man-pages/man4/random.4

.\" 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
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.\" %%%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>
.\"     Add a Usage subsection that recommends most users to use
.\"     /dev/urandom, and emphasizes parsimonious usage of /dev/random.
.\"
.TH RANDOM 4 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
random, urandom \- kernel random number source devices
.SH SYNOPSIS
#include <linux/random.h>
.sp
.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.
File \fI/dev/random\fP has major device number 1
and minor device number 8.
File \fI/dev/urandom\fP has major device number 1 and minor device number 9.
.LP
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.
.LP
When read, the \fI/dev/random\fP device will return random bytes
only within the estimated number of bits of noise in the entropy
pool.
\fI/dev/random\fP should be suitable for uses that need very
high quality randomness such as one-time pad or key generation.
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 flag
.BR O_NONBLOCK ,
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 .
.LP
A read from the \fI/dev/urandom\fP device will not block
waiting for more entropy.
If there is not sufficient entropy, a pseudorandom number generator is used
to create the requested bytes.
As a result, in this case the returned values are theoretically vulnerable to a
cryptographic attack on the algorithms used by the driver.
Knowledge of how to do this is not available in the current unclassified
literature, but it is theoretically possible that such an attack may
exist.
If this is a concern in your application, use \fI/dev/random\fP
instead.
.B O_NONBLOCK
has no effect when opening
.IR /dev/urandom .
When calling
.BR read (2)
for the device
.IR /dev/urandom ,
signals will not be handled until after the requested random bytes
have been generated.

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).

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
If you are unsure about whether you should use
.IR /dev/random
or
.IR /dev/urandom ,
then probably you want to use the latter.
As a general rule,
.IR /dev/urandom
should be used for everything except long-lived GPG/SSL/SSH keys.

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.

The kernel random-number generator is designed to produce a small
amount of high-quality seed material to seed a
cryptographic pseudo-random number generator (CPRNG).
It is designed for security, not speed, and is poorly
suited to generating large amounts of random data.
Users should be very economical in the amount of seed
material that they read from
.IR /dev/urandom
(and
.IR /dev/random );
unnecessarily reading large quantities of data from this device will have
a negative impact on other users of the device.

The amount of seed material required to generate a cryptographic key
equals the effective key size of the key.
For example, a 3072-bit RSA
or Diffie-Hellman private key has an effective key size of 128 bits
(it requires about 2^128 operations to break) so a key generator
needs only 128 bits (16 bytes) of seed material from
.IR /dev/random .

While some safety margin above that minimum is reasonable, as a guard
against flaws in the CPRNG algorithm, no cryptographic primitive
available today can hope to promise more than 256 bits of security,
so if any program reads more than 256 bits (32 bytes) from the kernel
random pool per invocation, or per reasonable reseed interval (not less
than one minute), that should be taken as a sign that its cryptography is
.I not
skillfully implemented.
.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:

.nf
    mknod \-m 666 /dev/random c 1 8
    mknod \-m 666 /dev/urandom c 1 9
    chown root:root /dev/random /dev/urandom
.fi

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:

.nf
    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
.fi

Also, add the following lines in an appropriate script which is
run during the Linux system shutdown:

.nf
    # 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
.fi

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 Interface
The files in the directory
.I /proc/sys/kernel/random
(present since 2.3.16) provide an additional interface to the
.I /dev/random
device.
.LP
The read-only file
.I entropy_avail
gives the available entropy.
Normally, this will be 4096 (bits),
a full entropy pool.
.LP
The file
.I poolsize
gives the size of the entropy pool.
The semantics of this file vary across kernel versions:
.RS
.TP 12
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:
This file is read-only, and gives the size of the entropy pool in
.IR bits .
It contains the value 4096.
.RE
.LP
The file
.I read_wakeup_threshold
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.
The file
.I write_wakeup_threshold
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.
.LP
The read-only files
.I uuid
and
.I boot_id
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
.nf
    struct rand_pool_info {
        int    entropy_count;
        int    buf_size;
        __u32  buf[0];
    };
.fi
.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
/dev/random
.br
/dev/urandom
.\" .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)

RFC\ 1750, "Randomness Recommendations for Security"