Apress the qmail handbook 2nd edition sep 2003 ISBN 1893115402

TheqmailHandbook
ISBN:1893115402
byDaveSill
Apress2002(492pages)
Thisguidebeginswitha
discussionofqmail’shistory,
architectureandfeatures,
andthengoesintoa
thoroughinvestigationofthe
installationandconfiguration
process.

TableofContents
TheqmailHandbook
Introduction
Chapter1 - Introducingqmail
Chapter2 - Installingqmail
Configuringqmail:The
Chapter3 Basics
Chapter4 - Usingqmail
Chapter5 - Managingqmail
Chapter6 - Troubleshootingqmail
Configuringqmail:
Chapter7 AdvancedOptions
Chapter8 - ControllingJunkMail
Chapter9 - ManagingMailingLists
Chapter10 - ServingMailboxes


Chapter11 - HostingVirtualDomain
andUsers

UnderstandingAdvanced
Chapter12 Topics
AppendixA - HowqmailWorks
AppendixB - RelatedPackages
AppendixC - HowInternetMailWorks
AppendixD - qmailFeatures
AppendixE - ErrorMessages
AppendixF - Gotchas
Index
ListofFigures
ListofTables
ListofListings


BackCover
Providesthoroughinstructionforinstalling,
configuring,andoptimizingqmail
Includescoverageofsecurenetworking,
troubleshootingissues,andmailinglist
administration
Coverswhatsystemadministratorswanttoknow
byconcentratingonqmailissuesrelevanttodaily
operation
Includesinstructionsonhowtofilterspambeforeit
reachestheclient
TheqmailHandbookwillguidesystemandmail
administratorsofallskilllevelsthroughinstalling,
configuring,andmaintainingtheqmailserver.Author
DaveSill,along-timeqmailuserandsystem
administrator,aswellastheauthorofthepopular

onlinetutorial“Lifewithqmail,”exposesreaderstoall
practicalaspectsofworkingwiththispopularmail
server.
Thisdefinitiveguidebeginswithadiscussionofqmail’s
history,architecture,andfeaturesandthengoesintoa
throughinvestigationoftheinstallationand
configurationprocess.Readerswilllearnhowtoinstall
qmailonseveraloperatingsystemsandgainvaluable
insightintoproperconfiguration,testingprocedures,
andperformancetuning,allofwhichareintegraltoa
properlyfunctioningproductionenvironmentmail
server.Readerswillalsolearnhowtoadministerusers
andmail,installfilters,andoverseedailyqmail


operationandmaintenance.Throughout,Sillfocuses
ontopicsessentialtoallmailadministrators,
elaboratinguponsuchsubjectsasconfiguringmailing
listmanagers,controllingspam,securenetworking,
scanningforviruses,hostingvirtualdomainsand
users,andcreatingdial-upclients.
TheqmailHandbookistheultimateresourcefor
administratorsanddevelopersneedingtomasterthe
functionalityofthepowerfulqmailsoftware.
AbouttheAuthor
DaveSillisaprofessionalsystemadministratorand
technicalsupportengineerwithmorethan15years
experience.He’sbeenusingqmailservicesinceitsfirst
publicreleasein1996andistheauthorofthepopular
onlineqmailguide“Lifewithqmail.”He’salsoanactive

contributortoonlineqmailsupportgroups,including
theqmailmailinglistandUsenetnewsgroup.


TheqmailHandbook
DAVESILL

Copyright©2002byDaveSill
Allrightsreserved.Nopartofthisworkmaybereproducedortransmitted
inanyformorbyanymeans,electronicormechanical,including
photocopying,recording,orbyanyinformationstorageorretrieval
system,withoutthepriorwrittenpermissionofthecopyrightownerand
thepublisher.
ISBN(pbk):1-893115-40-2
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EditorialDirectors:DanAppleman,GaryCornell,JasonGilmore,Karen
Watterson
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Thesourcecodeforthisbookisavailabletoreadersat
intheDownloadssection.Youwillneedto
answerquestionspertainingtothisbookinordertosuccessfully
downloadthecode.
Formymother
Acknowledgments
Thankstodanbernsteinforgivingusqmailandmanyotherpackages
includingthedaemontoolsanducspi-tcpsupportutilities.Thanksalsoto
themanypeoplewhohelpedmakemyonlineguide"Lifewithqmail"what
itistodayandtothemembersoftheqmailmailinglistwhohavehelped
melearnagreatdealaboutqmailovertheyears.
ThanksalsotothefinefolksatApress:JasonGilmoreandGaryCornell,

