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The hw_random framework is software that makes use of a
special hardware feature on your CPU or motherboard,
a Random Number Generator (RNG). The software has two parts:
a core providing the /dev/hw_random character device and its
sysfs support, plus a hardware-specific driver that plugs
into that core.
To make the most effective use of these mechanisms, you
should download the support software as well. Download the
latest version of the "rng-tools" package from the
hw_random driver's official Web site:
Those tools use /dev/hw_random to fill the kernel entropy pool,
which is used internally and exported by the /dev/urandom and
/dev/random special files.
Theory of operation:
CHARACTER DEVICE. Using the standard open()
and read() system calls, you can read random data from
the hardware RNG device. This data is NOT CHECKED by any
fitness tests, and could potentially be bogus (if the
hardware is faulty or has been tampered with). Data is only
output if the hardware "has-data" flag is set, but nevertheless
a security-conscious person would run fitness tests on the
data before assuming it is truly random.
The rng-tools package uses such tests in "rngd", and lets you
run them by hand with a "rngtest" utility.
/dev/hw_random is char device major 10, minor 183.
CLASS DEVICE. There is a /sys/class/misc/hw_random node with
two unique attributes, "rng_available" and "rng_current". The
"rng_available" attribute lists the hardware-specific drivers
available, while "rng_current" lists the one which is currently
connected to /dev/hw_random. If your system has more than one
RNG available, you may change the one used by writing a name from
the list in "rng_available" into "rng_current".
Hardware driver for Intel/AMD/VIA Random Number Generators (RNG)
Copyright 2000,2001 Jeff Garzik <>
Copyright 2000,2001 Philipp Rumpf <>
About the Intel RNG hardware, from the firmware hub datasheet:
The Firmware Hub integrates a Random Number Generator (RNG)
using thermal noise generated from inherently random quantum
mechanical properties of silicon. When not generating new random
bits the RNG circuitry will enter a low power state. Intel will
provide a binary software driver to give third party software
access to our RNG for use as a security feature. At this time,
the RNG is only to be used with a system in an OS-present state.
Intel RNG Driver notes:
* FIXME: support poll(2)
NOTE: request_mem_region was removed, for two reasons:
1) Only one RNG is supported by this driver, 2) The location
used by the RNG is a fixed location in MMIO-addressable memory,
3) users with properly working BIOS e820 handling will always
have the region in which the RNG is located reserved, so
request_mem_region calls always fail for proper setups.
However, for people who use mem=XX, BIOS e820 information is
-not- in /proc/iomem, and request_mem_region(RNG_ADDR) can
Driver details:
Based on:
Intel 82802AB/82802AC Firmware Hub (FWH) Datasheet
May 1999 Order Number: 290658-002 R
Intel 82802 Firmware Hub: Random Number Generator
Programmer's Reference Manual
December 1999 Order Number: 298029-001 R
Intel 82802 Firmware HUB Random Number Generator Driver
Copyright (c) 2000 Matt Sottek <>
Special thanks to Matt Sottek. I did the "guts", he
did the "brains" and all the testing.