What is an Alias (CNAME) record?
DO NOT USE ANY CNAMES UNLESS YOU UNDERSTAND THIS PERFECTLY.
You must be very careful when using CNAMEs! Please read this description carefully, as most people do not realize what CNAMEs actually do!
A CNAME is not the simple "deliver this A record instead of that one" mechanism that they at first appear to be when most people are first introduced to the concept. In fact, CNAMEs are intended to redirect all DNS based traffic for one domain to another. For this reason, it is a violation of the DNS protocol to have a CNAME and any other DNS record for the same domain name.
Here's why. It may seem a bit odd at first that if you have a CNAME redirecting "here.com" to "there.com", then that means you can't have a Mail Server (MX) record defined for "here.com" -- since that would be another DNS record for the same domain name "here.com" which is a violation. This is, in fact, the truth, and is also easily accounted for.
When an e-mail is sent to "root@here.com", the e-mail client requests the MX record for "here.com", and gets the CNAME instead. The e-mail client follows the CNAME to "there.com", and asks the DNS server at "there.com" for the MX record for "there.com"! The e-mail for "root@here.com" is then delivered to the mail server for "there.com", and that server had better be set up to handle e-mail for "here.com" or else that e-mail is going to get returned to sender!
It gets worse. Not all email clients understand this behavior of the DNS, so even if you set it up correctly, your email may get returned anyway! For this reason, if you set up a CNAME for the root of your domain, or if you set up a wildcard CNAME, you may not have any inappropriate MX records defined, and you will not receive technical support for e-mail related problems.
In addition, if your mail server is called "mail.anywhere.com", then you MUST NOT use a CNAME to define its IP address -- you MUST use an A record. Mail clients that send e-mail to "info@anywhere.com" will look for the MX record of "anywhere.com". They will discover that the mail server is called "mail.anywhere.com", and look for an A record. If a CNAME record is returned instead, the mail client will very likely not understand it, and return the e-mail to sender!
Once again, please read and reread this description until you are very clear what it means. The fact of the matter is that most people who use CNAMEs do not need them at all, and really should have used A records instead.
