Ok.
А вот из RFC2821.
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.
...
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.
...
5. Address Resolution and Mail Handling
Once an SMTP client lexically identifies a domain to which mail will
be delivered for processing (as described in sections 3.6 and 3.7), a
DNS lookup MUST be performed to resolve the domain name [22]. The
names are expected to be fully-qualified domain names (FQDNs):
mechanisms for inferring FQDNs from partial names or local aliases
are outside of this specification and, due to a history of problems,
are generally discouraged. The lookup first attempts to locate an MX
record associated with the name. If a CNAME record is found instead,
the resulting name is processed as if it were the initial name.
Написано что домен может быть псевдонимом, а требование ресолвинга в A
относится
только к HELO.
Однако есть RFC2181, где написано более чётко.
10.3. MX and NS records
The domain name used as the value of a NS resource record, or part of
the value of a MX resource record must not be an alias. Not only is
the specification clear on this point, but using an alias in either
of these positions neither works as well as might be hoped, nor well
fulfills the ambition that may have led to this approach. This
domain name must have as its value one or more address records.
Currently those will be A records, however in the future other record
types giving addressing information may be acceptable. It can also
have other RRs, but never a CNAME RR.
То есть прямой запрет на CNAME в MX.
Резюмируя, для записи MX вы можете использовать CNAME для домена в (поле
name),
но не для mail exchanger (поле data).
Denis Korost писал 2016-06-16 10:49:
В Thu, 16 Jun 2016 10:38:06 +0200
Max Kostikov <m...@kostikov.co> пишет:
Добрый день!
Я что-то тоже не припоминаю никаких номинальных ограничений.
Да и на практике CNAME в MX работают без видимых проблем.
https://www.ietf.org/rfc/rfc1912.txt
[Page 6]
RFC 1912 Common DNS Errors February
1996
Don't use CNAMEs in combination with RRs which point to other names
like MX, CNAME, PTR and NS. (PTR is an exception if you want to
implement classless in-addr delegation.) For example, this is
strongly discouraged:
podunk.xx. IN MX mailhost
mailhost IN CNAME mary
mary IN A 1.2.3.4
[RFC 1034] in section 3.6.2 says this should not be done, and [RFC
974] explicitly states that MX records shall not point to an alias
defined by a CNAME.
George L. Yermulnik писал 2016-06-16 09:19:
>> Единственный "запрет" на CNAME в отношении почты, который я помню -
>> для MX записей. И то там не запрет, а "особое" поведение в случае
>> CNAME.
>
> Про запрет знаю, а вот про "особое поведение" - не слышал. Ткнёте
> пальцем в соответствующее RFC?
--
Best regards,
Max Kostikov
BBM: 24CA5DF8 | https://kostikov.co
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