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In th is article, we w ill cover settingup a Linux machine to act as a mailhub for the local network, allowing

workstations to send and receive In-ternet mail without themselves beingdirectly connected to the outside world.

Befor e you try doing this , youshould already know how to installLinux and connect that Linux machineto the Internet. [ If y ou dont, well be

covering it in afutu reissueofPCNA - Ed.]

Sendmail

Although the sendmail utility comesready-built with almost any Linu x d is-tribution you care to name, there areadvantages to knowing how to com-pile it you rself. Firstly, a w idely-used,important p rogram such as sendmailinevitably attracts a lot of attention fromhackers. Updatesto the program incor-porating fixes for th e latest securityholes are regularly being made avail-able. These updates are available aspatches against the source code, wh ichyou need to add and then re-compile.(You can get sendm ail precomp iled, ofcourse, but its safer to compile yourown from source code that doesnt ap-pear to h ave been hacked.)

There is a sendmail Web page atww w.sendm ail.org,and many sitesm ir-ror the source code from there. Onceyou have the source, extract it into adirectory (/ usr/ src/ sendmailx.x.xx)and cd into the src directory therein. Inthere is a m akesendm ail shell script thatwill do all the work for you. It is easiest

to do the configuration in the sourcedirectory, before actually installing thebinary and configuration files.

Configuring

Tackling the sendmail configura-tion pr ocess from scratch is tough . Themain configura tion file, sendm ail.cf, isdesigned to be easy for the sendmailprogram to read. Unfortunately, thismeans that its somewhat cryptic tomere hu mans. Fortunately, it is a taskthat r ar ely has to be done f r om the

beginning. The sendmail distributionincludes several sample sendmail.cffiles, one or other of which can beadap ted to m ost configurations with afew changes. Any modern Linux dis-tribution will include these samplefiles in its send mail installation.

Recent versions of sendmail haveintroduced an important simplifica-tion into the configuration process, byshifting the user intervention from di-rect editing of the sendmail configura-tion file to making changes to files ofM4 macros, which is easier and moreimmediately und erstandable.

In this section, we w ill consider u seof the M4 macros. The section on con-figuring a sendmail client that con-nects to our mail hub will deal directlywith the / etc/ sendmail.cf file, as thatprocess is sufficiently simple to be eas-ily accomplished withou t the aid of theM4 macros.

Generating A Config

The sendmail configuration appa-ratu s is in the / cf subd irectory. Withinthis, the / m4 directory contains sup-por t r outines, which s hould not bechanged.

The / cf directory contains the con-

figuration files themselves. They have.mc suffixes, and m ust be ru n th roughM4 to become complete. The resultingoutput should have a .cf suffix.

The / ostype director y containsdefinitions describing a particular op-erating system type. These should al-wa ys be referenced u sing the OSTYPEmacro in the .mc file.

The / domain directory contains

definitions describing a particular do-main, referenced u sing the DOMAINmacro in the .mc file. The / mailer di-rectory holds descriptions of mailers,referenced using the MAILER macroin th e .mc file.

In the / sh directory are the shellfiles used when building the .cf filefrom th e .mc file in the cf subd irectory,while / feature holds special featuresthat you might want to include. Theyshould be referenced using the FEA-TURE m acro.

In / hack youll find local hacksfrom Berkeley (the hom e of sendm ail),of no more than voyeuristic interest, ifthat. And in / siteconfig is the site con-figuration, ie, tables of locally-con-nected UU CP sites.

Typi cal File

The root of the configuration proc-ess is wh at might becalled the base .mcfile. This forms the starting-point forM 4 directives which invoke othermacro files.All of the definitions in ourbase .mcfilein turn reference other .mcfiles. Order is important: follow that

given here.In w hat follows, I shall simply clar-ify w hat I have done h ere. For the full

Issu e 89 (N ovem ber 1997) Page 15 File: T1402.1

A mail hub is a machine that sits on y our LA N , acting as the interface between y our u sers and t he

Internet. It processes all mail, ensuring that messages sent from users get transmitted and that

incoming items are distributed to the correct user. Linux , the free version of Unix, allows y ou to

set up a Unix-based mail hub on an old Intel box for m inimum outlay and maximum flexibility.

By Paul Dunne

How To Set Up A

Linux Mail Hub

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information, see the README in thecf/ su bdirectory.

VERSION ID(@(#)you rcom pa ny .mc8.5 (Berks) 10/ 8/ 97)OSTYPE(mklinux) FEATURE(nou ucp )MAILER(local) MA ILER(sm tp )

The first line is for housekeeping,and puts the version line into the out-put file,soyou can keep track ofchanges.

