网络工程论文中英文对照外文翻译文献Word下载.docx

网络工程论文中英文对照外文翻译文献Word下载.docx

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AccordingtoFredCohen’swell-knowndefinition,acomputervirusisacomputerprogramthatcaninfectothercomputerprogramsbymodifyingtheminsuchawayastoincludea（possiblyevolved）copyofitself.Notethataprogramdoesnothavetoperformoutrightdamage（suchasdeletingorcorruptingfiles）inordertobecalleda“virus”.However,Cohenusesthetermswithinhisdefinition（e.g.“program”and“modify”）abitdifferentlyfromthewaymostanti-virusresearchersusethem,andclassifiesasvirusessomethingswhichmostofuswouldnotconsiderviruses.

Computervirusesarebitsofcodethatdamageoreraseinformation,files,orsoftwareprogramsinyourcomputer,muchlikevirusesthatinfecthumans,computervirusescanspread,andyourcomputercancatchaviruswhenyoudownloadaninfectedfilefromtheInternetorcopyaninfectedfilefromadiskette.Oncethevirusesisembeddedintoyourcomputer’sfiles,itcanimmediatelystarttodamageordestroyinformation,oritcanwaitforaparticulardateoreventtotriggeritsactivity.

Whatarethemaintypesofviruses?

Generally,therearetwomainclassesofviruses.ThefirstclassconsistsofthefileInfectorswhichattachthemselvestoordinaryprogramfiles.Theseusuallyinfectarbitrary.COMand/or.EXEprograms,thoughsomecaninfectanyprogramforwhichexecutionisrequested,suchas.SYS,.OVL,.PRG,&

.MNUfiles.

Fileinfectorscanbeeitherdirectactionorresident.Adirect-actionvirusselectsoneormoreotherprogramstoinfecteachothertimetheprogramwhichcontainsitisexecuted,andthereafterinfectsotherprogramswhen“they”areexecuted（asinthecaseoftheJerusalem）orwhencertainotherconditionsarefulfilled.TheViennaisanexampleofadirect-actionvirus.Mostothervirusesareresident.

Thesecondclassissystemorboot-recordinfectors:

thoseviruses,whichinfectexecutablecode,foundincertainsystemareasonadiskthatarenotordinaryfiles.OnDOSsystems,thereareordinaryboot-sectorviruses,whichinfectonlytheDOSbootsectorondiskettes.ExamplesincludeBrain,Stoned,Empire,Azusa,andMichelangelo.Suchvirusesarealwaysresidentviruses.

Finally,afewvirusesareabletoinfectboth（theTequilavirusisoneexample）.Thereareoftencalled“multipartite”viruses,thoughtherehasbeencriticismofthisname;

anothernameis“boot-and-file”virus.

Filesystemorclusterviruses（e.g.Dir-II）arethosethatmodifydirectorytableentriessothatthevirusisloadedandexecutedbeforethedesiredprogramis.Notethattheprogramitselfisnotphysicallyaltered;

onlythedirectoryentryis.Someconsidertheseinfectorstobeathirdcategoryofviruses,whileothersconsiderthemtobeasub-categoryofthefileinfectors.

Whataremacroviruses?

Manyapplicationsprovidethefunctionalitytocreatemacros.Amacroisaseriesofcommandstoperformsomeapplication-specifictask.Macrosaredesignedtomakelifeeasier,forexample,toperformsomeeverydaytasksliketext-formattingorspreadsheetcalculations.

Macroscanbesavedasaseriesofkeystrokes（theapplicationrecordwhatkeysyoupress）;

ortheycanbewritteninspecialmacrolanguages（usuallybasedonrealprogramminglanguageslikeCandBASIC）.Modernapplicationscombinebothapproaches;

andtheiradvancedmacrolanguagesareascomplexasgeneralpurposeprogramminglanguages.Whenthemacrolanguageallowsfilestobemodified,itbecomespossibletocreatemacrosthatcopythemselvesfromonefiletoanother.Suchself-replicatingmacrosarecalledmacroviruses.

MostmacrovirusesrununderWordforWindows.Sincethisisaverypopularwordprocessor,itprovidesaneffectivemeansforvirusestospread.MostmacrovirusesarewrittenusingthemacrolanguageWordBasic.WordBasicisbasedonthegoodoldBASICprogramminglanguage.However,ithasmany（hundredsof）extensions（forexample,todealwithdocuments:

edit,replacestring,obtainthenameofthecurrentdocument,opennewwindow,movecursor,etc.）.

WhatisaTrojanhorseprogram?

Atypeofprogramthatisoftenconfusedwithvirusesisa‘Trojanhorse’program.Thisisnotavirus,butsimplyaprogram（oftenharmful）thatpretendstobesomethingelse.

Forexample,youmightdownloadwhatyouthinkisanewgame;

butwhenyourunit,itdeletesfilesonyourharddrive.Orthethirdtimeyoustartthegame,theprogramE-mailyoursavedpasswordstoanotherperson.

Note:

simplydownloadafiletoyourcomputerwon’tactivateavirusorTrojanhorse;

youhavetoexecutethecodeinthefiletotriggerit.Thiscouldmeanrunningaprogramfile,oropeningaWord/Exceldocumentinaprogram（suchasWordorExcel）thatcanexecuteanymacrosinthedocument.

Whatkindoffilescanspreadviruses?

