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Security of Computer Network and Measures of Defense

  Abstract: Along with the deepening of the reform and opening-up policy and the entering of the WTO,international exchange of economy and culture has become more and more frequent,Internet has become an indispensable part of our life,itsrapid development brings great convenience to us,and all of the computers will be connectedby Internet in the 21 century,thus the meaning of information security has changed substantially。It not only changes from a general guarding to a common defence,but from a specific field to a public subject as well。However,the opening and sharing of the Internet resource,unefficient supervision as well as various virus make people's information and belongings being in an extremely dangerous environment. The fight between hackers and anti-hackers ,destruction andanti-destruction has already affected the stable running of the network and users' legal right,caused great economy damages and it could also threat our country's security. So a right understanding and in-time measures should be paid special attention to.The thesis firstly expatiates the current state of the network security and its importance,and discusses some major factors and threats that affect the network security. And the thesis also introduce some catalogues of the security techniques,some relevant information and their major characters,besides that,I enumerate some popular and effective methods of protecting our network,including the widely used firewall,and the meticulous techniques such as security scan techniques and techniques of intrusion detection. Lastly, some protective measures are stated.
  Key words: Network Security; firewall; security scan techniques; techniques of intrusion detection; prevention Strategy
  中图分类号:TP393文献标识码:A文章编号:1009-3044(2011)20-4819-06
  As we all know, the security of computer network is even more essential than any other field of our lives. For this type of industry is easy to be attacked by those who want to destroy our network and steal important information. Based on that, we are trying to figure out how to solve those serious issues . A large number of researchers have done plenty of experiments to settle this problem, besides that we could use what we've learned to explore this special industry and discover the most effective methods. For our students major in computer network,there are so many things to learn and so many problems to solve.
  Purpose of the paper
  1) To summarize and illustrate the key characters of the security of computer network.
  2) To find out the manners to solve and protect our network .
  1 Introduction
  1.1 Purpose of the Research
  Along with the popularity and development of the network, especially the widely use of the Internet,computers are used wider and deeper. It has become the most colourful fictitious world which is built by human beings.The rapid development of Internet has brought huge changes towards our work and lives. We get and share information though the Internet. Nowadays,Internet spreads everywhere in the world and always welcomes people to take part in to communicate and interchange with each other. At the same time,we have to notice that although the network is powerful,it is apt to be attacked. Statistics show that there are about 87% of the e-mails worldwide have been infected by virus and more than 90% of the websites have the experience of being attacked.There are over 60 million virus,which vary greatly and new types are produced each day.The Internet crimes,which also abides by the Moore Rules,double every 18 months. In China,great economic damages are caused by hackers each year. Our spiritual property is in great danger.While we are utilizing the superiority of the network, the network security could never be neglected. Under the circumstance of the complicated network,how to protect various websites and their application should be drew special attention to.
  1.2 Background of the Research
  As information technology develops, people are increasingly aware of the importance of network, through which,computers scattered in different parts are linked,and various important information is spread. A large number of computers linked together to form a local area network, in which,you can share programs documents and other resources among them, at the same time,multiple computers could share the same hardware, such as printers, modems, etc., what's more,we can also use the computer network to send and receive faxes, it's both convenient and economical.
  However, nearly all people who have ever contacted with the network know that in this fictitious world,there are always some people who want to break into other people's computer systems,they use a variety of vulnerabilities of the network and system, and illegally obtain unauthorized access to information. Unfortunately, now attacks and theft of information network systems do not need any advanced skills. There are a lot of network attack tools ,articles and other resources, which could be used and shared. Those attacks do not need to know how the program is running, simple implementation could cause huge threat. There are even some of the programs do not require human involvement, high-intelligent scanning could undermine the whole network. This makes the frequency of attacks a significant growth and density in recent years, and has also brought more and more security risks to network security.
  In such a network environment, if you want to better protect your network resources, protect our own interests are not violated, to have a certain network security measures is quite necessary. Articles in today's network security seems to be everywhere, however ,among them there are good and bad, under this circumstances, based on my computer learning through the years, as well as the huge help and support from my mentor,I finally complete the article.
