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Before moving on to chapters that outline the steps necessary to secure networks and computer systems it helps to first have an understanding of the kinds of attacks and threats that need to be defended against. Armed with this information it will be clearer in later chapters not just how to implement particular security measures, but also why such measures need to be implemented.

Before moving on to chapters that outline the steps necessary to secure networks and computer systems it helps to first have an understanding of the kinds of attacks and threats that need to be defended against. Armed with this information it will be clearer in later chapters not just how to implement particular security measures, but also why such measures need to be implemented.

== TCP and UDP Based Denial of Service (DoS) Attacks ==

== TCP and UDP Based Denial of Service (DoS) Attacks ==

Denial of Service (DoS) are undertaken with the express purpose of preventing users from accessing and using a service they should otherwise be able to access. Such attacks make malicious use of a variety of different standard protocols and tools. There is no single DoS attack method, and the term has come to encompass a variety of different forms of attack, a number of which are outlined below:

Denial of Service (DoS) attacks are undertaken with the express purpose of preventing users from accessing and using a service they should otherwise be able to access. Such attacks make malicious use of a variety of different standard protocols and tools. There is no single DoS attack method, and the term has come to encompass a variety of different forms of attack, a number of which are outlined below:

* '''Ping flood''' - This attack uses the Internet Message Protocol (ICMP) ''ping'' request to a server as a DoS method. The strategy either involves sending ping requests in such vast quantities that the receiving system is unable to respond to valid user requests, or sending ping messages which are so large (known as a ''ping of death'') that the system is unable to handle the request.

* '''Ping flood''' - This attack uses the Internet Message Protocol (ICMP) ''ping'' request to a server as a DoS method. The strategy either involves sending ping requests in such vast quantities that the receiving system is unable to respond to valid user requests, or sending ping messages which are so large (known as a ''ping of death'') that the system is unable to handle the request.

* '''Smurfing''' - As with Ping Flood attacks, smurfing makes use of the TCP Internet Message Protocol (ICMP) ''ping'' request to mount DoS attacks. In a typical smurfing attack the attacker sends a ping request to the broadcast address of network containing the IP address of the victim. The ping request is sent to all computers on the broadcast network, which in turn all reply to the IP address of the victim system thereby overloading the victim with ping responses. The primary method for preventing smurf attacks is to block ICMP traffic through routers so that the ping responses are blocked from reaching internal servers.

* '''Smurfing''' - As with Ping Flood attacks, smurfing makes use of the TCP Internet Message Protocol (ICMP) ''ping'' request to mount DoS attacks. In a typical smurfing attack the attacker sends a ping request to the broadcast address of network containing the IP address of the victim. The ping request is sent to all computers on the broadcast network, which in turn all reply to the IP address of the victim system thereby overloading the victim with ping responses. The primary method for preventing smurf attacks is to block ICMP traffic through routers so that the ping responses are blocked from reaching internal servers.

* '''TCP SYN Flood''' - Also known as the ''TCP Ack Attack'', this attack leverages the TCP three way handshake to launch a DoS attack. The attack begins with a client attempting to establish a TCP connection with the victim server. The client send a request to the server, which in turn returns an ACK package to acknowledge the connection. At this point in the communication the client should respond with a message accepting the connection. Instead the client sends another ACK which is responded to by the server with yet another ACK. The client continues to send ACKs to the server with the effect of causing the server to hold sessions open in anticipation of the client sending the final packet required to completion the connection. As a result the server uses up all available sessions serving the malicious client, thereby preventing access to other users.  

* '''TCP SYN Flood''' - Also known as the ''TCP Ack Attack'', this attack leverages the TCP three way handshake to launch a DoS attack. The attack begins with a client attempting to establish a TCP connection with the victim server. The client sends a request to the server, which in turn returns an ACK package to acknowledge the connection. At this point in the communication the client should respond with a message accepting the connection. Instead the client sends another ACK which is responded to by the server with yet another ACK. The client continues to send ACKs to the server with the effect of causing the server to hold sessions open in anticipation of the client sending the final packet required to complete the connection. As a result the server uses up all available sessions serving the malicious client, thereby preventing access to other users.  

* '''Fraggle''' - A fraggle attack is similar to a ''smurfing attack'' with the exception that the User Datagram Protocol (UDP) is used instead of using ICMP.  

* '''Fraggle''' - A fraggle attack is similar to a ''smurfing attack'' with the exception that the User Datagram Protocol (UDP) is used instead of using ICMP.  

* '''Land''' - Under a Land attack the attacker creates a fake SYN packet contain the same source and destination IP addresses and ports and sends it to the victim causing the system to become confused whn trying to respond to the packet.

* '''Land''' - Under a Land attack the attacker creates a fake SYN packet contain the same source and destination IP addresses and ports and sends it to the victim causing the system to become confused when trying to respond to the packet.

