At 09:32 04/01/2003 -0500, you wrote:

Hi Alan,

I have a pressing engagement, so I must be brief. If I miss something, feel free to ask -- I can follow up later.

It is possible to have redundant servers, most mid-sized and large ISP's, as well as e-mail providers and corporations do so. It takes careful planning, hardware and solid knowledge of how the software you use works.

You can certainly create a duplicate mail server, for redundancy, that can switch over to a primary role as necessary. You must ensure that the mail software on each machine is identical, as are the configurations. Using RSYNC, for instance, you can keep the contents of the mailboxes current (give or take N number of minutes between each rsync). However, that machine CANNOT act as a backup mail host -- it will simply deliver mail to an existing mailbox on the machine. Of course, the control and user files on the primary machine must be identical on the redundant host, and kept as current as possible (in case users change their passwords, for example). If you use a seperate LDAP or SQL host, you are better off.

There are other options for you. Consider creating a central NFS mount which stores the mailboxes (using RAID 0+1 or 10 preferably, read up on the difference if you don't), and have each host (primary and redundant) configured identically and connect to the NFS share. The benefit to this model is you can also perform load balancing, so each host can be a primary. Depending on how you configure your router, or if you choose to use something like "heartbeat" with IP failover, the failure of one machine will not effect the other. Of course, then the NFS share is a potential source of failure, and you must take that into account as well.

I do know some have been working on mail delivery to a database (like MySQL or Oracle) instead of to a file system (local or mounted), but I have not seen this used with Vpopmail before. That is another option.

Good luck, let me know how it works out for you.

-- Steve


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

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 '' and ''.  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

The VirtualHost entries would look like this:



Our router is then setup with "priority load
balancing": it knows that if the host header contains, 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,

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


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