On Thursday 12 January 2012 22:14:41 Jakob Curdes wrote: > Miles Fidelman wrote: > > - you can set up a 2ndry server (give it an MX record with lower > > priority than the primary server) - it will receive mail when the > > primary goes down; and you can set up the mail config to forward stuff > > automatically to the primary server when it comes back up -- people > > won't be able to get to their mail until the primary comes back up, but > > mail will get accepted and will eventually get delivered > > Just one additional note: in such a setup, you should not assume that > the secondary server only receives mail when the first one is down from > your side of view. > A client somewhere might have a different connectivity view and might > deliver mail to your secondary MX at any time. It is well-known that > spammer systems even try to deliver to the secondary in the hope that > protection there is lower. So, if you have a secondary, you must arrange > for mail delivered to that server to be passed on to the primary or a > separate backend server. And you need to protect it exactly as good as > your primary against virus, spam, and DOS attacks.
So: If you go through the hazzles to set up a second receiving host with the same quality and administration requirements as the first one, you will also want to reflect that by giving it an equally high score in the mx field. That way both servers will be used equally and you get load-balancing where you originally meant to buy hot-standby:-) Another comment from here: Email is such an old protocol that the immunity to network errors was built in. If a sending host can't reach the receiver, it will try again after some time. And then again and again until a timeout is reached. And that timeout is not 2-4 seconds like with many tcp-based protocols but 4 days giving the admins the chance on monday to fix the mailserver that crashed on friday evening. Of course, if you rely on "fast" mail for your business, the price of redundant smtp and redundant pop3/imap servers might pay off. For redundant pop3/imap the cyrus project (and probably the other too) seem to have a special daemon to sync mails and mail-actions across servers. Add a redundant master-slave replicating mysql (or postgres) for the account database or even ldap and you should get something that even scales beyond 2 machine. Completely off-topic for this list as I haven't thrown in any heartbeat, pacemaker, corosync or drbd at this point. Have fun, Arnold
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