The End of the NDR

By Larry Seltzer  |  Posted 2007-08-27 Print this article Print

Opinion: SMTP e-mail errors these days are much more often malicious than informative. We actually have to protect ourselves from them.

Im a paying subscriber to several of DynDNSs e-mail services. Today I got a newsletter from them announcing that they will stop generating local NDRs (non-delivery reports) from their services. NDRs, also known as bounce messages, are error messages from mail relay servers back to the message sender saying that the message cannot be delivered.
It has become common for spammers to forge originating e-mail addresses and to then send large spam runs against different servers. When this happens, DynDNS sometimes receives these messages, which cannot be delivered, or worse, get bounced back to the original forged sender, who now gets the spam in his or her inbox (aka, spam blow back)... We simply feel that this is not the right thing to do.
In one sense this is boring, dog-bites-man stuff. Everyone knows that NDRs are trouble. It wasnt always this way. Back in the early days when Al Gore invented the Internet, message delivery was far less reliable than it is now. The systems and connections between them were unreliable. Often a connection would be taken offline because, for example, someone needed that phone line to call Mom.
For this reason the standards that defined e-mail on the Internet (RFC 821/822, 2821/2822), created a requirement that a relay server must send a NDR if it experienced a problem delivering a message to a destination.
From RFC2821 section 3.7:
If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path).
Emphasis is original to the RFC. You MUST send the NDR. But DynDNS wont be doing it anymore, and good for them. They shouldnt, because these days "the originator of the undeliverable mail (as indicated by the reverse-path)" is almost certainly some innocent third party who did not send the e-mail. (Johannes Ullrich of the ISC basically agrees.) One upshot of this is that DynDNS will not be RFC-compliant, and this they clearly dont like. It bothers them a lot more than it bothers me. Look at where standards have got us with e-mail. Just because theyre "standards" doesnt mean we shouldnt protect ourselves from the consequences of their design flaws. NDRs have other nefarious uses. Many services stopped sending NDRs years ago because of DHAs (Directory Harvest Attacks), where a spammer attempts to construct a list of accurate addresses in a domain by using a dictionary of names and sending test messages to all of those names in the domain (,, etc.). If the spammer gets an NDR back, he knows its not a real address. If he doesnt get an NDR, its a real address. The downside to turning off NDRs is that customers and other outsiders may get the impression youre ignoring them when in fact they just typed the wrong name. But leave them on and you guarantee that eventually someone can create a highly accurate copy of your directory. They can use this not only for spamming but for targeted attacks that need to look credible, like an e-mail that needs to look like it came from the CFO. BATV is one way to address "blowback" and other bounce message abuse. Click here to read more. There are actually good. RFC-compliant ways to address bounce abuse. BATV (Bounce Address Tag Validation) is one way to block any bounces that didnt actually come from the users who purportedly sent them. Sadly, BATV has to be implemented in the MTA (mail transfer agent), which DynDNS doesnt generally control, so it wouldnt work for them. (Would it? Doesnt seem to me like it would.) When even good citizens like DynDNS start going non-compliant in order to do the right thing, its time for the standards people to get off their duffs and do their jobs. The RFC is broken. NDRs are hardly the only thing wrong with SMTP, but once its right to ignore the RFC things can only get worse. Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983. Check out eWEEK.coms for the latest security news, reviews and analysis. And for insights on security coverage around the Web, take a look at Security Center Editor Larry Seltzers blog Cheap Hack More from Larry Seltzer
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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