Eric Crist wrote: [ ... ]
One of my friends needs backup DNS/Mail in the even their connection goes down. How do I go about setting it up so that his user base (about 80 users) will not see any problems in mail transmission and reception if their primary servers go offline. I would like mine to automatically pickup the slack.
Setting up redundant DNS is trivial: DNS is designed to do that.
Setting up a backup MX is easy. It helps a lot if both mailservers are configured the same, and it is important that they have near-identical anti-spam and virus-filtering technologies.
Setting up a truly redundant POP/IMAP reader box is extremely hard.
To solve this problem for a local network, one normally uses a shared NAS box: in your case, this effectively requires one to set up a network-distributed filesystem, or some near-equivalent: for instance, a parallel database for mail stroage would serve as well. All sorts of nasty issues-- like the security of the data going between the two fileservers, or DBs, or whatever; significant added latency due to the storage mechanism confirming updates have propogated; etc appear.
My opinion is that it's better to go with a primary mail reader box and make very certain that box doesn't go down by using redundant hardware and a backup network link is easier and less likely to suffer from the "lets create a complex system with lots of moving parts which never gets fully tested and thus breaks in some weird way when the unexpected happens" syndrome. :-)
-- -Chuck
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