it-e-60 Secure Networks and Policies

What is a secure network? Can an Internet be made secure?[1] Although the concept of a
secure network is appealing to most users, networks cannot be classified simply as secure or not
secure because the term is not absoluteˆeach group defines the level of access that is permitted
or denied. For example, some organizations store data that is valuable. Such organizations define
a secure network to be a system prevents outsiders from accessing the organization's computers.
Other organizations need to make information available to outsiders, but prohibit outsiders from
changing the data. Such organizations may define a secure network as one that allows arbitrary
access to data, but includes mechanisms that prevent unauthorized changes. Finally, many large
organizations need a complex definition of security that allows access to selected data or services
the organization chooses to make public, while preventing access or modification of sensitive
data and services that are kept private.
Because no absolute definition of information secure exists, the first step an organization
must take to achieve a secure system is to define the organization's security policy. The policy
does not specify how to achieve protection. Instead, it states clearly and unambiguously the items
that are to be protected.

Defining an information security policy is complex. The primary complexity arises because an
information security policy cannot be separated from the security policy for computer systems
attached to the network. In particular, defining a policy for data that traverses a network does not
guarantee that data will be secure. Information security cannot prevent unauthorized users who
have accounts on the computer from obtaining a copy of the data. The policy must hold for the data
stored on disk, data communicated over a telephone line with a dialup modem, information printed
on paper, data transported on portable media such as a floppy disk, and data communicated over a
computer network.
Defining a security policy is also complicated because each organization must decide which
aspects of protection are most important, and often must compromise between security and ease
of use. For example, an organization can consider:
Data Integrity'
Data Availability'
Data Confidentiality and Privacy.

 

appealing

  • a. 引起兴趣的,动人的

    Continue reading it-e-60 Secure Networks and Policies

  • hibernate工程,正向还是反向?

    我用hibernate都是反向工程即先有数据库schema再生成dao,mapping file。

    我记得最早我这么做时就已经比较过了,但是现在又忘了为什么……

    这次做的项目先用的是mongodb,自己写的pojo,dal,现在要转为使用mysql,在想这回来个正向工程吧。查了一下,正向工程要先写mapping file或是在pojo上写注解,再用xdolet或者SchemaExport反向生成。想想,哎呀算了吧,看见xml就头疼!况且已经有了数据库设计的模型,转成sql很容易。最后还是决定反向工程。

    那答案就很清楚了,一般数据库设计都是先使用工具建好模型直接生成schema就可以。在反向自然是顺理成章的事。

    这篇文章也讨论了这个问题:http://www.iteye.com/topic/123

    从这四种自动生成工具来看, mapping file, java file and DDL,只要知道任何一种文件,都可以得到另外两种文件, 
    如: 
    1. 只有mapping file: 
    mapping file---hbm2java----java---SchemaExport----DDL 
    2.只有DDL 
    DDL---Middlegen---hbm----hbm2java----java 
    3.只有Java 
    java---XDoclet---hbm----SchemaExport----DDL

    Continue reading hibernate工程,正向还是反向?

    it-e-59 Internet Firewall Concept

    A packet filter is often used to protect an organization's computers and networks from
    unwanted Internet traffic. The filter is placed in the router that connects the organization to the
    rest of the Internet.
    A packet filter configured to protect an organization against traffic from the rest of the
    Internet is called an Internet firewall; the term is derived from the fireproof physical boundary
    placed between two structures to prevent fire from moving between them. Like a conventional
    firewall, an Internet firewall is designed to keep problems in the Internet from spreading to an
    organization's computers.
    Firewalls are the most important security tool used to handle network connections between
    two organizations that do not trust each other. By placing a firewall on each external network
    connection, an organization can define a secure perimeter that prevents outsiders from interfering
    with the organization's computers. In particular, by limiting access to a small set of computers, a
    firewall can prevent outsiders from probing all computers in an organization or flooding the
    organization's network with unwanted traffic.

