This report is meant to provide some background to the subject for people
interested in this new issue This study does not required advanced skills
of computing, however a basic knowledge of UNIX and networking would be
helpful to the reader.
I am grateful to a number of people for their assistance during the preparation of this dissertation. Among them I am especially thankful to my supervisor, Sharon Morgan, who has been guiding me for the plan and the layout of this document, George Cormack and Anna Stevenson who have corrected my grammar mistakes. I would also like to thank the worldwide Internet community which helped me to find some critical data about the Internet security.
CHAPTER 4: BREACHES AND HOW HACKERS WORK
CHAPTER 1: INTRODUCTION
Experts are forecasting that by the year 2000 some 200 million users will be accessing the Internet world-wide and the London-based Electronic Commerce Association (ECA) forecasts that electronic commerce will grow by 50 percent annually, reaching œ100 billion by 2002, with the number of companies accessing the web rising from 1.3 million at the end of 1997 to 8 million by 2001.
The nature of computing has changed tremendously over the last few years. As computers and networks have become cheaper and more powerful, they have also become more ubiquitous. One unfortunate side effect of these changes is that computer crime has become much more common.
A brief history of the Internet and its security problems is given in the first chapter. It introduces the Internet to the reader. Then, chapter 2 covers the main issue of Internet security in organisations. Chapter 3 presents the way hackers work and the weak points of the Internet security. Chapter 4 describes the methods used to improve this security.
The research of documentation has mainly been done on the Internet. However, some books and computer magazines have helped me complete an exhaustive search. I also interviewed a security consultant to a French Internet provider who gave me some technical advice about the implementation of the security on a UNIX system.
CHAPTER 2: Brief history of the Internet and its security problems
Before beginning to enter the problem of Internet security, it would be helpful to have an overview of the Internet. The best way to do this is to observe the history of the Internet.
Further details about the history of the Internet can be found in the Short History of the Internet  and The Roads and Crossroads of Internet's History .
CHAPTER 3: Internet security issues in organisations
As an increasing number of organisations come on the Internet, Internet security becomes a major issue. We are going to see, in this part, examples of problems that occur in organisations.
3.1 Hacked advertising
Many organisations open a web site to have a sort of window on the Internet, where they can expose their products, activities, etc ... A new kind of hacking consists of entering the website illegally and modifying the homepages. It is like entering in a shop and changing the displays. In all cases, when a web site is attacked, the new homepage produced by hackers does not promote the activity of the organisation. Several examples are here to illustrate this new emerging problem.
3.1.1 U.S. administrations
3.1.2 Other organisations
3.2 Industrial and military espionage
Nowadays, the bulk of Internet hosts are corporate sites. Some companies use Internet as a network to transmit data. There are a lot of examples of companies' hosts being hacked.
A good example is the Boeing site:
A few years ago, two college students hacked their way into Boeing's computers in search of password files. The U.S. Justice Department says that situation is a classic case of how hackers can drive up business costs. Boeing was also able to ascertain that these hackers had obtained root access to the federal courthouse system in Seattle. After the case was over, it cost Boeing $57,000 simply to check the integrity of their avionics data. 
Actually, the main risk is the theft of confidential data, but there is another risk, the risk of modified data. The hacker can change the current data into faulty ones. Then, the organisation will work with altered data and produce faulty results. This is why Boeing spent so much money to check all the integrity of its data.
In 1987, tracing an apparently innocuous 75-cent accounting error revealed an intruder who had given himself an account on the Lawrence Berkeley Lab's computer system. The account was traced to a West German programmer who was copying documents from military computers attached to the MILNET (the Internet segment reserved for military uses). The hacled documents were sold to the KGB. 
The Gartner Group's William Malik says that one of his clients, a large manufacturing company, lost a $900 million dollar bid to a competitor which had apparently cracked the company's computers and learned about its bid. 
Examples of this sort are abundant in the world of the Internet. However, a good number of these attacks are not reported to the public because they are concealed by the company or they are not detected by the network administrator of the firm.
