dhcpd.docx
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dhcpd.docx
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dhcpd
dhcpd(8)dhcpd(8)
NAME
dhcpd-DynamicHostConfigurationProtocolServer
SYNOPSIS
dhcpd[-pport][-f][-d][-q][-t|-T][-cfconfig-file]
[-lflease-file][-tftrace-output-file][-playtrace-playback-
file][if0[...ifN]]
DESCRIPTION
TheInternetSystemsConsortiumDHCPServer,dhcpd,implementsthe
DynamicHostConfigurationProtocol(DHCP)andtheInternetBootstrap
Protocol(BOOTP).DHCPallowshostsonaTCP/IPnetworktorequestand
beassignedIPaddresses,andalsotodiscoverinformationaboutthe
networktowhichtheyareattached.BOOTPprovidessimilarfunctional-
ity,withcertainrestrictions.
CONTRIBUTIONS
Thissoftwareisfreesoftware.Atvarioustimesitsdevelopmenthas
beenunderwrittenbyvariousorganizations,includingtheISCandVixie
Enterprises.Thedevelopmentof3.0hasbeenfundedalmostentirelyby
Nominum,Inc.
AtthispointdevelopmentisbeingshepherdedbyTedLemon,andhosted
bytheISC,butthefutureofthisprojectdependsonyou.Ifyouhave
featuresyouwant,pleaseconsiderimplementingthem.
OPERATION
TheDHCPprotocolallowsahostwhichisunknowntothenetworkadmin-
istratortobeautomaticallyassignedanewIPaddressoutofapoolof
IPaddressesforitsnetwork.Inorderforthistowork,thenetwork
administratorallocatesaddresspoolsineachsubnetandentersthem
intothedhcpd.conf(5)file.
Onstartup,dhcpdreadsthedhcpd.conffileandstoresalistofavail-
ableaddressesoneachsubnetinmemory.Whenaclientrequestsan
addressusingtheDHCPprotocol,dhcpdallocatesanaddressforit.
Eachclientisassignedalease,whichexpiresafteranamountoftime
chosenbytheadministrator(bydefault,oneday).Beforeleases
expire,theclientstowhichleasesareassignedareexpectedtorenew
theminordertocontinuetousetheaddresses.Oncealeasehas
expired,theclienttowhichthatleasewasassignedisnolongerper-
mittedtousetheleasedIPaddress.
Inordertokeeptrackofleasesacrosssystemrebootsandserver
restarts,dhcpdkeepsalistofleasesithasassignedinthe
dhcpd.leases(5)file.Beforedhcpdgrantsaleasetoahost,it
recordstheleaseinthisfileandmakessurethatthecontentsofthe
fileareflushedtodisk.Thisensuresthatevenintheeventofa
systemcrash,dhcpdwillnotforgetaboutaleasethatithasassigned.
Onstartup,afterreadingthedhcpd.conffile,dhcpdreadsthe
dhcpd.leasesfiletorefreshitsmemoryaboutwhatleaseshavebeen
assigned.
Newleasesareappendedtotheendofthedhcpd.leasesfile.Inorder
topreventthefilefrombecomingarbitrarilylarge,fromtimetotime
dhcpdcreatesanewdhcpd.leasesfilefromitsin-coreleasedatabase.
Oncethisfilehasbeenwrittentodisk,theoldfileisrenamed
dhcpd.leases~,andthenewfileisrenameddhcpd.leases.Ifthesys-
temcrashesinthemiddleofthisprocess,whicheverdhcpd.leasesfile
remainswillcontainalltheleaseinformation,sothereisnoneedfor
aspecialcrashrecoveryprocess.
BOOTPsupportisalsoprovidedbythisserver.UnlikeDHCP,theBOOTP
protocoldoesnotprovideaprotocolforrecoveringdynamically-
assignedaddressesoncetheyarenolongerneeded.Itisstillpossi-
bletodynamicallyassignaddressestoBOOTPclients,butsomeadminis-
trativeprocessforreclaimingaddressesisrequired.Bydefault,
leasesaregrantedtoBOOTPclientsinperpetuity,althoughthenetwork
administratormaysetanearliercutoffdateorashorterleaselength
forBOOTPleasesifthatmakessense.
BOOTPclientsmayalsobeservedintheoldstandardway,whichisto
simplyprovideadeclarationinthedhcpd.conffileforeachBOOTP
client,permanentlyassigninganaddresstoeachclient.
Wheneverchangesaremadetothedhcpd.conffile,dhcpdmustbe
restarted.Torestartdhcpd,sendaSIGTERM(signal15)tothe
processIDcontainedin/var/run/dhcpd.pid,andthenre-invokedhcpd.
BecausetheDHCPserverdatabaseisnotaslightweightasaBOOTPdata-
base,dhcpddoesnotautomaticallyrestartitselfwhenitseesachange
tothedhcpd.conffile.
Note:
Wegetalotofcomplaintsaboutthis.Werealizethatitwould
beniceifonecouldsendaSIGHUPtotheserverandhaveitreloadthe
database.Thisisnottechnicallyimpossible,butitwouldrequirea
greatdealofwork,ourresourcesareextremelylimited,andtheycan
bebetterspentelsewhere.Sopleasedon'tcomplainaboutthisonthe
mailinglistunlessyou'repreparedtofundaprojecttoimplementthis
feature,orpreparedtodoityourself.
