CH7 SecuringSitetoSiteConnectivitySecurityisaconcernwhenusingthepublicInternettoconductbusiness.VirtualPrivateNetworks(VPNs)areusedtoensurethesecurityofdataacrosstheInternet.AVPNisusedtocreateaprivatetunneloverapublicnetwork.DatacanbesecuredbyusingencryptioninthistunnelthroughtheInternetandbyusingauthenticationtoprotectdatafromunauthorizedaccess.

ThischapterexplainstheconceptsandprocessesrelatedtoVPNs,aswellasthebenefitsofVPNimplementationsandtheunderlyingprotocolsrequiredtoconfigureVPNs.

VPNsataGlance

Asmalltomediumsizedbusinessisgrowingandneedscustomers,teleworkers,andwired/wirelessemployeestobeabletoaccessthemainnetworkfromanylocation.Asthenetworkadministratorforthebusiness,youhavedecidedtoimplementVPNsforsecurity,networkaccessease,andcostsavings.

ItisyourjobtoensurethatallofthenetworkadministratorsstarttheVPNplanningprocesswiththesameknowledgeset.

FourbasicVPNinformationalareasneedtoberesearchedandpresentedtothenetworkadministrativeteam:

ConcisedefinitionofVPNs SomegeneralVPNfacts IPsecasaVPNsecurityoption WaysVPNsusetunneling

FundamentalsofVPNsOrganizationsneedsecure,reliable,andcosteffectivewaystointerconnectmultiplenetworks,suchasallowingbranchofficesandsupplierstoconnecttoacorporationsheadquarternetwork.Additionally,withthegrowingnumberofteleworkers,enterpriseshaveanincreasingneedforsecure,reliable,andcosteffectivewaystoconnectemployeesworkinginsmalloffice/homeoffice(SOHO)andotherremotelocations,withresourcesoncorporatesites.

Thefigureillustratesthetopologiesthatmodernnetworksusetoconnectremotelocations.Insomecases,theremotelocationsconnectonlytotheheadquarterslocation,whileinothercases,remotelocationsconnecttoadditionalsites.

OrganizationsuseVPNstocreateanendtoendprivatenetworkconnectionoverthirdpartynetworkssuchastheInternetorextranets.Thetunneleliminatesthedistancebarrierandenablesremoteuserstoaccesscentralsitenetworkresources.AVPNisaprivatenetworkcreatedviatunnelingoverapublicnetwork,usuallytheInternet.AVPNisacommunicationsenvironmentinwhichaccessisstrictlycontrolledtopermitpeerconnectionswithinadefinedcommunityofinterest.

ThefirstVPNswerestrictlyIPtunnelsthatdidnotincludeauthenticationorencryptionofthedata.Forexample,GenericRoutingEncapsulation(GRE)isatunnelingprotocoldevelopedbyCiscothatcanencapsulateawidevarietyofnetworklayerprotocolpackettypesinsideIPtunnels.ThiscreatesavirtualpointtopointlinktoCiscoroutersatremotepointsoveranIPinternetwork.

Today,asecureimplementationofVPNwithencryption,suchasIPsecVPNs,iswhatisusuallymeantbyvirtualprivatenetworking.

ToimplementVPNs,aVPNgatewayisnecessary.TheVPNgatewaycouldbearouter,afirewall,oraCiscoAdaptiveSecurityAppliance(ASA).AnASAisastandalonefirewalldevicethatcombinesfirewall,VPNconcentrator,andintrusionpreventionfunctionalityintoonesoftwareimage.

Asshowninthefigure,aVPNusesvirtualconnectionsthatareroutedthroughtheInternetfromtheprivatenetworkofanorganizationtotheremotesiteoremployeehost.Theinformationfromaprivatenetworkissecurelytransportedoverthepublicnetwork,toformavirtualnetwork.

ThebenefitsofaVPNincludethefollowing:

CostsavingsVPNsenableorganizationstousecosteffective,thirdpartyInternettransporttoconnectremoteofficesandremoteuserstothemainsitetherefore,eliminatingexpensive,dedicatedWANlinksandmodembanks.Furthermore,withtheadventofcosteffective,highbandwidthtechnologies,suchasDSL,organizationscanuseVPNstoreducetheirconnectivitycostswhilesimultaneouslyincreasingremoteconnectionbandwidth.

ScalabilityVPNsenableorganizationstousetheInternetinfrastructurewithinISPsanddevices,whichmakesiteasytoaddnewusers.Therefore,organizationsareabletoaddlargeamountsofcapacitywithoutaddingsignificantinfrastructure.

CompatibilitywithbroadbandtechnologyVPNsallowmobileworkersandtelecommuterstotakeadvantageofhighspeed,broadbandconnectivity,suchasDSLandcable,toaccesstotheirorganizationsnetworks.Broadbandconnectivityprovidesflexibilityandefficiency.Highspeed,broadbandconnectionsalsoprovideacosteffectivesolutionforconnectingremoteoffices.

SecurityVPNscanincludesecuritymechanismsthatprovidethehighestlevelofsecuritybyusingadvancedencryptionandauthenticationprotocolsthatprotectdatafromunauthorizedaccess.

TherearetwotypesofVPNnetworks:

Sitetosite Remoteaccess

SitetoSiteVPN

AsitetositeVPNiscreatedwhendevicesonbothsidesoftheVPNconnectionareawareoftheVPNconfigurationinadvance,asshowninthefigure.TheVPNremainsstatic,andinternalhostshavenoknowledgethataVPNexists.InasitetositeVPN,endhostssendandreceivenormalTCP/IPtrafficthroughaVPNgateway.TheVPNgatewayisresponsibleforencapsulatingandencryptingoutboundtrafficforalltrafficfromaparticularsite.TheVPNgatewaythensendsitthroughaVPNtunnelovertheInternettoapeerVPNgatewayatthetargetsite.Uponreceipt,thepeerVPNgatewaystripstheheaders,decryptsthecontent,andrelaysthepackettowardthetargethostinsideitsprivatenetwork.

