RNG device) to an entropy sink (e.g. kernel entropy pool)
rngd [-b | --background | -f | --foreground] [-p file | --pidfile=file]
[-B n | --rng-buffers=n] [--hrng=name] [-R name | --rng-driver=name]
[-r file | --rng-device=file] [-H n.nnn | --rng-entropy=n.nnn] [-Q
quality | --rng-quality=quality] [-T n | --rng-timeout=n] [-o file |
--random-device=file] [-s n | --random-step=n] [-W n | --fill-water-
mark=n[%]] [-t n | --feed-interval=n] [-?] [--help] [-V] [--version]
This daemon feeds entropy from a trusted source of true randomness
(called an entropy source) such as a hardware true random number gener-
ator (TRNG), to an entropy sink such as the kernel's entropy pool. It
does so only after checking the data to ensure that it is suitably ran-
Entropy measures the uncertainty (randomness) contained in an unit of
information. For the Linux kernel, this unit is a binary bit. The
kernel keeps all the entropy it gathers from different sources in an
entropy pool, from which it drawns uncertainty to generate random num-
bers. The primary purpose of rngd is to keep this pool as full of ran-
domness as possible.
rngd works on blocks of 20000 bits at a time, using the FIPS 140-2
tests to verify the randomness of the block of data. If that data is
deemed random, the block is fed back random-step*8 bits at a time to
the entropy sink, (in the case of the kernel, this is done until the
kernel's entropy pool is at least fill-watermark full).
If working with the kernel as an entropy sink, and the entropy pool is
full, the daemon will force-feed the kernel random-step*8 bits of data
every feed-interval seconds, to stir the entropy pool. If the entropy
source used by rngd is of very high quality, this will improve the
entropy of the pool.
The -f or --foreground options can be used to tell rngd to avoid fork-
ing on startup. This is typically used for debugging. The -b or
--background options, which fork and put rngd into the background auto-
matically, are the default.
The -R or --rng-driver options can be used to select the type of
entropy source. The -r or --rng-device options can be used to select
an alternate source of entropy, instead of the default /dev/hwrng. The
-o or --random-device options can be used to select an alternate
entropy output device, instead of the default /dev/random. Note that
this device must support the Linux kernel's /dev/random ioctl API.
The -H or --rng-entropy options should be used to inform rngd about the
entropy per bit of data received from the input source.
The -B or --rng-buffers options can be used to change the number of
Do not fork, nor detach from the controlling terminal.
-R name, --rng-driver=name (default: stream)
Entropy source driver. stream is a general purpose Unix stream
input driver, capable of receiving data from a file, named pipe,
or Unix character device. viapadlock is an user-space driver
for the TRNG embedded on some VIA CPUs that have the VIA PadLock
-o file, --random-device=file (default: /dev/random)
Kernel device used for entropy output.
-r file, --rng-device=file (default: /dev/hwrng)
Kernel device, fifo or file used for entropy input by the stream
entropy source driver.
Selects known-good defaults for some HRNGs. help lists all
known HRNGs. You can override these defaults selectively with
the other options.
-H n.n, --rng-entropy=n.n (default: 1.0)
Entropy per bit of input data. This is a floating point number
between 0 and 1 (inclusive).
-Q quality, --rng-quality=quality
Selects the quality of the random data an entropy source will
generate. Quality is: default, low, medium or high. Do not use
anything but high if the entropy sink will use the random data
directly, instead of using it to seed a PRNG/entropy pool.
Ignored by the stream entropy source driver.
-B n, --rng-buffers=n (default: 3)
Number of 20000 bit buffers to use.
-s n, --random-step=n (default: 64)
Number of bytes written to random-device at a time. This number
should be even, and between 8 and 2500. Setting it too high
will cause rngd to dominate the contents of the entropy pool.
Values above 256 are unlikely to increase performance.
-W n, --fill-watermark=n[%] (default: 50%)
Once we start doing it, feed entropy to random-device until at
least fill-watermark bits of entropy are available in its
entropy pool. Setting this too high will cause rngd to dominate
the contents of the entropy pool. Low values will hurt system
performance during entropy starves. fill-watermark can be spec-
ified either as an absolute number of bits, or as a percentage
of the total size of the entropy pool.
-t n, --feed-interval=n (default: 60)
If feed-interval is not zero, rngd will force-feed entropy to
the random device even when the entropy pool is full every feed-
interval seconds. Setting this too low may cause rngd to domi-
Give a short summary of all program options.
Print program version.
Depending on its settings, rngd can dominate the kernel's entropy pool,
by feeding it so much data, so often, that other sources of entropy are
mostly ignored or lost. Do not to that unless you trust rngd's source
of random data ultimately.
Also, there is only so much bandwidth available from a HRNG, and it is
often not much. Don't drain too much with too low a feed-interval, or
rngd may not have enough data on its buffers when the kernel gets low
rngd will dump some statistics to its output channel hourly, or when
sent a SIGUSR1 signal. The output channel is either stderr when in
foreground mode, or syslog when in background mode.
Bits received from HRNG source is the number of bits received by rngd
from the entropy source. Bits sent to kernel pool is the number of
bits sent by rngd to the kernel, and Entropy added to kernel pool is
the number of bits of entropy in that mass of data that was informed to
FIPS 140-2 successes and FIPS 140-2 failures counts the number of
20000-bit blocks either accepted or rejected by the FIPS 140-2 tests.
The other statistics show a breakdown of the FIPS 140-2 failures by
FIPS 140-2 test. See the FIPS 140-2 document for more information.
HRNG source speed measures the speed of the entropy source, for reading
a 20000-bit block of data. FIPS tests speed measures the speed of the
FIPS 140-2 tests for a 20000-bit block of data.
Lowest ready-buffers level records the lowest number of ready buffers
(i.e. that can be fed to the kernel) hit so far. If it gets to zero,
you may want to increase rng-buffers. If it is always above 1, you may
want to decrease rng-buffers.
Entropy starvations counts the number of times the kernel asked rngd
for entropy, and rngd had none to give. Time spent starving for
entropy records the time spent by rngd waiting for a buffer with good
entropy to become available, during such an episode.
rngd is multithreaded. If the threading implementation shows up as
multiple rngd processes, signals should be sent to the process listed
10 if there are problems with the parameters, or if rngd fails to lock
the pidfile, or if rng-device cannot be opened.
11 if an input/output error happens.
12 if an operating system or resource starvation error happens.
The /dev/random driver in Linux kernels up to 2.4.28 inclusive (and
probably later ones too) has broken entropy accounting, and fails to do
catrastrophic reseeds correctly. This has been fixed in Linux 2.6.
It is a very bad idea to shrink the capacity of the kernel entropy pool
with rngd running, as it may misbehave the next time the pool drains.
Please restart rngd every time the kernel entropy pool size is changed.
/dev/random, /dev/hwrng, /var/run/rngd.pid, /proc/sys/kernel/ran-
FIPS PUB 140-2 Security Requirements for Cryptographic Modules; NIST;
NIST Special Publication 800-22; NIST;
The Intel(R) Random Number Generator; Cryptography Research, Inc.;
Evaluation of VIA C3 Nehemiah Random Number Generator;
Cryptography Research, Inc.; 2003; http://www.cryptography.com/
Jeff Garzik <email@example.com>
Henrique de Moraes Holschuh <firstname.lastname@example.org>
rng-tools 2-unofficial-mt.14 February 2005 RNGD(8)
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