Entropy
From DikapediaV2
Reference: https://major.io/2007/07/01/check-available-entropy-in-linux/
Check available entropy in Linux Sometimes servers just have the weirdest SSL problems ever. In some of these situations, the entropy has been drained. Entropy is the measure of the random numbers available from /dev/urandom, and if you run out, you can’t make SSL connections. To check the status of your server’s entropy, just run the following:
# cat /proc/sys/kernel/random/entropy_avail
If it returns anything less than 100-200, you have a problem. Try installing rng-tools, or generating I/O, like large find operations. Linux normally uses keyboard and mouse input to generate entropy on systems without random number generators, and this isn’t very handy for dedicated servers.