    A firewall can lower the cost of providing security. Without a firewall to prevent access,
    outsiders can send packets to arbitrary computers in an organization. Consequently, to provide
    security, an organization must make all of its computer secure. With a firewall, however, a
    manager can restrict incoming packets to a small set of computers. In the extreme case, the set
    can contain a single computer. Although computers in the set must be secure, other computers in
    the organization do not need to be. Thus, an organization can save money because it is less
    expensive to install a firewall than to make all computer systems secure.

     

    1, perimeter  [pə'rimitə]
    n. 周长;周界;[眼科] 视野计

    2, interfering  [,intə'fiəriŋ]
    adj. 干涉的;多管闲事的
    v. 妨碍(interfer的ing形式)

    3, arbitrary  ['ɑ:bitrəri]
    adj. [数] 任意的;武断的;专制的

    Continue reading it-e-59 Internet Firewall Concept

    it-e-58 Network Security Report

    Any one responsible for the security of a trusted network will be concerned when connecting
    it to a distrusted network. In the case of connections to the Internet this concern may be based
    largely on anecdotal evidence gleaned from widespread media coverage of security breaches. A
    closer inspection of the facts and statistics behind some of the media coverage will, however, only
    serve to deepen that concern. For example, the US National Computer Security Agency (NCSA)
    asserts that most attacks to computer systems go undetected and unreported, citing attacks made
    against 9,000 Department of Defence computers by the US Defence Information Systems Agency

    (DISA). These attacks had an 88 percent success rate and went undetected by more than 95 percent
    of the target organizations. Only 5 percent of the 5 percent that detected an attack, a mere 22 sites,
    reacted to it.
    It is noteworthy that these sites belong to the US Department of Defence (DoD) and were
    not commercial sites, which may give security less priority than the DoD.
    NCSA also quote the FBI as reporting that in more than 80 percent of FBI investigated
    computer crimes, unauthorized access was gained through the Internet.
    Putting a value on the damage done by such attacks is difficult but a 1995 survey conducted
    by Ernst & Young, a New York based accounting firm, reported that one third of businesses
    connected to the Internet reported up to 100 000 USD in financial loss over a two year period due
    to malicious acts by computer users outside the firm. A little more than two percent of connected
    companies reported losses of more than 1M USD.
    There is amazement in the computer security industry at the level of ignorance to the
    problem. To understand the risks often involves a steep learning curve and they have few real
    parallels in everyday life, for example nobody worries that a burglar will be able to trick their
    front door into opening by posting cryptic messages through the letterbox. When there is a good
    "hacker" story to report the press goes into frenzy, but the general level of awareness is still
    surprisingly low. For example, the Sunday Times which prides itself on providing accurate
    coverage of IT issues published an article recently that claimed that most businesses worry too
    much about Internet security. The article goes on to explain that encryption is all that is needed to
    be completely secure. The article focuses purely on privacy of communication and completely
    misses the possibility of an attack originating from the Internet.

     

    1, anecdotal  [,ænik'dəutəl]
    adj. 轶事的;轶事一样的;多轶事的

    2, glean  [ɡli:n]
    vt. 收集(资料);拾(落穗)
    vi. 收集;拾落穗
    3, breach  [bri:tʃ]
    n. 违背,违反;缺口
    vt. 违反,破坏;打破
    4, citing 
    vbl. 引用,引证,举例
    5, burglar  ['bə:ɡlə]
    n. 夜贼,窃贼
    6, cryptic  ['kriptik]
    a. 秘密的(使用密码的,意义深远的)

