陕西师范大学_软件工程-郭浩东DHCP

陕西师范大学计算机网络实验报告年级：2014级姓名：郭浩东学号：41412179实验名称：利用wireshark分析DHCP一、实验目的加深对DHCP协议进行全面的了解和认识二、实验器材Wireshark三、实验内容1.AreDHCPmessagessentoverUDPorTCP?DHCPmessagesaresentoverUDP(UserDatagramProtocol).2.Drawatimingdatagramillustratingthesequenceofthefirstfour-packetDiscover/Offer/Request/ACKDHCPexchangebetweentheclientandserver.Foreachpacket,indicatedthesourceanddestinationportnumbers.Aretheportnumbersthesameasintheexamplegiveninthislabassignment?TheportnumbersarethesameastheexampleintheLab.3.Whatisthelink-layer(e.g.,Ethernet)addressofyourhost?TheLinkLayeraddressofmyworkstationis:00:90:4b:69:dd:344.WhatvaluesintheDHCPdiscovermessagedifferentiatethismessagefromtheDHCPrequestmessage?ThevalueswhichdifferentiatetheDiscovermessagefromtheRequestmessagearein“Option53:DHCPMessageType”.5.WhatisthevalueoftheTransaction-IDineachofthefirstfour(Discover/Offer/Request/ACK)DHCPmessages?WhatarethevaluesoftheTransaction-IDinthesecondset(Request/ACK)setofDHCPmessages?WhatisthepurposeoftheTransaction-IDfield?ThevalueoftheTransactionIDis0xe6746a7d.ThesecondTransactionIDis0xe4eff25f.ATransactionIDisusedsothattheDHCPservercandifferentiatebetweenclientrequestsduringtherequestprocess.6.AhostusesDHCPtoobtainanIPaddress,amongotherthings.Butahost’sIPaddressisnotconfirmeduntiltheendofthefour-messageexchange!IftheIPaddressisnotsetuntiltheendofthefour-messageexchange,thenwhatvaluesareusedintheIPdatagramsinthefour-messageexchange?ForeachofthefourDHCPmessages(Discover/Offer/Request/ACKDHCP),indicatethesourceanddestinationIPaddressesthatarecarriedintheencapsulatingIPdatagram.TheDHCPclientandserverbothuse255.255.255.255asthedestinationaddress.TheclientusessourceIPaddress0.0.0.0,whiletheserverusesitsactualIPaddressasthesource.7.WhatistheIPaddressofyourDHCPserver?TheIPaddressoftheDHCPserveris192.168.243.18.WhatIPaddressistheDHCPserverofferingtoyourhostintheDHCPOffermessage?IndicatewhichDHCPmessagecontainstheofferedDHCPaddress.TheDHCPserverofferedtheIPaddress192.168.243.92tomyclientmachine.TheDHCPmessagewith“DHCPMessageType=DHCPOffer”containedtheofferedIP.9.Intheexamplescreenshotinthisassignment,thereisnorelayagentbetweenthehostandtheDHCPserver.Whatvaluesinthetraceindicatetheabsenceofarelayagent?Istherearelayagentinyourexperiment?IfsowhatistheIPaddressoftheagent?10.ExplainthepurposeoftherouterandsubnetmasklinesintheDHCPoffermessage.Therouterlineindicatestotheclientwhatitsdefaultgatewayshouldbe.Thesubnetmasklinetellstheclientwhichsubnetmaskitshoulduse.11.IntheDHCPtracefilenotedinfootnote2,theDHCPserveroffersaspecificIPaddresstotheclient(seealsoquestion8.above).Intheclient’sresponsetothefirstserverOFFERmessage,doestheclientacceptthisIPaddress?Whereintheclient’sRESPONSEistheclient’srequestedaddress?Inmyexperiment,thehostrequeststheofferedIPaddressintheDHCPRequestmessage.12.Explainthepurposeoftheleasetime.Howlongistheleasetimeinyourexperiment?TheleasetimeistheamountoftimetheDHCPserverassignsanIPaddresstoaclient.Duringtheleasetime,theDHCPserverwillnotassigntheIPgiventotheclienttoanotherclient,unlessitisreleasedbytheclient.Oncetheleasetimehasexpired,theIPaddresscanbereusedbytheDHCPservertogivetoanotherclient.Inmyexperiment,theleasetimeis3days.13.WhatisthepurposeoftheDHCPreleasemessage?DoestheDHCPserverissueanacknowledgmentofreceiptoftheclient’sDHCPrequest?Whatwouldhappeniftheclient’sDHCPreleasemessageislost?TheclientsendsaDHCPReleasemessagetocancelitsleaseontheIPaddressgiventoitbytheDHCPserver.TheDHCPserverdoesnotsendamessagebacktotheclientacknowledgingtheDHCPReleasemessage.IftheDHCPReleasemessagefromtheclientislost,theDHCPserverwouldhavetowaituntiltheleaseperiodisoverforthatIPaddressuntilitcouldreuseitforanotherclient.14.ClearthebootpfilterfromyourWiresharkwindow.WereanyARPpacketssentorreceivedduringtheDHCPpacket-exchangeperiod?Ifso,explainthepurposeofthoseARPpackets.Yes,thereareARPrequestsmadebytheDHCPserver.BeforeofferinganIPaddresstoaclient,theDHCPserverissuesanARPrequestfortheofferedIPtomakesuretheIPaddressisnotalreadyinusebyanotherworkstation.