I am beginner in Security. My first assignment is about "Linux firewall using IPTABLES" . I know about Ubuntu, backtrack and kali Linux.
How to choose a Linux distribution to learn about security?
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For such a beginner and general subject (no offense! everybody has to begin somewhere!) such as iptables, any Linux distribution would be fine.
Choose one with which you feel comfortable, does not cause incompatibilities with your hardware and provide a large choice of packages so you will not be limited when having to install new software. DistroWatch is a good information source about current Linux distributions.
Afterward, depending on your studies, you may evolve toward a more specialized distribution. Some distribution provide already installed software or configuration hard to adapt on others (however the base system will still remain the same: Linux remains Linux, so the general experience and knowledge acquired on your favorite distribution will remain valid on others).
As a final word, I see a general tendency when dealing with security to always go toward Kali Linux, more for a fashion than objective reasons. Do not hesitate to try different distributions (virtual machines are great for such things!) since most provide a different feeling to the user.
Linux is meant to provide a free and open world so take advantage of this liberty to find a distribution with which you feel comfortable, most tools can be installed afterwards. The worse you could do would be to restrain yourself to a distribution making you feel uncomfortable: it would restrain your learning too.
I've never used much Ubuntu and never used BackTrack. But Kali is supposedly an improved distribution of BackTrack since it was (and is) developed by the same developers. (and BackTrack is no longer being maintained)
And besides that Kali already comes with a lot of tools to test security, make penetration tests, and it comes with iptables also, so you can use that for your assignment.
Kali it's used more to what is called "Offensive Security".
If you want to study offensive security Kali may be the best option due to it's many tools that come out of the box. However if you're starting to work with Linux, Kali may not be the most suitable distribution to learn the basics.
In any case, answering to your question the best way to choose a Linux distribution to learn about security I guess it could be searching for Linux distributions that come with security tools and reading the documentation for those distributions.
As the very good posts above have mentioned, there is no perfect fit for 'the right distribution' to learn Linux. The good thing though, is all of them will be very helpful in learning Linux given their common foundation. The reason nobody is willing to recommend a flavor of Linux is not because we all do not have preferences, but rather because the preferences are personal choices and not necessarily reasons for one being better/worse than the next.
So, where to start?
I would start with virtual machines. As long as the specs on your system are sufficient to be able to run at least one VM at a time, you can play with all the Linux distributions you would like. The simple act of installing a new flavor and configuring the network, iptables, etc., will be an immense learning opportunity. Personally I have installed about 15-20 different flavors of Linux as VM's this year. Most of them were to either test something specific, or just to see how something new worked. At this point I only have 2 VM's left standing as they were my last test systems. The beauty of it is I can install as many as I have storage for (It gets more restrictive regarding resources if I want them all running at the same time, but otherwise it is all about disk space).
Last but not least... unless you are actively learning penetration testing or ethical hacking, you do not need the tools that something like Kali provide. You mention learning iptables. Well I would then start by learning iptables. This can be done by reading the man page, and reading articles on configurations, etc. Ideally having a system to create/destroy/change rules would be good, but it is not a requirement to learn the capabilities of iptables. And you definitely do not need penetration tools if you are just learning how iptables works (the penetration/hacking comes later).