Re: Syn flooding attacks
The router could discard the SYN, remembering it, and let pass the retry SYN that usually occurs with valid connections and does not with invalid ones.
This is no good - all the crackers have to do is modify their programs to send two bogus SYNs, spaced apart, instead of just one. Vern
On Mon, 20 Oct 1997, Vern Paxson wrote:
The router could discard the SYN, remembering it, and let pass the retry SYN that usually occurs with valid connections and does not with invalid ones.
This is no good - all the crackers have to do is modify their programs to send two bogus SYNs, spaced apart, instead of just one.
Don't most SYN flood programs just send a constant stream of SYNs to the specified machine/port? The one I have for testing does that. So, sequential requests would get around this, no matter how many SYNs you were looking for. I think the best protection against SYN flooding is in the Kernel level of the OS. If you see a massive amount of SYN request coming in on one port from one machine or many, then you start applying cookies for those connections and decrease the hold time before you start dropping the connections due to un-answered SYN-ACKs. Don't most operating systems now support this feature (Win95 excluded)? Joe Shaw - jshaw@insync.net NetAdmin - Insync Internet Services
Joe Shaw writes:
Don't most SYN flood programs just send a constant stream of SYNs to the specified machine/port? The one I have for testing does that. So, sequential requests would get around this, no matter how many SYNs you were looking for. I think the best protection against SYN flooding is in the Kernel level of the OS. If you see a massive amount of SYN request coming in on one port from one machine or many, then you start applying cookies for those connections and decrease the hold time before you start dropping the connections due to un-answered SYN-ACKs. Don't most operating systems now support this feature (Win95 excluded)?
The whole "cookie" idea pretty much sucks, IMHO. It doesn't work particularly well. On the other hand, compressing your TCP state for half open connections is pretty cheap, and has the nice side effect of making your machine a much more efficient high volume server. Perry
participants (3)
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Joe Shaw -
Perry E. Metzger -
Vern Paxson