    Continue reading it-e-58 Network Security Report

    我来谈谈web框架

    首先声明:一家之言,权当笑尔。

    我做web有些时候了,刚开始做.net时遇到一个前辈写的框架,一堆xml配置,配置用什么.net控件,样式是什么。一个页面用一个xml配置就出来了。哎呀我的个天,那个xml真是看着头疼啊,做梦都是xml。一个不小心配错了就报错,但是找错又不直观。动态生成的控件,无法使用vs调试。每添加个新功能还是得改代码。我当时很纳闷,为什么要这样写呢?抛弃了.net提供的优点不说,把个逻辑混成一团分不开,我好郁闷,二次开发就是这么痛!现在想一想,大概是她想保持界面风格一致吧,受够排版的苦了?这也造成了我对xml极度的反感。

    后来做java,刚开始很喜欢框架,见个新框架就很激动,抱着这个action看又抱着那个action看。但是实际用的过程中比起.net组件化的架构确实要差一些,后来自己写了个框架sopo。意思是简单好用。

    JSF

    我最先用的是jsf,那时国内都是struts,而jsf正是刚提出来的时候,大概公司的技术领袖被sun的王婆卖瓜吹得晕乎了,决定使用jsf。我就开始jsf了,要自己开发jsf组件,开发一个还好,开发那么多我就烦了,这是什么个东西??我要开发个组件要配置一堆xml,经常配错就报莫名其妙的错误,你根本想不到是配置造成的。哎我嘞个去,又是这个xml。jsf组件开发也是MVC模型,写个组件起码要3个类(我写个tree组件有十几个类),它还有个绘制器的概念,意思是说哎呀你写个模型可以使用不同的绘制器来展示就可以了,像手机的用手机绘制器,浏览器用html绘制器。刚开始我还蛮认同的,后来一到写绘制器就心里就窝火:你XXX的能在html上站稳脚跟就不错了,还手机的,看我以后还用不用你@#¥@#¥。还有个特点就是慢,如今jsf都这么多年了,我打开一个jsf的网站,感觉就是-----慢。也难怪,本来一步就可以做好的事情,非要拆开一二三四五六,能不慢吗?这也反映了java普遍存在的问题---学院学究,摊子太大,脱离实际!

    后来换了家公司,他们也是上了jsf的套,正好找到我来救火,一看,匆匆忙忙组件都不会写,还用的richfaces。richfaces写组件还要依照他们规范来写客户端脚本。不过我倒觉得richfaces做的要好些,他的实现理念还比较创新,与ajax结合的比较好。我还发现了richfaces当时刷新请求机制的顺序问题,让richfaces的开发者改了。这家公司后来也怕了jsf,决定不再使用。

    Tapestry

    后来听说tapestry性能好,我就开始试用。它是基于模板的展现,但是我一直想找动态创建组件的方法,没有。看了源码,也没什么好办法实现!找资料,很少,而且我觉得tapestry4之前都比较复杂,虽然tapestry5似乎是意识到了这一点,大刀阔斧的删了很多鸡肋。但是一直没有火起来啊!用的人少,自己用的也不是很顺手。我对他的兴趣也越累越淡,弃之。

    webwork

    口口声声是说基于组件的,但是我想找到动态创建组建的方法,还是没有??页面上要写组件,代码里面还是要通过id才能获得@@。晕……其他的我也不想说了,不是我想要的。

    Strust2

    Struts出来的早,别人用着都好,MVC嘛,简单就是美,我认同硬道理--实际用的人多就是有道理的。struts2是基于webwork2的, MVC很好,我首先自己开发标签,(开发struts2标签见我的文章 http://kazge.com/archives/71.html)。感觉不爽。

    来看看广为流传的struts2的特点:

    Struts2 Action有以下特点:
    —  Action类完全是一个POJO,因此具有很好的代码复用性。
    —  Action类无需与Servlet API耦合,因此进行单元测试非常简单。
    —  Action类的execute方法仅返回一个字符串作为处理结果,该处理结果可映射到任何的视图,甚至是另一个Action。

