Jeffrey Goldberg wrote:
> 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.
True, that part is easy. However, when the primary server is brought back up it
should synchronize with the backup server to get updated. Also, I guess I have to
have the primary server make local deliveries AND forward each mail to the backup
server. Is this possible ?
> 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.
Ok - it's a minor issue. If the primary server breaks down I can have users change
the server entry manually. I have never "seen" such a network switch :o) Could you
please elaborate ?
> Finally, you've asked about mail access and delivery. While exim does do
> mail delivery, your question is really about a mail access question.
Ok. Currently, this is also a minor issue. I can have users switch the imap entry
manually as well.
> 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.
I'll be watching the O'Reilly section of Amazon :o)
/Michael
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