fornotonlytakingachanceonafirst-timeauthorbutactivelyrecruiting


him;GraceWong,formanagingtheproject;KimWimpsett,forturningmy
crudewritingsintoclearandconsistenttext;ToryMcLearnandSofia
Marchant,forlayingoutthebook;StephanieRodriguez,forher
marketingefforts;andthemanyothersbehindthesceneswhoIdidn't
dealwithdirectly.WorkingwithApresswasajoy:Theyweresupportive
andcommittedtoproducingahigh-qualitybook.
ThankstoCharlesCazabon,thetechnicalreviewer.Hissuggestions
werevaluableanddramaticallyimprovedthequalityofthefinished
product.Thiswillcomeasnosurprisetoanyonewhohasseenhis
contributionstotheqmaillist.
Finally,specialthankstomyfamilyandfriendswhoencouraged,
supported,andtoleratedmethroughouttheproject.Mywife,MaryJane,
convincedmetowritethisbookeventhoughsheknewitwouldbepainful
forthefamilyattimes.MychildrenAndy,Rachel,andErica
enthusiasticallysupportedmeandhelpedoutinmanyways.Andytested
theinstallationinstructionsinChapter2onfourLinuxdistributionsand
threeBSDdistributions.Myfathertookovermostofmychoresaround
thehouseandfarmforsixmonthsinadditiontohisusualcookingand
house/dog/kid-sittingduties.Mymotherhassupportedmethroughoutmy
life.Herstrengthisinspiring.Manyotherfriendsandfamilymembers
supportedthiseffort.Someareacknowledgedthroughoutthebookinthe
namesusedinexamples,butI'msureIleftsomeout.
—DaveSill,September2001
AbouttheAuthor
Davesillisaprofessionalsystemadministratorandtechnicalsupport
engineerwithmorethan15yearsofexperience.He'sbeenusingqmail
sinceitsfirstpublicreleasein1996andistheauthorofthepopular

onlineqmailguide,"Lifewithqmail."He'salsoanactivecontributorto
onlineqmailsupportgroupsincludingtheqmailmailinglistandUsenet
newsgroup.Heliveswithhiswife,children,andanassortmentofdogs,
cats,cows,chickens,andturkeysona31-acrefarmineastTennessee.
Whenhehassparetime,hebrewshisownbeerandtrainsinIsshinryu
karate.


AbouttheTechnicalReviewer
CharlesCazabonisasoftwaresystemsdeveloperwith15yearsof
experienceincomputingandinformationtechnology.Hehasbeenusing
andconfiguringqmailsince1998andistheauthorofseveralfree
softwareprograms,includinggetmail,queue-repair,andmemtester.Heis
alsoanactiveparticipantintheqmailmailinglist.HelivesinSaskatoon,
Canada,withhissignificantother,twosalamanders,sixhamsters,and
twomice.


Introduction
Thisbookdocumentshowtoinstall,configure,anduseqmail.Itwillbe
mostbeneficialtosystem,network,andmailadministrators,butitwill
alsobehelpfultouserswhowanttoreadandsende-mailmore
effectively.


WhatCanYouExpecttoLearnfromThisBook?
Youcanexpecttolearnthefollowing:
Whatqmailis,whatitcando,andwhatitcan'tdo
Howtoinstallandconfigureabasicqmailserver,including
varioussupportutilities

Howtouseqmailasaregularuser:controllingthedispositionof
incomingmessages,formattingoutgoingmessages,andworking
withmailboxesinmultipleformats
Howtomanageaqmailserver:settingupaliases,users,virtual
domains,andmailinglists;troubleshooting;performancetuning;
andcontrollingjunkmailandotherabuse
Howqmailworks:notjustwhatitdoes,buthowitdoesit


