The preferred way is to choose either www.example.com or example.com and let only one of them resolve, while the other should issue a 301 ("Moved permanently") back to the one that resolves.
The easier one to set up is to prefer the www version since it can be done with a simple CNAME and many DNS providers will offer a free automatic 301 forward for the non-www version. (But tastes differ on whether the non-www version looks better.)
Letting both versions resolve would give issues with Google indexing multiple versions of the same page, and reduces the ability to cache properly.
Re:Faster, More Awesome GitHub Pages
by tombrossman on Wednesday January 08, 2014 @05:50AM
(#7022796)
I'm looking to park a few low-traffic charity sites there and 'Set it and forget it' (like Ron Popeil). I think having both with and without the www working is a higher priority than SEO, so I'll have to compromise and set A records for example.com and CNAME for www.
If anyone figures out a better practice please leave a comment. My preference is no www (with www 301-ing) plus the CDN benefit, but this probably isn't possible.
AIUI: If your provider supports ALIAS then you set an ALIAS for example.com in addition to the CNAME for www. If your provider doesn't then you'll have to set an A for example.com to point to their IP, and the CDN won't work for example.com.
The way I read their documentation I set only a CNAME for www -> username.github.io. Does this mean I set no A record at all for the domain, to make sure I get the benefit of the CDN? If yes, how do I ensure requests for example.com resolve to www.example.com?