On Tue, 1 May 2007, Guy Fraser wrote:
It's a DoS because the system can have only a finite number of sockets
open (this is both a kernel limit and a postfix tuning parameter limit),
and greet pause ties them up doing nothing for a period of time. Recall
that postfix is written to support many operating systems and not all OSs
(especially the older ones, e.g., linux 2.4) support epoll (enabling
greater than 1024 elements in the select()). Consequently, on an active
server, legitimate connections will be denied because of a lack of an
available socket and thus you've denied service to a legit user.
Then you must also consider connection limiting and throttling DoS
as well. Your facts don't line up with reality. This system can and
does work well, when sendmail and the system are configured to make
allowances for the delay, even when each server is processing over a
million messages per month.
Your server; your rules; however, I am interested neither in debating
semantics nor foolish configurations. One million messages per month ->
10^6 / (30 * 86400) = 0.385 messages / second. This is a low traffic
site. As an aside, I would suggest that you whitelist servers from which
you've already accepted mail to avoid foolishly penalizing your intended
correspondents.
Steve Friedman
