On Fri, Mar 15, 2019 at 12:04:47PM -0400, Bill Cole via Exim-users wrote:
> > I've not looked hard to find one. The original RFC
> > for SRV doesn't mention CNAME.
>
> It would not be feasible to prohibit a CNAME reply for a specific query
> RRTYPE, since a CNAME has always been defined as applying to all queries
> for a name. This is fundamentally different from the prohibition on the
> RRDATA of a MX record being a name with a CNAME record.
Indeed, a CNAME response for the original query name is valid for all
domains [except for the root domain which can't be a CNAME].
Where CNAMEs are sometimes proscribed (and the restriction generally
ignored) is in domain names that appear in the RDATA portion of MX
and SRV records. For example, in RFC2782:
Target
The domain name of the target host. There MUST be one or more
address records for this name, the name MUST NOT be an alias (in
the sense of RFC 1034 or RFC 2181).
Despite the best intentions of the RFC authors implementations
generally tolerate CNAMEs in the RDATA (target names) of MX and SRV
records.
--
Viktor.
