mcookie(1) — Linux manual page


MCOOKIE(1)                    User Commands                   MCOOKIE(1)

NAME         top

       mcookie - generate magic cookies for xauth

SYNOPSIS         top

       mcookie [options]

DESCRIPTION         top

       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.

OPTIONS         top

       -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.

FILES         top



BUGS         top

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

SEE ALSO         top

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

REPORTING BUGS         top

       For bug reports, use the issue tracker at

AVAILABILITY         top

       The mcookie command is part of the util-linux package which can
       be downloaded from Linux Kernel Archive
       <>. This page
       is part of the util-linux (a random collection of Linux
       utilities) project. Information about the project can be found at
       ⟨⟩. If you have
       a bug report for this manual page, send it to This page was obtained from the
       project's upstream Git repository
       ⟨git://⟩ on
       2023-12-22. (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-14.) If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in
       this COLOPHON (which is not part of the original manual page),
       send a mail to

util-linux 2.39.594-1e0ad      2023-07-19                     MCOOKIE(1)