks to match. The
``b'' flag treats every blank in the target
as an optional blank. Finally the ``c''
flag, specifies case insensitive matching:
lowercase characters in the magic match both
lower and upper case characters in the
target, whereas upper case characters in the
magic only match uppercase characters in the
target.
pstring A Pascal-style string where the first byte
is interpreted as the an unsigned length.
The string is not NUL terminated.
date A four-byte value interpreted as a UNIX
date.
qdate An eight-byte value interpreted as a UNIX
date.
ldate A four-byte value interpreted as a UNIX-
style date, but interpreted as local time
rather than UTC.
qldate An eight-byte value interpreted as a UNIX-
style date, but interpreted as local time
rather than UTC.
beshort A two-byte value in big-endian byte order.
belong A four-byte value in big-endian byte order.
bequad An eight-byte value in big-endian byte
order.
befloat A 32-bit single precision IEEE floating
point number in big-endian byte order.
bedouble A 64-bit double precision IEEE floating
point number in big-endian byte order.
bedate A four-byte value in big-endian byte order,
interpreted as a Unix date.
beqdate An eight-byte value in big-endian byte
order, interpreted as a Unix date.
beldate A four-byte value in big-endian byte order,
interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
beqldate An eight-byte value in big-endian byte
order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
bestring16 A two-byte unicode (UCS16) string in big-
endian byte order.
leshort A two-byte value in little-endian byte
order.
lelong A four-byte value in little-endian byte
order.
lequad An eight-byte value in little-endian byte
order.
lefloat A 32-bit single precision IEEE floating
point number in little-endian byte order.
ledouble A 64-bit double precision IEEE floating
point number in little-endian byte order.
ledate A four-byte value in little-endian byte
order, interpreted as a UNIX date.
leqdate An eight-byte value in little-endian byte
order, interpreted as a UNIX date.
leldate A four-byte value in little-endian byte
order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
leqldate An eight-byte value in little-endian byte
order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
lestring16 A two-byte unicode (UCS16) string in little-
endian byte order.
melong A four-byte value in middle-endian (PDP-11)
byte order.
medate A four-byte value in middle-endian (PDP-11)
byte order, interpreted as a UNIX date.
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.
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 type specification
can be optionally followed by /[c][s]. The
``c'' flag makes the match case insensitive,
while the ``s'' flag update the offset to
the start offset of the match, rather than
the end. The regular expression is tested
against line N + 1 onwards, where N is the
given offset. Line endings are assumed to
be in the machine's native format. ^ and $
match the beginning and end of individual
lines, respectively, not beginning and end
of file.
search A literal string search starting at the
given offset. The same modifier flags can
be used as for string patterns. The
modifier flags (if any) must be followed by
/number the range, 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 \
escapes for special characters. The offset
works as for regex.
default This is intended to be used with the test x
(which is always true) and a message that is
to be used if there are no other matches.
Each top-level magic pattern (see below for an explanation
of levels) is classified as text or binary according to the
types used. Types ``regex'' and ``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.
The numeric types may optionally be followed by & 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 u to the type indicates that ordered
comparisons should be unsigned.
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. \n for new-line).
Numeric values may be preceded by a character indicating the
operation to be performed. It may be =, to specify that the
value from the file must equal the specified value, <, to
specify that the value from the file must be less than the
specified value, >, to specify that the value from the file
must be greater than the specified value, &, to specify that
the value from the file must have set all of the bits that
are set in the specified value, ^, to specify that the value
from the file must have clear any of the bits that are set
in the specified value, or ~, the value specified after is
negated before tested. x, to specify that any value will
match. If the character is omitted, it is assumed to be =.
Operators &, ^, and ~ don't work with floats and doubles.
The operator ! specifies that the line matches if the test
does not succeed.
Numeric values are specified in C form; e.g. 13 is decimal,
013 is octal, and 0x13 is hexadecimal.
For string values, the string from the file must match the
specified string. The operators =, < and > (but not &) 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 >\0 (because all non-empty strings
are greater than the empty string).
The special test x always evaluates to true. message The
message to be printed if the comparison succeeds. If the
string contains a 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 ``\b'', the message printed is the
remainder of the string with no whitespace added before it:
multiple matches are normally separated by a single space.
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:
!:mime MIMETYPE
i.e. the literal string ``!:mime'' followed by the MIME type.
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 >
characters preceding the offset. The number of > on the line indicates
the level of the test; a line with no > at the beginning is considered to
be at level 0. Tests are arranged in a tree-like hierarchy: If a the
test on a line at level n succeeds, all following tests at level n+1 are
performed, and the messages printed if the tests succeed, untile a line
with level n (or less) appears. For more complex files, one can use
empty messages to get just the "if/then" effect, in the following way:
0 string MZ
>0x18 leshort <0x40 MS-DOS executable
>0x18 leshort >0x3f extended PC executable (e.g., MS Windows)
Offsets do not need to be constant, but can also be read from the file
being examined. If the first character following the last > is a ( 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: (( x
[.[bslBSL]][+-][ y ]). The value of x is used as an offset in the file.
A byte, short or long is read at that offset depending on the [bslBSLm]
type specifier. 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 m type interprets the number as a middle endian
(PDP-11) value. To that number the value of y is added and the result is
used as an offset in the file. The default type if one is not specified
is long.
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
>0x18 leshort <0x40 MZ executable (MS-DOS)
# skip the whole block below if it is not an extended executable
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
>>(0x3c.l) string LX\0\0 LX executable (OS/2)
This strategy of examining has a drawback: You must make sure that you
eventually print something, or users may get empty output (like, when
there is neither PE\0\0 nor LE\0\0 in the above example)
If this indirect offset cannot be used directly, simple calculations are
possible: appending [+-*/%&|^]number inside parentheses allows one to
modify the value read from the file before it is used as an offset:
# 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
>0x18 leshort <0x40
>>(4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
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 `&' as a prefix to the offset:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# immediately following the PE signature is the CPU type
>>>&0 leshort 0x14c for Intel 80386
>>>&0 leshort 0x184 for DEC Alpha
Indirect and relative offsets can be combined:
0 string MZ
>0x18 leshort <0x40
>>(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
>>>&(2.s-514) string LE LE executable (MS Windows VxD driver)
Or the other way around:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 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
>>>(&0x7c.l+0x26) string UPX \b, UPX compressed
Or even both!
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 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
>>>&(&0x54.l-3) string UNACE \b, ACE self-extracting archive
Finally, 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.
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# search for the PE section called ".idata"...
>>>&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
>>>>(&0xe.l+(-4)) string PK\3\4 \b, ZIP self-extracting archive
SEE ALSOfile(1) - the command that reads this file.
BUGS
The formats long, belong, lelong, melong, short, beshort, leshort, date,
bedate, medate, ledate, beldate, leldate, and meldate are system-
dependent; perhaps they should be specified as a number of bytes (2B, 4B,
etc), since the files being recognized typically come from a system on
which the lengths are invariant.
OpenBSD 4.9 July 15, 2010 OpenBSD 4.9
