Chapter 15. Email

215

In this situation, the sendmail server needs to masquerade the machine names on the company network

so that their return address is user@bigcorp.com instead of user@devel.bigcorp.com.

To do this, add the following lines to

/etc/mail/sendmail.mc

.

FEATURE(always_add_domain)dnl

FEATURE(`masquerade_entire_domain')

FEATURE(`masquerade_envelope')

FEATURE(`allmasquerade')

MASQUERADE_AS(`bigcorp.com.')

MASQUERADE_DOMAIN(`bigcorp.com.')

MASQUERADE_AS(bigcorp.com)

After generating a new

sendmail.cf

using

m4

, this configuration will make all mail from inside the

network appear as if it were sent from

bigcorp.com

.

15.3.5. Stopping Spam with Sendmail

Email spam can be defined as unnecessary and unwanted email received by a user who never requested

the communication. It is a disruptive, costly, and widespread abuse of Internet communication stan 

dards.

Sendmail has made it relatively easy to block new spamming techniques being employed to send

junk email using your system. It even blocks many of the more usual spamming methods by default.

You would need to consciously activate them by changing your

/etc/mail/sendmail.mc

file in a

particular way to make your system susceptible.

For example, forwarding of SMTP messages, also referred to as SMTP relaying, has been disabled

by default since Sendmail version 8.9. Before this change occurred, Sendmail would direct your mail

host (

x.org

) to accept messages from one party (

y.com

) and send them to a different party (

z.net

).

Now, however, you have to specifically tell Sendmail to permit a domain to relay mail through your

domain. Simply edit the

/etc/mail/relay domains

file and restart Sendmail by typing the

ser 

vice sendmail restart

command as root to activate the changes.

However, many times, your users may be bombarded with spam from other servers throughout the In 

ternet that are beyond your control. In these instances, you can use Sendmail's access control features

available through the

/etc/mail/access

file. As root, add the domains that you would like to block

or specifically allow access, such as in this example:

badspammer.com

550 Go away and do not spam us anymore

tux.badspammer.com

OK

10.0

RELAY

Because

/etc/mail/access.db

is a database, you need to use

makemap

to activate your

changes by recreating the database map. This is easily done by running the

makemap hash

/etc/mail/access

/etc/mail/access

command as root.

This example shows that any email sent from

badspammer.com

would be blocked with a 550

RFC 821 compliant error code and message back to the spammer, except for email sent from the

tux.badspammer.com

sub domain, which would be accepted. The last line shows that any email

sent from the 10.0.*.* network can be relayed through your mail server.

As you might expect, this example only scratches the surface of what Sendmail can do in terms of

allowing or blocking access. See the

/usr/share/doc/sendmail/README.cf

for more detailed

information and examples.