CNAME для NS, возможно ли по RFC?

насколько я помню NS и MX не могу определяеться через CNAME.
это сделано для борьбы с двойным резолвингом или что-то типа того.

Здесь подобная тема поднималась уже когда-то несколько месяцев назад. Насчёт MX. можно, но не рекомендуют из-за доп. запросов:

RFC2821 [Simple Mail Transfer Protocol]:

...

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.

...

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. If no MX records are found, but an A RR is found, the A RR is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host.

Собственно, про MX:

http://www.linux.org.ru/view-message.jsp?msgid=2595124#2596201

На NS каких-либо ограничений тоже, вроде, нет. Единственное явное ограничение я нашёл на SRV RR.

А вот и подтверждение слов chocholl'а:

rfc2181: 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.

Searching for either NS or MX records causes "additional section processing" in which address records associated with the value of the record sought are appended to the answer. This helps avoid needless extra queries that are easily anticipated when the first was made.

Additional section processing does not include CNAME records, let alone the address records that may be associated with the canonical name derived from the alias. Thus, if an alias is used as the value of an NS or MX record, no address will be returned with the NS or MX value. This can cause extra queries, and extra network burden, on every query. It is trivial for the DNS administrator to avoid this by resolving the alias and placing the canonical name directly in the affected record just once when it is updated or installed. In some particular hard cases the lack of the additional section address records in the results of a NS lookup can cause the request to fail.

Так что, ну его нафиг :-). Не юзай синонимы в MX и NS.

P.S. Но статья, ссылка на которую есть в теме по ссылке, которую я дал выше, достаточно интересна.