Tag: DDoS

Mitigate DoS Attack using TCP Intercept on Cisco Router

This is really cool feature on Cisco router not usually mentioned until you dig a little deeper inside Cisco IOS. But first a bit of theory…

What is TCP SYN flood attack

TCP 3-way handshake

SYN flood DoS attack happens when many sources start to send a flood of TCP SYN packets usually with fake source IP.

This attack uses TCP 3-way handshake to reserve all server available resources with fake SYN requests not allowing legitimate users to establish connection to the server. SYN packet is the first step in TCP 3-way handshake. This is the step where client sends connection synchronization request to the server. Server receives TCP SYN from client, the server replies back with SYN ACK. SYN ACK acknowledges synchronization request.

In that moment server is waiting the client to complete the handshake by sending an ACK back to server to acknowledge the SYN ACK. With this third step, TCP session is successfully established and communication between server and client begins.

If the ACK is not received from the client side, server will wait for it for some time and then the session will timeout and get dropped. When the server deletes the session, his resources will be released.

TCP SYN flood attack

TCP SYN flood attack sends first packet of 3-way handshake SYN packet to server many times to cause the server to allocate resources for sessions that will never become established. It means that client who is attacking will never respond to server SYN ACK and the session will remain on the second step of 3-way.

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DDoS – Distributed Denial of Service attack

When a number of systems i.e. one or more than one web server floods the resources and bandwidth of a targeted system then a distributed denial of service attack (DDoS) takes place, Different types of methods are used by attackers in order to compromise the systems.

It is the malware that can carry out the mechanisms of DDoS attack; its best example was MyDoom. The mechanism of its DoS was actually triggered on a specific time and date. The DDoS of this kind involves hardcoding of the target IP address before the release of malware and there was no need of communication for launching the attack.

It can also happen that the system may be compromised with a trojan, giving a permission to the attacker downloading a zombie agent (sometimes the trojan already contain one). Attackers can destroy the systems with the help of automated tools that exploit the faults present in programs and listen for connections from far away hosts. The primary concern of this scenario is that the systems start serving as web servers.

One of the classic examples of DDoS tool is Stacheldraht DDoS tool. A layered structure is used and the attacker make use of a client program for connecting to the handlers, and these are compromised systems that send commands to the zombie agents, which give rise to DDoS attack. The handlers can control the agents with the use of automated routines in order to exploit program’s vulnerabilities that accept the connections running far away on the targeted hosts. Every single handler has a capacity to control up to 1000 agents.

These systems compromisers are referred to as botnets. Still the DoS tools like Stacheldraht utilizes the classic method of DoS attack centered on IP spoofing as well as amplification like fraggle attacks and smurf attacks (also referred to as bandwidth consumption attacks). Sometimes the SYN floods or resource starvation attacks may be used too. For the purpose of DoS modern tools can utilize DNS servers.

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DoS and DDoS – Denial of Service attacks

DoS – Denial of Service attack

The concept behind the design of DoS attack is interfering in the normal functions of a server, web site, or other resources of a network. The hackers and even the virus writers can use number of ways in order to get this job done. One of the most common methods is flooding a server with heavy traffic from network so that it becomes difficult to control it. As a result of this heavy traffic it is not possible to carry out the normal functions properly and sometimes this can further lead to server crash.

The only difference in case of DDoS attack is that multiple machines are used in order to conduct it. The master and zombie machines are used by hackers or virus writers in order to co-ordinate the attack across the other. These two machines usually exploit an application’s vulnerability on the machine, to install any malicious code like Trojan.

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