Configuring Sendmail - Workshop Wisdom

Configuring Sendmail
By Anurag Phadke

Here are some hands-on steps for configuring sendmail, the most popular Unix-based implementation of the Simple Mail Transfer Protocol for transmitting e-mail

E-mail has revolutionized mass communication by making it fast, reliable and affordable. Born in the days of ARPANET, even before the Internet was standardized, e-mail happened to be the only cause for people opting for an Internet connection. Starting from a few hundred mails per day in its youth at Berkeley University, more than 1 million e-mails make their trips round the globe every day. So just how simple or tough it is for servers to handle such a huge amount of load?

Here are some hands-on steps for configuring sendmail, the most popular Unix-based implementation of the Simple Mail Transfer Protocol for transmitting e-mail.

Down to basics
Sendmail developers have done just the right thing by doing away with things that caused trouble in the past such as "Relaying e-mails to each and every domain." Though the option still prevails, one needs to enable it in the conf files. That's one up against SPAM! Version 8.9 and above support MSA (Mail Submission Agent) which listens on connections made at port 587. This is a relatively new concept used by POP/IMAP clients like Eudora in place of SMTP.

Specific host can be enabled by /etc/mail/relay-domains file. Normally, entries in this file represent entire domains that are allowed to relay. However, if you want to list specific hosts in this file and not subdomains, you can use FEATURE (relay_hosts_only).

Installing and Configuring Sendmail
You can download the latest version of sendmail from ftp://ftp.sendmail.org/pub/sendmail

Here's the procedure for installing sendmail:

#cp sendmail.8.11.4.tar.gz / 'copies sendmail to / directory

#tar zxvf sendmail.8.11.4.tar.gz 'untars sendmail. By default, it gets untarred in the /sendmail-8.11.4 directory

#rm /sendmail.8.11.4.tar.gz 'removes sendmail from / directory

#cd /sendmail-8.11.4/ 'changes to /sendmail/ sendmail.x.x.x directory

#sh Build 'Compiles and Builds sendmail

Above commands will install sendmail along with its configuration files

in the default directories. Configuration and .mc files are installed in

/etc/mail/ directory.

Difference between .cf and .mc files is similar to the difference between English and Greek language. .cf is more of a cryptic language, can be easily parsed and cannot be understood by a normal person. On the other hand, .mc files are written in "English" though they are a bit slow at getting parsed out.

A first hand look at /etc/mail/sendmail.mc can be head spinning. So, we have tried to explain out some of the basic yet important features that must be included/edited in the sendmail.mc file.

dnl (delete through newline) omits the new line character, reducing the time require to parse through the file.

OSTYPE(`linux'): Defines the OS on which sendmail is running. Can be changed to other OS such as HPUX or Solaris.

MAILER(`SMTP'): Use SMTP (Send Mail Transfer Protocol) for sending emails over the Internet.

MAILER(`local') : Mails on the same server should be delivered locally.

define(`confTO_CONNECT', `1m'): Connect to the server at a 1 minute interval.

define(`ALIAS_FILE', `/etc/aliases'): Path where alias file resides.

define(`confMax_Message_size', '2000000'): Maximum size of the email that can be delivered.

define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun'): These are the privacy flags which issue authorization warnings, allow only root to run sendmail.

define(`confAUTH_OPTIONS', `A'): The ".mc" file allows

values to be assigned from one variable to another. In the above case, value of "A" is assigned to "confAUTH_OPTIONS". (confAUTH_OPTIONS=A)

FEATURE(use_CW_FILE): The list of domains for which mail should be delivered locally. This line is important if you are doing virtual hosting on your server.

FEATURE(`access_db'): Uses a macro file with the name "access_db". Explained below in detail.

Insert/Edit the lines in accordance with your requirement. The sendmail.mc file now needs to be converted to sendmail.cf in order to be understood by the machine. This can be achieved by the following command:

#m4 /etc/mail/sendmail.mc > /etc/sendmail.cf

Presence of sendmail-cf rpm is a prerequisite for using the m4 option.

