Re: [cobalt-users] RE: Raq4 - Fail Over/Backup Email for a domain..

[Jeff Lasman <jblists@xxxxxxxxxxxxx>]

Jerry Farquhar wrote:

> So.. back to my original issue.. is it possible to set the MX record to send
> with the same priority email messages to two separate domains?  Do you know
> of anyone who has tries it?

I don't see how you can use MX to send to two separate domains.

Here's how I'd do it...

First of all I'd run both DNS and email services on two geographically
dispersed servers...

Each server should run master DNS for the client's domain.  Neither
server runs as a slave.  The files are slightly different, though:

In addition to the usual a records, have one for mail1.example.com and
for mail2.example.com, pointing to each server.

However, each server should only have MX pointing to itself, so server
#1 should have an MX record pointing to mail1.example.com, and server #2
should have an MX record pointing to mail2.example.com.

The TTL on the MX records should be one minute or less for maximum
uptime.

Each server should be set to accept/relay email for the domain.  Create
entries for all the domain users in virtusertable, like this:

username@xxxxxxxxxx  <tab, not spaces>  username-relay

Also set up aliases (in the aliases file) as follows:

username-relay:  <tab>  username,username@xxxxxxxxxxxxxxxxx

(where username is the local user for the local server, and mailx is
mail2 on the mail1 server and mail1 on the mail2 server).

This way, the mail will be initially delivered to whichever server
serves DNS to the sending mailserver, and then from that box, be
redelivered both locally and to the foreign server.

If/when one server goes down, the other will get all the mail locally,
and continue to try to send it to the foreign server.

To make it easy for client users to pick up mail, each server's DNS can
also point pop.example.com to the IP# for it's own box, so users can
just set their email client to pick up mail from pop.example.com and get
served by the fastest server to respond if both are up, or the one
that's up, if one is down.

There are several disadvantages, the most important being that mail will
be on both servers, so mail will most likely get downloaded twice.  The
only way I can think of to keep this from happening is to have a
centralized mail store, which no matter how hardened would then become a
single-point of failure; using my method there is NO single point of
failure).

Another disadvantage is that DNS is often cached on local client systems
and networks; if one of the servers should go down, then client users
might have to reboot their systems, or the DNS on their local networks.

Have I forgotten anything?

Jeff
-- 
Jeff Lasman, nobaloney.net, P. O. Box 52672, Riverside, CA  92517 US
Internet & Unix/Linux/Sun/Cobalt Consulting +1 909 778-9980
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