Network-Based Attacks

During recent years, the number of different types of network attacks has been on the rise. This section covers some of the major types of network attacks and their purpose, in relation to their perpetrators and victims. Several common types of attacks are discussed, such as the following:

  • DoS attacks
  • DDoS attacks
  • Session hijacking (such as man-in-the-middle attacks)

It is extremely important to understand that everyone has the potential to be exploited by a network attack. Consequently, a full understanding of these threats is imperative.

DoS Attacks

DoS attacks are exercised to disrupt service to a single system or an entire network. An attacker uses this type of attack to overburden and overutilize system or network resources. DoS attacks can cause network devices to drop packets. They can also force applications to stop functioning properly. With a DoS attack, an attacker's goal is to prevent the system or network users from using its services.


DoS attacks can also be used to degrade the performance of IDS and IPS devices. They are often used to add other attacks, like penetration attempts and reconnaissance.

Most frequently, DoS attacks try to damage network connectivity, while trying to open counterfeited TCP or UDP connections. The target device tries to handle the additional connections to the best of its extent, thus consuming all of its available resources. Three common types of DoS attacks are designed to disrupt network connectivity:

  • TCP SYN flood attacks
  • land.c attacks
  • Smurf attacks

TCP SYN Flood Attacks

TCP SYN flood attacks are designed to take advantage of the methodology used in establishing a new TCP connection, referred as a TCP three-way handshake. Figure 1-4 illustrates how the TCP connections are established.

Figure 1-4. TCP Connection Establishment

In the example presented in Figure 1-4, the client tries to establish a TCP connection to the web server. First, it sends out a SYN (synchronize) packet to the server to synchronize the sequence numbers. It stipulates its initial sequence number (ISN). To initialize a connection, the client and server must synchronize each other's sequence numbers. The Acknowledgment (ACK) field is set to 0 because this is the first packet of the three-way handshake and there are no acknowledgements thus far. In the second packet, the server sends an acknowledgment and its own SYN (SYN-ACK) back to the client. The server acknowledges the request from the client, but also sends its own request for synchronization. The server increments the client's sequence number by one and, in addition, uses it as the acknowledgment number. To conclude the connection, the client sends an acknowledgment (ACK) packet to the web server. The client uses the same methodology the server used by providing an acknowledgment number.

In TCP SYN flood attacks, the attacker generates spoofed packets to appear as valid new connection requests. These packets are received by the server, but the connection never completes. On the other hand, the server tries to reply without successfully completing the connections. After several of these packets are sent to the server, the server may quit responding to new connections until its resources are available to process the additional requests or when the attack stops attacking. Figure 1-5 shows how SYN flood attacks work. The attacker sends numerous spoofed SYN packets to the web server.

Figure 1-5. SYN Flood Attack


Spoofing is the technique where the attacker sends IP packets with someone else's source address to hide their identity. Sometimes attackers use authorized external or internal IP addresses that are trusted by firewalls and other devices, in order to obtain access to internal resources.


The server keeps all spoofed connections open until they time out. This causes substantial performance degradation.


land.c Attacks

Another primitive example of a DoS attack is the land.c attack. In this type of attack, an attacker sends numerous SYN packets with the same source and destination IP addresses and the identical source and destination ports to its victim. The purpose of this attack is to make the victim send the reply packet to itself. Because the attacker repeatedly sends these packets, the victim can run out of resources by replying to itself. Technically, the attacker uses the server's own resources against itself.

Smurf Attacks

Attackers can also consume bandwidth by directing unnecessary traffic to the victim's network. A classic example of this type of attack is the smurf attack. There are two major components in a smurf attack:

  • The use of bogus Internet Control Message Protocol (ICMP) echo request packets
  • The routing of packets to IP broadcast addresses

Normally, ICMP handles errors and barters control messages. For example, the popular tool called ping uses ICMP. It is used to verify that a specific system on the network is responding. It does this by sending an ICMP echo request packet to such a system. Consequently, it expects the system to return an ICMP echo reply packet.

In smurf attacks, ICMP echo request packets are sent to IP broadcast addresses of remote subnets to degrade network performance. Figure 1-6 illustrates the essentials of smurf attacks. In smurf attacks, usually there is an attacker, an intermediary, and a victim (in this case, a web server). If the network is with a 24-bit subnet mask of, the broadcast address will be If the ICMP traffic is sent to the broadcast address, all the systems or nodes on the network will receive the ICMP echo request packets and, consequently, send ICMP echo reply packets in return. Additionally, the intermediary can also become the victim, because it receives an ICMP echo request packet sent to the IP broadcast address of its network. Attackers make this technique successful by using spoofed packets. By doing this, the victim responds with ICMP echo reply packets that consume available bandwidth.

Figure 1-6. Smurf Attack Example


DDoS Attacks

DoS attacks may be orchestrated collectively to structure a more sophisticated technique called distributed DoS (DDoS) attacks. DDoS attacks coordinate the use of several systems in different locations to attack a specific victim, making them very difficult to trace. In DDoS attacks, the attackers compromise numerous systems on the Internet by installing malicious code to launch coordinated attacks on victim sites. These compromised systems are often referred as bots (short for robots). These attacks characteristically deteriorate bandwidth and other network resources. Figure 1-7 explains how an attacker compromises numerous systems (agents) on the Internet and then launches a chained attack from them to a target system/network (victim).

Figure 1-7. DDoS Attacks

In general, attackers control the agents to generate these packets with spoofed source addresses.

Session Hijacking

Session hijacking occurs when an attacker intercepts a connection or session between two systems. The most common type of session hijacking attack uses TCP-based connections and source-routed packets. In other words, the hacker sits in on a given location on the network to take part in a session between two systems, routing all the TCP packets to pass through its system. Figure 1-8 shows a basic session hijacking example, which is known as a man-in-the-middle attack. An attacker sits in between the hosts and, by using a sniffer, intercepts and routes packets from Host A to Host B without them knowing that the information has been compromised by the attacker.

Figure 1-8. Session Hijacking Example

If an attacker is not able to use source routing, they can also use a method called blind hijacking. By using this technique, the attacker guesses the replies of the two hosts. The only disadvantage to the attacker is that they may send a command and never see its response. However, the attacker can impersonate the receiver of both devices.


Source routing is a technique by which the sender can specify the route that a packet should take through the network.

Part I: Product Overview

Introduction to Network Security

Product History

Hardware Overview

Part II: Firewall Solution

Initial Setup and System Maintenance

Network Access Control

IP Routing

Authentication, Authorization, and Accounting (AAA)

Application Inspection

Security Contexts

Transparent Firewalls

Failover and Redundancy

Quality of Service

Part III: Intrusion Prevention System (IPS) Solution

Intrusion Prevention System Integration

Configuring and Troubleshooting Cisco IPS Software via CLI

Part IV: Virtual Private Network (VPN) Solution

Site-to-Site IPSec VPNs

Remote Access VPN

Public Key Infrastructure (PKI)

Part V: Adaptive Security Device Manager

Introduction to ASDM

Firewall Management Using ASDM

IPS Management Using ASDM

VPN Management Using ASDM

Case Studies

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Cisco Asa(c) All-in-one Firewall, IPS, And VPN Adaptive Security Appliance
Cisco ASA: All-in-One Firewall, IPS, and VPN Adaptive Security Appliance
ISBN: 1587052091
EAN: 2147483647
Year: 2006
Pages: 231
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