It seems like spam is in the news every day lately, and frankly, some of the proposed solutions seem either completely hare-brained or worse than the problem itself. I'd like to reiterate a relatively modest proposal I first made over a year ago: Require legitimate DNS MX records for all outbound email servers.
MX records are one component of a domain's Domain Name System (DNS) information. They identify IP addresses that accept inbound email for a particular domain name. To get mail to, say, linux.com, a mail server picks an MX record from linux.com's DNS information and attempts to deliver the mail to that IP address. If the delivery fails because a server is out of action, the delivering server may work through the domain's MX records until it finds a server that can accept the mail. Without at least one MX record, mail cannot be delivered to a domain.
However, when the DNS was first put in place there was no requirement to be able to trace the identity of a sending mail server. This means that, as things stand, any IP address on the Internet can act as a mail server, even though it may be virtually anonymous and extremely difficult to trace. My proposal aims to close this loophole, so that only registered mail servers can send email.
Since this proposal depends on the existing DNS structure, it could be enacted (presumably with a grace period for organizations to get their mail servers registered) without requiring any initial technological changes whatsoever.
Over time, mail servers could be configured to reject mail that comes from other mail servers that have no MX record. Furthermore, since the MX record would be tied to the legal owner of the domain in question, additional filtering could be done to reject mail from servers that are owned by known spammers. In the longer term, this would decrease the complexity and increase the accuracy of mail filtering software. It also gives spammers fewer places to hide.
Of course this solution is useful only if the contact information for the domain in question is reliable, but this is an area that has been tackled already to some degree. ICANN-accredited domain registrars are required to include a contractual provision that contact information for a registered domain must be valid and up-to-date.
A downside to requiring MX records is that a mail server with an MX record is presumed to be capable of receiving mail. This would create headaches for organizations that currently depend on a division between outbound and inbound mail servers. This "headache," however, is more palatable than the current "life-threatening illness" that spam has become. In addition, one of the root causes of much of the spam we receive is the fact that mail servers can so easily be configured to send millions of messages without any legitimate mechanism for returning those same messages to the original sending IP address.
There are a number of steps organizations could take to mitigate the initial effects of having to register outbound mail servers, if they currently don't accept inbound mail on those servers:
Give the MX record of an outbound mail server a very low priority. A sending mail server is less likely to pick a receiving mail server that has an MX record with a low priority.
Register a separate domain for "outbound-only" MX records. If inbound mail is typically addressed to "email@example.com" and you have outbound servers registered under "someplace_outbound_mail.com", then your outbound servers will never be selected for normal inbound mail.
With these mitigating measures in place, it would be safe to keep outbound mail servers closed to inbound mail. Current mail server technology is reliable enough to deal with the situation where one (or more) of many listed mail servers is found to be unavailable.
In the longer term though, it would be better to discontinue the practice of separating outbound and inbound mail server IP addresses. That was a legitimate technique when there was no other practical way to handle load-balancing and security, but today multi-homing and firewalling products can handle the load and security issues without requiring a specific division of mail server IP addresses. Discontinuing this practice would leave spammers sticking out like a sore thumb.
One final attraction of mail server registration: Enforcement is simple. No army of "men in black" is needed to chase down the lawbreakers — if you choose not to register your mail servers, or repeatedly send spam from those you do, then nobody will accept your mail.
Simple as that.
By John Fitzgibbon, Software Engineer
|Cybersquatting||Policy & Regulation|
|DNS Security||Registry Services|
|IP Addressing||White Space|
Minds + Machines