RANDOM(7D)RANDOM(7D)NAME
random, urandom - Strong random number generator device
SYNOPSIS
/dev/random
/dev/urandom
DESCRIPTION
The /dev/random and /dev/urandom files are special files that are a
source for random bytes generated by the kernel random number generator
device. The /dev/random and /dev/urandom files are suitable for appli‐
cations requiring high quality random numbers for cryptographic pur‐
poses.
The generator device produces random numbers from data and devices
available to the kernel and estimates the amount of randomness (or
"entropy") collected from these sources. The entropy level determines
the amount of high quality random numbers that are produced at a given
time.
Applications retrieve random bytes by reading /dev/random or /dev/uran‐
dom. The /dev/random interface returns random bytes only when suffi‐
cient amount of entropy has been collected. If there is no entropy to
produce the requested number of bytes, /dev/random blocks until more
entropy can be obtained. Non-blocking I/O mode can be used to disable
the blocking behavior. The /dev/random interface also supports poll(2).
Note that using poll(2) will not increase the speed at which random
numbers can be read.
Bytes retrieved from /dev/random provide the highest quality random
numbers produced by the generator, and can be used to generate long
term keys and other high value keying material.
The /dev/urandom interface returns bytes regardless of the amount of
entropy available. It does not block on a read request due to lack of
entropy. While bytes produced by the /dev/urandom interface are of
lower quality than bytes produced by /dev/random, they are nonetheless
suitable for less demanding and shorter term cryptographic uses such as
short term session keys, paddings, and challenge strings.
Data can be written to /dev/random and /dev/urandom. Data written to
either special file is added to the generator's internal state. Data
that is difficult to predict by other users may contribute randomness
to the generator state and help improve the quality of future generated
random numbers.
/dev/random collects entropy from providers that are registered with
the kernel-level cryptographic framework and implement random number
generation routines. The cryptoadm(1M) utility allows an administrator
to configure which providers will be used with /dev/random.
ERRORS
EAGAIN
O_NDELAY or O_NONBLOCK was set and no random bytes are
available for reading from /dev/random.
EINTR
A signal was caught while reading and no data was
transferred.
ENOXIO
open(2) request failed on /dev/random because no entropy
provider is available.
FILES
/dev/random
/dev/urandom
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Evolving │
└────────────────────┴─────────────────┘
SEE ALSOcryptoadm(1M), open(2), poll(2), attributes(5)NOTES
/dev/random can be configured to use only the hardware-based providers
registered with the kernel-level cryptographic framework by disabling
the software-based provider using cryptoadm(1M). You can also use cryp‐
toadm(1M) to obtain the name of the software-based provider.
Because no entropy is available, disabling all randomness providers
causes read(2) and poll(2) on /dev/random to block indefinitely and
results in a warning message being logged and displayed on the system
console. However, read(2) and poll(2) on /dev/urandom continue to work
in this case.
An implementation of the /dev/random and /dev/urandom kernel-based ran‐
dom number generator first appeared in Linux 1.3.30.
A /dev/random interface for Solaris first appeared as part of the Cryp‐
toRand implementation.
Sep 1, 2008 RANDOM(7D)