OSTYPE

The first macro defines our operat-ing system. You must define an oper-ating s ys tem environment, or theconfigura tion file build will fail. For us,the OS is of course Linux; we use thefile in ostyp es called mklinu x.mc. Thiscontains details such as default file lo-cations and other OS-specific material.It should n ot need to be changed.

FEATURE

The only feature we u se is nouucp,which says dont do anything special

with UUCP addresses at all.Theres another feature, nullclient.

This could be used to d o wha t we w illdo later on by hand, ie, gener ate astripp ed-down configuration file thatdoes nothing but forward all mail to acentral hub via a local SMTP-basednetwork. The argument is the nam e ofthat hub.

MAILER

The MAILER macros use macro

files to specify rules to handle one ormore mailers. Here, w e invoke defini-tions for a local mailer and an SMTPmailer. There are fewer mailers sup-ported in this version than the pre-v io u s v er sio n, o w in g m o st ly t o asimpler world. As a general rule, putthe MAILER definitions last in your.mc file, and always put MAILER-(smtp) before MAILER(uucp ) - severalfeatures an d definitions will modifythe definition ofm ailers,and th e SMTPmailer modifies the UUCP mailer.

The local parameter specifies the

local and prog mailers. You w ill almostalways need these; the only exceptionis if you relay all your mail to anothersite. This m ailer is included automat-ically.

SMTP is the Simple M ail TransportProtocol m ailer. This does not hidehosts behind a gateway or anotherother such hack; it assumes a worldwhere everyone is running the nameserver. This file actually defines fourmailers: smtp for regular (old-style)

SMTP to other servers, esmtp for ex-tended SMTP to other servers,smtp8"to do SMTP to other servers withoutconverting 8-bit data to MIME (essen-tially, this is your statement that youknow the other end is 8-bit clean evenif it doesnt say so), and relay fortransmission to our RELAY_HOST,LUSER_RELAY or MAILER_H UB.

sendmail.cf

The final sendmail configurationfile is produced by invoking M4 withthe .mc file given above as its argu-

File: T1402.2 Issu e 89 (N ovem ber 1997) Page 16

### Defined Macros (1)

# The name of the mail hub

DRwotan.dunne.com

# The hub as it is known to the outside world

DHtiny1.demon.co.uk

# The local official domain name

Dj$w

# Our domain name

DDdunne.com

# Identity of the error message sender

DnMailer-Daemon

# Look of the Unix From line

DlFrom $g $d

# The characters that separate address components

Do.:%@!^=/[]

# Default form for the senders address

Dq

### Defined Classes (2)

# All possible names for local machine

Cw localhost donner

### Options (3)

# default delivery mode (in background)

Odbackground

# temporary file permissions-0600 for secure mail

OF0600

# default UID & GID

Ou1

Og1

# level at which to syslog errors

OL9

# Wait for SMTP replies.

Or1h

# default messages to old style

OoTrue

# Replace unquoted spaces with a dot

OB.

### Header Declarations (4)

HFrom: $q

HReceived: by $j id $i; $b

H?x?Full-Name: $?x$x$.

H?D?Date: $a

H?M?Message-Id:

### Priorities (5)

Pspecial-delivery=100

Pfirst-class=0

Plist=-30

Pbulk=-60

Pjunk=-100

### Mailer Delivery Agent Definitions (6)

# Mailer to forward all mail to the hub machine

Mhub, P=[IPC], S=10, R=0, F=xmDFMuCX, A=IPC $h

# Sendmail requires these, but we wont use them

Mlocal, P=/bin/mail, S=0, R=0, F=lsDFMShP, A=deliver $u

Mprog, P=/bin/sh, S=0, R=0, F=lsDFMeu, A=sh -c $u

### The Rules Sets (7)

S0 select delivery agent

R@$+ $#error $: Missing user name

R$+ $#hub $@$R $:$1 forward to hub

S3 preprocessing for all rule sets

R$*$* $n handle error addresses

R$*$* $2$4 de-nest brackets

R$*$* $2 basic RFC822 parsing

S10 rewrite the sender for the hub

R$- $@$1@$H user -> user@hub

R$-@$w $@$1@$H user@local -> user@hub

R$-@$=w $@$1@$H user@othernames -> user@hub

R$-@$=w.$D $@$1@$H user@domain -> user@hub

S1 dummy ruleset 1 (unused)

Figure 1 - A sample sendmail.cf file.

