eHelpStopping spam is a constant battle. And while it is true that not all spam can be successfully filtered, Cal Poly Pomona is committed to curbing as much of it as possible.
The latest addition to Cal Poly Pomona’s anti-spam arsenal is an email filtering technique called reverse Domain Name System (DNS) lookup. Any time an email is sent across the Internet, a 32-bit number called an IP address is used to identify the server sending the emails.
Typically, a spammer uses an originating email address that doesn't match the IP address of the mail server. The spammer may do this to make it purposely difficult to trace the spam messages, or as a way of "spoofing" a valid sender, such as ebay.com. As this is often the case with spam, reverse DNS lookup was developed as a means by which servers could differentiate between valid emails sent from mail servers with IP addresses that match their DNS addresses, and those that don't match.
Reverse DNS lookup works as follows.
When an email is sent to a Cal Poly Pomona email address, the university’s gateway email servers conduct a reverse DNS lookup, wherein the sender’s IP address is then resolved as the server’s hostname. As an added precaution, the reverse DNS lookup is followed by a forward DNS lookup, which, as the name suggests, is just the opposite process. If the sender’s IP address matches in both the reverse and forward DNS lookups, then the email is allowed through. If, however, the sender’s IP address does not match in both the reverse and forward DNS lookups, then the email is blocked, and is automatically returned to the sender by Cal Poly Pomona’s server along with an error alert message, serving as notification of the email’s failure.
Since Cal Poly Pomona introduced reverse DNS lookup for its email servers, the amount of spam received by the university’s email users has been greatly reduced, and the load on the campus mail gateways has been significantly lessened.
Statistically, before the controls, the university was processing 20.3 million email messages monthly. As of early September, 2005, the university is processing 9 million email communications monthly.
This means that the transit time for email sent to and from Cal Poly Pomona email addresses has also been significantly cut.
In other words, you can expect to receive far less spam, and faster mail delivery.
While the pros of reverse DNS lookup far outweigh the cons, it is possible that legitimate email sent to Cal Poly Pomona email users may fail reverse DNS lookup and be subsequently blocked by the university’s email servers. This will only happen, of course, if the reverse and forward DNS lookups of a sender’s IP address do not match.
These cases, however, are expected to be rare, as reverse DNS lookup has become a standard within higher education and industry. The majority of the organizations or entities that provide email service have adapted their email practices accordingly.
It is important to remember that even if a legitimate email were to fail Cal Poly Pomona’s reverse DNS lookup process, an error alert message will automatically be sent by the university’s server to the email’s sender, serving as notification of the email’s failure.
This error message may appear as follows:
“host [server name, IP address]said: 450 Client host rejected: cannot find your hostname.
There is an easy corrective action the sender’s Information Technology (IT) contact can take to prevent such occurrences in the future.
In the event that you are ever contacted by an individual who indicates that an email sent to you was returned and describes an error message in any way similar to the one above, simply notify the Help Desk, where specialists trained in reverse DNS can work with the sender’s technical staff to resolve the issue.
If the sender’s IT contact has any questions, Cal Poly Pomona’s I&IT Help Desk personnel are available for consultation.
More detailed information on the technical aspects of reverse DNS lookup can be accessed at: http://www.dnsstuff.com.