3.3 Business integrity
Credit card security is one of the biggest issue of the Internet because the potential to literally create a world wide commerce opens a very large hole to credit card fraud.
Nowadays, there are myriad virtual shops on the Internet that sell books, computer accessories, clothes and so forth ... All of these virtual shops use the credit card payment method. This means that the customer sends his credit card number to the virtual shop which then debits the amount of money from the account of the bank's credit card. But there are two problems: First, the transmission of the credit card number can be pick up by a third party without knowledge from either the merchant and the customer. Secondly, a majority of these commercial sites are not secure at all. Even if the transmission of the credit card number through the Internet is safe, the way to store it in the web-site is not. Credit card numbers in files inside a company's database can also be taken. Several firms specialising in Internet commerce have been hacked in search of credit card number or customers' files.
It's already happened at the Internet service provider Netcom of San Jose (California, USA). Kevin Mitnick, perhaps the world's most famous hacker, has stolen about 17,000 credit card numbers before to being caught by clever high-tech detectives. 
While commerce on the Internet is becoming a multimillion pound business, it is also becoming a large concern for credit card corporations and their customers.
3.4 System disabling
Systems providing TCP-based services (WWW, email, newsgroup, gopher, etc ...) to the Internet community may be unable to provide those services while under a hacker's attack and for some time after the attack has ceased. The service itself is not harmed by the attack; usually only the ability to provide the service is impaired. In some cases, the system may exhaust its memory, crash, or be rendered otherwise inoperative. These attacks block the system temporarily, so during the inoperative time, the company owner of the system might lose money. A description of this sort of attack is in the next section. 
CHAPTER 4: Breaches and how hackers work
How do hackers work ? What are the weak points of the Internet ? In this chapter we will see the different methods used by hackers to reach their goals and some of the weaknesses of networks. Of course, the study can not be exhaustive, but it will cover the main used methods.
First of all, we will learn what the definition of a "hacker" is. Subsequently, a short explanation about the network file system will be useful for the understanding of the rest of this report.
4.1.1 Definition of a hacker
On Internet, calling someone a "hacker" is usually a statement that said person holds a great deal of knowledge and expertise in the field of computing-networking, and is someone who is capable of exercising this expertise with great finesse. The hacker holds the belief that system-cracking for fun and exploration is ethically OK as long as the person commits no theft, vandalism, or breach of confidentiality. A cracker is one who breaks security into other peoples' computer systems, for a variety of reasons. Particularly antisocial crackers have a vandalistic streak, and delete filestores, crash machines, and stop running processes in pursuit of their "kicks". In the "real world", various media people have taken the word "hacker" and coerced it into meaning the same as "cracker". This usage is wholly inappropriate among the Internet community.
4.1.2 Network file system
Every time a user requests access to a file, the operating system decides whether that user should have access to the file in question. The system makes this decision based on who owns the file, who is asking for access to the file, and what access permissions the owner has set up. The access permissions define who will have access to the file.
There are two main reasons why one should care about protecting his/her files from other users. The first reason is that one may wish to protect the contents of his/her files from others. One may consider the contents of the files private, and does not want others to be able to read or modify the contents of the files. The second reason is that if others can modify the files, they can obtain access to your account. For example, if a malicious user has write access to your home directory, they can create or modify your ".rhosts" file to give anyone unlimited access to your account.
The main goal for an intruder is to gain root access. Root access allows a user to do all that he wants on a system, he can delete, modify or add new files. Most computer breakins where the intruder gains "root access" to a computer begin when the intruder breaks into a regular user's account. Once on the machine as a regular user, the intruder can then launch an attack to gain root access on a machine by using a security hole in the operating system. 
4.2 Social engineering
This is a term used among cracker for cracking techniques that rely on weaknesses in the human being attached to a computer system rather than software; the aim is to trick people into revealing passwords or other information that compromises a target system's security.
Classic tricks include phoning up a mark who has the required information and posing as a field service tech or a fellow employee with an urgent access problem. A common variation is to do this by phone, talk or IRC (Internet Relay Chat).
