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Network Security

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Threats and Preventive Measures

A basic understanding of computer networks is requisite in order to understand the principles of network security. Quite simply, a computer network is a system of interconnected computers sharing data and physical devices. The data can range from application programs to databases and the physical devices may range from a modem to a plotter.

Network security is a complicated subject, historically only tackled by well-trained and experienced experts. However, as more and more people become ``wired'', an increasing number of people need to understand the basics of security in a networked world.

Network Security

It's very important to understand that in security, there are two extremes: absolute security and absolute access. The closest we can get to an absolutely secure machine is one unplugged from the network, power supply, locked in a safe, and thrown at the bottom of the ocean. Unfortunately, it isn't terribly useful in this state. A machine with absolute access is extremely convenient to use: it's simply there, and will do whatever you tell it, without questions, authorization, passwords, or any other mechanism. Unfortunately, this isn't terribly practical, either: the Internet is a bad neighborhood now, and it isn't long before some bonehead will tell the computer to do something like self-destruct, after which, it isn't terribly useful to you.

This is no different from our daily lives. We constantly make decisions about what risks we're willing to accept. When we get on an airplane, we're accepting the level of risk involved as the price of convenience. However, most people have a mental picture of what an acceptable risk is, and won't go beyond that in most circumstances. If I happen to be upstairs at home, and want to leave for work, I'm not going to jump out the window. Yes, it would be more convenient, but the risk of injury outweighs the advantage of convenience.

Every organization needs to decide for itself where between the two extremes of total security and total access they need to be. A policy needs to articulate this, and then define how that will be enforced with practices and such. Everything that is done in the name of security, then, must enforce that policy uniformly.

Types of Network Threats and How to avoid them

Now, after having a basic idea of networking and network security, we can actually get into the more detailed aspects of it. First of all, we need to get into the types of threats there are against networked computers, and then some things that can be done to protect a network against various threats.

But before we begin, we need to know how does an attacker gain access to the computer network? The most common source of network attacks is generally through any connection that the networked computers have to the outside world. This includes Internet connections, dial-up modems, and even physical access. (How do you know that one of the temps that you've brought in to help with the data entry isn't really a system cracker looking for passwords, data phone numbers, vulnerabilities and anything else that can get him access to your equipment?) Along with that, the computer network is also vulnerable to insider attacks from the company staff itself. There is very little that can be done about this situation.

In order to be able to adequately address security, all possible avenues of entry must be identified and evaluated. The security of that entry point must be consistent with the stated policy on acceptable risk levels.

Unauthorized access

This simply means that people who shouldn't use the network computer services are able to connect and use them. For example, people outside your company might try to connect to your company accounting machine or to the NFS server. There are various ways to avoid this attack by carefully specifying who can gain access through these services. We can prevent network access to all except the intended users.

Exploitation of known weaknesses in programs

Some programs and network services were not originally designed with strong security in mind and are inherently vulnerable to attack. The BSD remote services (rlogin, rexec, etc.) are an example. The best way to protect against this type of attack is to disable any vulnerable services or find alternatives. With Open Source, it is sometimes possible to repair the weaknesses in the software.

Denial of service

Denial of service attacks cause the service or program to cease functioning or prevent others from making use of the service or program. These may be performed at the network layer by sending carefully crafted and malicious datagrams that cause network connections to fail. They may also be performed at the application layer, where carefully crafted application commands are given to a program that cause it to become extremely busy or stop functioning.

Preventing suspicious network traffic from reaching the network hosts and preventing suspicious program commands and requests are the best ways of minimizing the risk of a denial of service attack. It's useful to know the details of the attack method, so the network security personnel should educate themselves about each new attack as it gets publicized.


This type of attack causes a host or application to mimic the actions of another. Typically the attacker pretends to be an innocent host by following IP addresses in network packets. For example, a well-documented exploit of the BSD rlogin service can use this method to mimic a TCP connection from another host by guessing TCP sequence numbers. To protect against this type of attack, the authenticity of datagrams and commands must be verified and datagram routing with invalid source addresses should be prevented. We can also introduce unpredictablility into connection control mechanisms, such as TCP sequence numbers and the allocation of dynamic port addresses.


This is the simplest type of attack. A host is configured to "listen" to and capture data not belonging to it. Carefully written eavesdropping programs can take usernames and passwords from user login network connections. Broadcast networks like Ethernet are especially vulnerable to this type of attack. To protect against this type of threat, we should avoid use of broadcast network technologies and enforce the use of data encryption. IP firewalling is very useful in preventing or reducing unauthorized



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