Le 19/07/2023 à 23:03, Michael Peddemors a écrit :
In theory, that is how it is SUPPOSED to work, in practice (and we
have lots of history where customers ran into this problem when one
went down), I believe that it was Outlook that didn't try an
alternative IP address for a 20 min internal cache for instance,
before a requery of the DNS was done, at which time it again would
choose which IP to connect to. As well, SOME modems would get the two
results, and return only one to the client. And lots of libraries we
see, do the DNS query, get two IP results, but then only use the first
one returned, etc..
The windows cache is supposed (and is confirmed on my side ) to work the
same as other DNS cache: It will cache all the A records.
Outlook being a good IMAP client is another story :-)
Not arguing how it is supposed to work, just forewarning those to be
ready when it doesn't work like the manual says.. (Everyone hates
phone calls about email being down).
If you want to be certain, only a true load balancer will fit the bill.
Oh, and another PS.. IF you are going to do round robin, suggest you
make two (2) MX records, and put two IPs in both, and then equal
weight the two MX's.
That is exactly what should not be done.
Never put more than one IPv4 or IPv6 behind a FQDN pointed by a MX.
It will kill the proper HA algorithm build in the SMTP/MX protocol. You
will introduce some unnecessary delivery delays/retry backoff in case of
one server failure.
Put as many MX records has you have SMTP gateways. Or group some
gateways behind some LB VIP if you have/need a high count of gateways.
Keeps a more even load, given those that only prefer the first MX
returned, and those that prefer the last (spammers)
There is no ordering, round robin apply here too.
MX are for MTA to MTA communications. Talking about MUA/clients, they
don't care/use MX.
Emmanuel.
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