  1.3 Thesis Structure
  The thesis is divided into the following sections:
  Chapter One, article introduction.
  Chapter Two , analysis of computer network security, network security situation and major threats which should be faced ,and the factors that affect network security.
  Chapter Three,describes common network security precautions, including the mature firewall technology, and its types ,principles, and other characteristics, along with that,is the use of some other core technologies such as security scanning, and security of application system.
  Chapter Four, paper conclusion.
  2 Computer Network Security
  2.1 The concept of computer network security
  International Organization for Standardization defines"computer security" as: "the data processing system to establish and adopt the technology and governance of security, protection of computer hardware, software, data is not due to accidental and malicious destruction of reasons, change and leakage." The definition of computer security includes both physical security and logical security aspects ,the logical security of the content may be understood as we often say that information security, refers to information's confidentiality, integrity and availability of protection of the network security. While the meaning is the expansion of information security, that is, network security is protection of confidentiality, integrity and availability of the network information. Computer network security will be the specific meaning varies with users, different users have various familiarity and demands towards network security. From the perspective of ordinary users, they may just want their privacy or confidentiality of information are protected against eavesdropping, tampering and forgery when they are transmitted onto the network;in addition , while the network providers care about the network and information security, they must also consider how to deal with sudden natural disasters, military combat and other damage to the network hardware, as well as how to recover and maintain the continuity of network communications when the network is abnormal .
  In essence, network security, including security of hardware and software which compose the net system,and information transmitted on the network, this would not be destroyed by accidental or malicious attacks. There are both the technical aspects of network security and the governance issues,the two aspects reinforce each other, and neither of them could be absent. Network intrusion and attacks on human behavior makes network security faces new challenges.
  2.2 The Status of Computer Network Security
  Computer network security means that the hardware, software and data in the network system are protected from accidental or malicious reasons of destruction, change and disclosure, meanwhile the system could work continuously and reliably, and the network service is not interrupted. The complexity and diversity of computer and network technology, makes it an area that needs continuous update and improvement. The current methods of hackers have exceeded the types of computer viruses, and most of the attacks are fatal. In the Internet network, the Internet itself is not restricted by temporal and geographical factors,so every time there is a new means of attack , it can be spread around the world in a week, these attacks which use vulnerabilities of the network and system will attack computer systems and make the network breakdown. Worms, backdoors, Rootkits, DOS, and Sniffer are several familiar means of hacker attacks. But all these attacks show their astonishing power, nowadays, it seems to be getting worse. The new variations of these attacks, are more intelligent than the previous methods, and they target directly at Internet-based protocols and operating system. Controling program from a Web program to the kernel-level Rootlets, hackers' attack methods retread continuously,challenginguser's ability to keep the information security.
  2.3 The Main Threats to Network Security
  It is generally believed that, the threats to the computer network system security mainly from three areas: computer viruses, hacker attacks and denial of service attacks.
  2.3.1 Attacks from the Computer Virus
  At present, there are more than 14,000 kinds of active virus, they invade the computer network,cause destruction of network resources, so that the network does not work, and the entire network may even breakdown.
  2.3.2 Attacks from the Hackers
  Attack means of hackers are constantly updated, there are different system security problems almost every day. Updated security tools, however,are too much slow, in most cases,people need to be involved to the discover previously unknown safety issues, this always makes them too slow to react to the security problems. When the security tools discovered and make efforts to correct the safety issues, other problems emerge. Therefore, the hacker can always use the advanced means which can't be identified by the security tools to attack.
  2.3.3 Denial of Service Attack
  Such as "point in the mail bombs", which takes the form that users would receive a lot of unwanted e-mail in a very short time, thus affecting the normal business operation. When it is serious, system will shutdown, and network breakdown will be caused.
  Specifically, the "denial of service"attack could be divided into two types:the first one is the attack based on vulnerabilities,which can also be called logic attack. In this type of attack,attackers will first find out the system vulnerabilities then they send data packets which are designed carefully,andthose packets make the system breakdown or make the capability fall rapidly;and the second one is based on the flux,which can be called flooding attack . In this method ,the attackers will send plenty of data packets,in order to use up the bandwidth and system resources of the target computer.
