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This section is a Work In Progress and may be incomplete or require additional testing and documentation.

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Why This Matters

Entropy is randomness collected by the operating system from various sources (keyboard timings, mouse movements, disk activity, network traffic). This randomness is crucial for:
  • Generating cryptographic keys
  • Creating secure random numbers
  • SSL/TLS operations
  • SSH key generation
  • Password salt generation
Insufficient entropy can weaken cryptographic operations and make your system more vulnerable to attacks.

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What is rng-tools

The rng-tools package provides utilities for using hardware random number generators and feeding entropy into the system’s random number pool (/dev/random).

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Installation and Configuration

1

Install rng-tools

On Debian based systems:
sudo apt-get install rng-tools
2

Configure hardware device

Set the hardware device used to generate random numbers by adding this to /etc/default/rng-tools:
HRNGDEVICE=/dev/urandom
Or use this command:
echo "HRNGDEVICE=/dev/urandom" | sudo tee -a /etc/default/rng-tools
3

Restart the service

sudo systemctl stop rng-tools.service
sudo systemctl start rng-tools.service
4

Verify the service is running

sudo systemctl status rng-tools.service

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Checking Entropy Levels

You can check your current entropy level: 
cat /proc/sys/kernel/random/entropy_avail
A value above 1000 is generally good. Values below 200 may cause cryptographic operations to slow down while waiting for more entropy.

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Testing Randomness

To test the quality of random data: 
cat /dev/random | rngtest -c 1000
This reads 1000 blocks from /dev/random and performs statistical tests on the randomness.
For production systems, especially virtualized environments, consider using hardware random number generators (if available) or entropy-gathering daemons like haveged instead of or in addition to rng-tools.

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Additional Resources

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Notes

Virtual Machines

VMs often have low entropy. Consider using virtio-rng or haveged for better performance.

Hardware RNG

Modern CPUs have hardware RNG (RDRAND). rng-tools can utilize this if available.

/dev/random vs /dev/urandom

/dev/urandom is generally recommended for most use cases and won’t block on low entropy.

Monitoring

Monitor entropy levels regularly, especially during high-load periods.
This section needs additional testing and validation. The configuration above is a starting point but may require adjustments based on your specific system and security requirements.

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