---
name: "mcookie"
section: "1"
platform: "linux"
package: "util-linux"
version: "2.42.4-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/mcookie"
---

# mcookie(1)

generate magic cookies for xauth

## NAME

mcookie - generate magic cookies for xauth

## SYNOPSIS

**mcookie** \[options\]

## DESCRIPTION

**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

- **-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**

  `Display version and exit.`

## FILES

*/dev/urandom*

*/dev/random*

## BUGS

`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*` <https://github.com/util-linux/util-linux/issues>.`

## AVAILABILITY

`The `**mcookie**` command is part of the util-linux package which can be downloaded from `*Linux Kernel Archive*` <https://www.kernel.org/pub/linux/utils/util-linux/>.`