Reverse DNS check
 

Would it be possible for an add-on that would check the IP address of someone registering for a Reverse DNS entry to authenticate that IP address?

In the past week, our site was spammed by two separate registered users and when I did a Reverse DNS entry check the result said there was no DNS entry (PTR) for those IP addresses.

As an anti-spam measure, if a Reverse DNS check found no record an error message could be displayed to the user preventing him from registering and posting.

I asked my webhost and he said:
Can it be done? I know that large email service providers do this as an anti-spam measure. Can vBulletin do it?

Not sure if this is useful info, but check out this: http://us2.php.net/gethostbyaddr

Many perfectly valid IP addresses have no rDNS, you would be blocking legitimate users.

I think Paul may be right and I've re-considered my stance. Despite the quoted recommendation, many legitimate members don't have reverse DNS entries (PTR records) due to no fault of their own or because they use a proxy to remain anonymous for security. Check some of your key members and you'll probably find many don't have PTR records. I think this idea will have to be shelved.