Enough has been said about bandit, it is a terminal-based series of challenges.
Visit bandit.labs.overthewire.org to begin.
If you get stuck, lots and lots of walkthroughs are available on the internet. But don’t resort to them instantly, rather try to use the hints to solve the problems on your own to learn.
Last night, I made an attempt to solve the problems from Bandit Level 0 to Bandit 15. The walkthrough is in Bangla Language. I recommend watching it in 2x speed.
The session was interrupted due to internet connection problems.
Quak Quaks of the Ugly Duckling 