    前两点我很赞同,第三点我很痛恨,因为涉及到xml.我还要用到过导航继承,那个配置哦,配得我头都是昏的。我真搞不懂,不就是个导航吗?有必要搞得这么复杂吗?实现个页面要配置几个地方,这是个问题啊。曾经有人问我一个struts2的程序为什么那样写,就是个登陆的功能,这个跳到那,那儿再跳到远方……他看半天没看懂¥%¥%。哎---无语。

    struts2还提供多种“特性”,什么多种模板语言啊,titles啊,实际你用起来,觉得完全就是忽悠人,就算是吃饱了撑着一个项目里面用多个模板语言,它实际支持的很不好,这毛病那毛病,你用了就知道。

    许多细节地方我都不说了,反正我用struts2很恼火。

    SpringMVC

    这个和spring结合的话倒是很方便,不过它的映射仍然需要配置或者是写注释。每写个页面要写个模板,控制器,配置mapping。我比较反感的是不能省略配置映射这一步---哪那么多配置的!!不过相对于其他来说,还是比较简洁的。如果以“保持团队编程风格一致”这个理由(我可是很不认同这个观点)来说非得选个框架的话,我就会选它。

     

    我相信许多熟手都会自己掌控住一个灵活的适合系统的框架。通过对底层几种技术的柔和应用来构建系统,而不需要这些另外的框架。毕竟,那只是个框架,它只提供了一种工具而已。

    Continue reading 我来谈谈web框架

    it-e-57 Internet Security

    In recent years, Internet changes our life a lot. We use e-mail and Internet phone to talk with
    our friends, we get up-to-date information through web and we do shopping in the cyber-market.
    Internet has many advantages over traditional communication channels, e.g. it's cost effective, it
    delivers information fast and it is not restricted by time and place. [1]The more people use Internet,
    the more concerns about Internet security.
    In person-to-person community, security is based on physical cues. To name but a few, we
    use our signature to authenticate ourselves; we seal letters to prevent others inspection and
    modification; we receive receipt with the shop's chop to make sure we paid; we get information
    from a reliable source. But in the Internet society, no such physical cue is available. There are
    two areas that we concern about in Internet communication. The first one is secrecyˆhow do we
    ensure no one reads the data during its transmission? The second one is authenticationˆhow do
    we be sure that the identity of someone claiming "who it is". Imagine one day you receive an

    e-mail, which the e-mail sender is "Bill Gates". How do you confirm the e-mail is actually sent
    by Bill Gates?
    Encryption is the way to solve the data security problem. In real life, if Tom wants to talk with
    Mary secretly, he can choose a room with nobody there and talk with Mary quietly, or he can talk
    with Mary using codes understandable by Tom and Mary only. We take the second approachˆ
    encryptionˆto transmit data through Internet. There are two kinds of encryption techniquesˆ
    symmetric key encryption and asymmetric key encryption.
    For symmetric key encryption, both parties should have a consensus about a secret encryption key.
    When A wants to send a message to B, A uses the secret key to encrypt the message. After receiving the
    encrypted message ,B uses the same (or derived)secret key to encrypt the message.The advantage of
    using symmetric key encryption lies in its fast encryption and decryption processes(when compared
    with asymmetric key encryption at the same security level). The disadvantages are , first, the encryption
    key must be exchanged between two parties in a secure way before sending secret messages. Secondly,
    we must use different keys with different parties. For example, if A communicates with B, C, D and E,
    A should use 4 different keys. Otherwise, B will know what A and C as well as A and D has been
    talking about. The drawbacks of symmetric key encryption make it unsuitable to be used in the Internet,
    because it's difficult to find a secure way to exchange the encryption key.
    For asymmetric key encryption, there is a pair of keys for each party: a public key and a
    private key. The public key is freely available to the public, but only the key owner gets hold of
    the private key. Messages encrypted by a public key can only be decrypted by its corresponding
    private key, and vice versa. When A sends message to B, A first gets B's public key to encrypt
    the message and sends it to A. After receiving the message, B uses his private key to decrypt the
    message. The advantage comes in the public key freely available to the public, hence free from
    any key exchange problem. The disadvantage is the slow encryption and decryption process.
    [2]Almost all encryption schemes used in the Internet uses asymmetric key encryption for
    exchanging the symmetric encryption key, and symmetric encryption for better performance.
    Asymmetric key cryptography seems to attain secrecy in data transmission, but the authentication
    problem still exists. Consider the following scenario: when A sends a message to B, A gets B's
    public key from the Internetˆbut how can A know the public key obtained actually belongs to B?
    Digital certificate emerges to solve this problem.
    Digital certificate is an identity card counterpart in the computer society. When a person
    wants to get a digital certificate, he generates his own key pair, gives the public key as well as
    some proof of his identification to the Certificate Authority (CA). CA will check the person's
    identification to assure the identity of the applicant.[3] If the applicant is really the one "who
    claims to be", CA will issue a digital certificate, with the applicant's name, e-mail address and the
    applicant's public key, which is also signed digitally with the CA's private key. When A wants to
    send B a message, instead of getting B's public key, A now has to get B's digital certificate. A
    first checks the certificate authority's signature with the CA's public key to make sure it's a
    trustworthy certificate. Then A obtain B's public key from the certificate, and uses it to encrypt