  2.4 Factors Affect Network Security
  2.4.1 Operating Systems not Have Related Security Configuration
  Currently, there are three types of commonly used operating systems, unix linux windows NT/2000/2003 server / XP, these operating systems are all accord with theC2-level,but there are also some certain security risks and vulnerabilities. One of the reasons that the operating system is exposed to the intruder is that we are unaware of these vulnerabilities, and fail to take appropriate measures.
  2.4.2 Network Operating System Vulnerabilities
  Many popular operating systems all have network security vulnerabilities, such as UNIX, MS NT and Windows .Hackers often use the vulnerabilities ofoperating system itself andinvade our system.
  2.4.3 Denial of Service Attack
  Denial of service attack means attackers find a way to stop target machine from providing services, it is one of the means commonly used by hackers to attack. In fact, the consumptive attack to the network bandwidth is only a small part of denial of service attack, as long as it can cause trouble to the target, it can make some certain services be suspended or even cause the host crashes, it belongs to a kind of denial of service attacks, denial of service attack has not got a rational solution, the reason is that the shortcomings of the network protocol security itself, and thus the denial of service attacks have become the ultimate way of the attackers.
  2.4.4 Improper Use of Security Products
  Now there are many anti-virus products, all of them have both advantages and disadvantages, and some of these antivirus products are not compatible. However, some users want to achieve the highest level of security, have installed a variety of security products, and this leads to a burden of the computer operation. So a proper anti-virus should be carefully selected for your system.
  2.4.5 Lack of Strict Network Security Management System
  Complete and accurate safety evaluation is the basis for hacking prevention system. It will make a scientific and accurate analysis and evaluation towards the security performance of the current or undergoing network , and protect the security of the actualize security policies' realization, economic viability and enforceability of the network. Network security assessment analysis is to check the network to find whether there were loopholes which can be exploited by hackers, and the security situation assessment, analysis, and recommendations enhance the process of network.
  3 Common Computer Network Security Precautions
  Network information security involves all aspects of the problem ,and it is a complex system. A complete network information security system should include at least three types of measures:The first one is law and policy, regulations and safety education and other external soft environment. The second one is the technical aspects, such as encrypted storage of information transmission, authentication, firewall, network anti-virus and so on. The last one is management measures, including technical and social ones. The main measures among them are: to provide real-time ability to change the security policy, real-time monitoring enterprise security state, to check the existing security system leaks, to nip in the bud. These three tips are indispensable, in which legal policy is the cornerstone of security, technology is guarantee of the safety, techniques are the safeguard of safety, and management and audit is the defence line of safety. The following measures are common means to prevent computer network .
  3.1 Firewalls
  In a computer network, the firewall is a barrier set between a trustful internal network and the distrustful outside world,it is used to protect network resources and users' reputation ,and to make a network from attack from another one. Logically, a firewall is a separator, a limiter, and also a parser to effectively monitor the activities between the internal network and the Internet to ensure the security of the internal network. As a center of "containment point", it can centralize the security management of the local area network, shield illegal request, and prevent cross-authority access as well as make a security alarm.
  3.1.1 Types of Firewall
  ICSA ,the International Computer Security Committee,divides firewall into three categories :Packet Filtering Firewall, Application Gateway firewall, and Status Detection firewall.
  3.1.1.1 Packet Filtering Firewall
  Acting on the protocol suite of network layer and transport layer, based on packet header source address, destination address and port number, protocol type and other signs to determine whether to allow the packet to go through, only send the packet which meets filtering logic to the appropriate destination exit end , the rest of the data packets will be dropped from the data stream.
  3.1.1.2 Application Gateway firewall
  Application Proxy is completely different from packet filtering firewall,as the first one controls all the information flow at the network layer while Application Proxy has been handled to the application layer, carries the function of firewall out at the application layer. Its proxy function terminates the clients' connection at the firewall and initializes a new protective internal network . The built-in mechanism can provide additional security, that's because it isolates the internal from the external networks, this makes it hard for the hackers outside to detect on the internal network of the firewall,more importantly, it enables the network administrators to control the network services comprehensively.