Organization
Chapter1,"Introducingqmail,"describesqmailanditsfeatures.Readitif
you'renotsureexactlywhatqmailisorwhatitcandoforyou.Italso
describestheoverallorganizationoftheqmailsuite,comparesqmailto
otherUnixmailers,andlistsothersourcesofqmailinformationand
support.
Chapter2,"Installingqmail,"describesstep-by-steptheinstallationof
qmailonawiderangeofoperatingsystemdistributions,including
commercialUnixvariants,Linux,andvariousBerkeleySoftware
Distributions(BSDs).
Chapter3,"Configuringqmail:TheBasics,"showshowtoconfigure
qmailforavarietyofbasicfunctions.
Chapter4,"Usingqmail,"covershowusersreadandsendmessages.
Chapter5,"Managingqmail,"coverstheqmailctlscript,queue
management,andadministrativecommands.
Chapter6,"Troubleshootingqmail,"showshowtomonitortheqmail
processes,understandthelogfiles,analyzemessageheaders,conduct
tests,anddiagnosecommonproblems.
Chapter7,"Configuringqmail:AdvancedOptions,"showshowto
configureqmailforavarietyoftypicalconfigurations,migrateSendmail
systemstoqmail,andusesource-codemodifications.Italsoshowshow

tousetheQMTPandQMQPprotocols,enablesecurenetworking,and
improvetheperformanceofyourqmailsystem.
Chapter8,"ControllingJunkMail,"coversmethodsfordealingwith
unwantedmailatboththesystemanduserlevels.
Chapter9,"ManagingMailingLists,"detailsinstallingandusingthree
popularmailinglistmanagerswithqmail:ezmlm,Majordomo,and
Mailman.


Chapter10,"ServingMailboxes,"showshowtoprovideremoteaccessto
users'mailboxesviathePOP3andIMAPprotocols.
Chapter11,"HostingVirtualDomainsandUsers,"coverstwopopular
qmailadd-onsformanagingvirtualdomainsandvirtualusers:VmailMgr
andVpopmail.
Chapter12,"UnderstandingAdvancedTopics,"explainsfromaqmail
perspectivesomeadvancedtopicssuchasscalableserver"farms,"
accessinguserinformationviaLDAPorSQL,andtheVariableEnvelope
ReturnPath(VERP)mechanismthatqmailusesforreliableautomatic
bouncehandling.
Theappendicescover:
Howqmailworks
Relatedpackages
HowInternetmailworks
qmail'sfeatures
Errormessages
Gotchas


Audience
Thisbookisaimedatanyoneinterestedinrunningqmail,fromtherank

amateur(newbie)whojustinstalledLinuxonasparecomputerallthe
wayuptotheexperiencedsystemadministratorormailadministrator.
However,installing,configuring,andmaintainingamailerisacomplex
task.Ifyou'renotanexperiencedsystemadministrator,youprobably
shouldn'tattempttoswitchanexistingmailsystemwiththousandsof
userstoqmailuntilyou'recomfortablewithusingandmanagingUnix
systems.
Ifyou'reacompleteUnix/Linuxnewbie,youshouldstartwithagood
introductiontoUnixforuserssuchasTheUnixOperatingSystemby
KaareChristian.Whileyou'rereadingthatbook,experimentonyourown
system.Untilyouactuallydothetasksyou'vereadabout,youwon't
reallyunderstandwhatyou'redoingandyou'llprobablyforgetmostofit
beforeyoureallyneedit.
Ifyou'reanexperiencedUnix/Linuxuser,butyou'renotfamiliarwith
systemadministration,manygoodbooksareavailable.Thebestis
probablyUnixSystemAdministrationHandbookbyNemeth,etal.,which
coversmostofthecommonUnixvariants,includingSolaris,HP-UX,Red
HatLinux,andFreeBSD.Ifpossible,selectonespecifictothevariantof
UnixorLinuxthatyou'llbeusing.AlthoughallflavorsofUnixlookpretty
similartousers,theydiffersubstantiallyinthedetailsofsystem
administration.


