Ask Slashdot: Is Reverse DNS a Worthy Standard For Fighting Spam?

drmartin66 makes it to the front page with this question: "Last weekend I installed a new spam filter server for a client, and enabled connection rejection if the sending server did not have a Reverse DNS record. Since then, I have had a number of emails rejected from regulator bodies that do not have a Reverse DNS record, and are refusing to have one created for their email server. What is your opinion of Reverse DNS records? Are they (or should they be) a standard, and required? Or are they useless for spam fighting?"

rDNS

[alphatel]

Like all things spam, marking the message as bad automatically is generally discouraged. If you simply increase the SCL value by some reasonable number, and continue to raise SCL based on other soft violations (like spamhaus, surbl, etc), you will rarely put good senders in the junk email folder, and very frequently be able to reject most spam content.

Better Question...

[RedACE7500]

What reason would anyone have to be running an SMTP server without a PTR record?

It's a poor differentiator

[sjames]

Filtering based on lack of rDNS is an old technique that actually did a good job of detecting spam without an excess of false positives for about a week in the late '90s. It has for some reason become enshrined as policy by a great many people now. These days it is occasionally a better indicator of NOTspam since the spammers all make sure they have rDNS set up and have done so since that week or so in the '90s.

Consider, if someone in a striped shirt wrote your business a bad check a decade ago, would you maintain a policy of not doing business with people who wear striped shirts?

Re:Absolutely required.

[Anonymous Coward]

I didn't really speak to that. I just wanted to correct the commenter that claimed that all one need do is sign up to a virtual host to somehow magically hijack someone else's reverse DNS zone. It wouldn't be that simple. One would either need to trick the authority responsible for delegating the zone or hack a server in the chain of authority.

The reason many mail servers require matching forward and reverse DNS is because it provides a level of assurance that your ISP is aware of and approves of your providing an outside service to the Internet -- in this case a mail server. It's not a guarantee that spam won't come from your server, but gives your server an added level of credibility.