AsitetositeVPNisanextensionofaclassicWANnetwork.SitetositeVPNsconnectentirenetworkstoeachother,forexample,theycanconnectabranchofficenetworktoacompanyheadquartersnetwork.Inthepast,aleasedlineorFrameRelayconnectionwasrequiredtoconnectsites,butbecausemostcorporationsnowhaveInternetaccess,theseconnectionscanbereplacedwithsitetositeVPNs.

RemoteaccessVPNs

WhereasitetositeVPNisusedtoconnectentirenetworks,aremoteaccessVPNsupportstheneedsoftelecommuters,mobileusers,andextranet,consumertobusinesstraffic.AremoteaccessVPNiscreatedwhenVPNinformationisnotstaticallysetup,butinsteadallowsfordynamicallychanginginformation,andcanbeenabledanddisabled.RemoteaccessVPNssupportaclient/serverarchitecture,wheretheVPNclient(remotehost)gainssecureaccesstotheenterprisenetworkviaaVPNserverdeviceatthenetworkedge.

RemoteaccessVPNsareusedtoconnectindividualhoststhatmustaccesstheircompanynetworksecurelyovertheInternet.Internetconnectivityusedbytelecommutersistypicallyabroadband,DSL,wireless,orcableconnection,asindicatedinthefigure.

VPNclientsoftwaremayneedtobeinstalledonthemobileusersenddeviceforexample,eachhostmayhaveCiscoAnyConnectSecureMobilityClientsoftwareinstalled.Whenthehosttriestosendanytraffic,theCiscoAnyConnectVPNClientsoftwareencapsulatesandencryptsthistraffic.TheencrypteddataisthensentovertheInternettotheVPNgatewayattheedgeofthetargetnetwork.Uponreceipt,theVPNgatewaybehavesasitdoesforsitetositeVPNs.

Note:TheCiscoAnyConnectSecureMobilityClientsoftwarebuildsonpriorCiscoAnyConnectVPNClientandCiscoVPNClientofferingstoimprovethealwaysonVPNexperienceacrossmorelaptopandsmartphonebasedmobiledevices.ThisclientsupportsIPv6.

FundamentalsofGenericRoutingEncapsulationGenericRoutingEncapsulation(GRE)isoneexampleofabasic,nonsecure,sitetositeVPNtunnelingprotocol.GREisatunnelingprotocoldevelopedbyCiscothatcanencapsulateawidevarietyofprotocolpackettypesinsideIPtunnels.GREcreatesavirtualpointtopointlinktoCiscoroutersatremotepoints,overanIPinternetwork.

GREisdesignedtomanagethetransportationofmultiprotocolandIPmulticasttrafficbetweentwoormoresites,thatmayonlyhaveIPconnectivity.ItcanencapsulatemultipleprotocolpackettypesinsideanIPtunnel.

Asshowninthefigure,atunnelinterfacesupportsaheaderforeachofthefollowing:

Anencapsulatedprotocol(orpassengerprotocol),suchasIPv4,IPv6,AppleTalk,DECnet,orIPX Anencapsulationprotocol(orcarrier),suchasGRE Atransportdeliveryprotocol,suchasIP,whichistheprotocolthatcarriestheencapsulatedprotocol

GREisatunnelingprotocoldevelopedbyCiscothatcanencapsulateawidevarietyofprotocolpackettypesinsideIPtunnels,creatingavirtualpointtopointlinktoCiscoroutersatremotepointsoveranIPinternetwork.IPtunnelingusingGREenablesnetworkexpansionacrossasingleprotocolbackboneenvironment.Itdoesthisbyconnectingmultiprotocolsubnetworksinasingleprotocolbackboneenvironment.

GREhasthesecharacteristics:

GREisdefinedasanIETFstandard(RFC2784). IntheouterIPheader,47isusedintheprotocolfieldtoindicatethataGREheaderwillfollow. GREencapsulationusesaprotocoltypefieldintheGREheadertosupporttheencapsulationofanyOSILayer3

protocol.ProtocolTypesaredefinedinRFC1700as"EtherTypes". GREitselfisstatelessbydefaultitdoesnotincludeanyflowcontrolmechanisms. GREdoesnotincludeanystrongsecuritymechanismstoprotectitspayload. TheGREheader,togetherwiththetunnelingIPheaderindicatedinthefigure,createsatleast24bytesofadditional

overheadfortunneledpackets.

GREisusedtocreateaVPNtunnelbetweentwosites,asshowninFigure1.ToimplementaGREtunnel,thenetworkadministratormustfirstlearntheIPaddressesoftheendpoints.Afterthat,therearefivestepstoconfiguringaGREtunnel:

Step1.Createatunnelinterfaceusingtheinterfacetunnelnumbercommand.

Step2.SpecifythetunnelsourceIPaddress.

Step3.SpecifythetunneldestinationIPaddress.

Step4.ConfigureanIPaddressforthetunnelinterface.

Step5.(Optional)SpecifyGREtunnelmodeasthetunnelinterfacemode.GREtunnelmodeisthedefaulttunnelinterfacemodeforCiscoIOSsoftware.

ThesampleconfigurationinFigure2illustratesabasicGREtunnelconfigurationforrouterR1.

TheconfigurationofR2inFigure3mirrorstheconfigurationofR1.

Theminimumconfigurationrequiresspecificationofthetunnelsourceanddestinationaddresses.TheIPsubnetmustalsobeconfiguredtoprovideIPconnectivityacrossthetunnellink.BothtunnelinterfaceshavethetunnelsourcesetasthelocalserialS0/0/0interfaceandthetunneldestinationsetasthepeerrouterserialS0/0/0interface.TheIPaddressisassignedtothetunnelinterfacesonbothrouters.OSPFhasalsobeenconfiguredtoexchangeroutesovertheGREtunnel.