Conventions
Thisbookusescertaintypographicalconventionstohelpconvey
informationclearlyandconcisely.
Doublequotes("")areusedtoindicateanunusualmeaningfora
commonword,suchas"bounce."
Italicsareusedtointroducenewterms,likeinjection,orsimplyfor
emphasis.Italicsarealsousedtoindicatevariables,like/user/forauser

nameor/concurrencylocal/foraconfigurationsetting.
Textthatappearsinafixed-widthtypeface,suchasqmail-sendor
<>,representsafilename,commandname,
username,e-mailaddress,domainname,codesample,orUniform
ResourceLocator(URL).
Adirectivetorunasinglecommandthatshouldnotproduceanyoutput
lookslike:
touch.qmail
Ifacommandmustbeperformedbythesuperuser(UID0),thehash(#)
shellpromptisused:
#touch/var/qmail/alias/.qmail-root
Ifacommandshouldbeperformedbyanon-privilegeduser,thedollar
sign($)shellpromptisused:
$touch.qmail
Ifanexamplemixesuserinputandcommandoutput,userinputisprinted
inbold:
$date
SatMay507:06:49EDT2001
$


Note Examplesthatincludeoutputendwithalineconsistingsolely
oftheshellprompt($)toshowthattheoutputincludedis
complete.


WebSite
Forthelatestinformationonerrataortodownloadthescriptsusedin
Chapter2,visitthebook'sWebsiteat

Chapter1:Introducingqmail


Highlights
Andywantstosendane-mailmessagetohisfriendJosh.Heopenshis
mailclient,clicksonNewMail,entersJosh'saddressintheTofield,fills
intheSubjectfieldwithashortdescriptionofthemessage,andtypesthe
messageintothelargeeditingareaoftheform.Whenhe'sdone,he
clicksontheSendbutton.Asfarashe'sconcerned,themessageissent,
butbehindthescenes,complicatedmachinerywhirstolife.Athousand
tinystepswillbeexecutedonAndy'sbehalfbyprocessesonvarious
systemsbetweenAndyandJosh—whocouldbeinthesameroomor
halfaworldaway.
TheInternetMessageTransferAgent(MTA)isthekeyplayerinthe
behind-the-scenese-mailinfrastructure—it'sthemachinerythatmovesemailfromthesender'ssystemtotherecipient'ssystem.
BeforetheInternetexplosionintheearly1990s,oneMTA,Sendmail,
wasresponsiblefordeliveringalmostallofthemail.ButSendmailwas
designedforanInternetunlikethemodernInternet.AtthetimeSendmail
wascreated,therewereonlyahandfulofsystemsontheentireInternet,
andmostofthepeopleonlinekneweachother.Itwasafriendly,
cooperativecommunitythatconsistedmostlyofthepeoplewhowrotethe
softwarethatmadetheInternetworkormanagedthehardwarethatit
connected.Securitywasnotamajorconcern:Therewasnotmuchthat
neededprotection,andtherewerefewpotential"badguys"fromwhichto
beprotected.
ThemodernInternetisverydifferent.It'smillionsoftimeslarger,so
knowingalltheotheradministratorsandusersisimpossible.Infact,it's
accessiblebyanyonewithaccesstoapubliclibrary.Billionsofdollarsin
businessandconsumercommercetakesplaceannuallyoverthe

Internet.Largecorporationsexistwhoseentirebusinessmodelrelieson
theirInternetpresence.Assuch,thestakesarehigh,andit'snolonger
possibletotreatsecuritycasually.Ontopofallthis,serversarebeing
subjectedtostaggeringloads—atypicalmailservertodaymightsend
moremessagesinonedaythanamailservertenyearsagosentinone


year.
TheSendmaildevelopershaveworkedhardovertheyearstoenhance
itssecurityandperformance,butthere'sonlysomuchthatcanbedone
withoutafundamentalredesign.In1995,DanielJ.Bernstein,thena
mathematicsgraduatestudentattheUniversityofCalifornia,Berkeley,
begandesigningandimplementinganMTAforthemodernInternet:
qmail.
WhileSendmailisonehuge,complexprogramthatperformsitsvarious
functionsasthesuperuser(theall-powerfulUnixrootaccount),qmailis
asuiteofsmall,focusedprogramsthatrununderdifferentaccountsand
don'ttrusteachother'sinputtobecorrect.
WhileSendmailplodsthroughalistofrecipientsdeliveringonemessage
atatime,qmailspawnstwentyormoredeliveriesatatime.Andbecause
qmail'sprocessesaremuchsmallerthanSendmail's,itcandomorework
faster,withfewersystemresources.Further,Sendmailcanlose
messagesinsomeofitsdeliverymodesifthesystemcrashesatthe
wrongtime.Forreliability,speed,andsimplicity,qmailhasonecrashproofdeliverymode.