    message and sends to B.
    Authentication is an important part everyday life. The lack of strong authentication has
    inhibited the development of electronic commerce. It is still necessary for contracts, legal
    documents and official letters to be produced on paper. Strong authentication is then, a key
    requirement if the Internet is to be used for electronic commerce. Strong authentication is
    generally based on modern equivalents of the one time pad. For example tokens are used in place
    of one-time pads and are stored on smart cards or disks.
    [4] Many people pay great amounts of lip service to security, but do not want to be bothered
    with it when it gets in their way. It's important to build systems and networks in such a way that
    the user is not constantly reminded of the security system around him. Users who find security
    policies and systems too restrictive will find ways around them. Security is everybody's business,
    and only with everyone's cooperation, an intelligent policy, and consistent practices, will it be
    achievable.

     

    1, cue  [kju:]
    n. 提示,暗示;线索
    vt. 给…暗示

    2, chop  [tʃɔp]
    n. 厚肉片,排骨
    v. 剁碎,砍,切

    3, secrecy  ['si:krəsi]
    n. 保密;秘密;隐蔽

    4, symmetric  [si'metrik]
    a. 对称的
    5, asymmetric  [,eisi'metrik]
    a. 不对称的
    6, consensus  [kən'sensəs]
    n. 一致,合意,交感
    7, cryptography  [krip'tɔɡrəfi]
    n. 密码学;密码使用法

    Continue reading it-e-57 Internet Security

    it-e-56 Content Filtering Sifts out Viruses

    It might be a stretch to call the recent "Melissa" virus a positive event because it disrupted
    thousands o government and commercial computer systems. But it did put the focus on content
    filtering, a network security technology that observers say has been undervalued. In the long run,
    many believe this new focus will prove beneficial to users.
    As a macro virus attached to an e-mailed Microsoft Corp. Word document, Melissa would
    not have been picked up by traditional security solutions such as firewalls or intrusion-detection
    systems, which are designed to detect items that break certain global access rules. The only way
    to detect a virus such as Melissa is to examine what is inside the e-mail, which can only be done
    through content filtering, supporters of the technology said.
    Filtering products have been around for years, but manufacturers have been struggling to
    meet the needs of organizations that employ varying security policies among different users.
    Recently, however, vendors have released filtering products that can be tailored to the needs of
    user groups, and industry has begun working on standards that allow these products to work with
    firewalls.
    Content filtering encompasses several areas of protection. As well as guarding against
    viruses, it includes:
    E-mail filtering, which controls incoming e-mail that contains spam, file attachments that
    are too large or hoax e-mails. It also can be used internally to make sure confidential information
    is not accidentally or intentionally sent outside of the organization.
    Uniform Resource Locator filtering, which blocks access to inappropriate sites not connected
    to a user's work needs.
    Malicious-code protection, which prevents hostile code embedded in Java and ActiveX
    scripts in otherwise innocuous World Wide Web pages from reaching the user's browser and