TheindividualGREtunnelcommanddescriptionsaredisplayedinFigure4.

Note:WhenconfiguringGREtunnels,itcanbedifficulttorememberwhichIPnetworksareassociatedwiththephysicalinterfacesandwhichIPnetworksareassociatedwiththetunnelinterfaces.RememberthatbeforeaGREtunneliscreated,thephysicalinterfaceshavealreadybeenconfigured.ThetunnelsourceandtunneldestinationcommandsreferencetheIPaddressesofthepreconfiguredphysicalinterfaces.TheipaddresscommandonthetunnelinterfacesreferstoanIPnetworkspecificallymanufacturedforthepurposesoftheGREtunnel.

ThereareseveralcommandsthatcanbeusedtomonitorandtroubleshootGREtunnels.Todeterminewhetherthetunnelinterfaceisupordown,usetheshowipinterfacebriefcommand,asshowninFigure1.

ToverifythestateofaGREtunnel,usetheshowinterfacetunnelcommand.ThelineprotocolonaGREtunnelinterfaceisupaslongasthereisaroutetothetunneldestination.BeforeimplementingaGREtunnel,IPconnectivitymustalreadybeineffectbetweentheIPaddressesofthephysicalinterfacesonoppositeendsofthepotentialGREtunnel.Thetunneltransportprotocolisdisplayedintheoutput,alsoshowninFigure1.

IfOSPFhasalsobeenconfiguredtoexchangeroutesovertheGREtunnel,verifythatanOSPFadjacencyhasbeenestablishedoverthetunnelinterfaceusingtheshowipospfneighborcommand.InFigure2,notethatthepeeringaddressfortheOSPFneighborisontheIPnetworkcreatedfortheGREtunnel.

InFigure3,usetheSyntaxCheckertoconfigureandverifyaGREtunnelonR2followedbyR1.

GREisconsideredaVPNbecauseitisaprivatenetworkthatiscreatedbytunnelingoverapublicnetwork.Usingencapsulation,aGREtunnelcreatesavirtualpointtopointlinktoCiscoroutersatremotepointsoveranIPinternetwork.TheadvantagesofGREarethatitcanbeusedtotunnelnonIPtrafficoveranIPnetwork,allowingfornetworkexpansionbyconnectingmultiprotocolsubnetworksacrossasingleprotocolbackboneenvironment.GREalsosupportsIPmulticasttunneling.Thismeansthatroutingprotocolscanbeusedacrossthetunnel,enablingdynamicexchangeofroutinginformationinthevirtualnetwork.Finally,itiscommonpracticetocreateIPv6overIPv4GREtunnels,whereIPv6istheencapsulatedprotocolandIPv4isthetransportprotocol.Inthefuture,theseroleswilllikelybereversedasIPv6takesoverasthestandardIPprotocol.

However,GREdoesnotprovideencryptionoranyothersecuritymechanisms.Therefore,datasentacrossaGREtunnelisnotsecure.Ifsecuredatacommunicationisneeded,IPsecorSSLVPNsshouldbeconfigured.

IPsecVPNsofferflexibleandscalableconnectivity.Sitetositeconnectionscanprovideasecure,fast,andreliableremoteconnection.WithanIPsecVPN,theinformationfromaprivatenetworkissecurelytransportedoverapublicnetwork.ThisformsavirtualnetworkinsteadofusingadedicatedLayer2connection,asshowninthefigure.Toremainprivate,thetrafficisencryptedtokeepthedataconfidential.

IPsecisanIETFstandardthatdefineshowaVPNcanbeconfiguredinasecuremannerusingtheInternetProtocol.

IPsecisaframeworkofopenstandardsthatspellsouttherulesforsecurecommunications.IPsecisnotboundtoanyspecificencryption,authentication,securityalgorithms,orkeyingtechnology.Rather,IPsecreliesonexistingalgorithmstoimplementsecurecommunications.IPsecallowsnewerandbetteralgorithmstobeimplementedwithoutamendingtheexistingIPsecstandards.

IPsecworksatthenetworklayer,protectingandauthenticatingIPpacketsbetweenparticipatingIPsecdevices,alsoknownaspeers.IPsecsecuresapathbetweenapairofgateways,apairofhosts,oragatewayandhost.Asaresult,IPseccanprotectvirtuallyallapplicationtrafficbecausetheprotectioncanbeimplementedfromLayer4toLayer7.

AllimplementationsofIPsechaveaplaintextLayer3header,sotherearenoissueswithrouting.IPsecfunctionsoverallLayer2protocols,suchasEthernet,ATM,orFrameRelay.

IPseccharacteristicscanbesummarizedasfollows:

IPsecisaframeworkofopenstandardsthatisalgorithmindependent. IPsecprovidesdataconfidentiality,dataintegrity,andoriginauthentication. IPsecactsatthenetworklayer,protectingandauthenticatingIPpackets.

IPsecsecurityservicesprovidefourcriticalfunctions,asshowninthefigure:

Confidentiality(encryption)InaVPNimplementation,privatedatatravelsoverapublicnetwork.Forthisreason,dataconfidentialityisvital.Itcanbeattainedbyencryptingthedatabeforetransmittingitacrossthenetwork.Thisistheprocessoftakingallthedatathatonecomputerissendingtoanotherandencodingitintoaformthatonlytheothercomputerwillbeabletodecode.Ifthecommunicationisintercepted,itcannotbereadbyahacker.IPsecprovidesenhancedsecurityfeatures,suchasstrongencryptionalgorithms.

