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Article: 1393
From: Anthony Howe
Date: 2007-01-11 13:27:54 -0500
Subject: Re: single MX resolves to multiple hosts?

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Grant Taylor wrote:
> Ben Spencer wrote:
>> Milter-spiff failed the related email message. I suspect it is because
>> the connecting mail server was 1.2.3.4 and milter-spiff's lookups
>> returned 1.2.3.5.
> 
> I have no comment on the operation of milter-spiff in this situation as 
> I don't have enough information.  However if this is indeed the case, 
> (milter-spiff not properly handling multiple IP addresses for an MX) 
> this is a bug that should be addressed and I'm sure that Anthony would 
> have something to say.

There are two libsnert trouble tickets for this issue already.

Libsnert's DNS client code will fetch an MX record list and only use the 
first one found if multihome. Generally this is sufficent for what is 
was originally written for, milter-sender, since if you have two or more 
MX entries (which is recommended Best Current Practice) and each 
multihomed, then milter-sender would try each of the MX in turn upto the 
attempt limit.

This technique falls down for milter-sender in the case of only one MX 
record, which is a bad idea already, that is multihomed.

RFC 2821 section 5. Address Resolution and Mail Handling talks about 
multihomed MX records and in my mind leaves some wiggle room as to 
correct strategy.

However, the Dns.c fails to address the needs of milter-spiff and others 
that need support for multihomed MX records

In the trouble tickets concerning this issue, I've conceded that missing 
support for multihomed MX records is a bug, but it requires a major 
change / rewrite to the Dns.c code, which will NOT appear in 
libsnert/1.63 (unelss I find a nice simple hack) when I'm already 4 
months overdue with the next round of updates, which already very extensive.

-- 
Anthony C Howe          Skype: SirWumpus                    SnertSoft
+33 6 11 89 73 78         AIM: SirWumpus    Sendmail Milter Solutions
http://www.snert.com/     ICQ: 7116561
     http://www.snertsoft.com/

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