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
.\" 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 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.
The file
.I /dev/random\fP
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.
.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
Linux 3.17 and later provides the simpler and safer (see below)
.BR getrandom (2)
interface which requires no special files.
.LP
When read, the
.I /dev/urandom
device returns random bytes using a pseudorandom
number generator seeded from the entropy pool.
Reads from this device are nonblocking.
When read during early boot time, this device may return
data prior to the entropy pool being initialized.
If this is of concern in your application, use
.BR getrandom (2)
or \fI/dev/random\fP instead.

.LP
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 very
high quality randomness, and can afford indeterminate delays.
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 .

The flag
.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
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.

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 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.
Normally, this will be 4096 (bits),
a full entropy pool.
.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
.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 BUGS
During early boot time, reads from
.I /dev/urandom
may return data prior to the entropy pool being initialized.
.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"