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 The World | bzs@TheWorld.com | http://www.TheWorld.com Purveyors to the Trade | Voice: 800-THE-WRLD | Dial-Up: US, PR, Canada Software Tool & Die | Public Access Internet | SINCE 1989 *oo*