I'm trying to configure GitHub with my naked domain.com (APEX in GitHub) and www.domain.com.
I have username.github.io working OK.
My A record is pointing to 192.30.252.153 and 192.30.252.154.
I then have two CNAME records set:
www.domain.com→domain.comio.domain.com→username.github.io
This configuration, however, is not working.
Accessing domain.com results in a 404 on GitHub. Meanwhile, accessing www.domain.com results in an ERR_NAME_NOT_RESOLVED.
Existing documentation is confusing and there's no clear way of configuring things end-to-end.
Can someone tell me what I'm missing?
                        

Here's what I've done to get domain.com and www.domain.com working you'll need this:
I think the wildcard A record may be optional in your case but I'm not certain.
Edit: Removed wildcard A record. @mheadroom pointed out that GitHub recommends against this entry because it would allow anyone to host a GitHub Pages site on one of your subdomains! https://help.github.com/articles/tips-for-configuring-a-cname-record-with-your-dns-provider/