mcookie — generate magic cookies for xauth


mcookie [options]


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 getrandom() system call, /dev/urandom, /dev/random, or the libc pseudo-random functions, in this preference order. See also the option −−file.


−f, −−file file

Use this file as an additional source of randomness (for example /dev/urandom). 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 device.

The number argument may be followed by the multiplicative suffixes 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 meaning as "KiB") or the suffixes KB=1000, MB=1000*1000, and so on for GB, TB, PB, EB, ZB and YB.

−v, −−verbose

Inform where randomness originated, with amount of entropy read from each source.

−V, −−version

Display version information and exit.

−h, −−help

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

  mcookie.1 --
Public Domain 1995 Rickard E. Faith (