Chapter1FirewallOverview
FWtranslatessourceIPaddress(orportid)ofthere-
ceivedpackettopresetIPaddress(orportid),andthen
forwardsthepacketwhosesourceaddressismodied.
–Translatingdestinationaddress
FWtranslatesdestinationIPaddressorportidofthe
receivedpacket(FWinterfaceaddressinusualcases)
topresetIPaddressorportid(actualIPaddressorport
id),andthenforwardsthepacketwhosedestination
addressismodied.
–Bi-directionalNAT
FWtranslatessourceaddressanddestinationaddress
(orportid)ofthepacketatthesametime.
�Accesscontrolmodule.AccesscontrolruledenesifFW
permitsthepacketsmatchingrulestopassthrough.When
receivingonepacket,FWmatchesitwithrulesinaccess
ruletableonebyoneaccordingtopolicysequencenum-
berandprocessesthepacketaccordingtooperation(per-
mitordeny)speciedbycorrespondingpolicy.Ifcorre-
spondingaccesspolicyfailstobematched,thepacketwill
beforwardedtodestinationinterface.ZXR108900Series
SwitchFWwillprocesthispacketaccordingtodefaultprop-
erty(permitordeny)oftheareawheredestinationinter-
facelocates.
4.SessionEstablishment
Asforthepacketwithnosessionformatching,ZXR108900
SeriesSwitchFWwillcreateonenewrecordinsessiontableac-
cordingtopacketprocessinginformationinsteps1-3,includ-
ingpacketdestinationaddress,sourceaddress,route,address
translationpolicy,accessruleandotherinformation.Packets
ofthissessionreceivedafterthisnewrecordwillbeprocessed
accordingtorecordinthesessiontable.
5.ProcessingbeforeRouting
Whenpolicychangesduringcommunicationprocess,FWwill
re-invokepacketlteringmoduleandaccesscontrolmodule
tomatchthepacketwithpolicy.
6.RouteQuerying
ZXR108900SeriesSwitchFWmoduleselectspacketforward-
inginterfaceaccordingtoroutingtableorMACaddresstable
learnedoneachinterface.Ifpacketaddressistranslated,FW
willsearchNATtabletondtheactualaddressforrouting.
MatchingAccess
ControlRules
Accesscontrolrulesareasetofpoliciescustomizedbyuser .These
rulescandenewhatpackets(meetingcertainconditions)can
passFWandwhatpackets(meetingsomeotherconditions)willbe
deniedbyFW.Datacontainedineachaccesspolicyinclude:source
addressanddestinationaddressofthepacket,service(protocol
typeandportid)andoperations(forwardingordropping)per-
formedtothepacketsmeetingconditions.
Inaccesspolicy,policysourcedenesthesourceofpacket,which
canbeoneormultipleobjects(suchashost,subnet,scopeandso
on).Whensourceaddressofthepacketbelongstothescopeof
policysource,itisbelievedtomeetconstraintconditionsofpolicy
source.
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