.\" Copyright (c) 1997 John S. Kallal (kallal@voicenet.com)

.\"

.\" %%%LICENSE_START(GPLv2+_DOC_ONEPARA)

.\" This is free documentation; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

.\" published by the Free Software Foundation; either version 2 of

.\" the License, or (at your option) any later version.

.\" %%%LICENSE_END

.\"

.\" Some changes by tytso and aeb.

.\"

.\" 2004-12-16, John V. Belmonte/mtk, Updated init and quit scripts

.\" 2004-04-08, AEB, Improved description of read from /dev/urandom

.\" 2008-06-20, George Spelvin <linux@horizon.com>,

.\"             Matt Mackall <mpm@selenic.com>

.\"

.TH RANDOM 4 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

random, urandom \- kernel random number source devices

.SH SYNOPSIS

#include <linux/random.h>

.PP

.BI "int ioctl(" fd ", RND" request ", " param ");"

.SH DESCRIPTION

The character special files \fI/dev/random\fP and

\fI/dev/urandom\fP (present since Linux 1.3.30)

provide an interface to the kernel's random number generator.

The file

.I /dev/random

has major device number 1 and minor device number 8.

The file

.I /dev/urandom

has major device number 1 and minor device number 9.

.PP

The random number generator gathers environmental noise

from device drivers and other sources into an entropy pool.

The generator also keeps an estimate of the

number of bits of noise in the entropy pool.

From this entropy pool, random numbers are created.

.PP

Linux 3.17 and later provides the simpler and safer

.BR getrandom (2)

interface which requires no special files;

see the

.BR getrandom (2)

manual page for details.

.PP

When read, the

.I /dev/urandom

device returns random bytes using a pseudorandom

number generator seeded from the entropy pool.

Reads from this device do not block (i.e., the CPU is not yielded),

but can incur an appreciable delay when requesting large amounts of data.

.PP

When read during early boot time,

.IR /dev/urandom

may return data prior to the entropy pool being initialized.

.\" This is a real problem; see

.\" commit 9b4d008787f864f17d008c9c15bbe8a0f7e2fc24

If this is of concern in your application, use

.BR getrandom (2)

or \fI/dev/random\fP instead.

.PP

The \fI/dev/random\fP device is a legacy interface which dates back to

a time where the cryptographic primitives used in the implementation

of \fI/dev/urandom\fP were not widely trusted.

It will return random bytes only within the estimated number of

bits of fresh noise in the entropy pool, blocking if necessary.

\fI/dev/random\fP is suitable for applications that need

high quality randomness, and can afford indeterminate delays.

.PP

When the entropy pool is empty, reads from \fI/dev/random\fP will block

until additional environmental noise is gathered.

If

.BR open (2)

is called for

.I /dev/random

with the

.BR O_NONBLOCK

flag, a subsequent

.BR read (2)

will not block if the requested number of bytes is not available.

Instead, the available bytes are returned.

If no byte is available,

.BR read (2)

will return -1 and

.I errno

will be set to

.BR EAGAIN .

.PP

The

.B O_NONBLOCK

flag has no effect when opening

.IR /dev/urandom .

When calling

.BR read (2)

for the device

.IR /dev/urandom ,

reads of up to 256 bytes will return as many bytes as are requested

and will not be interrupted by a signal handler.

Reads with a buffer over this limit may return less than the

requested number of bytes or fail with the error

.BR EINTR ,

if interrupted by a signal handler.

.PP

Since Linux 3.16,

.\" commit 79a8468747c5f95ed3d5ce8376a3e82e0c5857fc

a

.BR read (2)

from

.IR /dev/urandom

will return at most 32\ MB.

A

.BR read (2)

from

.IR /dev/random

will return at most 512 bytes

.\" SEC_XFER_SIZE in drivers/char/random.c

(340 bytes on Linux kernels before version 2.6.12).

.PP

Writing to \fI/dev/random\fP or \fI/dev/urandom\fP will update the

entropy pool with the data written, but this will not result in a

higher entropy count.

This means that it will impact the contents

read from both files, but it will not make reads from

\fI/dev/random\fP faster.

.SS Usage

The

.IR /dev/random

interface is considered a legacy interface, and

.IR /dev/urandom

is preferred and sufficient in all use cases, with the exception of

applications which require randomness during early boot time; for

these applications,

.BR getrandom (2)

must be used instead,

because it will block until the entropy pool is initialized.

.PP

If a seed file is saved across reboots as recommended below (all major

Linux distributions have done this since 2000 at least), the output is

cryptographically secure against attackers without local root access as

soon as it is reloaded in the boot sequence, and perfectly adequate for

network encryption session keys.

Since reads from

.I /dev/random

may block, users will usually want to open it in nonblocking mode

(or perform a read with timeout),

and provide some sort of user notification if the desired

entropy is not immediately available.

.\"

.SS Configuration

If your system does not have

\fI/dev/random\fP and \fI/dev/urandom\fP created already, they

can be created with the following commands:

.PP

.in +4n

.EX

mknod \-m 666 /dev/random c 1 8

mknod \-m 666 /dev/urandom c 1 9

chown root:root /dev/random /dev/urandom

.EE

.in

.PP

When a Linux system starts up without much operator interaction,

the entropy pool may be in a fairly predictable state.

