I just wanted to share the solution i found for a problem i had because it was quite hard to find how to solve it only by searching on google. So here goes:
If you are running a server on a Linux based operating system - in my case i was using Ubuntu Server 15.10 - and you get the error "Authentication servers are down. Please try again later, sorry!" while trying to connect to your server, i know two ways to fix this:
1) Edit your server.properties file and set the value of online-mode to false.
2) Run this command in the terminal of your Linux based OS:
P.S. I would not recommend the first option, since it disables the verification process your server does when a player connects to it. The consequence of this is (i) all the players on the server will have the default minecraft skin, and (ii) anybody could use a cracked version of the game and connect to your server with with your player name and do whatever he wants with your inventory.
Big thanks to cubethethird for posting the 2nd solution on his website.
That's it! I hope this helps some peeps out there having trouble with minecraft on linux