Re: comcast ipv6 PTR

[Barry Shein <bzs () world std com>]

On October 15, 2013 at 01:23 fmartin () linkedin com (Franck Martin) wrote:
> If you want to block spam on IPv6, then you can start by rejecting connections to SMTP from any IPv6 that do not 
> have a PTR. No need to analyze the format of the PTR.
>
> It is in several recommendations that a sending email IP must have a PTR.
>
> That ISPs will not do a PTR on all IPv6 but only on static IPv6, improves the spam blocking feature above. No need 
> to maintain list of dynamic IP space...

Well yes we don't accept email delivery from any host w/o reverse dns.

At any rate I was pointing out that PTR records with easily id'd
patterns, where sites choose to use them, can be useful for spam
blocking. It's a weak defense but any survey of spam blocking would
conclude that everything other than special case (e.g., tight
whitelisting) is a weak defense.

But if no one uses RDNS for hosts which they believe should not be
sending email directly -- a policy decision, and the most likely
effect, rendering them unable to send email to many though not all
sites -- then yes, that would have the same effect on email MTAs which
first reject hosts lacking RDNS and then look for various patterns in
the RDNS response.

It's really two different, if related, cases.

Is there any reason other than email where clients might demand RDNS?

For example, web sites that may not talk to a host w/o RDNS? I don't
know any off hand though it sounds plausible.

-- 
        -Barry Shein

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