Security of any operating system is one of the primary responsibilities of any Linux system administrator. I must say that, its also one of the toughest tasks, for a Linux system administrator. You cannot be very certain, that the machine that's under you is a secure machine(there exists no machine on the internet, which can be called as completely secure).
Oh yeah..You can make your machine, the most secure machine in the world, just by pulling out the Network Interface Cable(Joking...).
Although it was just a joke, if you think a bit deeper you will come to know, that its quite true. You cant make a system completely secure. Because just imagine the number security patches, a software vendor roles out each and everyday(Thanks to open source community, as they are doing a great job in this area).
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