Overview
ThischapterintroducestheconceptoftheMTAanddiscussesone
particularMTA,qmail:
First,we'llexaminetheroleoftheMTAintheInternete-mail

infrastructure.
Next,we'lllookatqmail—whatitdoesandwhyyoumightwantto
useit.
qmail'smaindesigngoalsweresecurity,reliability,performance,
andsimplicity.We'llseehowqmail'screatorwasabletoachieve
thesegoals.
We'llalsocompareqmailtootherpopularUnixMTAssuchas
Sendmail,Postfix,Courier,andExim.
Next,we'lllookatqmail'sfeatures,history,architecture,and
distributionlicense.
Finally,we'lllistvarioussourcesofinformationonqmailsuchas
documentation,Websites,andmailing-listarchives.We'llalso
coverqmailsupportchannels:mailinglistsandhiredconsultants.


WhatIsqmail?
qmailisanInternetMTAforUnixandUnix-likeoperatingsystems.An
MTA'sfunctionistwofold:toacceptnewmessagesfromusersand
deliverthemtotherecipient'ssystems,andtoacceptmessagesfrom
othersystems,usuallyintendedforlocalusers.
Usersdon'tusuallyinteractdirectlywithMTAs;theyuseMailUserAgents
(MUAs)—thefamiliarmailprogramssuchasOutlookExpress,Eudora,
Pine,orMuttthatusersrunontheirdesktopsystems.Figure1-1shows
howalloftheseagentsinteractwitheachother.


Figure1-1:Howthesender,recipient,MUA,andMTA
interact
qmailisadrop-inreplacementfortheSendmailsystemprovidedwith
mostUnixoperatingsystems.Whatthatmeansisthattheuserofa

systemwillnotnecessarilynoticeaswitchfromSendmail,orsomeother
MTA,toqmail.Thisdoesnotmeanthatthesystemadministratorwon't
seeadifference.AlthoughallMTAsperformthesamefunctions,they
differwidelyininstallation,configuration,andfunctionality.Don'tassume
thatyourabilitytomanageSendmailwillletyougetuptospeedquickly
withqmail:Itwon't.Infact,detailedknowledgeofanotherMTAmight
evenslowyoudownbecauseyou'llbeunlearningthatsysteminaddition
tolearningqmail.



WhyUseqmail?
YouroperatingsystemincludedanMTA,probablySendmail,soifyou're
readingthisbookyou'reprobablylookingforsomethingbetter.Someof
theadvantagesofqmailoverbundledMTAsincludesecurity,
performance,reliability,andsimplicity.

Security
qmailwasdesignedwithhighsecurityasagoal.Sendmailhasalong
historyofserioussecurityproblems.WhenSendmailwaswritten,the
Internetwasamuchfriendlierplace.Everyonekneweveryoneelse,and
therewaslittleneedtodesignandcodeforhighsecurity.Today'sInternet
isamuchmorehostileenvironmentfornetworkservers.
qmailcreatorBernsteinissoconfidentthatqmailissecurethathe
guaranteesit.Inhisguarantee
(heevenoffers$500
tothefirstpersonwhocanfindasecuritybuginqmail.Hefirstmadethis
offerinMarchof1997,andthemoneyremainsunclaimed.
qmail'ssecuredesignstemsfromsevenrules,discussedinthefollowing
sections.

