Hello,

I am the SysAdmin for a small webhosting/colocation
company, and we are in the midst of re-designing our
network, particularly our mail servers.
 
While in one of our discussions regarding failover
load-balancing, an interesting question was brought
up:

Is it possible to have the mail boxes in different
locations on the server, and direct the user to their
mailbox based on the domain, or IP address the request
is coming in on?

Let me explain what I mean, using our web servers as
an example (yes, I know that HTTP and mail are
different protocols, but please bear with me)...

We have two Linux web servers, let's say IP addresses
of '192.168.1.1' and '192.168.1.2'.  Each server is
setup to listen on each IP address.

Each domain we setup is setup on one of the servers as
it's "primary" server, where the files are stored
served from the local hard drive.

The site is then setup on the other server, which
would server the site off an NFS mount from an NFS
server (the NFS server uses 'rsync' at midnight each
night to grab the updates/changes from the web servers
themselves).

The VirtualHost entries would look like this:

Primary:
<VirtualHost 192.168.1.1:80>
    DocumentRoot
/websites/[0-9,a-z]/customerdomain.com
    ...
</VirtualHost>

Secondary:
<VirtualHost 192.168.1.1:80>
    DocumentRoot
/nfs/websites/[0-9,a-z]/customerdomain.com
    ...
</VirtualHost>

Our router is then setup with "priority load
balancing": it knows that if the host header contains
192.168.1.1, to go to Server A (which serves up
the site locally), but if he is down, go to Server B
(which serves the site off the NFS mount).  Of course,
the advantage to this setup is that we could lose
either server, and still be fine (some people just
would not be able to FTP their site chanes), *or* we
could lose the NFS server, and still be okay.  Of
course, if we lost a server *and* the NFS server, then
we piss off half of our customers :-)

So, now for the questions:

  1. Is something similar possible for mail servers
(it's not so much the contents of the email
directories I would care about as much as the
existence of the mailboxes themselves)
  2. If not possible (and I am almost sure it is not),
what is the reason?  I would guess it is the different
way the protocols work.  Is there material you could
point me towards to learn about this (the RFC's??)

(I do know from previous posts that Vpopmail/Qmail
does support IP-based domain hosting, but I am not
sure that this means it can serve up the contents of a
mail directory in a location other than, say,
'/home/vpopmail/domains/customerdomain.com/user/Maildir'...)

TIA for your patience regarding this slightly
off-topic post... :-)

Alan


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