DataIntegrityThereceivercanverifythatthedatawastransmittedthroughtheInternetwithoutbeingchangedoralteredinanyway.Whileitisimportantthatdataisencryptedoverapublicnetwork,itisjustasimportanttoverifythatithasnotbeenchangedwhileintransit.IPsechasamechanismtoensurethattheencryptedportionofthe

packet,ortheentireheaderanddataportionofthepacket,hasnotbeenchanged.IPsecensuresdataintegritybyusingchecksums,whichisasimpleredundancycheck.Iftamperingisdetected,thepacketisdropped.

AuthenticationVerifytheidentityofthesourceofthedatathatissent.Thisisnecessarytoguardagainstanumberofattacksthatdependonspoofingtheidentityofthesender.Authenticationensuresthattheconnectionismadewiththedesiredcommunicationpartner.Thereceivercanauthenticatethesourceofthepacketbycertifyingthesourceoftheinformation.IPsecusesInternetKeyExchange(IKE)toauthenticateusersanddevicesthatcancarryoutcommunicationindependently.IKEusesseveraltypesofauthentication,includingusernameandpassword,onetimepassword,biometrics,presharedkey(PSK),anddigitalcertificates.

AntiReplayProtectionThisistheabilitytodetectandrejectreplayedpacketsandhelpspreventspoofing.Antireplayprotectionverifiesthateachpacketisuniqueandnotduplicated.IPsecpacketsareprotectedbycomparingthesequencenumberofthereceivedpacketswithaslidingwindowonthedestinationhostorsecuritygateway.Apacketthathasasequencenumberthatisbeforetheslidingwindowisconsideredtobelateoraduplicatepacket.Lateandduplicatepacketsaredropped.

TheacronymCIAisoftenusedtohelprememberthefirstthreeofthesefunctions:confidentiality,integrity,andauthentication.

Confidentiality

VPNtrafficiskeptconfidentialwithencryption.PlaintextdatathatistransportedovertheInternetcanbeinterceptedandread.Encryptthedatetokeepitprivate.Digitallyencryptingthedatarendersitunreadableuntilitisunencryptedbytheauthorizedreceiver.

Forencryptedcommunicationtowork,boththesenderandthereceivermustknowtherulesthatareusedtotransformtheoriginalmessageintoitscodedform.Rulesarebasedonalgorithmsandassociatedkeys.Inthecontextofencryption,analgorithmisamathematicalsequenceofstepsthatcombinesamessage,text,digits,orallthreewithastringofdigitsthatarecalledakey.Theoutputisanunreadablecipherstring.Theencryptionalgorithmalsospecifieshowanencryptedmessageisdecrypted.Decryptionisextremelydifficultorimpossiblewithoutthecorrectkey.

Inthefigure,Gailwantstosendanelectronicfundstransfer(EFT)acrosstheInternettoJeremy.Atthelocalend,thedocumentiscombinedwithakeyandrunthroughanencryptionalgorithm.Theoutputisencryptedciphertext.TheciphertextisthensentthroughtheInternet.Attheremoteend,themessageisrecombinedwithakeyandsentbackthroughtheencryptionalgorithm.Theoutputistheoriginalfinancialdocument.

ConfidentialityisachievedthroughtheencryptionoftrafficasittravelsthroughaVPN.Thedegreeofsecuritydependsonthekeylengthoftheencryptionalgorithmandthesophisticationofthealgorithm.Ifahackertriestohackthekeythroughabruteforceattack,thenumberofpossibilitiestotryisafunctionofthekeylength.Thetimetoprocessallofthepossibilitiesisafunctionofthecomputerpoweroftheattackingdevice.Theshorterthekey,theeasieritistobreak.Forexample,wherearelativelysophisticatedcomputermaytakeapproximatelyoneyeartobreaka64bitlongkey,thesamecomputermaytakeanywherefrom10to19yearstodecrypta128bitlongkey.

Thedegreeofsecuritydependsonthekeylengthoftheencryptionalgorithm.Askeylengthincreases,itbecomesmoredifficulttobreaktheencryption.However,alongerkeyrequiresmoreprocessorresourceswhenencryptinganddecryptingdata.

DESand3DESarenolongerconsideredsecuretherefore,itisrecommendedthatAESbeusedforIPsecencryption.ThegreatestsecurityforIPsecencryptionofVPNsbetweenCiscodevicesisprovidedbythe256bitoptionofAES.Inaddition,

512bitand768bitRivestShamirAdleman(RSA)keyshavebeencrackedandCiscorecommendsusing2048bitkeyswiththeRSAoption,ifusedduringtheauthenticationphaseofIKE.

SymmetricEncryption

Encryptionalgorithms,suchasAES,requireasharedsecretkeytoperformencryptionanddecryption.Eachofthetwonetworkingdevicesmustknowthekeytodecodetheinformation.Withsymmetrickeyencryption,alsocalledsecretkeyencryption,eachdeviceencryptstheinformationbeforesendingitoverthenetworktotheotherdevice.Symmetrickeyencryptionrequiresknowledgeofwhichdevicestalktoeachothersothatthesamekeycanbeconfiguredoneachdevice,asdepictedinFigure1.

Forexample,asendercreatesacodedmessagewhereeachletterissubstitutedwiththeletterthatistwolettersdowninthealphabetAbecomesC,BbecomesD,andsoon.Inthiscase,thewordSECRETbecomesUGETGV.Thesenderhasalreadytoldtherecipientthatthesecretkeyisshiftby2.WhentherecipientreceivesthemessageUGETGV,therecipientcomputerdecodesthemessagebyshiftingbacktwolettersandcalculatingSECRET.Anyoneelsewhoseesthemessageseesonlytheencryptedmessage,whichlookslikenonsense,unlessthepersonknowsthesecretkey.

Hereisasynopsisforsymmetricalgorithms:

Usessymmetrickeycryptography Encryptionanddecryptionusethesamekey Typicallyusedtoencryptthecontentofthemessage Examples:DES,3DES,andAES

Howdotheencryptinganddecryptingdevicesbothhaveasharedsecretkey?Onecoulduseemail,courier,orovernightexpresstosendthesharedsecretkeystotheadministratorsofthedevices.Another,moresecuremethodisasymmetricencryption.

