mcookie - Man Page

generate magic cookies for xauth

Examples (TL;DR)


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(2) 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.

-h,  --help

Display help text and exit.

-V,  --version

Print version and exit.





It is assumed that none of the randomness sources will block.

See Also

md5sum(1), X(7), xauth(1), rand(3)

Reporting Bugs

For bug reports, use the issue tracker at


The mcookie command is part of the util-linux package which can be downloaded from Linux Kernel Archive.


2023-10-23 util-linux 2.40-rc1