|MAGIC(5)||File Formats Manual||MAGIC(5)|
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:
nconsecutive blanks, the target needs at least
nconsecutive 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.
/[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 + 1onwards, where
Nis the given offset. Line endings are assumed to be in the machine's native format.
$match the beginning and end of individual lines, respectively, not beginning and end of file.
/numberthe 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.
usemagic 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.
^then the endianness of the magic is switched; if the magic mentioned
leshortfor example, it is treated as
beshortand vice versa. This is useful to avoid duplicating the rules for different endianness.
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.
Numeric values may be preceded by a character indicating the
operation to be performed. It may be
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,
specify that the value from the file must be greater than the specified
&, 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
x, to specify that any value will match.
If the character is omitted, it is assumed to be
~ don't work with
floats and doubles. The operator
that the line matches if the test does not
Numeric values are specified in C form; e.g.
13 is decimal,
0x13 is hexadecimal.
For string values, the string from the file must match the
specified string. The operators
> (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.
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:
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 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, until 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
meldateare 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.
|September 20, 2017||OpenBSD-current|