We now need to focus our attention on some of the other files that exist in the /mail/etc directory.

access: It contains vital information about handling the received emails. A basic access file along with the response that it should return is mentioned below.

spammer@bahbah.com ERROR:"550 We hate spammers" 'shall return an email back to we the above mentioned error.

spam@bah.com Reject 'Rejects email from spam@bah.com

blah.com OK 'Accepts email from blah.com

[127.0.0.1] RELAY 'Relays messages originating from 127.0.0.1 (local machine)

Save the file. You shall now need to update sendmail with the newly created access file using the following command:

#makemap hash /etc/mail/access < /etc/mail/access

virtusertable: While doing virtual hosting, many hosts offer the option of forwarding e-mails to a different address or provide unlimited e-mail id's such as any@bah.com, info@bah.com and forward them to a single e-mail address. The file virtusertable is used for this purpose. After editing this file, you once again need to run the "makemap" command as shown for the "access" file.

info@bah.com foo-info 'Forwards emails to the username foo-info on the server.

notexist@bahbah.com error:nouser No such user here 'Sends an email to the sender with the subject No such user here

@blahblah.org me@bah.org 'Forwards emails from the domain blahblah.org to me@bah.org

Make sure that you have configured your machine using proper IP address and DNS entries. If not, run "linuxconf" from the shell and make appropriate changes in the Basic Host Configuration menu. You also need to edit the /etc/hosts file in accordance to the entry made in the Basic Host Configuration.

Sample: /etc/hosts file

#IP address localhost localhost.localdomain

127.0.0.1 bahbah bahbah.com

Now that you are through with the setup of "sendmail", you need to know how to start, restart, stop and know the status of sendmail on your server. For this copy the code given inside the box, to /etc/init.d/sendmail. The command and response following it is mentioned below:

#./etc/init.d/sendmail start

Starting sendmail: [ OK ]

To test if the server started up alright, telnet to yourself on port 25 by

typing

#telnet 127.0.0.1 25

If it is functioning properly you will see something like this:

Trying 127.0.0.1...

Connected to 127.0.0.1

Escape character is '^]'.

220 127.0.0.1 ESMTP Sendmail 8.11.2/8.11.2; Sat, 21 Jul 2001 03:50:33 +0530

If you are able to see this, then Sendmail has been successfully setup on your server.

Anurag Phadke Can be reached at cbca@mantraonline.com

Check Spamming
A good Web host, ensures that his customers are always protected from services that they didn't opt for. When a single person receives "SPAM" he/she can simply delete it, but when all the users on a particular machine are greeted with "SPAM" it not only places load on the server but also consumes disk space and queues up e-mail. One can prevent spamming by updating the "access_db" file regularly. http://mail-abuse.org/rbl/ keeps a regular track of spammers and is a good reference point.

Using Sendmail for Web Hosting
Comments/Feedback forms are essential for maintaining good customer relationship over the Web. Your customers can use "sendmail" installed on the server via the "cgi-bin" directory. The line(MAIL, "|/usr/sbin/sendmail -t") || return 0; tells your cgi script that sendmail is installed in "/usr/sbin/sendmail" directory and can be used for sending mails from the website.

#!/bin/bash

# sendmail This shell script takes care of starting and

stopping sendmail.

# chkconfig: 2345 80 30

# description: Sendmail is a Mail Transport Agent, which is the program \ that moves mail from one machine to another.

# processname: sendmail

# config: /etc/sendmail.cf

# pidfile: /var/run/sendmail.pid

# Source function library.

. /etc/init.d/functions

# Source networking configuration.

. /etc/sysconfig/network

# Source sendmail configureation.

if [ -f /etc/sysconfig/sendmail ] ; then

. /etc/sysconfig/sendmail

else

DAEMON=no

QUEUE=1h

# Check that networking is up.

[ ${NETWORKING} = "no" ] && exit 0

[ -f /usr/sbin/sendmail ] || exit 0

RETVAL=0

prog="sendmail"

start() {

# Start daemons.

echo -n $"Starting $prog: "

/usr/bin/newaliases > /dev/null 2>&1

for i in virtusertable access domaintable

mailertable ; do

if [ -f /etc/mail/$i ] ; then

makemap hash /etc/mail/$i

< /etc/mail/$i

done

daemon /usr/sbin/sendmail $([ "$DAEMON" = yes

] && echo -bd) \

$([ -n "$QUEUE" ] && echo -q$QUEUE)

RETVAL=$?

echo

[ $RETVAL -eq 0 ] && touch /var/lock/subsys/

sendmail

return $RETVAL

}

stop() {

# Stop daemons.

echo -n $"Shutting down $prog: "

killproc sendmail

RETVAL=$?

echo

[ $RETVAL -eq 0 ] && rm -f /var/lock/subsys/

sendmail

return $RETVAL

}

# See how we were called.

case "$1" in

start)

start

;;

stop)

stop

;;

restart|reload)

stop

start

RETVAL=$?

;;

condrestart)

if [ -f /var/lock/subsys/sendmail ]; then

stop

start

RETVAL=$?

;;

status)

status sendmail

RETVAL=$?

;;

echo $"Usage: $0 {start|stop|restart|condrestart|

status}"

exit 1

esac

exit $RETVAL