Re: [OT] CNAMEs en MX [ era Servidor de correo de backup ]
Victor Calzado Mayo, a las 10:20 del martes 8 abr 2003, comentó:
> Hola
> On Tuesday 08 April 2003 09:02, Angel Luis Mateo Martínez wrote:
> > El lun, 07 de 04 de 2003 a las 23:49, Manuel Samper escribió:
> > > Aaggh! que patinazo. Borra eso de la IP; un registro MX siempre debe
> > > contener un "hostname" (que debe poder resolver a una IP con un registro
> > > A, por supuesto).
> >
> > Esta es una pregunta que siempre me he hecho... ¿por qué el MX debe
> > apuntar a un registro A? ¿Por qué no debe apuntar a un CNAME?
> 2.3.5 Domain
>
> A domain (or domain name) consists of one or more dot-separated
> components. These components ("labels" in DNS terminology [22]) are
> restricted for SMTP purposes to consist of a sequence of letters,
> digits, and hyphens drawn from the ASCII character set [1]. Domain
> names are used as names of hosts and of other entities in the domain
> name hierarchy. For example, a domain may refer to an alias (label
> of a CNAME RR) or the label of Mail eXchanger records to be used to
> deliver mail instead of representing a host name. See [22] and
> section 5 of this specification.
RFC2821:
3.6 Domains
Only resolvable, fully-qualified, domain names (FQDNs) are permitted
when domain names are used in SMTP. In other words, names that can
be resolved to MX RRs or A RRs (as discussed in section 5) are
permitted, as are CNAME RRs whose targets can be resolved, in turn,
to MX or A RRs. Local nicknames or unqualified names MUST NOT be
used. There are two exceptions to the rule requiring FQDNs:
- The domain name given in the EHLO command MUST BE either a primary
host name (a domain name that resolves to an A RR) or, if the host
has no name, an address literal as described in section 4.1.1.1.
[...]
4.1.4 Order of Commands
[...]
The SMTP client MUST, if possible, ensure that the domain parameter
to the EHLO command is a valid principal host name (not a CNAME or MX
name) for its host. If this is not possible (e.g., when the client's
address is dynamically assigned and the client does not have an
obvious name), an address literal SHOULD be substituted for the
domain name and supplemental information provided that will assist in
identifying the client.
An SMTP server MAY verify that the domain name parameter in the EHLO
command actually corresponds to the IP address of the client.
However, the server MUST NOT refuse to accept a message for this
reason if the verification fails: the information about verification
failure is for logging and tracing only.
Parece ser que el "must" a propósito del EHLO es el origen de toda la
cuestión... y si aplicamos la recomendación de "ser liberal en lo que se
acepta y estricto en lo que se envía", pues lo estricto aquí es atenerse
al "must" sobre el hostname en el EHLO. En cualquier caso, los
rfc 821/2821 son lo suficientemente extensos como para que surjan
contradicciones según el punto de vista del que los lee.
Que conste que no estoy defendiendo nada, únicamente indico que para
andar "sobre seguro", es lógico encontrarse con afirmaciones categóricas
acerca de no apuntar MX a CNAMES.
Saludos
Manuel Samper
PD: Perdón por mandar a la lista textos en inglés. Pero para el que no
lo entienda, le aseguro que no se pierde nada interesante.
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