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ment. The command looks like this:

m4 m4/ cf.m4 config.mc > config.cf

wher e config.mc is the macr o fileweve developed above, and config.cfthe output - the sendmail configura-t io n file - w h ich w ill e nd u p a s/ etc/ send mail.cf.

A Sendmail Inst allat ion

Having completed the configura-tion, log in as r oot and type makeinstall to install the new sendmail -having remembered to back up yourold program in case anything shouldbe out of order. The following are thefiles installed by send ma il.

/usr/sbin/sendmail

This is the actual s endmail pr o-gram. There may be a symbolic link insendmails historic location, / usr/ lib,pointing here, but / usr/ sbin/ is nowthe actual location.

/etc/sendmail.cfThis is the configur ation file that w e

generated.

/usr/bin/newaliases

Th is is a sym bolic link to / usr/ sbin/ sendmail. When invokedby this name,sendmailwillrebuild thealiases database.

/var/spool/mqueue

This is the post office, where in-go-ing and outgoing mail is kept awaitingdelivery. It should have mode 700, toprev ent inquisitive users from peeking

at other usersm ail.

/etc/aliases

This is the systemw ide aliases file.

/usr/lib/sendmail.hf

This is the help file for send mail.

/etc/sendmail.st

This optional file can be u sed bysendm ail to record statistics.

/usr/bin/mailq

Th is is a sym bolic link to / usr/ sbin/ sendmail. When invoked

under this name, sendmail prints thecontents of the m ail queue.

Starting Sendmail

You will most likely want to havethe sendmail daemon started everytime the machine boots up. This isdone by add ing a line to the appropri-ate rc file in / etc/ rc.d. The exact con-figuration of / etc/ rc.d varies betweenLinux distributions (I suppose if youare conservative, you may just have asimp le BSD-style set-up, with only / etc/ rc an d / etc/ rc.local), so I can t

give exact instructions that are guar an-teed to be a pp licable. The easiest thingis probably to pu t it in / etc/ rc.d/ rc.lo-cal. The line shou ld look like this:

if [ -x /usr/sbin/sendmail ]

echo sendmail

/usr/sbin/sendmail -bd -q1h

fi

This checks to see if the file is there,then tells the system console what itsd o in g , a n d st ar ts u p se nd m a il indaemon mode (-bd),and sets it to proc-ess the mail queue every hour (-q1h).

The Mai l Q ueue

Them ail queue livesin / var/ spool- / mqueue (unless you have changedthe default, which you really shouldnot). All mail messages are held as twofile s h er e, o n e file b ein g n a m eddfXXXnnnnn, the other qfXXXnnnnn,where XXX is a three-letter sequence,nnnnn a five-number sequence, bothbeing used simply to give every mes-sage a unique identifier. The qf file isthe queue control file, containing theemail mess age header and variousprocessing information; the df file isthe data file, and contains the body ofthe email message. There are otherfiles, but they are transient and usu allyof interest only to sendmail.

Logging

Sendmail uses the syslog(8) facility.Usually, this is set up to log all send-mail messages to / var/ log/ maillog,which by default will record all mailthat passes through sendm ail.

Security

Sendm ailh as a reputation as a secu-rity nightmare, but this is largely un-

deserved, p articularly w ith version 8,wh ich solved a lot of the problems thatprevious versions did have. Much ofsendm ails security is dow n to th e sys-tem adminis tr ator. Some s pecificpoints to watch for are:

q Make sure the aliases file isnt wri-table except by trusted system per-sonnel. This includ es both the textand database version.

q Make sure tha t other files that send-

mail reads, such as the mailertable,are only writable by trusted systempersonnel.

q The queue directory should not beworld writable.

Clients

Providing email service to othermachines in the network can be donein two ways. The first is to use SMTPto act as a mail hub that s ends andreceives Internet (and optionally local)mail on behalf of the other machines.Secondly, a POP service can be set up,

where local users use client softwareon their comp uters to collect their mailvia the POP3 protocol, and send mailvia SMTP to the server.

/etc/aliases

Users on the local network m ust beidentifiable by the sendmail processrunn ing on the server m achine. In thecase of POP mailboxes, this is done bycreating a n ormal user account. In thecase of Linux clients collecting mailthrough sendmail themselves, this isdone by adding the appropriate alias

to / etc/ aliases. For example, on mylocal network, any mail arriving at mymail server (tiny1.demon.co.uk) forbob is s ent on to bob@donner .-du nne.com on m y Internet network bythe following line in / etc/ aliases:

bob: bob@don ner.du nne.com

The simp lest way to make sure thatmail comes back to the r ight place is toset the Reply-To head er in allou tgoingmail to point to the account on th e mailhub, not the originating machine. Thiscan be done in the options settings of

your POP3 mailer, or w ill be han dledfor you by sendmail on a Linux client.