Another classic social engineering trick is for a hacker to send email claiming to be a system administrator. The hacker will claim to need your password for some important system administration work, and ask you to email it to him/her. It is possible for a hacker to forge email, making it look like it came from somebody you know to be a legitimate system administrator. Often the hacker will send this message to every user on a system, hoping that one or two users will fall for the trick.
Another form of social engineering goes back to guessing your password. People who can find out things about you, can use that information to guess your password. For example, the names of your children, their birthdays or the license plate number on your car are all likely candidates for guessing as passwords. Hackers can go very far to guess passwords.
4.3 Password cracking
Passwords are very important because they are the first line of defence against interactive attacks on a system. It can be stated simply: if a cracker cannot interact with a remote system, and he has no access to read or write the information contained in the password file, then he has almost no chance to mount a successful attack against the system. This is also why, if a cracker can at least read the password file of a remote host, it is so important that he is not able to break any of the passwords contained therein. If he can, then it is also fair to assume that he can log on to the system and can then break into "root" via an operating system hole.
The most common way a hacker will try to get a password is via a "dictionary attack". In a dictionary attack, the attacker takes a dictionary of words and names, and tries each one to see if it is the right password. They do this with programs which can guess hundreds or thousands of words per second. This makes it easy for them to try lots of variations: word spelled backwards, different capitalisations, adding a digit to the end, and so on. In addition, the hacker community has built large dictionaries which are designed to "crack" passwords. Using words from foreign languages, or names of things, people or towns is no protection against current password crackers. The most famous program of passwords cracking is "Crack4.1" with its general 50,000-word dictionary.
4.4 Packet and password sniffing
If a hacker can not guess your password, there are other ways he/she can try to get it. One way which has become very popular is called "password sniffing". It turns out that most networks use what is known as "broadcast" technology. What that means is that every message that a computer on the network transmits can be read by any other computer on that network. In practice, all the computers except the recipient of the message will notice that the message is not meant for them, and ignore it. However, many computers can be programmed to look at every message on the network. If one does this, one can look at message which are not intended for him/her.
Hackers have programs which do this, and then scan all the messages
which traverse a network looking for passwords. If someone logs in to a
computer across a network, and some other computer on the network has been
compromised this way, the person may unwittingly give his/her password
to the attacker. This is a serious threat to users who login to computers
from remote sites. If someone logs in on the console of a computer, his/her
password never crosses a network where it can be sniffed. But if someone
logs in from some other network or from an Internet service provider, he/she
is dependent on the security of these networks. The well known programs
of password sniffing are:
- Esniff.c (Source for a basic ethernet sniffer)
- Solaris Sniffer (A more powerful version of E-Sniff modified for Solaris 2)
4.5 IP spoofing
The IP address of a host is presumed to be valid and is therefore trusted by TCP and UDP services. A problem is that, using IP source routing, an attacker's host can masquerade as a trusted host or client. Briefly, IP source routing is an option that can be used to specify a direct route to a destination and return path back to the origination. The route can involve the use of other routers or hosts that normally would not be used to forward packets to the destination. An example of how this can be used such that an attacker's system could masquerade as the trusted client of a particular server is as follows:
An even simpler method for spoofing a client is to wait until the client system is turned off and then impersonate the client's system. In many organisations, staff members use personal computers and TCP/IP network software to connect to and utilise UNIX hosts as a local area network server. The personal computers often use NFS to obtain access to server directories and files (NFS uses IP addresses only to authenticate clients). An attacker could, after hours, configure a personal computer with the same name and IP address as another's, and then initiate connections to the UNIX host as if it were the "real" client. This is very simple to accomplish and likely would be an insider attack.