AsymmetricEncryption

Asymmetricencryptionusesdifferentkeysforencryptionanddecryption.Knowingoneofthekeysdoesnotallowahackertodeducethesecondkeyanddecodetheinformation.Onekeyencryptsthemessage,whileasecondkeydecryptsthemessage,asdepictedinFigure2.Itisnotpossibletoencryptanddecryptwiththesamekey.

Publickeyencryptionisavariantofasymmetricencryptionthatusesacombinationofaprivatekeyandapublickey.Therecipientgivesapublickeytoanysenderwithwhomtherecipientwantstocommunicate.Thesenderusesaprivatekeythatiscombinedwiththepublickeyoftherecipienttoencryptthemessage.Also,thesendermustshareitspublickeywiththerecipient.Todecryptamessage,therecipientwillusethepublickeyofthesenderwithitsownprivatekey.

Hereisasynopsisforasymmetricalgorithms:

Usespublickeycryptography Encryptionanddecryptionuseadifferentkey Typicallyusedindigitalcertificationandkeymanagement Examples:RSA

DataIntegrity

DiffieHellman(DH)isnotanencryptionmechanismandisnottypicallyusedtoencryptdata.Instead,itisamethodtosecurelyexchangethekeysthatencryptdata.(DH)algorithmsallowtwopartiestoestablishasharedsecretkeythatisusedbyencryptionandhashalgorithms.

IntroducedbyWhitfieldDiffieandMartinHellmanin1976,DHwasthefirstsystemtoutilizepublickeyorasymmetriccryptographickeys.Today,DHispartoftheIPsecstandard.Also,aprotocolknownasOAKLEYusesaDHalgorithm.OAKLEYisusedbytheIKEprotocol,whichispartoftheoverallframeworkcalledInternetSecurityAssociationandKeyManagementProtocol.

EncryptionalgorithmssuchasDES,3DES,andAES,aswellastheMD5andSHA1hashingalgorithmsrequireasymmetric,sharedsecretkeytoperformencryptionanddecryption.Howdotheencryptinganddecryptingdevicesgetthesharedsecretkey?Theeasiestkeyexchangemethodisapublickeyexchangemethodbetweentheencryptinganddecryptingdevices.

TheDHalgorithmspecifiesapublickeyexchangemethodthatprovidesawayfortwopeerstoestablishasharedsecretkeythatonlytheyknow,althoughtheyarecommunicatingoveraninsecurechannel.Likeallcryptographicalgorithms,DHkeyexchangeisbasedonamathematicalsequenceofsteps.

TheintegrityandauthenticationofVPNtrafficishandledbyhashalgorithms.Hashesprovidedataintegrityandauthenticationbyensuringthatunauthorizedpersonsdonottamperwithtransmittedmessages.Ahash,alsocalledamessagedigest,isanumberthatisgeneratedfromastringoftext.Thehashissmallerthanthetextitself.Itisgeneratedbyusingaformulainsuchawaythatitisextremelyunlikelythatsomeothertextwillproducethesamehashvalue.

Theoriginalsendergeneratesahashofthemessageandsendsitwiththemessageitself.Therecipientparsesthemessageandthehash,producesanotherhashfromthereceivedmessage,andcomparesthetwohashes.Iftheyarethesame,therecipientcanbereasonablysureoftheintegrityoftheoriginalmessage.

Inthefigure,GailsentAlexanEFTof$100.JeremyhasinterceptedandalteredthisEFTtoshowhimselfastherecipientandtheamountas$1000.Inthiscase,ifadataintegrityalgorithmwereused,thehasheswouldnotmatch,andthetransactionwouldbeinvalid.

VPNdataistransportedoverthepublicInternet.Asshown,thereispotentialforthisdatatobeinterceptedandmodified.Toguardagainstthisthreat,hostscanaddahashtothemessage.Ifthetransmittedhashmatchesthereceivedhash,theintegrityofthemessagehasbeenpreserved.However,ifthereisnomatch,themessagewasaltered.

VPNsuseamessageauthenticationcodetoverifytheintegrityandtheauthenticityofamessage,withoutusinganyadditionalmechanisms.

HashbasedMessageAuthenticationCode(HMAC)isamechanismformessageauthenticationusinghashfunctions.AkeyedHMACisadataintegrityalgorithmthatguaranteestheintegrityofamessage.AnHMAChastwoparameters,amessageinputandasecretkeythatisknownonlytothemessageoriginatorandintendedreceivers.ThemessagesenderusesanHMACfunctiontoproduceavalue(themessageauthenticationcode)thatisformedbycondensingthesecretkeyandthemessageinput.Themessageauthenticationcodeissentalongwiththemessage.ThereceivercomputesthemessageauthenticationcodeonthereceivedmessageusingthesamekeyandHMACfunctionasthesenderused.Thenthereceivercomparestheresultthatiscomputedwiththereceivedmessageauthenticationcode.Ifthetwovaluesmatch,themessagehasbeencorrectlyreceivedandthereceiverisassuredthatthesenderisamemberofthecommunityofusersthatsharethekey.ThecryptographicstrengthoftheHMACdependsuponthecryptographicstrengthoftheunderlyinghashfunction,onthesizeandqualityofthekey,andonthesizeofthehashoutputlengthinbits.

TherearetwocommonHMACalgorithms:

MD5Usesa128bitsharedsecretkey.Thevariablelengthmessageand128bitsharedsecretkeyarecombinedandrunthroughtheHMACMD5hashalgorithm.Theoutputisa128bithash.Thehashisappendedtotheoriginalmessageandforwardedtotheremoteend.

SHASHA1usesa160bitsecretkey.Thevariablelengthmessageandthe160bitsharedsecretkeyarecombinedandrunthroughtheHMACSHA1hashalgorithm.Theoutputisa160bithash.Thehashisappendedtotheoriginalmessageandforwardedtotheremoteend.

