Do I need CAL for external users on dedicated AD

I'm wondering if I need Active Directory CAL for external users. The environment will be 2 SharePoint servers, 1 SQL server, and 1 Active Directory, that stores external users, who only connect to SharePoint (on extranet).

Then, on their ActiveDirectory, I would use DirSync to do a kind of copy, so my internal users would be able to connect to extranet sharepoint site by using their credentials.

I found some articles on google that say : "You don't need AD CAL because users won't access the AD Server" ... but I'm not sure I can rely on them.