Electronic mail on the Internet is also particularly easy to spoof and, without enhancements such as digital signatures, generally can not be trusted. As a brief example, consider the exchange that takes place when Internet hosts exchange mail. The exchange takes place using a simple protocol consisting of ASCII-character commands. An intruder easily could enter these commands by hand by using TELNET to connect directly to a system's Simple Mail Transfer Protocol (SMTP) port (port 25 on UNIX systems). The receiving host trusts that the sending host is who it says it is, thus the origin of the mail can be spoofed easily by entering a sender address that is different from the true address. As a result, any user, without privileges, can falsify or spoof e-mail.
Other services, such as Domain Name Service, can be spoofed, but with more difficulty than electronic mail. These services still represent a threat that needs to be considered when using them. 
4.6 Trojan Horses
A Trojan horse is a program which hides itself in another apparently benign program. When the victim runs the apparently benign program he or she also ends up running the hidden Trojan program.
There are examples of UNIX Trojan horse programs on the Internet. For example in one incident, a well known anonymous ftp archive was broken into. The attackers modified a popular program available from this site, allowing them to break into computers which subsequently down-loaded and installed this program.
A worm is an autonomous agent capable of propagating itself without the use of another program or any action by a person. The most famous worm attack occurred in November 1988, when a student launched a program on the Internet which was able to develop itself through the hosts network. Within 8 hours between 2 and 3 thousand computers were infested. Computers began to shut down because worm programs reappeared over network connections faster than they could be deleted.
What exactly did the worm do? The worm infested only computers running one particular UNIX operating system. Each worm began by creating a list of remote target machines from information found in the current host.
In parallel the worm would:
Worms attacks are rare, but it is still a method used by hackers when a new bug is found on an OS. This has the "advantage" of being able to hack a lot of sites in little time.
4.8 Trap Door or back door
A trap door or back door is an entry point into a computer system that bypasses the normal security measures, a hidden software or hardware mechanism that permits system protection mechanisms to be circumvented. It is activated in some non-apparent manner.
It can be a hole in the security of a system deliberately left in place by designers or maintainers. The motivation for such holes is not always sinister; some operating systems, for example, come out of the box with privileged accounts intended for use by field service technicians or the vendor's maintenance programmers.
In 1983, Ken Thompson (one of the author of UNIX) revealed the existence of a back door in early UNIX versions that may have qualified as the most clever security hack of all time. The C compiler contained code that would recognise when the "login" command was being recompiled and insert some code recognising a password chosen by Thompson, giving him entry to the system whether or not an account had been created for him. Normally such a back door could be removed by removing it from the source code for the compiler and recompiling the compiler. But to recompile the compiler, you have to use the compiler. So Thompson also arranged that the compiler would recognise when it was compiling a version of itself, and insert into the recompiled compiler the code to insert into the recompiled `login' the code to allow Thompson entry and, of course, the code to recognise itself and do the whole thing again the next time around. And having done this once, he was then able to recompile the compiler from the original sources; the hack perpetuated itself invisibly, leaving the back door in place and active but with no trace in the sources.
Almost every time, when a hacker leaves a system, he leaves behind one or several backdoors to be able to come back whenever he wants. It is why when a system has been hacked it is safer to check all the system files or re-install the operating system.
4.9 TCP-SYN flooding
When a system (called the client) attempts to establish a TCP connection to a system providing a service (the server), the client and server exchange a set sequence of messages. This connection technique applies to all TCP connections (telnet, Web, email, etc...)
The client system begins by sending a SYN message to the server. The
server then acknowledges the SYN message by sending SYN-ACK message to
the client. The client then finishes establishing the connection by responding
with an ACK message. The connection between the client and the server is
then open, and the service-specific data can be exchanged between the client
and the server.
Here is a view of this message flow:
Client and server can now
send service-specific data
The potential for abuse arises at the point where the server system has sent an acknowledgement (SYN-ACK) back to client but has not yet received the ACK message. This is a half-open connection. The server has built in its system memory a data structure describing all pending connections. This data structure is of finite size, and it can be made to overflow by intentionally creating too many partially-open connections.