Viruseshavethepotentialtoinfectanytypeofexecutablecode,notjustthefilesthatarecommonlycalled“programfiles”.Forexample,somevirusesinfectexecutablecodeinthebootsectoroffloppydiskorinsystemareasofharddrives.Anothertypeofvirus,knownasa“macro”virus,caninfectwordprocessingandspreadsheetdocumentsthatusemacros.Andit’spossibleforHTMLdocumentscontainingJavaScriptorothertypesofexecutablecodetospreadvirusesorothermaliciouscode.

Sincevirusescodemustbeexecutedtohaveanyeffect,filesthatthecomputertreatsaspuredataaresafe.Thisincludesgraphicsandsoundfilessuchas.gif,.jpg,.mp3,.wav,.etc.,aswellasplaintextin.txtfiles.Forexample,justviewingpicturefileswon’tinfectyourcomputerwithavirus.Theviruscodehastobeinaform,suchasan.exeprogramfileoraWord.docfilewhichthecomputerwillactuallytrytoexecute.

Howdovirusesspread?

ThemethodologyofvirusinfectionwasprettystraightforwardwhenfirstcomputervirusessuchasLehighandJerusalemstartedappearing.Avirusisasmallpieceofcomputercode,usuallyformseveralbytestoafewtensofbytes,thatcando,well,somethingunexpected.Suchvirusesattachthemselvestoexecutablefiles—programs,sothattheinfectedprogram,beforeproceedingwithwhatevertasksitissupposedtodo,callstheviruscode.Oneofthesimplestwaystoaccomplishthatistoappendtheviruscodetotheendofthefile,andinsertacommandtothebeginningoftheprogramfilethatwouldjumprighttothebeginningoftheviruscode.Afterthevirusisfinished,itjumpsbacktothepointoforiginationintheprogram.Suchviruseswereverypopularinthelateeighties.Theearlieronesonlyknewhowtoattachthemselvesto.Comfiles,sincestructureofa.COMfileismuchsimplerthanthatofan.EXEfile—yetanotherexecutablefileformatinventedforMS-DOSoperatingsystem.ThefirstvirustobecloselystudiedwastheLehighvirus.Itattacheditselftothefilethatwasloadedbythesystematboottime—COMMAND.COM.thevirusdidalotofdamagetoitshost,soafterthree-fourreplicationsitwasnolongerusable.Forthatreason,thevirusnevermanagedtoescapetheuniversitynetwork.

Whenyouexecuteprogramcodethat’sinfectedbyavirus,theviruscodewillalsorunandtrytoinfectotherprograms,eitheronthesamecomputeroronothercomputersconnectedtoitoveranetwork.Andthenewlyinfectedprogramswilltrytoinfectyetmoreprograms.

Whenyoushareacopyofaninfectedfilewithothercomputerusers,runningthefilemayalsoinfecttheircomputer;

andfilesfromthosecomputersmayspreadtheinfectiontoyetmorecomputers.

Ifyourcomputerifinfectedwithabootsectorvirus,thevirustriestowritecopiesofitselftothesystemareasoffloppydisksandharddisks.Thentheinfectedfloppydisksmayinfectothercomputersthatbootfromthem,andtheviruscopyontheharddiskwilltrytoinfectstillmorefloppies.

Someviruses,knownas‘multipartite’viruses,andspreadbothbyinfectingfilesandbyinfectingthebootareasoffloppydisks.

Whatdovirusesdotocomputers?

Virusesaresoftwareprograms,andtheycandothesamethingsasanyotherprogramrunningonacomputer.Theaccrualeffectofanyparticularvirusdependsonhowitwasprogrammedbythepersonwhowrotethevirus.

Somevirusesaredeliberatelydesignedtodamagefilesorotherwiseinterferewithyourcomputer’soperation,whileotherdon’tdoanythingbuttrytospreadthemselvesaround.Buteventheonesthatjustspreadthemselvesareharmful,sincetheydamagefilesandmaycauseotherproblemsintheprocessofspreading.

Notethatvirusescan’tdoanydamagetohardware:

theywon’tmeltdownyourCPU,burnoutyourharddrive,causeyourmonitortoexplode,etc.warningsaboutvirusesthatwillphysicallydestroyyourcomputerareusuallyhoaxes,notlegitimateviruswarnings.

ModernvirusescanexistonanysystemformMSDOSandWindow3.1toMacOS,UNIX,OS/2,WindowsNT.Someareharmless,thoughhardtocatch.TheycanplayajingleonChristmasorrebootyourcomputeroccasionally.Otheraremoredangerous.Theycandeleteorcorruptyourfiles,formatharddrives,ordosomethingofthatsort.Therearesomedeadlyonesthatcanspreadovernetworkswithorwithoutahost,transmitsensitiveinformationoverthenetworktoathirdparty,orevenmesswithfinancialdataon-line.

What’sthestoryonvirusesandE-mail?

Youcan’tgetavirusjustbyreadingaplain-textE-mailmessageorUsenetpost.Whatyouhavetowatchoutforareencodedmessagecontainingembeddedexecutablecode（i.e.,JavaScriptinHTMLmessage）ormessagethatincludeanexecutablefileattachment（i.e.,anencodedprogramfileoraWorddocumentcontainingmacros）.

InordertoactivateavirusorTrojanhorseprogram,youcomputerhastoexecutesome