MCOOKIE(1) User Commands MCOOKIE(1)
mcookie - generate magic cookies for xauth
mcookie generates a 128-bit random hexadecimal number for use with the
X authority system. Typical usage:
xauth add :0 . `mcookie`
The "random" number generated is actually the MD5 message digest of
random information coming from one of the sources /dev/urandom,
/dev/random, or the libc pseudo-random functions, in this preference
-f, --file file
Use this file as an additional source of randomness. When file
is '-', characters are read from standard input.
-m, --max-size number
Read from file only this number of bytes. This option is meant
to be used when reading additional randomness from a file or
The number argument may be followed by the multiplicative suf-
fixes KiB=1024, MiB=1024*1024, and so on for GiB, TiB, PiB, EiB,
ZiB and YiB (the "iB" is optional, e.g., "K" has the same mean-
ing as "KiB") or the suffixes KB=1000, MB=1000*1000, and so on
for GB, TB, PB, EB, ZB and YB.
Inform where randomness originated, with amount of entropy read
from each source.
Display version information and exit.
Display help text and exit.
It is assumed that none of the randomness sources will block.
md5sum(1), X(1), xauth(1), rand(3)
The mcookie command is part of the util-linux package and is available
from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-
util-linux December 2014 MCOOKIE(1)
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