Creating half-open connections is easily accomplished with IP spoofing. The attacking system sends SYN messages to the victim server system; these appear to be legitimate but in fact reference a client system that is unable to respond to the SYN-ACK messages. This means that the final ACK message will never be sent to the victim server system.
The half-open connections data structure on the victim server system will eventually fill; then the system will be unable to accept any new incoming connections until the table is emptied out. Normally there is a time-out associated with a pending connection, so the half-open connections will eventually expire and the victim server system will recover. However, the attacking system can simply continue sending IP-spoofed packets requesting new connections faster than the victim system can expire the pending connections.
In most cases, the victim of such an attack will have difficulty in accepting any new incoming network connection. In these cases, the attack does not affect existing incoming connections nor the ability to originate outgoing network connections. However, in some cases, the system may exhaust memory, crash, or be rendered otherwise inoperative.
The location of the attacking system is obscured because the source addresses in the SYN packets are often implausible. When the packet arrives at the victim server system, there is no way to determine its true source. Since the network forwards packets based on destination address, the only way to validate the source of a packet is to use input source filtering. 
There are also plenty of other attack methods but they are very technical for a non-advanced UNIX user. Here is a short list of some of them:
Sendmail attack: attack via the mail system on port 25
NIS and NFS attack
FTP attack: attack via the ftp port (21)
Telnet attack: attack via the telnet port (23)
Rlogin and rsh attack
This is not an exhaustive list, because there are probably many attacks that have not been disclosed.
CHAPTER 5: Ways to secure the insecured
In this part, we are going to see which methods can be used to improve the security on Internet. The two main hopes of Internet security remains in firewalls and encryption.
5.1 Internet firewalls
Firewalls have been called condoms for corporate networks. They provide digital protection associated with the rapid growth of internetworking and commercialisation of the Internet. As with condoms, many people have heard of firewalls and some people use them. However, the number of security incidents arising from Internet connections strongly suggests that not enough people are using them properly.
5.1.1 What is a firewall ?
A firewall is a form of access-control technology that prevents unauthorised access to information resources by placing a barrier between an organisation's network and an unsecured network (e.g. Internet). A firewall is also used to prevent the unauthorised export of proprietary information from a corporate network. In other words, a firewall functions as a gateway, controlling traffic in both directions.
The typical firewall is an inexpensive micro-based UNIX box kept clean of critical data, with a bunch of modems and public network ports on it but just one carefully watched connection back to the rest of the cluster. The special precautions may include threat monitoring or call-back.
Some firewalls permit only Email traffic through them, thereby protecting the network against any attacks other than attacks against the Email service. Other firewalls provide less strict protections, and block services that are known to be problems. 
Generally, firewalls are configured to protect against unauthenticated interactive logins from the "outside" world. This, more than anything, helps prevent vandals from logging into machines on the internal network. More elaborate firewalls block traffic from the outside to the inside, but permit users on the inside to communicate freely with the outside. The firewall can protect you against any type of network borne attack.
Firewalls are also important since they can provide a single "choke point" where security and audit can be imposed. Unlike in a situation where a computer system is being attacked by someone dialling in with a modem, the firewall can act as an effective "phone tap" and tracing tool.
Figure 5.1.1 Firewall's location
5.1.2 What can a firewall not do ?
Firewalls can not protect against attacks that do not go through the firewall. Many corporations that connect to the Internet are very concerned about proprietary data leaking out of the company through that route. Unfortunately for those concerned, a magnetic tape can just as effectively be used to export data. Firewall policies must be realistic, and reflect the level of security in the entire network. For example, a site with top secret or classified data doesn't need a firewall at all: they shouldn't be hooking up to the Internet in the first place, or the systems with the really secret data should be isolated from the rest of the corporate network.
Firewalls can not protect very well against things like viruses. There are too many ways of encoding binary files for transfer over networks, and too many different architectures and viruses to try to search for them all. In other words, a firewall cannot replace security, consciousness on the part of the users. In general, a firewall cannot protect against a data-driven attack (attacks in which something is mailed or copied to an internal host where it is then executed). This form of attack has occurred in the past against various versions of Sendmail.