  3.1.1.3 Status Detection firewall
  Status Detection firewall processes the data in the packet directly,and judge the data according to the two groups. And then decides whether to allow the packet to go through.
  3.1.2 The Principle of Building a Firewall
  3.1.2.1 Firewall Itself Is Safe
  As a product of information system security, the firewall itself should be safe, never gives any chances to the invaders. If it is like the defense of Macedonia,though the front side is unbreakable, the attackers can easily bypass the line into the internal system,and network systems will not have any security to speak of.
  3.1.2.2 Management and Training
  Evaluation of management and training is an important aspect of a firewall to see whether it is good or not. As I've said before, in the calculation of the cost of the firewall, we can not only calculate the acquisition cost, but consider the total cost of ownership as well. Staff training and routine maintenance costs are usually occupy the larger proportion of TCO. An excellent security products provider must offer good training and after service to its customers.
  3.1.2.3 Scalability
  In the early stages of network construction, due to the small size of the internal information system, the losses caused by the attack is also smaller, so there is no need to buy too complex and expensive firewall products. But with the network expansion and the increase in network applications, the cost of network risk will increase dramatically, and at that time, we need to use firewall products with improved security. If the early acquisition of the firewall is not scalable, or the expansioncost is pretty high, this is a waste of investment. Nice products should leave enough flexibility to the users, in the case of less demanding of security level, you can just buy the basic system, and as requirements increase, users still have room to further increase the options. This can not only protect users' investment,but also expand the product coverage for the firewall products providers.
  3.1.2.4 firewall security
  The most difficult aspects of firewall products is the assessment of the safety performance of the firewall, that means whether firewall is effectively shielded from external intrusion. This is the same as the security of the firewall itself, which could not be easily judged by ordinary users . Even if a firewall is installed, if there isn't actual external invasion,there is no way to know the merits of the product's performance. But testing the security performance of products in practice is extremely dangerous, so when you are in the choice of firewall products, you should try to choose a product which has larger share of the market and is adopted by authority certification testing of.
  3.1.3 Functions of Firewall
  1) Supporting certain security policies, filtering out unsafe and illegal users' services, that is ,filtering the network data sends into and out, managing accessing behavior goes into and out. All the above firewall technologies are the key ones, apart from which,we must also strengthen the compositive application of the encryption technology, security audit, security kernel, authentication, and the negative balance technology.
  2) Monitoring the Network Security and Giving Alarm.
  3) Using Network Address Translation (NAT) technology, corresponding the limited IP address either dynamic or with static with the internal IP address,to alleviate the problem of address space shortage.
  4) Firewall is the gateway that no matter information goes in or out,it should pass through the firewall,and it is suitable to gather information which is used and misused by the system and network. With this gateway, the firewall could record between networks. It is the best location to audit and record the cost of the Internet. The network administrator can provide the cost of Internet connectivity situation, find out the location of potential bandwidth bottlenecks,and provide department-level accounting according to the accounting model.
  5) Can be connected to a separate network, and physically separated from the internal network and deploy WWW servers and FTP servers, act as a place to release internal information to external sites.
  3.1.4 Points of Selecting Firewalls
  Since 1986 the U.S. company Digital installed the world's first commercial firewall system, and put forward the concept of a firewall, the firewall technology has been rapidly developed. Companies from both home and abroad have launched firewall products of different functions. In choosing the firewall, the following should be noted:
  1) Should be a protector of the whole network. All of the information in and out should pass through the firewall, at the same time, only the flux which meets the local security policy could pass through the firewall.
  2) Could make up the shortcomings of other operating systems.
  3) Providing users with different platforms. Firewall is a perfect platform to change address,Internet logs, audit, and even accounting.
  4) Providing better after service to users.