ProgramsandFilesAreNotAddresses,SoDon'tTreat
ThemasAddresses
Sendmailblurredthedistinctionbetweenaddresses(usersoraliases)
andthedispositionofmessagessenttothoseaddresses—usually
mailboxfilesormail-processingprograms.Ofcourse,Sendmailtriesto
limitwhichfilesandprogramscanbewrittento,butseveralserious
securityvulnerabilitieshaveresultedfromfailuresinthismechanism.
Onesimpleexploitconsistedofsendingamessagetoanonexistentuser
onaSendmailsystemwithareturnaddressof:
"|/bin/mail

ThiswouldcauseSendmailtogenerateabouncemessageandattempt
tosendittothereturnaddress.Inthiscase,thereturnaddresswasa
commandthatmailedacopyofthevictim'spasswordfiletotheattacker.
Inqmail,addressesareclearlydistinguishedfromprogramsandfiles.It's
notpossibletospecifyacommandorfilenamewhereqmailexpectsan
addressandhaveqmaildelivertoit.

DoasLittleasPossibleinsetuidPrograms
TheUnixsetuid()mechanismiscleveranduseful.Itallowsaprogram
runbyoneusertotemporarilyassumetheidentityofanotheruser.It's
usuallyusedtoallowregularuserstogainhigherprivilegestoexecute
specifictasks.
Tip Checkoutthemanpagesformoreinformationaboutsetuid().
Thecommandmansetuidshoulddisplaythesetuid()
documentation.
That'sthegoodnewsaboutsetuid().Thebadnewsisthatit'shardto
writesecureandportablesetuid()programs.Whatmakesithardto

securesetuid()programsisthattheyrunanenvironmentspecifiedby
theuser.Theusercontrolsthesettingsofenvironmentvariables,
resourcelimits,command-linearguments,signals,filedescriptors,and
more.Infact,thelistisopen-endedbecausenewoperatingsystem
releasescanaddcontrolsthatdidn'texistbefore.Andit'sdifficultfor
programmerstodefendagainstfeaturesthatdon'tyetexist.
Inqmail,there'sonlyonemodulethatusessetuid():qmail-queue.
Itsfunctionistoacceptanewmailmessageandplaceitintothequeue
ofunsentmessages.Todothis,itassumestheidentityofthespecial
userID(UID)thatownsthequeue.

DoasLittleasPossibleasRoot
Thesuperuser,anyuseraccountwiththeUID0(zero),hasessentially
unlimitedaccesstothesystemonmostUnixoperatingsystems.By
limitingtheusageoftherootUIDtothesmallsetoftasksthatcanonly


bedoneasroot,qmailminimizesthepotentialforabuse.
Twoqmailmodulesrunasroot:qmail-startandqmail-lspawn.
qmail-startneedsrootaccesstostartqmail-lspawnasroot,and
qmail-lspawnneedstorunasrootsoitcanstartqmail-local
processesundertheUIDoflocalusersacceptingdeliveryofmessages.
(The"Architecture"sectionofthischaptercoverstheseinmoredetail.)

MoveSeparateFunctionsintoMutuallyUntrusting
Programs
MTAsperformarangeofrelativelyindependenttasks.SomeMTAssuch
asSendmailaremonolithic,meaningtheyconsistofasingleprogram
thatcontainsallthecodetoimplementallofthesetasks.Asecurity
problemsuchasabufferoverflowinoneofthesefunctionscanallowan

attackertotakecontroloftheentireprogram.
qmailusesseparateprogramsthatrununderasetofqmail-specific
UIDs,compartmentalizingtheiraccess.Theseprogramsaredesignedto
mistrustinputfromeachother.Inotherwords,theydon'tblindlydowhat
they'retold:Theyvalidatetheirinputsbeforeoperatingonthem.
Compromisingasinglecomponentofqmaildoesn'tgranttheintruder
controlovertheentiresystem.

Don'tParse
Parsingistheconversionofhuman-readablespecificationsintomachinereadableform.It'sacomplex,error-proneprocess,andattackerscan
sometimesexploitbugsinparsingcodetogainunauthorizedaccessor
control.
qmail'smodulescommunicatewitheachotherusingsimpledata
structuresthatdon'trequireparsing.Modulesthatdoparseareisolated
andrunwithuser-levelprivileges.

KeepItSimple,Stupid