    being executed.
    The most obvious benefit of filtering products is the ability to boost network security, but
    many organizations also use them for internal network control. For example, URL filters can be
    used to block access to certain sites and to maintain productivity. Message Inspector, an e-mail
    filter produced by Elron Software Inc. uses context-sensitive filtering to weed out offensive or
    sensitive communications in e-mail, newsgroups or FTP sites. Message Inspector is an example
    of the newer generation of filter products that examine messages for words and phrases used in
    conjunction with each other to narrow the range of filter targets. This strategy differs from the
    one employed by traditional products that block communications based on factors such as
    keywordsˆa process that can generate a lot of "false positive" alarms.
    Melissa presented a clear idea of why people need a gateway-based virus-detection product,
    "Viruses need to be kept out of the enterprise altogether because you just can't update all
    virus-detection software on desktops in time to catch them."
    The International Computer Security Association is working on what it calls the Common
    Content Inspection program to define a general application program interface (API) that would
    enable filter vendors to fit their products to a range of firewalls.

     

    1, disrupt  [dis'rʌpt]
    vt. 破坏;使瓦解;使分裂;使中断;使陷于混乱
    adj. 分裂的,中断的;分散的

    2, hoax  [həuks]
    v. 欺骗,哄骗,愚弄
    n. 愚弄人,恶作剧

    3, boost  
    n. 推进,支援
    v. 推进,提高

    Continue reading it-e-56 Content Filtering Sifts out Viruses

    it-e-55 Backdoor

    Backdoor programs are typically more dangerous than computer viruses, as they can be
    used by an intruder to take control of a PC and potentially gain access to an entire network.
    Backdoor programs, also referred to as Trojan horses, are typically sent as attachments to
    e-mails with innocent-looking file names, tricking users into installing them. They often enable
    remote users to listen in on conversations using the host computer's microphone, or even see
    through its video camera if it has one. Back Orifice (BO) 2000 is a backdoor program designed
    for malicious use. Its main purpose is to maintain unauthorized control over another machine for
    reconfiguration and data collection. It takes the form of a client/server application that can
    remotely control a machine without the user's knowledge to gather information, perform system
    commands, reconfigure machines and redirect network traffic.
    With BO an intruder has to know the user's IP address to connect, or could scan an entire
    network looking for the victim. Once connected, the intruder can send requests to the BO 2000
    server program, which performs the actions the intruder specifies on the victim's computer,
    sending back the results.
    BO is installed on the server machine simply through the execution of the server application.
    This executable file is originally named bo2k.exe, but it can be renamed. The configuration wizard
    will step through the various configuration settings, including the server file (the executable), the
    network protocol, port number, encryption, and password. Once this process is complete, running
    bo2kgui.exe executes the user interface for BO.
    It is very difficult to detect BO, because it is so highly configurable. In addition, backdoor
    programs are multi-dimensional, so several detection methods are recommended to achieve maximum
    protection and awareness of the installation of BO 2000 on a machine or series of machines on a
    network.
    We recommend coupling the use of an updated version of anti-virus software to detect
    which machines on the network have BO installedˆand intrusion detection software to identify
    attacks over the network.
    Users are urged to follow three important precautions:
    Do not accept files from Internet chat systems.
    If you are connected to the Internet, do not enable network sharing without proper security
    in place.
    Do not open e-mail attachments: never run any executable files sent to you (.exe files or .zip
    files with a.exe in them). It is safer if these are run through a virus checker first, but they could be

    new backdoor programs or viruses that a virus scanner will not detect. It is safe to open Word
    documents and Excel spreadsheets if the Microsoft Auto-Run feature is turned off. Allowing
    macros to run automatically can spread e-mail viruses such as Melissa. Many people send each
    other animations in e-mail: it is easy to put a backdoor program into one of these and users
    cannot tell when they infect their computers with Back Orifice 2000.

