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Re: FTP denial of service attack
From: deraadt () CVS OPENBSD ORG (Theo de Raadt)
Date: Tue, 7 Dec 1999 22:41:45 -0700


I don't know of any ftp clients which make use of this feature (multiple
data channels supported concurrently) as the original ftp clients were all
line-based and only suported one transfer at a time.  Maybe this is
reasonable, but it would be a shame for the default defense to this attack
to mean you can't use FTP to it's full potential (i.e. start a transfer
from the current session but keep using the current `login' session, maybe
to start other transfers, as requried).  Triming the number of concurrent
data sessions to a maximum of 1-5 (by default) would probably be enough,
with the capability to set this higher/lower as required.

The OpenBSD ftpd has never permitted more than 1 connection at a time
in PASV mode, thus this particular denial of service attack does not
work.

I caused myself some difficulties by accidentally starting up 400 perl
instances, though..

One of the Linux's out there also ships with our ftpd, so they will
not have a problem with this either.  It's either Debian or Suse...


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