Help with Facebook Site URL / Site Domain (using Domain Forwarding)

I've posted this on Facebook developers forum, but things seem to move slowly over there and support is scarce so I'm trying stackoverflow:

I have a website using FB login button. My architecture is kind of simple:

I have a domain (let's call it example.com) on whois.com. My application is running in an EC2 instance inside Amazon with an elastic IP (let's call it 1.2.3.4). I configured domain forwarding on whois.com, so all requests to example.com* are forwarded to 1.2.3.4*. Everything works fine except that I cannot config开发者_运维知识库ure Site URL and Site Domain correctly. I get "API Error Code: 191" with every configuration I've tried...

It works if I use the EC2 public DNS address, but I obviously don't want to use that...

Anyone know what should be the correct Site URL and Site Domain?

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Site domain has to match the domain that appears in the address bar of the user's browser when they are interacting the Facebook-related parts of your site.