On Tue, 29 Jun 1999, Michael Jenner wrote:
> I'm looking for a way to setup a mailsystem with two servers, one
> is the primary and the second is a backup server. If the primary
> fails the clients should automatically turn to the second server
> - for incoming as well as outgoing mail. Mails (/var/spool/mail -
> or xx) should be available from both hosts.
You are talking about three different things. The first one is easy, and
the second two are harder. I'm only able to answer the easy question, but
will try to clarify the harder two.
For incoming mail, there is a mechanism built into the standards for
backup. These are the MX records for name service. If you lookup up the
name Cranfield.ac.uk as an MX record (as mail systems do when they send
mail to us), you will find something like
cranfield.ac.uk preference = 5, mail exchanger = euler.central.cranfield.ac.uk
cranfield.ac.uk preference = 10, mail exchanger = neptune.pegasus.cranfield.ac.uk
This makes euler our first preference and neptune a backup. When euler is
down or refusing SMTP connections, other mail servers will then try to
pass mail to neptune. Neptune will pass on what it can, but might queue
things that need to be processed on euler.
The second issue is how to have backups for out-going mail. Mail clients
usually don't respect MX records to the same mechanism doesn't work. I
know that some sites have managed to do what you want (large ISPs for
example), but I don't know how. I suspect that they might be using a
network switch at a particular address and that that picks one of the real
hosts for doing things.
Finally, you've asked about mail access and delivery. While exim does do
mail delivery, your question is really about a mail access question.
The obvious answer is to have the delivery onto some file system that is
then served up to several hosts each running their own POP or IMAP servers
for mail access. The difficulty with this is that the most common way to
have a file system available across several machines, NFS, is generally
considered unsafe wrt to the file locking that needs to be done.
(Remember, that several different processes may be trying to modify the
mailboxes, exim (or other delivery agent) and the mail access server (POP3
or IMAP servers).
Here I would like to ask a "me too" and would be interested to know how
others do this. But that is not really an exim issue, but if someone is
using exim deliveries to a system that gets NFS mounted to other systems
running UW-imapd, I would like to hear about it.
Oh, why hasn't O'Reilly come out with a good "Mail systems management"?
If we weren't under staffed at Cranfield (anyone want a job at a nice
friendly university computer center? [serious]), I would take a year off
and research and write the book myself (though there are others far more
qualified; I'd have to research a lot). I know that PH is working on a
book, but I'm not quite sure what it's scope is supposed to be.
-j
--
Jeffrey Goldberg +44 (0)1234 750 111 x 2826
Cranfield Computer Centre FAX 751 814
J.Goldberg@??? http://WWW.Cranfield.ac.uk/public/cc/cc047/
Relativism is the triumph of authority over truth, convention over justice.
--
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