RE: [cobalt-users] Backup MX Server

[Nico Meijer <nico.meijer@xxxxxxxxx>]

Hi John,

> In this scenario, the backup MX server would permit any mail going to
> domain.com in itself and continue to relay it to the primary mail server
> until a time-out period (a few days) has exceeded.  The backup server should
> not challenge usernames nor need usernames to be entered.  I don't want to
> have my users create user names on their primary mail server than have to go
> to the backup mail server and re-create users names there.

You needn't worry, it's not that hard and you don't need to enter users twice.

Let's assume your backup MX is running sendmail. The two main things to 
keep in mind are "class w" and "class r". These are very important.

Class w contains hostnames that sendmail will receive mail for; ie: deliver 
locally.
Class r contains hostnames that sendmail will relay mail for; ie: deliver 
remotely (via smtp, perhaps).

Let's take domain.com as an example.

Your primary mailserver will need to receive mail for this domain (class 
w), which on a raq is done via the GUI when creating/editing a virtual site 
(checkbox "Receive mail for [whatever, I forgot]"). On a 'normal' (no pun 
intended) system, you would add "domain.com" (and any hosts in that domain 
that have an MX record) to "/etc/sendmail.cw" or 
"/etc/mail/local-host-names", depending on the version you're running. This 
is class w.

Your fallback mailserver must *not* receive mail (ie: deliver locally) for 
domain.com. Your mail would get stuck at this mailserver. It must only 
allow relaying for this domain, which in sendmail is done by adding 
"domain.com" (and again any hosts in that domain that have an MX record) to 
"/etc/mail/relay-domains". This is class r.

Please do not confuse this with /etc/mail/access. This file manages the 
hosts/IPs/domains to relay mail "from", not "to".

One important thing. If you should name your fallback mailserver 
"mx1.mydomain.net", "mx1.mydomain.net" must be added to class w on this 
fallback mailserver. This is not an option.

You could also allow sendmail to relay mail for any domain for which it is 
listed as an MX record. Doing this is of course suicide, because then I'd 
demand to know it's hostname and try get some changes to my DNS setup done 
overnight. Heck, I'll even throw in a few beers! ;-)

For further reading, see (among others):
http://www.sendmail.org/faq/section4.html#4.14
http://www.sendmail.org/faq/section4.html#4.5

In other words: sendmail.org is your friend. ;-)

Good luck... Nico