网络工程论文中英文对照外文翻译文献.docx
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网络工程论文中英文对照外文翻译文献.docx
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网络工程论文中英文对照外文翻译文献
网络工程论文中英文对照外文翻译文献
ComputerViruses
Whatarecomputerviruses?
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
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