COMMANDLINE
Thenamesofthenetworkinterfacesonwhichdhcpdshouldlistenfor
broadcastsmaybespecifiedonthecommandline.Thisshouldbedone
onsystemswheredhcpdisunabletoidentifynon-broadcastinterfaces,
butshouldnotberequiredonothersystems.Ifnointerfacenamesare
specifiedonthecommandlinedhcpdwillidentifyallnetworkinter-
faceswhichareup,eliminatingnon-broadcastinterfacesifpossible,
andlistenforDHCPbroadcastsoneachinterface.
Ifdhcpdshouldlistenonaportotherthanthestandard(port67),the
-pflagmayused.Itshouldbefollowedbytheudpportnumberon
whichdhcpdshouldlisten.Thisismostlyusefulfordebuggingpur-
poses.
Torundhcpdasaforegroundprocess,ratherthanallowingittorunas
adaemoninthebackground,the-fflagshouldbespecified.Thisis
usefulwhenrunningdhcpdunderadebugger,orwhenrunningitoutof
inittabonSystemVsystems.
Tohavedhcpdlogtothestandarderrordescriptor,specifythe-d
flag.Thiscanbeusefulfordebugging,andalsoatsiteswhereacom-
pletelogofalldhcpactivitymustbekeptbutsyslogdisnotreliable
orotherwisecannotbeused.Normally,dhcpdwilllogalloutput
usingthesyslog(3)functionwiththelogfacilitysettoLOG_DAEMON.
Dhcpdcanbemadetouseanalternateconfigurationfilewiththe-cf
flag,oranalternateleasefilewiththe-lfflag.Becauseofthe
importanceofusingthesameleasedatabaseatalltimeswhenrunning
dhcpdinproduction,theseoptionsshouldbeusedonlyfortesting
leasefilesordatabasefilesinanon-productionenvironment.
Whenstartingdhcpdupfromasystemstartupscript(e.g.,/etc/rc),it
maynotbedesirabletoprintouttheentirecopyrightmessageon
startup.Toavoidprintingthismessage,the-qflagmaybespeci-
fied.
TheDHCPserverreadstwofilesonstartup:
aconfigurationfile,anda
leasedatabase.Ifthe-tflagisspecified,theserverwillsimply
testtheconfigurationfileforcorrectsyntax,butwillnotattemptto
performanynetworkoperations.Thiscanbeusedtotesttheanew
configurationfileautomaticallybeforeinstallingit.
The-Tflagcanbeusedtotesttheleasedatabasefileinasimilar
way.
The-tfand-playoptionsallowyoutospecifyafileintowhichthe
entirestartupstateoftheserverandallthetransactionsitpro-
cessesareeitherloggedorplayedbackfrom.Thiscanbeusefulin
submittingbugreports-ifyouaregettingacoredumpeverysooften,
youcanstarttheserverwiththe-tfoptionandthen,whentheserver
dumpscore,thetracefilewillcontainallthetransactionsthatled
uptoitdumpingcore,sothattheproblemcanbeeasilydebuggedwith
-play.
The-playoptionmustbespecifiedwithanalternateleasefile,using
the-lfswitch,sothattheDHCPserverdoesn'twipeoutyourexisting
leasefilewithitstestdata.TheDHCPserverwillrefusetooperate
inplaybackmodeunlessyouspecifyanalternateleasefile.
CONFIGURATION
Thesyntaxofthedhcpd.conf(5)fileisdiscussedseparately.This
sectionshouldbeusedasanoverviewoftheconfigurationprocess,and
thedhcpd.conf(5)documentationshouldbeconsultedfordetailedrefer-
enceinformation.
Subnets
dhcpdneedstoknowthesubnetnumbersandnetmasksofallsubnetsfor
whichitwillbeprovidingservice.Inaddition,inordertodynami-
callyallocateaddresses,itmustbeassignedoneormorerangesof
addressesoneachsubnetwhichitcaninturnassigntoclienthostsas
theyboot.Thus,averysimpleconfigurationprovidingDHCPsupport
mightlooklikethis:
subnet239.252.197.0netmask255.255.255.0{
range239.252.197.10239.252.197.250;
}
Multipleaddressrangesmaybespecifiedlikethis:
subnet239.252.197.0netmask255.255.255.0{
range239.252.197.10239.252.197.107;
range239.252.197.113239.252.197.250;
}
IfasubnetwillonlybeprovidedwithBOOTPserviceandnodynamic
addressassignment,therangeclausecanbeleftoutentirely,butthe
subnetstatementmustappear.
LeaseLengths
DHCPleasescanbeassignedalmostanylengthfromzerosecondsto
infinity.Whatleaselengthmakessenseforanygivensubnet,orfor
anygiveninstallation,willvarydependingonthekindsofhostsbeing
served.
Forexample,inanofficeenvironmentwheresystemsareaddedfromtime
totimeandremovedfromtimetotime,butmoverelativelyinfre-
quently,itmightmakesensetoallowleasetimesofamonthofmore.
Inafinaltestenvironmentonamanufacturingfloor,itmaymakemore
sensetoassignamaximumleaselengthof30minutes-enoughtimeto
gothroughasimpletestprocedureonanetworkappliancebeforepack-
agingitupfordelivery.
Itispossibletospecifytwoleaselengths:
thedefaultlengththat
willbeassignedifaclientdoesn'taskforanyparticularlease
length,andamaximumleaselength.Thesearespecifiedasclausesto
thesubnetcommand:
subnet239.252.197.0netmask255.255.255.0{
range239.252.197.10239.252.197.107;
default-lease-time60
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