Note:CiscoIOSalsosupports,256bit,384bit,and512bitSHAimplementations.

Authentication

IPsecVPNssupportauthentication.Whenconductingbusinesslongdistance,itisnecessarytoknowwhoisattheotherendofthephone,email,orfax.ThesameistrueofVPNnetworks.ThedeviceontheotherendoftheVPNtunnelmustbeauthenticatedbeforethecommunicationpathisconsideredsecure,asindicatedinthefigure.Therearetwopeerauthenticationmethods:

PSKAsecretkeythatissharedbetweenthetwopartiesusingasecurechannelbeforeitneedstobeused.Presharedkeys(PSKs)usesymmetrickeycryptographicalgorithms.APSKisenteredintoeachpeermanuallyandisusedtoauthenticatethepeer.Ateachend,thePSKiscombinedwithotherinformationtoformtheauthenticationkey.

RSAsignaturesDigitalcertificatesareexchangedtoauthenticatepeers.Thelocaldevicederivesahashandencryptsitwithitsprivatekey.Theencryptedhash,ordigitalsignature,isattachedtothemessageandforwardedtotheremoteend.Attheremoteend,theencryptedhashisdecryptedusingthepublickeyofthelocalend.Ifthedecryptedhashmatchestherecomputedhash,thesignatureisgenuine.

IPsecusesRSA(publickeycryptosystem)forauthenticationinthecontextofIKE.TheRSAsignaturemethodusesadigitalsignaturesetupinwhicheachdevicedigitallysignsasetofdataandsendsittotheotherparty.RSAsignaturesuseacertificateauthority(CA)togenerateauniqueidentitydigitalcertificatethatisassignedtoeachpeerforauthentication.TheidentitydigitalcertificateissimilarinfunctiontoaPSK,butprovidesmuchstrongersecurity.EachinitiatorandrespondertoanIKEsessionusingRSAsignaturessendsitsownIDvalue,itsidentitydigitalcertificate,andanRSAsignaturevalueconsistingofavarietyofIKEvalues,allencryptedbythenegotiatedIKEencryptionmethod(suchasAES).

TheDigitalSignatureAlgorithm(DSA)isanotheroptionforauthentication.

Asstatedearlier,theIPsecprotocolframeworkdescribesthemessagingtosecurethecommunications,butitreliesonexistingalgorithms.

TherearetwomainIPsecprotocolsdepictedinFigure1:

AuthenticationHeader(AH)AHistheappropriateprotocoltousewhenconfidentialityisnotrequiredorpermitted.ItprovidesdataauthenticationandintegrityforIPpacketsthatarepassedbetweentwosystems.However,AHdoesnotprovidedataconfidentiality(encryption)ofpackets.Alltextistransportedinplaintext.Usedalone,theAHprotocolprovidesweakprotection.

EncapsulatingSecurityPayload(ESP)AsecurityprotocolthatprovidesconfidentialityandauthenticationbyencryptingtheIPpacket.IPpacketencryptionconcealsthedataandtheidentitiesofthesourceanddestination.ESPauthenticatestheinnerIPpacketandESPheader.Authenticationprovidesdataoriginauthenticationanddata

integrity.AlthoughbothencryptionandauthenticationareoptionalinESP,ataminimum,oneofthemmustbeselected.

Figure2illustratesthecomponentsofIPsecconfiguration.TherearefourbasicbuildingblocksoftheIPsecframeworkthatmustbeselected.

IPsecframeworkprotocolWhenconfiguringanIPsecgatewaytoprovidesecurityservices,anIPsecprotocolmustbeselected.ThechoicesaresomecombinationofESPandAH.Realistically,theESPorESP+AHoptionsarealmostalwaysselectedbecauseAHitselfdoesnotprovideencryption,asshowninFigure3.

Confidentiality(IfIPsecisimplementedwithESP)Theencryptionalgorithmchosenshouldbestmeetthedesiredlevelofsecurity:DES,3DES,orAES.AESisstronglyrecommended,withAESGCMprovidingthegreatestsecurity.

IntegrityGuaranteesthatthecontenthasnotbeenalteredintransit.Implementedthroughtheuseofhashalgorithms.ChoicesincludeMD5andSHA.

AuthenticationRepresentshowdevicesoneitherendoftheVPNtunnelareauthenticated.ThetwomethodsarePSKorRSA.

DHalgorithmgroupRepresentshowasharedsecretkeyisestablishedbetweenpeers.Thereareseveraloptions,butDH24providesthegreatestsecurity.

Itisthecombinationofthesebuildingblocksthatprovidestheconfidentiality,integrity,andauthenticationoptionsforIPsecVPNs.

Note:ThissectionintroducedIPsectoprovideanunderstandingofhowIPsecsecuresVPNtunnels.ConfiguringIPsecVPNsarebeyondthescopeofthiscourse.

RemoteaccessVPNSolutionsVPNshavebecomethelogicalsolutionforremoteaccessconnectivityformanyreasons.VPNsprovidesecurecommunicationswithaccessrightstailoredtoindividualusers,suchasemployees,contractors,andpartners.Theyalsoenhanceproductivitybyextendingthecorporatenetworkandapplicationssecurelywhilereducingcommunicationcostsandincreasingflexibility.

UsingVPNtechnology,employeescanessentiallytaketheirofficewiththem,includingaccesstoemailsandnetworkapplications.VPNscanalsoallowcontractorsandpartnerstohavelimitedaccesstothespecificservers,webpages,orfilesrequired.Thisnetworkaccessallowsthemtocontributetobusinessproductivitywithoutcompromisingnetworksecurity.