  3.1.5 Firewall Limitations
  Generally speaking,there are three limitations of a firewall:
  1) Firewall cannot prevent attacks which don't pass through it. Such as the users ofinternal private network connect Internet through a modem dial-up, and the "bad guys" could get in without the firewall. Even more frightening, is the attacks from the internal private network.
  2) Always requires a special closed network topology to support, the improvement of network security is often at the expense of flexibility, diversity and openness of network services.
  3) Technically, firewall has no efforts on preventing the safety threats from internal. For example it can't keep traitors away or damage what users paid no attention to.
  3.2 Security Scan Techniques
  Among the network security technologies,another important technique is security scan techniques. Along with firewall and security monitoring system,they could produce a high security network. This technology is from the tools which were used by hackers when they tried to invade. Commercial security scanning tools provide powerful support for vulnerabilities of the network .
  3.2.1 Classification
  3.3.4 Security of the Operating System
  Almost all of the operating system have security vulnerabilities, and the more popular the system is,the more vulnerabilities there are. For this problem,you should increase pack continuously,besides that,these things are required:Check the system settings (storage methods of sensitive data, access control, password selection / update);system-based security monitoring system.
  3.4 Techniques of Intrusion Detection
  3.4.1 Overview of Intrusion Detection
  Intrusion detection is a technology to ensure the security of the computer system,it is designed and configured to discover and report the unauthorized and abnormal phenomena of the system,and to detect the acts of violating the security policy . In the intrusion detection system,ituses the audit records, and can identify any undesirable activities,so that we can restrict these activities and protect the system. The intrusion detection technology in schools, government agencies, enterprises,could better be replaced by hybrid intrusion detection,intrusion detection system both based on network and host in the network,will become a complete tridimensional structure of active defense system, and some intrusion detection devices can be linked with the firewall
  3.4.2 Principia of Intrusion Detection
  Intrusion detection system is a new network security technology, the aim of it is to provide real-time intrusion detection and take necessary protective measures, such as recording of evidence for tracking and recovery, disconnecting from the network and so on. Real-time intrusion detection is important because it can deal with attacks from the internal network firstly,and it can shorten the time of hackers' intrusion.
  3.4.3 Key Techniques of Intrusion Detection
  1) Intelligent Intrusion Detection.
  2) Distributed Intrusion Detection.
  3) Application layer intrusion detection.
  4) Intrusion detection based on analysis of protocol and behavior.
  5) Combine with the operating system .
  6) Establish evaluation system of intrusion detection system.
  7) Combine with other network security technologies.
  3.4.4 Challenges of Intrusion Detection
  1) How to improve the speed of intrusion detection system to meet the requirements of network communication.
  2) How to reduce the failure of report and misinformation of intrusion detection system,and how to improve its security and accuracy.
  3)How to improve the interactive performance of intrusion detection system to improve the safety performance of the whole system.
  3.5 Hacker Attack and Prevention Techniques
  3.5.1 IP Spoofing Attacks and Prevention
  In order to perform IP spoofing,hackers always do the followings:make the trusted host lose its working ability, while sampling the target host's TCP sequence number,guessing the serial number of the data. Then, disguising as a trusted host, and building the target host application based on address verification connection. If succeed,hackers can use a simple command to place back door of the system for unauthorized operation.
  An easy way to prevent this is to give up the address-based authentication. Never allow the use of remote call command of R * class;Remove the hosts file; empty / etc / hosts equiv file.
  3.5.2 Port Scanning and Preventions
  It is easy to find the remote port scanning tools online, such as Superscan、IP Scanner、Fluxay etc.. From which we can clearly understand that,which ports of the host that is in uncommon use are opened;whether it supports FTP, Web services;and whether the FTP service supports "anonymous",IIS version, and whether there are IIS vulnerabilities which can be easily breached showed .
  There are two methods to prevent port scanning:
  Close unused potentially and hazardous ports.
  Check the ports , whenever there are evidences of port scanning,close it immediately.
  3.5.3 Buffer Overrun and Preventions
  Buffer overflow is occurred when the programmer did not check program memory space when they were programming , and this causes the leaks of memory. It is a means of attacking systems, in virtue of programming code out of its length in the buffer,causing overrun, thereby undermining the stack. Many ways can be used to deal with buffer overflow, the following four are the most common methods.
