In <[log in to unmask]> [log in to unmask] writes:

> We are trying to setup an "alternate" mail host for our primary mail
> servers so that when one goes down, mail can be delivered to another to
> avoid mail bounces (undeliverable after xx hours/days) to be queued for
> the affected host.  The obvious method is a prioritized list of MX
> records for the covered hosts.
>
> I tried this once between two hosts and ended up with a forwarding loop
> so I'm a bit hesitant to experiment by trial and error.
>
> The basic question is this:  if we have DNS setup like
>    hosta.foo.edu  MX   0 hosta.foo.edu
>                   MX  10 hostb.foo.edu
>    hostb.foo.edu  MX   0 hostb.foo.edu
>                   MX  10 hosta.foo.edu

The main factor here is does the user send mail to "xxx@hosta" or "xxx@hostb"?
If a user/mailer sends to xxx@hosta it first does a lookup on "hosta.foo.edu"
which gets back "mx 0 hosta.foo.edu" and "mx 10 hostb.foo.edu". Then;
-it tries to reach the lowest preference (0) host - hosta. If it gets through,
 mail is delivered/end-of-story. If it can't reach hosta for whatever reason,
 it then tries the next entry [DNS rules state keep trying hostnames on the
 MX list until you reach your own hostname or the end of the list]. In this
 case the next name on the list would be hostb [assuming the sender isn't on
 hostb to begin with].

-Once the mail gets to hostb, as long as hostb doesn't recognize the name
'hosta' as an alias for itself, it queues the mail for final delivery. It
 does a lookup on 'hosta' and gets the same two MX records; it tries to reach
 the lowest preference (hosta)- again if it can't reach hosta, it goes on
 down the list. Now it finds itself as the next in the list; this causes it
 to give up and queue the mail for another try later.

> It's clear how "the world" sees this and treats these entries, but what
> about hosta and hostb?  If hostb receives mail for hosta, will it follow
> the MX chain and resend to itself?  Is this a default MTA behavior or
> does it require some tweaking of the MTA?  If the latter (and the reason
> for this message being cross-posted) how is this conveyed to sendmail
> (Solaris), LSMTP (NT), and/or NetMail (MPE/iX)?

All mailers are required to follow DNS rules as above. The DNS records for
'hostb' would never be looked at unless mail is addressed to "xxx@hostb".
[NetMail follows the same rules as all other SMTP MTAs (are supposed to)].

If you want to prevent the undeliverable/undeliverable-yet messages, then
just the MX entries for hosta would do; assuming hostb doesn't send out those
undeliverable-yet messages. To end-users systems, mail would have been
accepted by a defined proxy and their mailers would treat the mail as having
been delivered (to another MTA at least, if not delivered to final destination
).

Lots of sites use this same scheme to handle e-mail delivery when they only
have part-time (dial-up) Internet connections; If they're on-line when the
mail is sent, they get it directly. If not, then it goes to the next choice
on the MX list; usually a system(s) at their ISP. While they're not online,
mail gets queued up on their ISP's system, and next time they connect the
ISPs system (during one of its retries) reaches their system and delivers
all the mail for them in its queue.

                 -Chris


______________________/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_
  Chris Bartram        Sales (US):   800 Net-Mail    Fax:+1 703 451-3720
   ______                         +1 703 569-9189 E-Mail: [log in to unmask]
  /__ |  \__________   Sales (Europe):+44(1480)414131 Fax:+44(1480)414134
 /  / | / ________     Sales (Pacific Rim):+61 3 489 8216 (same for fax)
|  /_ |<  ______       Tech Support:+1 703 569-9189  Fax:+1 703 451-3720
 \ __)| \ ___          E-mail: [log in to unmask]   Personal(me): [log in to unmask]
  \______/Associates,  6901 Old Keene Mill Rd Suite 500 Springfield VA 22150
_________________Inc._/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_
Gopher: gopher.3k.com   Anon-FTP: ftp.3k.com  WWW: http://www.3k.com/