TMDA backscatter

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On Oct 8, 2007, at 4:37 AM, Frank Ellermann wrote:

SM wrote:

TMDA may cause backscatter.

After an SPF PASS, the "backscatter" by definition can't hit an innocent bystander. By the same definition any "backscatter" after an SPF FAIL hits an innocent bystander, and therefore is net abuse.

There is a real risk SPF might be used as basis for acceptance, rather than just for qualifying DSNs. As a basis for acceptance, this can cause email to fail. The macro expansion of SPF records permits the _same_ DNS record within a spam campaign to generate a large number of subsequent and different DNS transactions to be sent by recipients to "innocent bystanders". Much of the danger of auto responses has to do with DDoS concerns. Unfortunately, SPF represents a far graver concern than that caused by auto-responses.

A safer approach would be to format all DSNs per RFC3464 and remove original message content. This reduces incentives for abusing the automated responses. Mailman made a mistake where an error caused a DSN that returned original content without first verifying the validity of the return path. Had TMDA been a requisite for initial acceptance, just those white-listed would have been prone to this error.

-Doug


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