Issu e 89 (N ovem ber 1997) Page 17 File: T1402.3PC N etwork Advisor

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The Sendmail.cf File

Now, lets examine the sendmail.cffile itself - its listed in Figure 1.This filecan be kept quite simple when its onlytask is to relay all mail to another m a-chine for further processing. Here is aminimal sendmail.cf file for installingon Linux boxes that talk to the mailhub. There isnt the space for a fullrun -dow n on th e syntax of the file. Forthe full gen, consult the irreplaceable

book Sendmail, published by OReilly& Associates.Sendmail commands are usually

one letter in length, and must be at thebeginning of a line. Generally, there isno space between a command and itsarguments.

The first part, Macros, shows vari-ables (macros in sendmail p arlance)being defined by use of the D, DefineMacro, command. All the macros de-fined here are explained by commentson the line before them - a wise practisewhich should not be confined to exam-ple files.

The second section, Classes, is for aspecial type ofvariable,a class,that canhold multiple values. The commandhere is C. The class we are defining is

W, which holds a list ofa lternative hostnames for the machine (that is, otherthan the FQDN).

The third section sp ecifies sendm ailoptions. These can be given on thecommand line, but as there are rathera lot, it makes more sense to have themin the file.

The four th par t s pecifies whatheaders must be in every mail mes-sage. These are the h eaders that send-m a il w ill a d d if t h e M UA h a s n o t

already done so.The fifth section is a set of prioritysettings.Sendm ailw ill by default proc-ess the mail in its queue in order ofdecreasing priority, beginning withspecial-delivery. The level of prior-ity is set by the MUA with the Prece-dence: header.

The sixth section defines a set ofmailers that sendmail will use to actu-ally deliver mail. Remember, sendmailis a Mail Transport Agent - it doesntdo the delivery itself. These lines allbegin with M. Localan d prog are man-datory. The real work here is done by

the special mailer [IPC], which invokesinternal sendm ail routines rather thanan external mailer program, to send allmail to a smart host using SMTP.

The s eventh par t is the hear t of sendmail, the rule sets. These definethe re-writing of ad dresses. The basicidea here is that there are tw o sides, aright-hand side and a left-hand side,where the RHS is a pattern to matchagainst input, and th e LHS is the trans-formation to effect upon the input if amatch is made.LHSand RHSare sepa-r ated by tabs ; comments ar e in thethird colum n.

WindowsUnder Wind ows I use Eudora Lite,

which is a freeware, cut-down versionof Eudora Pro. It is a fine email clientin its own right and is available fromwww.qualcomm.com. Setting it up issimply a matter of pointing it at themail hub, then telling it the POP usern a m e a n d p a ss w or d . O n e g lit ch Ifound was that not all options ar esaved to EUDORA.INI. Specifically, Ih ad to set U seWin Sock =1 a ndUseDialup=0 by editing the ini file, aschanging these options from the menu

had no effect.

File: T1402.4 Issu e 89 (N ovem ber 1997) Page 18

The AuthorPau l Dunn e (paul@tiny1.demon .-

co.uk) is a freelance w riter andUnix/ Internet consultant.

PCNA

Edit ing /etc/sendmai l.cf By Hand

So, you h ave an existing sendmail installation an d dont want to go to thebother of fussing about w ith getting the sendm ail source, figuring out M4,etc? Well, while I cant hop e to cover a ll the d etails of the sendm ail configu-ration file syntax here,I can tell you the minimum changes you need to maketo transform a generic sendmail.cf into one you can use.

Because of the complexity of this file, I will list here only th e thing s that areabsolutely essential to chan ge. I will presu me tha t there is already a su itable

sendmail.cf on the machine (provided either by the Linux d istribution, orfrom the sendmail sources).

The W macro contains any other n ames that this host is known by, besidesthe FQDN. For example:

Cwlocalhost wotan.dunne.com

TheS macrocan contain the name ofa smart relay host, to which allnon-localmail is forwarded without further ado. Some sites can deliver mail to thelocal network, but cannot look up hosts on the Internet w ith DNS. Usuallysuch sites are connected to the outside world w ith UUCP. To ensure deliveryof all mail, such sites need to forward all non-local mail over th e UUCP linkto a smart (or well-connected) host.

These are allth e chan ges you absolutely have to mak e. The other pa rameters

change how sendmail behaves, but should w ork on your site w ithoutmodification.

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