Haveged
The haveged project is an attempt to provide an easy-to-use, unpredictable random number generator based upon an adaptation of the HAVEGE algorithm. Haveged was created to remedy low-entropy conditions in the Linux random device that can occur under some workloads, especially on headless servers.
Installation
Start and enable haveged.service.
List available entropy
If you are not sure, whether you need haveged, run:
# cat /proc/sys/kernel/random/entropy_avail
This command shows you how much entropy your server has collected. If it is rather low (<1000), you should probably install haveged. Otherwise cryptographic applications will block until there is enough entropy available, which eg. could result in slow wlan speed, if your server is a Software access point.
You should use this command again to verify how much haveged boosted your entropy pool after the installation.
Alternative
Unless you have a specific reason to not trust any hardware random number generator on your system, you should try to use them with the rng-tools first and if it turns out not to be enough (or if you do not have a hardware random number generator available), then use Haveged.
Virtual machines
As discussed at Is it appropriate to use haveged as a source of entropy on virtual machines?, it can be contested whether haveged provides quality entropy within a virtual environment. Haveged relies on the rdtsc instruction, which may be virtualized within a virtual machine resulting in lower quantity entropy. On some hypervisors, it is possible to disable the virtualization of rdtsc, which would in theory allow haveged to provide higher quality entropy.