TherearetwoprimarymethodsfordeployingremoteaccessVPNs:

SecureSocketsLayer(SSL) IPSecurity(IPsec)

ThetypeofVPNmethodimplementedisbasedontheaccessrequirementsoftheusersandtheorganizationsITprocesses.

BothIPsecandSSLVPNtechnologiesofferaccesstovirtuallyanynetworkapplicationorresource.SSLVPNsoffersuchfeaturesaseasyconnectivityfromnoncompanymanageddesktops,littleornodesktopsoftwaremaintenance,andusercustomizedwebportalsuponlogin.

CiscoIOSSSLVPNistheindustrysfirstrouterbasedSSLVPNsolution.Itoffersanywhereconnectivitynotonlyfromcompanymanagedresource,butalsofromemployeeownedPCs,contractororbusinesspartnerdesktops,andInternetkiosks.

TheSSLprotocolsupportsvariouscryptographicalgorithmsforoperations,suchasauthenticatingtheserverandclienttoeachother,transmittingcertificates,andestablishingsessionkeys.CiscoSSLVPNsolutionscanbecustomizedforbusinessesofanysize.Thesesolutionsdelivermanyremoteaccessconnectivityfeaturesandbenefits,including:

Webbased,clientlessaccessandcompletenetworkaccesswithoutpreinstalleddesktopsoftware.Thisfacilitatescustomizedremoteaccessbasedonuserandsecurityrequirements,anditminimizesdesktopsupportcosts.

Protectionagainstviruses,worms,spyware,andhackersonaVPNconnectionbyintegratingnetworkandendpointsecurityintheCiscoSSLVPNplatform.Thisreducescostandmanagementcomplexitybyeliminatingtheneedforadditionalsecurityequipmentandmanagementinfrastructure.

UseofasingledeviceforbothSSLVPNandIPsecVPN.ThisreducescostandmanagementcomplexitybyfacilitatingrobustremoteaccessandsitetositeVPNservicesfromasingleplatformwithunifiedmanagement.

CiscoIOSSSLVPNisatechnologythatprovidesremoteaccessbyusingawebbrowserandthewebbrowsersnativeSSLencryption.Alternatively,itcanprovideremoteaccessusingtheCiscoAnyConnectSecureMobilityClientsoftware.

TheCiscoASAprovidestwomaindeploymentmodesthatarefoundinCiscoSSLVPNsolutions,asshowninthefigure:

CiscoAnyConnectSecureMobilityClientwithSSLRequirestheCiscoAnyConnectClient CiscoSecureMobilityClientlessSSLVPNRequiresaninternetbrowser

TheCiscoASAmustbeconfiguredtosupporttheSSLVPNconnection.

CiscoAnyConnectSecureMobilityClientwithSSL

ClientBasedSSLVPNsprovideauthenticateduserswithLANlike,fullnetworkaccesstocorporateresources.However,theremotedevicesrequireaclientapplication,suchastheCiscoVPNClientorthenewerAnyConnectclienttobeinstalledontheenduserdevice.

InabasicCiscoASAconfiguredforfulltunnelingandaremoteaccessSSLVPNsolution,remoteusersusetheCiscoAnyConnectSecureMobilityClient,showninFigure1,toestablishanSSLtunnelwiththeCiscoASA.AftertheCiscoASAestablishestheVPNwiththeremoteuser,theremoteusercanforwardIPtrafficintotheSSLtunnel.TheCiscoAnyConnectSecureMobilityClientcreatesavirtualnetworkinterfacetoprovidethisfunctionality.Theclientcanuseanyapplicationtoaccessanyresource,subjecttoaccessrules,behindtheCiscoASAVPNgateway.

CiscoSecureMobilityClientlessSSLVPN

TheclientlessSSLVPNdeploymentmodelenablescorporationstoprovideaccesstocorporateresourcesevenwhentheremotedeviceisnotcorporatelymanaged.Inthisdeploymentmodel,theCiscoASAisusedasaproxydevicetonetworkresources.Itprovidesawebportalinterfaceforremotedevicestonavigatethenetworkusingportforwardingcapabilities.

InabasicCiscoASAclientlessSSLVPNsolution,remoteusersemployastandardwebbrowsertoestablishanSSLsessionwiththeCiscoASA,asshowninFigure2.TheCiscoASApresentstheuserwithawebportaloverwhichtheusercanaccessinternalresources.Inthebasicclientlesssolution,theusercanaccessonlysomeservices,suchasinternalwebapplications,andbrowserbased,filesharingresources,asshowninFigure3.

ManyapplicationsrequirethesecurityofanIPsecremoteaccessVPNconnectionforauthenticationandencryptionofdata.WhendeployingVPNsfortelecommutersandsmallbranchoffices,easeofdeploymentiscriticaliftechnicalresourcesarenotavailableforVPNconfigurationonaremotesiterouter.

TheCiscoEasyVPNsolutionfeatureoffersflexibility,scalability,andeaseofuseforbothsitetositeandremoteaccessIPsecVPNs.TheCiscoEasyVPNsolutionconsistsofthreecomponents:

CiscoEasyVPNServerACiscoIOSrouterorCiscoASAFirewallactingastheVPNheadenddeviceinsitetositeorremoteaccessVPNs.

CiscoEasyVPNRemoteACiscoIOSrouterorCiscoASAFirewallactingasaremoteVPNclient. CiscoVPNClientAnapplicationsupportedonaPCusedtoaccessaCiscoVPNserver.

UsingtheCiscoEasyVPNservermakesitpossibleformobileandremoteworkersusingaVPNClientontheirPCs,orusingCiscoEasyVPNRemoteonanedgerouter,tocreatesecureIPsectunnelstoaccesstheirheadquarters'intranet,asshowninthefigure.