     

    1, potentially  [pə'tenʃəli]
    adv. 可能地,潜在地

    Continue reading it-e-55 Backdoor

    it-e-54 Computer Viruses

    Just as human viruses invade a living cell and then turn it into a factory for manufacturing
    viruses, computer viruses are small program that replicate by attaching a copy of themselves to
    another program. Once attached to the host program, the virus then lock for other programs to
    "infect". In this way, the virus can spread quickly throughout a hard disk or an entire organization if

    it infects a LAN (Local Area Network) or a multi-users system.
    [2] Skillfully written virus can infect and multiply for weeks or months without being detected.
    During that time, system backups duplicate the viruses, or copies of data or programs made and
    passed to other systems to infect. At some pointˆdetermined by how the virus was programmedˆ
    the virus attacks. The timing of the attack can be linked to a number of situations, including: a
    certain time or date; the presence of a particular user ID; the use or presence of a particular file; the
    security privilege level of the user; and the number of times of a file is used.
    Likewise, the mode of attack varies, so-called "being" viruses might simply display a
    message, like the one that infected IBM's main computer system last Christmas with a season's
    greeting.
    Malignant viruses, on the other hand, are designed to damage your system. One common
    attack is to wipe out data, to delete files, or to perform a format of disk.
    There are four main types of viruses: shell, intrusive, operating system, and source code.
    Shell viruses wrap themselves around a host and do not modify the original program.
    Shell program are easy to write, which is why about half of all viruses are of this type. In
    addition, shell viruses are easy for programs like Data Physician to remove.
    Intrusive viruses invade an existing program and actually insert a portion of themselves
    into the host program. Intrusive viruses are hard to write and difficult to remove without
    damaging the host file.
    Shell and intrusive viruses most commonly attack executable program fileˆthose with.
    COM or. EXE extensionˆalthough data are also at some risk.
    Operating system viruses work by replacing parts of operating system with their own
    logic. [4]Very difficult to write, these viruses have the ability, once booted up, to take
    total control of your system. According to Digital Dispatch, known versions of operating
    system viruses have hidden large amounts of attack logic in falsely marked bad disk
    sectors. Others install RAM-resident programs or device drivers to perform infection or
    attack functions invisibly from memory.
    [5]Source code viruses are intrusive programs that are inserted into a source program
    such
    as those written in Pascal prior to the program being compiled. These are the least
    common viruses because they are not only hard to write, but also have a limited number
    of hosts compared to the other types.
    New computer viruses are written all the time, and it's important to understand how your
    system can be exposed to them and what can do to protect your computer. Follow the suggestions
    listed below to substantially decrease the danger of infecting your computer system with a
    potentially dangerous computer virus.
    Be very cautious about inserting disks from unknown sources into your computer.
    Always scan the disk's files before operating any of them.
    Only download Internet files from reputable sites.

    Do not open e-mail attachments (especially executable files) from strangers.
    Purchase, install, and use an anti-virus software program. The program you choose must
    provide three functions:
    Detection.
    Prevention.
    Removal.
    As new viruses are created everyday, upgrade your anti-virus software regularly.

    1, invade  [in'veid]
    vt. 侵略;侵袭;侵扰;涌入
    vi. 侵略;侵入;侵袭;侵犯

    2, replicate  ['replikit, 'replikeit]
    vt. 复制;折叠
    vi. 重复;折转
    adj. 复制的;折叠的
    n. 复制品;八音阶间隔的反覆音

    3, wipe  [waip]
    n. 擦拭,用力打
    v. 擦,消除,拭去

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