3        The hw_random framework is software that makes use of a
   4        special hardware feature on your CPU or motherboard,
   5        a Random Number Generator (RNG).  The software has two parts:
   6        a core providing the /dev/hw_random character device and its
   7        sysfs support, plus a hardware-specific driver that plugs
   8        into that core.
  10        To make the most effective use of these mechanisms, you
  11        should download the support software as well.  Download the
  12        latest version of the "rng-tools" package from the
  13        hw_random driver's official Web site:
  17        Those tools use /dev/hw_random to fill the kernel entropy pool,
  18        which is used internally and exported by the /dev/urandom and
  19        /dev/random special files.
  21Theory of operation:
  23        CHARACTER DEVICE.  Using the standard open()
  24        and read() system calls, you can read random data from
  25        the hardware RNG device.  This data is NOT CHECKED by any
  26        fitness tests, and could potentially be bogus (if the
  27        hardware is faulty or has been tampered with).  Data is only
  28        output if the hardware "has-data" flag is set, but nevertheless
  29        a security-conscious person would run fitness tests on the
  30        data before assuming it is truly random.
  32        The rng-tools package uses such tests in "rngd", and lets you
  33        run them by hand with a "rngtest" utility.
  35        /dev/hw_random is char device major 10, minor 183.
  37        CLASS DEVICE.  There is a /sys/class/misc/hw_random node with
  38        two unique attributes, "rng_available" and "rng_current".  The
  39        "rng_available" attribute lists the hardware-specific drivers
  40        available, while "rng_current" lists the one which is currently
  41        connected to /dev/hw_random.  If your system has more than one
  42        RNG available, you may change the one used by writing a name from
  43        the list in "rng_available" into "rng_current".
  47        Hardware driver for Intel/AMD/VIA Random Number Generators (RNG)
  48        Copyright 2000,2001 Jeff Garzik <>
  49        Copyright 2000,2001 Philipp Rumpf <>
  52About the Intel RNG hardware, from the firmware hub datasheet:
  54        The Firmware Hub integrates a Random Number Generator (RNG)
  55        using thermal noise generated from inherently random quantum
  56        mechanical properties of silicon. When not generating new random
  57        bits the RNG circuitry will enter a low power state. Intel will
  58        provide a binary software driver to give third party software
  59        access to our RNG for use as a security feature. At this time,
  60        the RNG is only to be used with a system in an OS-present state.
  62Intel RNG Driver notes:
  64        * FIXME: support poll(2)
  66        NOTE: request_mem_region was removed, for two reasons:
  67        1) Only one RNG is supported by this driver, 2) The location
  68        used by the RNG is a fixed location in MMIO-addressable memory,
  69        3) users with properly working BIOS e820 handling will always
  70        have the region in which the RNG is located reserved, so
  71        request_mem_region calls always fail for proper setups.
  72        However, for people who use mem=XX, BIOS e820 information is
  73        -not- in /proc/iomem, and request_mem_region(RNG_ADDR) can
  74        succeed.
  76Driver details:
  78        Based on:
  79        Intel 82802AB/82802AC Firmware Hub (FWH) Datasheet
  80                May 1999 Order Number: 290658-002 R
  82        Intel 82802 Firmware Hub: Random Number Generator
  83        Programmer's Reference Manual
  84                December 1999 Order Number: 298029-001 R
  86        Intel 82802 Firmware HUB Random Number Generator Driver
  87        Copyright (c) 2000 Matt Sottek <>
  89        Special thanks to Matt Sottek.  I did the "guts", he
  90        did the "brains" and all the testing.