Microsoft Chairman Bill Gates on Tuesday announced a proposed open standard to deter e-mail spoofing, a k a caller ID for e-mail. Microsofts proposal pitched to the security experts at the RSA Conference falls right in line with the emerging industry consensus that changes must be made to the e-mail infrastructure in order to make serious progress in the battle against spam.
The actual “Caller ID for E-Mail” specification calls for a system that has more in common with competing proposals than it has separating them. Like the SPF (Sender Policy Framework) specification, the basic mechanism involves requiring domains that send messages to add records to their DNS that will allow recipients to determine the addresses of the authorized sending servers. Recipients can then see if the sender of a message is really authorized to do so.
SMTP authentications systems like Caller ID wont stop spam in and of themselves, but they will make it possible for reputation systems from vendors such as Brightmail Inc., as well as simpler blacklists and whitelists, to be more effective. With a far smaller amount of spam surviving such challenges, filtering would also become more effective.
Authentication would also put up a huge barrier to e-mail worms like MyDoom, Bagle and Sobig that have comprised the majority of malware attacks in recent years. These worms all use SMTP engines built into the attack code itself. With Caller ID, none of them would authenticate properly in their current form, and its not clear that virus authors would have an effective way to send authenticated mail on arbitrary systems they infected.
There are differences between Caller ID and Sender Policy Framework, but they arent fundamental ones. The format of the records is different, and the method of comparing message headers to those records also differs. There are advantages and disadvantages to both.
At the same time, these differences wont change the basic fact that to implement either standard, owners of e-mail domains will have to upgrade their SMTP mail server (often known as a Message Transfer Agent or MTA) to one that supports the standard.
Microsoft should be able to provide this capability in their Exchange Server by means of a plug-in, although its not clear which versions of Exchange they would support. Like SPF, Caller ID would require changes in the Message Transfer Agent to deal with forwarded mail accounts (accounts from which mail is simply moved on to another account, rather than being collected by the users mail client). And like SPF, Caller ID would authenticate just the domain of the sender, not the user within that domain.
With so much in common, why doesnt Microsoft just adopt SPF? According to George Webb, Group Manager of Microsofts Antispam Technology and Strategy Team, the company decided that the differences are important.
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The Differences Between Caller
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One difference between the schemes is that with Caller ID the entire message is read, allowing the comparison to the valid sender addresses to be performed against the full header information. This could allow for a better analysis, and opens up the possibility that a modified e-mail client could be involved in the decision-making.
The downside is that under Caller ID the Message Transfer Agent necessarily reads more spam than with Sender Policy Framework, wasting bandwidth and memory.
Microsoft also argues that its use of XML to encode the DNS record information is more forward-looking, in that XML was designed to encode systems like this.
Some critics have argued that the verbosity of XML will cause problems with the DNS protocol which is typically performed with User Datagram Protocol (UDP) and limited to 512-byte packets. Microsoft responded that its kept the XML vocabulary terse and the tags very small, and that there is a mechanism in the specification by which commands can span UDP datagrams.
Sender Policy Framework has already been adopted by a large number of domains (7,693 as of February 24), including some famous destinations such as Symantec.com, Motleyfool.com and Oreilly.com.
However, of all the domains currently advertising Sender Policy Framework records in their DNS, the one that really matters is Aol.com. With Microsoft implementing the Caller ID plan and Yahoo touting its own Domain Keys proposal, the good news is that all three of the largest mail providers in the United States are committed to SMTP authentication.
The bad news is that they are committed to three different implementations.
Bill Gates is correct that the Internet will become a test laboratory for the next few months, as major ISPs, ISVs and corporations evaluate the alternatives.
The only mechanism for determining the winner will be standards groups such as the Internet Engineering Task Forces AntiSpam Research Group (ASRG), which moves at a speed unworthy of the Internet. And then theres the loosely-knit association between Microsoft, AOL, and Yahoo! on spam control.
In other words, its up to the big boys now.
Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983. Be sure to check out
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