Varun Varma posted in linux-india-help:
The DNS resolver subsystem will send a query to lookup the MXs for mail.abc.net and the DNS would point out that mail.abc.net is a CNAME for abc.net. Depending on how the DNS is setup, either the client side resolver would then resend the query as MX records for abc.net or the DNS server would fetch this for the client and directly return the MX records for abc.net. Either way, the MX entries for abc.net would be returned.
Interesting. I didn't know that the DNS RFCs allow this. Do you have any authoritative references on the validity of this trick?
This "trick" is part of the SMTP RFC - Section 5 of RFC2821 [Simple Mail Transfer Protocol]:
ftp://ftp.rfc-editor.org/in-notes/rfc2821.txt
A more detailed explanation appears in the now obsoleted RFC974[MAIL ROUTING AND THE DOMAIN SYSTEM]:
ftp://ftp.rfc-editor.org/in-notes/rfc974.txt
If you are looking at the DNS RFCs, you are looking in the wrong place. Actually, even if you are looking at the DNS RFCs and understand what CNAMEs are, you will see why this would work.
Please take the time to read these, it will clarify your doubts about DNS and Mail routing.
BTW, is it my imagination or does a "dig mx outblaze.com" only return one MX, the MX has only one A record associated with it? What I am missing here?
When you lookup a DNS entry, you get one. What is so surprising about it?
What is so surprising is that all production mail systems have atleast one backup MX associated with them. I am sure that Suresh's book would cover that.
Is there any RFC that says one MUST have multiple MXes, each of them with multiple A records?
Not that I am aware of. Are you aware of any RFC that says that you MUST backup data from/cluster production servers? Or you don't do that because no RFC says that it's a MUST...
-- Regards, Varun Varma --------------------------------------- Mindframe Software & Services Pvt. Ltd. http://www.mindsw.com ---------------------------------------
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