The first problem is detecting a DOS attack as it can be mistaken with a heavy
utilization of the machine. DDOS attacks using reflection are extremely hard to
block due to the enormous number and diversity of machines a malicious user
can involve in the attack (virtually any machine can be turned into a zombie).
In addition, as the attacker is often only indirectly involved (he attacks through
the zombies and the reflective network), it is often impossible to identify the
Figure 2.1: A DDOS attack: The attacker sends the order to the computers he
personally controls (masters) which then forward it to the zombies, which DOS
as many machines as possible and spoof their IP to be the victim’s, who will
receive all the replies.
source of the attack. Because of these factors, there exists no general way of
blocking DOS attacks.
A widely used technique to hinder DOS attacks is “pricing”. The host will
submit puzzles to his clients before continuing the requested computation, thus
ensuring that the clients go through an equally expensive computation. DOS
attacks are most efficient when the attacker consumes most of his victim’s resources
whilst investing very few resources himself. If each attempt to flood his
victim results in him having to solve a puzzle beforehand, it becomes more difficult
to launch a successful DOS attack. “Pricing” can be modified so that when
the host perceives to be under an attack, it gives out more expensive puzzles,
and therefore reduces the effect of the attack. Although this method is effective
against a small number of simultaneous attackers, it more or less fails against
very distributed attacks. Other drawbacks are that some legitimate clients, such
as mobile devices, might perceive puzzles too hard and/or would waste limited
battery power to them.
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