Offence CNAME as MX??

[Simon Waters]

Odhiambo Washington wrote:
> 
> I did that, yes, but it means that mail.wananchi.com has no PTR record. Is that bad
> also??

The usual goal with PTR records is that;

Lookup the name, get an IP address.

Lookup the PTR, and get a secondname.

Lookup the A for the secondname and get the same IP address.

i.e

name1 -> IP1
IP1 -> name2
name2 -> IP1

name1 does not have to be equal to name2 (Although obviously in
simple cases it will be).

Some people would add multiple PTR records to that IP1 maps to
both name1 and name2, but this generally doesn't help, as very
little software understands this.

MX'es shouldn't point at CNAMEs, but as far as I know only
archaic versions of sendmail have a problem with this. Since the
"right way" is no harder than the wrong way, it seems silly to
break the standards by using a CNAME, and thus excluding a few
rather dated sendmail sites.

A (very) few overly paranoid sites expect the SMTP handshake to
use the canonical name of the host. Thus if we have;

mail IN A 1.2.3.4
ns2 IN A 1.2.3.4
4 PTR ns2

The mail program should refer to itself as "ns2.domain.tld", I
forget what obscure mailer does this, but the admins are usually
grateful when told there is a setting in the software to make
their site less paranoid, as they are usually bouncing more mail
than they want to *8-)