There are plenty of sorts of firewalls. However the main part of them are a piece of software installed on the router of the company or on another host. But there are also hardware firewalls. It is an electronic board which is plugged inside the computer. There are different role for a firewall. Some are packet filtering router, dual-home gateway, bastion host, etc ... There is also a wide range of firewalls for each operating system: UNIX, Novell Netware, Windows NT, LINUX, and so on ...
Nowadays, firewalls are a good rampart against hackers. However, if a firewall is not installed properly, it could be worth than not having one due to a false sense of security.
5.2 Password protection and generation of safe passwords
As the password protection is one of the main problem of Internet security, there are two major ways to improve the password security: Shadow password and generation of "secure" passwords.
5.2.1 Shadow password
Shadow password is a system where the plaintext of the password file is hidden from all users except root, hopefully stopping all password cracking attempts at source. It provides a good degree of password file robustness.
5.2.2 Generation of secure passwords
First, it is interesting to see how many possible passwords there are. Most people worried that programs like "Crack" will eventually grow in power until they can do a completely exhaustive search of all possible passwords, to break into a specific users' account (usually root).
Valid passwords are created from a set of 62 chars [A-Za-z0-9] and they
can be made of even more characters such as #'œ$%^& etc ... There are
also to be between 5 and 8 characters long. With only the 62 common characters
the size of the set of all valid passwords is:
625+626+627+628 = 2.2E+14
A figure which is far too large to usefully undertake an exhaustive search with current technologies. Moreover, if one can use some of all the 95 non-control characters in passwords, this increases the search space for a cracker to cover even further.
Any password derived from any dictionary word (or personal information), modified in any way, constitutes a potentially guessable password.
For example password based on:
login name: cme45212
first name: sandra, stefan
standard surname: smith
backward words: htims, 21254emc, retupmoc
words of dictionary: computer
capitalised word: Computer, CoMpuTer
words of cracking dictionary: PORSCHE911, 12345678, qwerty, abcxyz, mr.spoke
foreign language words: salut45, gutentag
A good password is easy to remember, but hard to guess. The best way to make a password "un-crackable" is to make it appear random. Remember, it is always possible to insert digits and punctuation in a password. The favourite way to think of passwords which appear random, but which are easy to remember is to:
1.Take a phrase, or a line from a poem or a song. It needs to be at
least 8 words long
2.Take the first letter from each word, and use it as a character in your password.
3.Take advantage of punctuation.
4.If you can not think of one that is long enough, you can use a shorter one and pad it at the beginning and end with digits.
For example, the phrase: "One for all, and all for one" yields the relatively un-crackable password: "Ofa,&af1"
A good way for a network administrator to know if users have secured passwords is to use password cracking programs on his own system as will do a cracker. "Crack" is a good tool for insuring that UNIX system's users have not selected easily guessed passwords which appear in standard dictionaries. 
Encryption is the cryptographic methods and the technology which permits users to send messages that can be understood (decrypted) only by the intended recipient, improving controls on routing messages over the Internet, and improving operating system quality to decrease program flaws and other security vulnerabilities.
There are two main types of encryption: asymmetric encryption (also called public-key encryption) and symmetric encryption.
5.3.1 Asymmetric or public key encryption
This is a cryptographic system that uses two keys: a public key known to everyone and a private or secret key known only to the recipient of the message.
Example: When John wants to send a secure message to Jane, he uses Jane's public key to encrypt the message. Jane then uses her private key to decrypt it.
An important element to the public key system is that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key if you know the public key.
Public-key systems, such as Pretty Good Privacy (PGP, see further), are becoming popular for transmitting information via the Internet. RSA system used for securing payment through the world wide web is now a standard. These systems are extremely secure and relatively simple to use. The only difficulty with public-key systems is that you need to know the recipient's public key to encrypt a message for him or her. However when paying through the WWW, the browser manages this task itself by asking the remote server its public key. 
