On 29 Jun 99, at 12:58, 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.
> 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.
I am working on it ... do not hurry, but those interested can write
me. It is just a daemon running on the same machine of the client
that recognize a number of smart-hosts, and try sending in turn. It
should work plain, but require any user to configire his machine
this way ...
> 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
It is not a real backup ... If the filesystem that hosts mail is
bnroken, all the system is broken.
You should modify the deliver part so it
1) every single (of the pair) machine is configured as being the
primary server for the domain, so it does not relay to the other
lower in MX chain
2) Both machines have the same mailboxes, and they do deliver
locally and, after having rewritten the headers, deliver to the orther
machine (so one machine will be mxa.example.net the other
mxb.example.net and first one will accept mail for
mxa.example.net and example.net, all mail delivered to
user@??? will be both delivered locally AND to
user@???, while mail in arraive as
user@??? will be just delivered.
3) At this point every message is on both machines, if either one
fails the user would be able to get his message from the other one,
and this can be ealily done with a standard exim.
Now the hardest part: A message that has been removed from one
machine ust be removed also from the other.
If done from the user it could be easy, I do here but this way it is
just for me, that read both from home and from work, so i download
from home from the mx10 machine and from work from the MX5
one, but i know how to remotely mark messages and sort them.
I wopuld suggest this way: the pop3/imap server should be
modified to write somewhere the msgid of the deleted messages.
This file should be made available to the other machine.
Should be a daemon that every time (say 3 hours) do read this file
and purge al local messages that have previously been delivered on
the other machine.
This way the third part (having two machines for reading inconing
mail that are a full backup) is accomplished. Post here if you did
the pathc to the pop/imap server.
> 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.
>
>
> --
> *** Exim information can be found at http://www.exim.org/ ***
>
Leonardo Boselli
nucleo informatico e telematico
Dipartimento Ingegneria Civile
Universita` di Firenze
V. S. Marta 3 - I-50139 Firenze
tel +39()0554796431 fax +39()055495333
http://www.dicea.unifi.it/~leo
--
*** Exim information can be found at
http://www.exim.org/ ***
