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.
----------
[*] 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will be withdrawn one day, but there's no sign of it yet.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.