5.3.2 Symmetric encryption
It is a type of encryption where the same key is used to encrypt and decrypt the message. DES encryption (Data Encryption Standard) is the most famous form of symmetric encryption. It is currently used by US administrations to send data through a network. However, They use the public key encryption system to send the key of DES encryption to the recipient of the encrypted file.
This is a program for encrypting messages developed by Philip Zimmerman. PGP is one of the most common ways to protect messages on the Internet because it is effective, easy to use, and free. PGP is based on the public-key method, which uses two keys: one is a public key that you disseminate to anyone from whom you want to receive a message, the other is a private key that you use to decrypt messages that you receive.
To encrypt a message using PGP, you need the PGP encryption package, which is available for free from a number of sources (the official repository is at the Massachusetts Institute of Technology).
PGP is such an effective encryption tool that the US government actually brought a lawsuit against Zimmerman for putting it in the public domain and hence making it available to enemies of the U.S. After a public outcry, the US lawsuit was dropped, but it is still illegal to use PGP in many other countries.
Encryption is the most effective way to achieve data security, ensure data integrity and confidentiality.
5.4 IP spoofing and SYN-flooding
With the current IP protocol technology, it is impossible to eliminate IP-spoofed packets. However, there are steps to be taken to reduce the number of IP-spoofed packets entering and exiting the network.
The best method is to install a filtering router that restricts the input to the external interface (known as an input filter) by not allowing a packet through if it has a source address from the internal network. In addition, it could be good to filter outgoing packets that have a source address different from the internal network to prevent a source IP spoofing attack from originating from the internal site.
The combination of these two filters would prevent outside attackers from sending the site packets pretending to be from the internal network. It would also prevent packets originating within the internal network from pretending to be from outside this network. These filters will not stop all TCP SYN attacks, since outside attackers can spoof packets from any outside network, and internal attackers can still send attacks spoofing internal addresses.
IP spoofing and SYN-flooding are currently the two major unresolvable problems of Internet security.
5.5 Security auditing tools
There are plenty of tools available through the Internet to check the security of a system. Some tools scan hosts for known vulnerabilities: SATAN is the most famous program, others tools check the file integrity such as Tripwire. The network administrator is strongly advised to use these tools before hackers.
Here is a short list of the other well known tools: ISS, C2 security, COPS, Tiger (part of the TAMU Security Package), MD5. 
5.6 Security through obscurity
This is a way to consider that any system can be secure so long as nobody outside of its implementation group is allowed to find out anything about its internal mechanisms. The technique is hiding account, passwords in binary files or scripts with the presumption that "nobody will ever find it".
This is a philosophy favoured by many bureaucratic US agencies. The main critic of this technique maintains that it is pseudosecurity because it does not solve the real problems of security but instead hides them.
It can also tie the manager into trusting a small group of people for as long as they live. If the employees get an offer of better pay from somewhere else, the knowledge goes with them, whether the knowledge is replaceable or not. Once the secret gets out, that is the end of the security.
However, this technique can complement other security steps.
5.7 IP Restriction
IP Restriction is a very common thing to do to limit a user to parts of the server. By allowing only a few IP address to other parts of the server, a hacker will not be granted access to areas where he or she can cause damage. 
5.8 Education and awareness
One of the major threat to the security of a system is not the technical holes of a host computer; it is the lack of awareness. By "lack of awareness," I mean that Internet users are often under the impression that the only way a hacker can break into their account or the system is through some secret back door left open by careless administrators. Another misunderstanding is the belief that if there is nothing of value in a user's account, no one would bother to break in. What an intruder finds valuable about an account is access to the system. This single access allow the intruder to get root access via a hole in the operating system or can be used as a gateway to hack other sites. The user is then responsible for this.
- A good step is to take strict measures to make users aware of the
importance of their password by encourage them to:
changing their password after the first login.
- not sharing their account
- protecting their password i.e. be careful while typing the password
- changing their password regularly, and especially after logging into the account from a remote machine.
- choosing secure passwords
Another good way to imply users for the security is to make them sign a charter of responsibility and good behaviour on the Internet.
