401k after tax conbribution (mega backdoor) eligibility

My work 401k allows AFTER TAX contribution and I want to know who is eligible of doing that? 

 

Do I need to meet below requirements in order to do so?

1. Gross income exceed the contribution limit for Roth IRA (example: $240K for married couple)

2. Have max out the 401K contribution limit of $23000 or $305000 if older than 50.

 

I read a few places saying the above but not everywhere. And I can't find the answer from IRS site.  And, my work 401k let me know do so even I do not meet the above two points.  I want to know if I am violating the tax rule. Please hep.

 

Thanks in advance.