Re: [EXIM] How exim uses DNS

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Author: Vadim Vygonets
Date:  
To: exim-users
Subject: Re: [EXIM] How exim uses DNS
Quoth Carl Horne on Mon, Jun 21, 1999:
> 1) I have a box thats setup to route mail to several
> different servers as well as to the internet. The box uses an internal
> DNS so it can route to any internal box. Because of a firewall I want
> all outgoing mail to be routed through this box. Also internal routing
> should go through this box.


You must use the domainlist router to route mail inside your
domain, without putting MX records. Hint: use bydns_a or similar
lookup.

Or, if you really need to deliver mail to any host specified as
the domain part of the e-mail address, add the mx_domains option
to the lookuphost router. For example,

lookuphost:
driver = lookuphost
transport = remote_smtp
mx_domains = ! *.your.dom : *

I assumed here that you use Exim 3.0x. If you use an older
version, consult your manual.

> 2) On a sendmail box (I am
> trying to get rid of) I have if it does a DNS lookup and there is no A
> record it uses the MX record on internal lookups. If there is a A
> record it uses the A record. Is there a way to get exim to follow this
> type of logic?


Hmm. In general, every host should have an MX record, and the
mailer daemons must ask for the MX records before looking up A
records (see RFC 1123). Confusion may arise in the cases where
the DNS file for the domain contains records such as:

*.your.dom.        IN    MX 10    mail.your.dom.
host.your.dom.        IN    A    123.45.67.89


Here, if you look up the MX record for host.your.dom, you don't
get any (you probably get the A record back, but I'm not sure
about that -- not a DNS guru), because the A record for
host.your.dom "hides" the woldcard MX record.

> It seems to me that exim uses MX records even for reverse lookups.


I don't think so. For reverse lookups, reverse PTR records
should be used, such as (for the example above):

89.67.45.123.IN-ADDR.ARPA.    IN    PTR    host.your.dom.


Vadik.

-- 
If you think C++ is not overly complicated, just what is a protected
abstract virtual base pure virtual private destructor, and when
was the last time you needed one?
        -- Tom Cargill, C++ Journal, Fall 1990.


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