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All People > Patrick-Dussault > Patrick Dussault's Blog > 2016 > December

Linux and Entropy

Posted by Patrick-Dussault Employee Dec 23, 2016

Entropy determine how predictable the second random number might be.
So if you have low entropy around 200, then the random numbers are
more predictable than if you have a pool of 2000. This might have the
impact on the kernel to face delay to get the most unpredictable
random number.


To get enough entropy on Linux system, close to the max of 4096bits, run
the rngd command as following :


# rngd -r /dev/urandom


You'll see the entropy pool increasing.


# cat /proc/sys/kernel/random/entropy_avail
# cat /proc/sys/kernel/random/entropy_avail



If you have launched a program in your PuTtY SSH session on Linux or Unix

box, and unfortunatly, you need to leave and you do not want to interrupt the execution

of the program, you can use the disown command to make your program not get the SIGHUP

when closing the session and keep it running in the background :



1 - Hit "ctrl+z" to pause the program and get back to the shell;

2 - Type "bg" to run it in the background;

3 - Type "disown -h [job-spec]" where [job-spec] is the job number;


To illustrate that :


$ XPSImport export.xml -fo -passphrase password -vT -l export.log -e export_error.log

(hit ctrl+z)

[1]+ Stopped XPSImport

$ bg

[1]+ XPSImport &

$ disown -h %1


then you can close or exit your session and the process will keep

running on the machine in the background.