Entropy

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Table of Contents

• Entropy
  • Alternative entropy sources

Entropy

Not available from C2S/CIS standard.

Entropy is the measure of the random numbers available from /dev/urandom.

It is important for a secure operating system to have sufficient quantities of entropy available for various crypotographic and non-cryptographic purposes, such as:

• generation of cryptographic keys
• TCP port randomisation (NAT, outbound connection)
• TCP sequence number selection
• writing random files for testing network functionality and throughput
• overwriting hard disks prior to reuse or resale or encryption

Alternative entropy sources

Rationale

It is generally recommended wherever entropy is used heavily to supply additional entropy sources.

Solution

Haveged

Haveged was created to remedy low-entropy conditions in the Linux random device that can occur under some workloads, especially on headless servers.

# Add haveged daemon to autostart
systemctl enable haveged

# For temporary change:
echo "1024" > /proc/sys/kernel/random/write_wakeup_threshold

# For permanent change (edit /etc/rc.local):
/usr/local/sbin/haveged -w 1024

Comments

To check the status of your server’s entropy, just run the following:

cat /proc/sys/kernel/random/entropy_avail

To check the maximum limit of entropy:

cat /proc/sys/kernel/random/poolsize

Useful resources

• Haveged