Install RNG-Tools.

Check system entropy levels with this watch command:

watch -n 1 cat /proc/sys/kernel/random/entropy_avail

Then install RNG-Tools via aptitude like this:

sudo aptitude install rng-tools

Start, stop and control RNG-Tools on an Ubuntu/Debian system:

sudo service rng-tools start
sudo service rng-tools stop
sudo service rng-tools restart
sudo service rng-tools force-reload

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Add RNG-Tools as a startup service.

Add rng-tools as an Ubuntu service from starting on boot:

# sudo update-rc.d rng-tools defaults
sudo systemctl enable elasticsearch.service

Remove rng-tools as an Ubuntu service from starting on boot:

# sudo update-rc.d rng-tools remove

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Configure RNG-Tools.

Once installed open up the RNG-Tools config file:

sudo nano /etc/default/rng-tools

And add values for HRNGDEVICE and RNGDOPTIONS like this:

HRNGDEVICE=/dev/urandom
RNGDOPTIONS="--fill-watermark=90% --feed-interval=1"

Add those values to the bottom of each respective commented out area for easier contextual reference:

# Configuration for the rng-tools initscript
# $Id: rng-tools.default,v 1.1.2.5 2008-06-10 19:51:37 hmh Exp $

# This is a POSIX shell fragment

# Set to the input source for random data, leave undefined
# for the initscript to attempt auto-detection.  Set to /dev/null
# for the viapadlock and tpm drivers.
#HRNGDEVICE=/dev/hwrng
#HRNGDEVICE=/dev/null
HRNGDEVICE=/dev/urandom

# Additional options to send to rngd. See the rngd(8) manpage for
# more information.  Do not specify -r/--rng-device here, use
# HRNGDEVICE for that instead.
#RNGDOPTIONS="--hrng=intelfwh --fill-watermark=90% --feed-interval=1"
#RNGDOPTIONS="--hrng=viakernel --fill-watermark=90% --feed-interval=1"
#RNGDOPTIONS="--hrng=viapadlock --fill-watermark=90% --feed-interval=1"
#RNGDOPTIONS="--hrng=tpm --fill-watermark=90% --feed-interval=1"
RNGDOPTIONS="--fill-watermark=90% --feed-interval=1"

Note that on Ubuntu 14.04 and above that RNGDOPTIONS with --feed-interval doesn’t work. So just set that to something like this:

RNGDOPTIONS="--fill-watermark=90% "