The security is the business of everybody on a system: the administrator as well as the users. And the role of the administrator is to educate his users to security.
CHAPTER 6: Conclusion
As Internet security is a recent issue, the legislation about it is quite rare. The US federal networking council wrote a draft on the Internet security in 1995. This defines where the responsibilities of users of the Internet lie: the user, the management of multi-user hosts and Internet facilities, the system administrators, the Federal Networking Council, the Vendors and System Developers, the Computer Network and Service Providers. Further details about the US legislation can be found on the website of the US federal networking council .
The best solution for the Internet security is encryption. But many countries' governments do not approve this technique because it is a danger to the state security and governments can not control the information. So, it is why in the U.S. the public key encryption is only allowed with a 48-bit key. In other countries such as France and Singapore, encryption is banned. Now, corporations want to choose the strong security features that they need to protect information being communicated in electronic commerce.
While there is not and never will be a fool proof secure network, we can protect ourselves from the majority of the problems associated with the Internet. As the Internet continues to grow in popularity, it will surely grow with statistics of fraud, break in, and plain mischief. If you don't want to take the risk, turn off your computer now.
"The only system which is truly secure is one which is switched off and unplugged, locked in a titanium lined safe, buried in a concrete bunker, and is surrounded by nerve gas and very highly paid armed guards. Even then, I wouldn't stake my life on it." Gene Spafford
This document has dealt with the Internet security issues and has described the main techniques hackers use to achieve their kicks. The last chapter has described the methods used to improve the security.
The documentation was sometimes particularly difficult to find, especially that dealing with tricks hackers use to break into hosts. Moreover, I learned a lot of interesting things on the subject and it made me aware of this new important issue.
 Sterling B Short History of the Internet http://www.forthnet.gr/forthnet/isoc/short.history.of.internet
 Gromov G R The Roads and Crossroads of Internet 's History http://www.internetvalley.com/intval.html
 - Internet site of the TV channel CNN http://www.cnn.com 29 - December 1996
 - Hacked page published in the 2600 magazine http://www.2600.com/hacked_pages/
 - Internet site of the newspapers BangkokPost http://www.bangkokpost.net - 3 march 1997
 - Hacked page published in the 2600 magazine http://www.2600.com/hacked_pages/
 - Hacked page published in the 2600 magazine http://www.2600.com/hacked_pages/
 - Internet site of the TV channel CNN http://www.cnn.com - 2 march 1997
 Clifford The Cuckoo's Egg, Doubleday, Stoll, 1989
 - Newsweek, 2/6/95, p.36
 - Internet security: IBM, Mastercard and DPS complete first end-to-end secure electronic transaction over Internet EDGE, Jan 6 1997
 Wagner M Web attacks bring down servers, but business stay Computer World, Jan 6 1997
 Bryant R UNIX security, SAMS publishing, p53
 - IP Spoofing, CERT advisory team, 1995 http://www.deter.com/unix/papers/cert_ip_spoof.txt
 Gromov G R The Roads and Crossroads of Internet 's History http://www.internetvalley.com/intval.html
 Joncheray L A Simple Active Attack Against TCP, 1995 http://www.physnet.uni-hamburg.de/provos/security/iphijack.txt
 Ranum M Thinking About Firewalls http://www.deter.com/unix/papers/firewall_ranum.ps.gz
 Klein D Foiling the Cracker http://www.abc.se/~jp/articles/computer/security/passwd2.txt
 - RSA encryption system official site http://www.rsa.com
 Farmer D The COPS Security Checker System,1994 http://www.deter.com/unix/papers/cops_dan_farmer.txt
 Chapman D B Network (In)Security Through IP Packet Filtering, 1992 http://www.deter.com/unix/papers/packet_filt_chapman.ps.gz
 - A Framework for Action, Federal Networking Council http://www.fnc.gov/fisp_sec_contents.html
Please, for any queries or comments: firstname.lastname@example.org Copyright Xtream.