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There is no one single fix for security, and the systems, users and data (SUD) must be protected at many different levels, whether it be from the lowest level of the Internet model with physical security, to the highest level with data encryption. It is also never possible to truly guard every layer completely, as there are often weaknesses in each of the layers that can be exploited. In fact, one of the most difficult attacks to defend against is a social engineering attack, where users are prompted for their login details from a respected source, which is a spoofed one. As will be seen in this chapter, each of the devices which are used to protect SUD’s are typically there to deter potential intruders, and not provide an ultimately bar to intrusions. Often there is a trade-off between implementing extensive security, which will reduce the performance of intermediate devices, or to implement simplier security, which will have a lesser effect on security devices. This chapter investigates some of the devices which are used at the Internet and transport layers of the Internet model. The main devices are: - Stateless/Packet-filter firewalls. These are typically routers which filter data packets at the Internet (network) and transport layers, and thus filter network addresses and TCP/UDP ports. These are typically known as screening firewalls. These devices tend to be stateless, where each packet is looked at independently for all others. - Stateful firewalls (PIX/ASA). These are more complex devices than packet filter firewalls, and remember the state of a connection. They can also filter at most of the layers of abstration, such as for IP addresses, TCP ports, Application Layer commands, URLs, and so on. They tend to have an These are specialised devices which focus on insolating the inside network from the outside, and in creating DMZ’s. - Proxy servers. These are used to act as a buffer between an external network and an internal one, and are used to isolate nodes from external untrusted hosts. Proxy servers are often known as application gateways. - NAT (Network Address Translation) devices. These are used to translate net-work address from an internal network to an external one. They have many applications, such as being able to assign a wide range of private IP addresses, and in isolating nodes from direct external access. Figure 6.1 shows some of the icons used.
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This table shows the occurances of the letters in the text (ignoring the case of the letters):
This table shows how the text matches a normal probability to text (where 'E' has the highest level of occurance and 'Z' has the least). The grey rows show what would be expected for the order, and the red one shows what your text gives for the order: