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Custom Domains And Email Delivery, #2

There are a lot of details involved, in setting up a custom domain. Some details are not so apparent to every blog owner, when choosing a DNS model.

From time to time, I get confused queries about whether there is a practical difference between the "asymmetrical" and "symmetrical" DNS models.

Let's compare the "Symmetrical" and "Asymmetrical" DNS address models.

Blog Published To Domain Root - Symmetrical DNS Configuration

The most obvious configuration uses a symmetrical DNS structure - dual "CNAME" referrals. This will interfere with any auxiliary services, hosted outside Google. Here, you may publish to "mydomain.com" or to "www.mydomain.com", at your discretion.
mydomain.com. 3600 IN CNAME ghs.google.com.
www.mydomain.com. 3600 IN CNAME ghs.google.com.

Blog Published To Domain Root - Asymmetrical DNS Configuration, aka "Google Apps"

If you have a domain with email, FTP, and other possible auxiliary services, or if your registrar can't provide "CNAME" referral for the domain root, you use an asymmetrical structure, which uses Google Apps to let you add auxiliary services, with quadruple DNS server redundancy. Here, you may publish to "www.mydomain.com" only.
mydomain.com. 3600 IN A 216.239.32.21
mydomain.com. 3600 IN A 216.239.34.21
mydomain.com. 3600 IN A 216.239.36.21
mydomain.com. 3600 IN A 216.239.38.21
www.mydomain.com. 3600 IN CNAME ghs.google.com.

If you are setting up a new domain, 9 time out of 10 you can flip a coin. If you are prepared to publish to "www.mydomain.com", either model is acceptable. With an existing domain, and services like email being used, the difference between the asymmetrical and symmetrical models may be crucial to the success of the migration of the blog to the new URL.

Many existing domains, for established companies, have auxiliary services associated with the domain root ("naked domain"). One of the most common services involves email servers, which accept incoming email. Domain email services are defined with "MX" records.

mydomain.com. 900 IN MX 10 mx1.mydomain.com.
mydomain.com. 900 IN MX 10 mx2.mydomain.com.

With some DNS servers, definition of a "CNAME" referral for "mydomain.com" redirects all services, including "MX", to the "CNAME" target. As soon as you add a symmetrical DNS to support a Google custom domain, incoming corporate email delivery stops working.

mydomain.com. 3600 IN CNAME ghs.google.com.
www.mydomain.com. 3600 IN CNAME ghs.google.com.

Many registrars will not allow "CNAME" referral for the domain root, for this reason. This is one reason why Google developed the asymmetrical model - and the Google Apps "216.239.nn.21" server series.

mydomain.com. 3600 IN A 216.239.32.21
mydomain.com. 3600 IN A 216.239.34.21
mydomain.com. 3600 IN A 216.239.36.21
mydomain.com. 3600 IN A 216.239.38.21
www.mydomain.com. 3600 IN CNAME ghs.google.com.

For an existing domain, the asymmetrical model is almost always preferred. And with an asymmetrical model, be prepared to publish to "www.mydomain.com" - and (if desired) select "Redirect mydomain.com to www.mydomain.com.".

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