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.