Rate Limiting as an Anti-Spam Tool

By Larry Seltzer  |  Posted 2004-06-21 Print this article Print

Opinion: ISPs need to start putting a cap on the number of e-mail messages sent by users—a policy that would leave nearly all users unaffected but would be a powerful deterrent for spammers.

The spit is hitting the fan these days over the impending standards for SMTP authentication. Im a big fan of it, but many are arguing against it—some constructively and some not. One of the main dismissive put-downs against authentication, especially with respect to zombied PCs, is that spammers and worms will simply switch to using the users authenticated ISP account to send spam. There are a number of reasons why Im not so worried about this. One of them is that ISPs, at least consumer ISPs, will need to begin limiting the number of e-mail messages sent by users. This is one of those policies that will leave 99-something percent of all users unaffected but generate extreme rage among the others.

Hard limits are not a good way to implement changes like this. What is needed is what we call "rate limiting"—limiting the amount of mail users can send over some period of time. When you do this, you implicitly limit the total amount of mail they can send and, as a practical matter, can make high-volume spamming impractical.

As I noted in a recent column, many ISPs are monitoring the use of port 25, the SMTP port, on their clients connections. They dont talk about specifics, but Im sure ISPs that exercise this route look not just at the volume but at headers and perhaps for evidence that the system is being remotely controlled.

Blocking actual use of the ISP mail servers is another matter. I spoke to a couple of ISPs that do monitor their mail servers looking for abuse. Clearly, there are some things users can do that would block most acceptable usage policies, such as sending viruses or pornography, but it basically seems to me that they are keeping their policies vague.

I think the time has come to be more specific. Set a rate limit for outbound mail for consumer accounts. There are systems available to enforce it. And it would be yet another sign to users whose computers have been taken over that they need to clean them out.

Next Page: Choosing strong passwords.

Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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