* '''Teardrop''' - A teardrop type of DoS attack exploits a weakness in the TCP/IP implementation of some operating systems. The attack works by sending messages fragmented into multiple UDP packages. Ordinarily the operating system is able to reassemble the packets into a complete message by referencing data in each UDB packet. The teardrop attack works by corrupting the offset data in the UDP packets making it impossible for the system to rebuild the original packets. On systems which are unable to handle this corruption a crash is the most likely outcome of a teardrop attack.

* '''Teardrop''' - A teardrop type of DoS attack exploits a weakness in the TCP/IP implementation on some operating systems. The attack works by sending messages fragmented into multiple UDP packages. Ordinarily the operating system is able to reassemble the packets into a complete message by referencing data in each UDB packet. The teardrop attack works by corrupting the offset data in the UDP packets making it impossible for the system to rebuild the original packets. On systems that are unable to handle this corruption a crash is the most likely outcome of a teardrop attack.

* '''Bonk''' -  An effective attack on some Windows systems involving the transmission corrupted UDP packets to the DNS port (port 53) resulting in a system crash.

* '''Bonk''' -  An effective attack on some Windows systems involving the transmission corrupted UDP packets to the DNS port (port 53) resulting in a system crash.

== Distributed Denial of Service (DDoS) Attacks ==

== Distributed Denial of Service (DDoS) Attacks ==

The Denial of Service (DoS) attacks outlined above involve the use of a single client to launch an attack on a system or service. Distributed Denial of Service Attacks use the same basic attack methodologies as outline above with the exception that the attacks are initiated from multiple client systems.  

The Denial of Service (DoS) attacks outlined above involve the use of a single client to launch an attack on a system or service. Distributed Denial of Service Attacks use the same basic attack methodologies as outline above, with the exception that the attacks are initiated from multiple client systems.  

The way this typically works is that malicious parties will use viruses to subtly gain control over large numbers of computers (typically poorly defended home computers connected to broadband internet connections). Unbeknown to the owner of the computer (which generally continues to function as normal) the system is essentially a zombie waiting to be given instructions. Once the malicious party has gathered an army of zombie computers they are instructed to participate in massive Distributed DoS attacks on unsuspecting victims. A large enough volume of zombie systems can, and indeed have been know to bring down even the largest and most scalable enterprise infrastructure, and even bring parts of the internet itself to a grinding halt.

The way this typically works is that malicious parties will use viruses to subtly gain control over large numbers of computers (typically poorly defended home computers connected to broadband internet connections). Unbeknown to the owner of the computer (which generally continues to function as normal) the system is essentially a zombie waiting to be given instructions. Once the malicious party has gathered an army of zombie computers they are instructed to participate in massive Distributed DoS attacks on unsuspecting victims. A large enough volume of zombie systems can, and indeed have been known to bring down even the largest and most scalable enterprise infrastructure, and even bring parts of the internet itself to a grinding halt.

== Back Door Attacks ==

== Back Door Attacks ==

Back Door attacks utilize programs which provide a mechanism for entering a system without going through the usual authentication process. This can either take the form of hidden access points intentionally put into application by the original developers to aid in maintaining and debugging the software (which were then left in when the software was deployed by customers) or a malicious program that is placed on a system via a virus, or other method which opens up the system to unauthorized access.

Back Door attacks utilize programs that provide a mechanism for entering a system without going through the usual authentication process. This can either take the form of hidden access points intentionally put into application by the original developers to aid in maintaining and debugging the software (which were then left in when the software was deployed by customers) or a malicious program that is placed on a system via a virus, or other method which opens up the system to unauthorized access.

A number of back door programs have been discovered over the years, some which are listed below:

A number of back door programs have been discovered over the years, some which are listed below:

* '''Back Orifice''' - This rather distastefully named tool was developed by a group known as the the Cult of the Dead Cow Communications. The primary purpose of Back Orifice is to provide remote access to a server for the purposes of performing administrative tasks.

* '''Back Orifice''' - This rather distastefully named tool was developed by a group known as the Cult of the Dead Cow Communications. The primary purpose of Back Orifice is to provide remote access to a server for the purposes of performing administrative tasks.

* '''NetBus''' - Similar to Back Orifice, NetBus is also designed to enable remote administrative access to Windows system.

* '''NetBus''' - Similar to Back Orifice, NetBus is also designed to enable remote administrative access to Windows system.

== IP and DNS Spoofing Attacks ==

== IP and DNS Spoofing Attacks ==

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The basis of spoofing involves masquerading a trusted system in order to gain unauthorized access to a secure environment. IP spoofing involves modifying data to make it appear to originate from the IP address of a system that is trusted by a server or firewall. Using this approach, a host is able to pass through the IP filtering that would otherwise serve to prevent access.  

The basis of spoofing involves masquerading as a trusted system in order to gain unauthorized access to a secure environment. IP spoofing involves modifying data to make it appear to originate from the IP address of a system that is trusted by a server or firewall. Using this approach, a host is able to pass through the IP filtering that would otherwise serve to prevent access.