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postfix-users: Re: is this DNS setup RFC-correct ?

Re: is this DNS setup RFC-correct ?

From: mouss <mouss_at_nospam>
Date: Thu Oct 28 2010 - 19:34:33 GMT
To: ML postfix <postfix-users@postfix.org>

Le 25/10/2010 18:20, Leonardo Rodrigues a écrit :
>
> Hi,
>
> This question is not postfix-related .... so its somehow
> off-topic. But it's related to mail systems, so i cant imagine any
> other place to get some help on this subject.
>
> I have a domain (domain.com.br) with lots of subdomains in the
> format nnn.domain.com.br.
>
> nnn.domain.com.br (about 40 different subdomains) have only 2
> (two) DNS entries:
>
> nnn IN MX 10 server.nnn.domain.com.br.
> nnn IN TXT "v=spf1 include:domain.com.br -all"
>
> there's absolutely no A entries for the subdomains.
>
> i'm having an isolated problem with a customer that cannot send
> mails to one of my subdomains (everything else is working fine,
> several thousand of messages flowing daily, this really seems to be an
> isolated problem). Altough, their sysadmin (from the domain which
> cannot send us mail) is telling what i'm doing (having an MX entry but
> no A entries) is RFC-illegal, altough he cannot point me which RFC i'm
> not complaining with.
>
> i really dont know if this setup (MX record but no A record) is
> RFC-OK. The only think i know is that it's been working for years and
> i receive thousand of emails daily on my several subdomains with no
> problem at all.
>
> question is .... do a domain (or in my case, a subdomain) *MUST*
> have an A record ? Is the config i'm doing (DNS speaking) correct or
> incorrect (RFC-speaking) ??

to receive email, you need either an MX OR an A record. you do not need
both.

Obviously, Wietse has no problem receiving email:

$ host -t a porcupine.org
porcupine.org has no A record
$ host -t mx porcupine.org
porcupine.org mail is handled by 10 spike.porcupine.org.
$ host -t a spike.porcupine.org.
spike.porcupine.org has address 168.100.189.2

nor do I (for @ml.netoyen.net):

$ host -t a ml.netoyen.net
ml.netoyen.net has no A record
$ host -t mx ml.netoyen.net
ml.netoyen.net mail is handled by 10 tichka.netoyen.net.
$ host -t a tichka.netoyen.net
tichka.netoyen.net has address 91.121.33.217

while many domains have an A record, this is related to the web, not
smtp. it's just so that http://example.com/ works (for mobile devices,
"www." is 4 chars...).

Anyway, the "common" way to configure MX servers is

example.com. MX 10 joe.example.org.
joe.example.org. A 192.0.2.1

that is
- the domain has an MX record which is a "hostname"
- that hostname has an A record