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How to configure the router to minimize a Denial of Service (DoS) attack

What is Denial of Service or Distributed Denial of Service?

DoS attack, denial-of-service attack, is an explicit attempt to make a computer resource unavailable by either injecting a computer virus or flooding the network with useless traffic. There are two types of DoS attacks: computer attack and network attack. Common forms of denial os services attacks are:

Ping of death

Ping of death is caused by an attacker deliverately sending a ping packet, normally 64 bytes, that is larger than the 65,535 bytes. Many computer systems cannot handle an IP packet larger than the maximum IP packet size of 65,535, and often causes computer systems crash. It is illegal to send a ping packet of size greater than 65,535, but a packet of such size can be sent if it is fragmented. When a receiving computer reassembles the packet, a buffer overflow occurs, which often causes computer to crash. This exploit has affected a wide variety of systems including Unix, Linux, Mac, Windows and routers; but the fixes have been applied since 1997 making this exploit mostly historical.

Ping of flood

Ping of flood is caused by an attacker overwhelming the victim's network with ICMP Echo Request (ping) packets. This is a fairly easy attack to perform without extensive network knowledge as many ping utilities support this operation. A flood of ping traffic can consume singificant bandwidth on low to mid-speed networks bringing down a network to a crawl.

Smurf Attack

Smurf attack exploits the target by sending repeated ping request to broadcast address of the target network. The ping request packet often uses forged IP address (return address), which is the target site that is to receive the denial of service attack. The result will be lots of ping replies flooding back to the innocent, spoofed host. If number of hosts replying to the ping request is large enough, the network will no longer be able to receive real traffic.

SYN Floods

When establishing a session between TCP client and server, a hand-shaking message exchange occurs betwen a server and client. A session setup packet contains a SYN field that identifies the sequence in the message exchange. An attacker may send a flood of connection request and do not respond to the replies, which leaves the request packets in the buffer so that legitimate connection request can't be accommodated.

Teardrop Attack

Teardrop attack exploits by sending IP fragment packets that are difficult to reassemble. A fragment packet identifies an offset that is used to assemble the entire packet to be reassembled by the receiving system. In the teardrop attack, the attacker's IP puts a confusing offset value in the sebsequent fragments and if the receiving system doesn't know how to handle such situation, it may cause the system to crash.

Mail Bomb

Unauthorized users send large number of email messages with large attachments to a particular mail server, filling up disk space resulting in denied email services to other users.

Distributed DoS (DDoS) attack

DDoS (Distributed Denial Of Service) is a tactic used to attack a victim from multiple compromised computers. Attacker installs a virus or trojan software on compromised systems, and use them to flood a victim's network in a way that the victim's server cannot handle it.

DDoS involves 3 parties: an offender, helpers and a victim. The offender is the one who plots the attack, and helpers are the machines that are compromised by the offender to launch attack against a victim (the target). The offender commands the helpers to attack the victim's host at the precisely same time. Due to this co-ordinated nature between the offender and helpers, the DDoS is also known as co-ordinated attack.

Core issue

A DoS attack happens when a large number of connections in the victim server's TCP connection list prevents legitimate users from gaining access 
to the victim Internet servers
Use the rate-limiting mechanism to minimize the DoS attack. 

You can use the rate-limit command to configure the committed access rate (CAR) and distributed CAR (DCAR) policies on an interface.


In order to configure the rate-limiting mechanism, apply this command on a router:

rate-limit {input | output} [access-group [rate-limit] ] conform-action exceed-action


  • bps—Average rate (in bits per second)
  • burst-normal—Normal burst size (in bytes)
  • burst-max—Excess burst size (in bytes)
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Last update:
‎06-22-2009 04:12 PM
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