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At 4:29 PM -0500 1/6/2000, Paul S Vail wrote:
Re: load balancing
From: Men & Mice Support
Date: Thursday, January 6, 2000
Time: 10:00:00 pm
>>>I have another question. When setting up the domain (be it normal or
>>>virtual), the qdns wizard creats a MX record with the parameter 2 as
>>>mail.xxx.xxx rather than the name server as is apparently depicted above.
>>> Then, the mail.xxx.xxx is used as a CNAME, which is picked off by DNS
>>>Expert as a no-no (yet it works o.k.) thoughts?
>>
>>The assistant will suggest using the name "mail.xxx.xxx" (but you can
>>choose another), and it will also create an A record for that name.
>>In this case, you should not create a CNAME record for it.
>>
>>To create the above using the assistant, you'd override the suggested
>>mail server names with "primary" and "pokie", then manually create
>>the CNAME record for "mail" (if desired).
>>
>>The point of the CNAME record named "mail" (above) is so that you can
>>tell users to enter "mail.example.com" as their POP3/IMAP and SMTP
>>servers. However, you should never use a CNAME alias in an MX record
>>- doing so will cause some mail to bounce, and you may never find
>>about it.
>
>I use the same server for both smtp and pop, and did start out naming it
>mail.xxx.xxx for simplicity.
>
>I'm still not clear about the benefits of using CNAME over an A record,
>unless IP numbers switched about, for simple service.
Paul,
If you don't fully understand CNAME records, then you shouldn't use them. That said, let me try to spell out the ramifications of each:
A record:
1) Associates name with address. Name can then be used in service records (NS and MX).
2) Some services expect A record to match PTR record, but having multiple PTR records for an address is counterproductive; for the most part, this is not an important consideration.
3) If address changes, A record must be changed to match. If many records resolve to the address in question, then all must be changed individually. This can be a nightmare if you have many domains (but can be solved with AppleScript).
CNAME record:
1) Associates one name with another - says "alias x to y", so that x takes on all characteristics of y (in theory).
2) Because of (1), no other record can be named x - there can't be A records, MX records, NS records, or anything else named x.
3) Also because of (1), there can be no subdomains of x, meaning that if name.x is an alias, it is an error for any record to be named ns1.name.x.
4) Because of both (2) (and (3) as well, for most configurations), if a name is delegated from another name (e.g. example.com), it can't be an alias (can't have a CNAME record) because it must have other records and will usually have subdomains (e.g. www.example.com)
5) It is an error for a service record (NS or MX) to point to an alias. Such records must point to names that have A records.
6) If an address changes, it doesn't directly affect an alias, because the alias doesn't resolve to an address. When the canonical name (defined with an A record) is changed, the alias will automatically change with it.
____________________________________________________________________
Chris Buxton cbuxton@menandmice.com
Men & Mice http://www.menandmice.com
Makers of: QuickDNS Pro
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