Hi Louie, on Tue, 25 May 2004 14:28:40 +0800 you wrote:
> I only have 1 mx which is "MX 0" if i would setup another MX would it
> mean i'll have another MX/MTA right?
Depends how you set things up. If you set up another server you have
several options:
- equal priority MX
- lower priority MX
- don't add an MX record to the DNS, but add IP addresses for both
machines to your DNS name (e.g. mail.example.com =
192.168.0.1/192.168.0.2) and do simple round-robin
- cluster the machines in a high-availability "hot-spare" manner, where 1
machine always has the "live" IP address, but the other machine can take
it over if required.
> but how about when my 1st mx is
> down, what will happen to those email that is on the second MX?
Typically, in a "normal" secondary MX scenario, the secondary MX just
holds onto the mail until the primary is up, then sends it all to the
primary.
> will i still retrieve it?
Depends what you mean. Will you still reliably get your mail? Yes, as long
as the primary isn't down for longer than your secondary's retry limit.
Will you (or your users) be able to *collect* the mail from the secondary?
Not with a simple "secondary MX" scenario; if what you're trying to do is
add a second, redundant mailserver which is transparent from the outside
world then you may want to do the clustering I mentioned earlier. You
might find the Linux High Availability Project an interesting start -
http://www.linux-ha.org/ .
> Am planning a second MX which is using exim again, is there any
> config/settings i should add on the secondary Exim server?
To do a "basic" secondary MX, just add the domains you want to handle to
the domainlist "relay_to_domains". Also, at the bare minimum, you should
also add a recipient verification callout (add
/callout=15s/callout_defer_ok or something similar to your RCPT ACL where
you have "verify = recipient"). Otherwise, you will end up generating
unnecessary bounces on your secondary MX for addresses that don't exist on
the primary. Alternatively, you can copy your userlists or whatever from
the primary so that the secondary knows which are valid users.
HOWEVER, I *strongly* recommend you do not set up a secondary MX without
being crystal clear about why you are doing it. Make sure you can answer
the following questions clearly:
- What's the problem that you're trying to solve?
- Does the scenario you're proposing solve it?
In many cases, people have secondary MXes when they don't really need them
at all and they don't help very much. In particular, if you have *any*
kind of spam or virus checking, you must ensure that both your primary and
secondary are set up in an identical way, or your secondary just turns
into a spam-generating leaky backdoor to your mail system for spammers to
abuse. And even if you don't have *any* spam/virus checking of any kind,
still see my note above about user lists - these days, all responsible
administrators will do their utmost to ensure that they reject mails at
SMTP time on the "front line", to avoid generating "collateral spam".
Tim