  1) Write strict code.
  2) Non-implementation of the stack data segment.
  3) Use the borderline check of the program compiler.
  4) Pointer integrity checking.
  3.5.4 Denial of Service
  Denial of Service attack,is a general term of the common attacks on the net, the aim is to make the computer or network not to provide normal services. In DoS attacks, the most common ones are the network bandwidth and connectivity attacks. Ways of solving:
  1) Shortening the time of SYN Timeout.
  2) Setting the blacklist.
  3) Using the appropriate software (firewall), masking some suspicious clients, could also reduce the load on the system.
  3.5.5 Network Monitoring and Prevention
  Network monitoring technology was originally provided to the management of network security administrators, it can be used to monitor the status of the network, data flow and information transmitted over the network. When the information transmitted in the form clear text over the network,the use of monitoring technology for attack is not a difficult task, as long as the network interface is set to monitor mode, we can capture the information transmitted over the Internet continuously.
  Network monitoring is difficult to be detected, because the host which is working for receiving the information transferred on the local area network passively,do not exchange information with other hosts forwardly, and do no change data packets which are transferred online.
  Ways of solving:
  1) Segmenting the network logically or physically.
  2) Switching hubs instead of shared hub.
  3) Using encryption technology.
  4) Setting off VLAN.
  3.5.6 Password Cracking and Prevention
  The first line of defense against intruders is the password system. Almost all multi-user systems require users not only to provide a name or identifier (ID), but also to provide a password. In the real system, the invaders are always trying to get the system-accepted password by guessing or getting the password files,so that they can enter the system. After invaders landed, they can find loopholes in the system in order to get further privileges.
  Ways of solving:
  Setting complex passwords, uppercase and lowercase letters plus numbers and special symbols are recommended.
  3.5.7 Common Steps of Hacker Attack
  3.5.7.1 Collecting Information
  To judge the operating system ,open network services and other related configuration information of the host.
  3.5.7.2 Confirming Target
  Often is the server, the host system or small network.
  3.5.7.3 Vulnerability Detection
  Using the collected target information, take further analysis of system vulnerabilities.
  3.5.7.4 Attacking
  Find out the trust relationship of the breached system in the network,install the detectors in the target system, get privileged user access of the breached system .
  3.5.7.5 Mopping-up
  Setting up back door in the compromised systems,as a springboard to attack other machines; clearing the log, and destroying traces of the invasion.
  In the above,I have summarized some common network and system vulnerabilitieswhich appear all the time,and I also made some explanation towards these vulnerabilities and their characters,meanwhile,some detailed protective methods are included,to make people have a better understanding about these vulnerabilities so that we can better protect our network. However ,the information security system actually includes laws,policies and a development platform of technology and market. When we are building such a system,we should pay special attention to our unique security products,if we want to solve the security problem thoroughly,the only way is to improve the entire network security techniques by promoting security industries of the nation.
  4 Chapter Conclusion
  Network security is a comprehensive topic,whichinvolves techniques,management,usage and other aspects,there are both security problems of information system itself ,and physical and logical technological measures,one skill could only deal with one problem,and it is not all-purpose;it's asystem project,should not only depend on virus-killing products、firewalls and vulnerability detection to protect,but we should realize that computer network is aman-machine project,what we should protect is the computer network,while the mainody is humans. What we need to do is that the development and explosion of the hardware of the computer network, establish a safe network system,and forster a consciousness of network security,only in this way could we protect our system better.Lower the damages to the lowest level,so we can produce an efficient,general and safe network security. So ,we should have strict security policies,clear security strategies and qualified administrators,thus chould we ensure the completeness and accuracy of the information safely and instantly and provide powerful security service for the Internet,this is an uegent need of network security,and it is related to the development of the Internet.
  From four years' study in college,and the research of my thesis,I have obtained further understanding of network security and its relevant techniques,as well as that,I figure out the urgent need of computer talents,if we want to be the safeguard of the network ,we should carry out more researches seriously and professionally.
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