This reduces the actual amount of noise in the entropy pool

below the estimate.

In order to counteract this effect, it helps to carry

entropy pool information across shut-downs and start-ups.

To do this, add the lines to an appropriate script

which is run during the Linux system start-up sequence:

.PP

.in +4n

.EX

echo "Initializing random number generator..."

random_seed=/var/run/random-seed

# Carry a random seed from start-up to start-up

# Load and then save the whole entropy pool

if [ \-f $random_seed ]; then

    cat $random_seed >/dev/urandom

else

    touch $random_seed

fi

chmod 600 $random_seed

poolfile=/proc/sys/kernel/random/poolsize

[ \-r $poolfile ] && bits=$(cat $poolfile) || bits=4096

bytes=$(expr $bits / 8)

dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

.EE

.in

.PP

Also, add the following lines in an appropriate script which is

run during the Linux system shutdown:

.PP

.in +4n

.EX

# Carry a random seed from shut-down to start-up

# Save the whole entropy pool

echo "Saving random seed..."

random_seed=/var/run/random-seed

touch $random_seed

chmod 600 $random_seed

poolfile=/proc/sys/kernel/random/poolsize

[ \-r $poolfile ] && bits=$(cat $poolfile) || bits=4096

bytes=$(expr $bits / 8)

dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

.EE

.in

.PP

In the above examples, we assume Linux 2.6.0 or later, where

.IR /proc/sys/kernel/random/poolsize

returns the size of the entropy pool in bits (see below).

.\"

.SS /proc interfaces

The files in the directory

.I /proc/sys/kernel/random

(present since 2.3.16) provide additional information about the

.I /dev/random

device:

.TP

.I entropy_avail

This read-only file gives the available entropy, in bits.

This will be a number in the range 0 to 4096.

.TP

.I poolsize

This file

gives the size of the entropy pool.

The semantics of this file vary across kernel versions:

.RS

.TP

Linux 2.4:

This file gives the size of the entropy pool in

.IR bytes .

Normally, this file will have the value 512, but it is writable,

and can be changed to any value for which an algorithm is available.

The choices are 32, 64, 128, 256, 512, 1024, or 2048.

.TP

Linux 2.6 and later:

This file is read-only, and gives the size of the entropy pool in

.IR bits .

It contains the value 4096.

.RE

.TP

.I read_wakeup_threshold

This file

contains the number of bits of entropy required for waking up processes

that sleep waiting for entropy from

.IR /dev/random .

The default is 64.

.TP

.I write_wakeup_threshold

This file

contains the number of bits of entropy below which we wake up

processes that do a

.BR select (2)

or

.BR poll (2)

for write access to

.IR /dev/random .

These values can be changed by writing to the files.

.TP

.IR uuid " and " boot_id

These read-only files

contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.

The former is generated afresh for each read, the latter was

generated once.

.\"

.SS ioctl(2) interface

The following

.BR ioctl (2)

requests are defined on file descriptors connected to either \fI/dev/random\fP

or \fI/dev/urandom\fP.

All requests performed will interact with the input

entropy pool impacting both \fI/dev/random\fP and \fI/dev/urandom\fP.

The

.B CAP_SYS_ADMIN

capability is required for all requests except

.BR RNDGETENTCNT .

.TP

.BR RNDGETENTCNT

Retrieve the entropy count of the input pool, the contents will be the same

as the

.I entropy_avail

file under proc.

The result will be stored in the int pointed to by the argument.

.TP

.BR RNDADDTOENTCNT

Increment or decrement the entropy count of the input pool

by the value pointed to by the argument.

.TP

.BR RNDGETPOOL

Removed in Linux 2.6.9.

.TP

.BR RNDADDENTROPY

Add some additional entropy to the input pool,

incrementing the entropy count.

This differs from writing to \fI/dev/random\fP or \fI/dev/urandom\fP,

which only adds some

data but does not increment the entropy count.

The following structure is used:

.IP

.in +4n

.EX

struct rand_pool_info {

    int    entropy_count;

    int    buf_size;

    __u32  buf[0];

};

.EE

.in

.IP

Here

.I entropy_count

is the value added to (or subtracted from) the entropy count, and

.I buf

is the buffer of size

.I buf_size

which gets added to the entropy pool.

.TP

.BR RNDZAPENTCNT ", " RNDCLEARPOOL

Zero the entropy count of all pools and add some system data (such as

wall clock) to the pools.

.SH FILES

.I /dev/random

.br

.I /dev/urandom

.SH NOTES

For an overview and comparison of the various interfaces that

can be used to obtain randomness, see

.BR random (7).

.SH BUGS

During early boot time, reads from

.I /dev/urandom

may return data prior to the entropy pool being initialized.

.\" .SH AUTHOR

.\" The kernel's random number generator was written by

.\" Theodore Ts'o (tytso@athena.mit.edu).

.SH SEE ALSO

.BR mknod (1),

.BR getrandom (2),

.BR random (7)

.PP

RFC\ 1750, "Randomness Recommendations for Security"

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.