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On 9 Mar 00, at 10:47, Claus Färber wrote:
> Well, the question here is, what's better: A security hole allowing
> remote attackers to find out which email address is valid without
> waiting for the bounce (and giving a valid return address in advance)
> or a security hole allowing remote attackers to start a DoS attack by
> sending messages which eventually double bounce?
>
> I believe it's better not to accept that mail in the first place.
Correction: It would be better if it was for free. Unfortunately,
checking whether a username exists (against large database) can
be arbitrarily slow; unless you're planning to overload your busy
box, forget it; qmail-smtpd just carries on the conversation,
qmail-queue writes down the message, fsync()es it and you're done.
Adding one zillion system calls to validate the username is a DoS
attack waiting to happen, too. Which one would you rather have?
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