CiscoEasyVPNServer

TheCiscoEasyVPNServermakesitpossibleformobileandremoteworkersusingVPNClientsoftwareontheirPCstocreatesecureIPsectunnelstoaccesstheirheadquarters'intranetwherecriticaldataandapplicationsexist.ItenablesCiscoIOSroutersandCiscoASAFirewallstoactasVPNheadenddevicesinsitetositeorremoteaccessVPNs.RemoteofficedevicesusetheCiscoEasyVPNRemotefeatureortheCiscoVPNClientapplicationtoconnecttotheserver,whichthenpushesdefinedsecuritypoliciestotheremoteVPNdevice.Thisensuresthatthoseconnectionshaveuptodatepoliciesinplacebeforetheconnectionisestablished.

CiscoEasyVPNRemote

TheCiscoEasyVPNRemoteenablesCiscoIOSroutersorsoftwareclientstoactasremoteVPNclients.ThesedevicescanreceivesecuritypoliciesfromaCiscoEasyVPNServer,minimizingVPNconfigurationrequirementsattheremotelocation.ThiscosteffectivesolutionisidealforremoteofficeswithlittleITsupportorforlargecustomerpremisesequipment(CPE)deploymentswhereitisimpracticaltoindividuallyconfiguremultipleremotedevices.

ThefigureshowsthreenetworkdeviceswithEasyVPNRemoteenabled,allconnectingtoanEasyVPNserverforconfigurationparameters.

CiscoVPNClient

TheCiscoVPNClientissimpletodeployandoperate.Itallowsorganizationstoestablishendtoend,encryptedVPNtunnelsforsecureconnectivityformobileemployeesortelecommuters.

ToinitiateanIPsecconnectionusingtheCiscoVPNclient,alltheusermustdoisopentheCiscoVPNclientwindow,asshowninFigure1.TheCiscoVPNclientapplicationliststheavailablepreconfiguredsites.TheuserdoubleclicksasitetoselectitandtheVPNclientinitiatestheIPsecconnection.Intheuserauthenticationdialogbox,theuserisauthenticatedwithausernameandpassword,asshowninFigure2.Afterauthentication,theCiscoVPNClientdisplaysaconnectedstatus.

MostoftheVPNparametersaredefinedontheCiscoIOSEasyVPNServertosimplifydeployment.AfteraremoteclientinitiatesaVPNtunnelconnection,theCiscoEasyVPNServerpushestheIPsecpoliciestotheclient,minimizingconfigurationrequirementsattheremotelocation.

ThissimpleandhighlyscalablesolutionisidealforlargeremoteaccessdeploymentswhereitisimpracticaltoconfigurepoliciesindividuallyformultipleremotePCs.Thisarchitecturealsoensuresthatthoseconnectionsareusinguptodatesecuritypoliciesandeliminatestheoperationalcostsassociatedwithmaintainingaconsistentpolicyandkeymanagementmethod.

Note:ConfiguringtheCiscoVPNclientisbeyondthescopeofthiscourse.Checkwww.cisco.comformoreinformation.

BothIPsecandSSLVPNtechnologiesofferaccesstovirtuallyanynetworkapplicationorresource,asshowninthefigure.SSLVPNsoffersuchfeaturesaseasyconnectivityfromnoncompanymanageddesktops,littleornodesktopsoftwaremaintenance,andusercustomizedwebportalsuponlogin.

IPsecexceedsSSLinmanysignificantways:

Numberofapplicationsthataresupported Strengthofencryption Strengthofauthentication Overallsecurity

Whensecurityisanissue,IPsecisthesuperiorchoice.Ifsupportandeaseofdeploymentaretheprimaryissues,considerSSL.

IPsecandSSLVPNarecomplementarybecausetheysolvedifferentproblems.Dependingonitsneeds,anorganizationcanimplementoneorboth.ThiscomplementaryapproachallowsasingledevicesuchasanISRrouteroranASAfirewallappliancetoaddressallremoteaccessuserrequirements.WhilemanysolutionsoffereitherIPsecorSSL,Cisco

remoteaccessVPNsolutionsofferbothtechnologiesintegratedonasingleplatformwithunifiedmanagement.OfferingbothIPsecandSSLtechnologiesenablesorganizationstocustomizetheirremoteaccessVPNwithoutanyadditionalhardwareormanagementcomplexity.

VPNsareusedtocreateasecureendtoendprivatenetworkconnectionoverathirdpartynetwork,suchastheInternet.AsitetositeVPNusesaVPNgatewaydeviceattheedgeofbothsites.TheendhostsareunawareoftheVPNandhavenoadditionalsupportingsoftware.

AremoteaccessVPNrequiressoftwaretobeinstalledontheindividualhostdevicethataccessesthenetworkfromaremotelocation.ThetwotypesofremoteaccessVPNsareSSLandIPsec.SSLtechnologycanprovideremoteaccessusingaclientswebbrowserandthebrowsersnativeSSLencryption.UsingCiscoAnyConnectsoftwareontheclient,userscanhaveLANlike,fullnetworkaccessusingSSL.

GREisabasic,nonsecuresitetositeVPNtunnelingprotocolthatcanencapsulateawidevarietyofprotocolpackettypesinsideIPtunnels,thusallowinganorganizationtodeliverotherprotocolsthroughanIPbasedWAN.TodayitisprimarilyusedtodeliverIPmulticasttrafficorIPv6trafficoveranIPv4unicastonlyconnection.

IPsec,anIETFstandard,isasecuretunneloperatingatLayer3oftheOSImodelthatcanprotectandauthenticateIPpacketsbetweenIPsecpeers.Itcanprovideconfidentialitybyusingencryption,dataintegrity,authentication,andantireplayprotection.Dataintegrityisprovidedbyusingahashalgorithm,suchasMD5orSHA.AuthenticationisprovidedbythePSKorRSApeerauthenticationmethod.

Thelevelofconfidentialityprovidedbyencryptiondependsonthealgorithmusedandthekeylength.Encryptioncanbesymmetricalorasymmetrical.DHisamethodusedtosecurelyexchangethekeystoencryptdata.