Re: [Exim] Canonicalizing Addresses

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Szerző: Philip Hazel
Dátum:  
Címzett: John Lawless
CC: Bernard Stern, Exim List
Tárgy: Re: [Exim] Canonicalizing Addresses
On Fri, 17 Sep 1999, John Lawless wrote:

>      OK.  So, MX is best for foreign hosts but bydns_a can sometimes be
> necessary for internal routing.


Yes, but think of foreign *mail domains* rather than hosts.

Perhaps this way of looking at it will help:

You use MX records when you want to find out which hosts to use for
mail to a specific mail domain. MX records map mail domains to host
names. [*]

You use A records when you want to find the IP address for a specific
host name.

In other words, which you use should depend on the type of name you have
in your hand.

Given a random email address on the net, you cannot assume that the
domain is actually a host name, so you must go for MX records. If, on
the other hand, the domain is one of "yours", you might know that it is
in fact also a host name - or you might want to route mail for a specific
domain to a specific host. In both these cases, you would use A records.

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[*] There is one historical "backwards compatibility" wrinkle: if there
are *no* MX records for a given domain, then you are permitted to try
for a host of the same name. I keep hoping this historical confusion
will be withdrawn one day, but there's no sign of it yet.


-- 
Philip Hazel            University of Cambridge Computing Service,
ph10@???      Cambridge, England. Phone: +44 1223 334714.