Linux Mail Server

Don't Be a Spam Source (Part 4):

To create a mail server, you must allow some level of relaying. Select the correct feature from the following list of sendmail features to relax the relay restrictions just enough to get the job done:

FEATURE(`promiscuous_relay') Tells sendmail to relay mail from all sources.
FEATURE(`relay_entire_domain') Tells sendmail to relay mail from any local domain; that is, any domain defined in class M.
FEATURE(`relay_based_on_MX') Tells sendmail to relay mail for any host for which the local host is the MX server.
FEATURE(`relay_local_from') Tells sendmail to relay mail that contains the local domain in the MAIL FROM: header.
FEATURE(`accept_unresolvable_domains') Tells sendmail to accept mail from a host, even if it cannot be found in DNS or the host table. Normally, mail from hosts that do not exist in the domain name system is rejected.

To turn a system that blocks all relaying into a mail server, create a configuration file that allows the appropriate level of relaying. For example, you could create a variation on the foobirds.m4 DOMAIN file.The following listing shows such a variation.


			    divert(0)

				VERSIONID(`foobirds.m4 03/16/2002')

				define(`confFORWARD_PATH', `$z/.forward.$w+$h:$z/.forward+$h:$z/

				Ä.forward.$w:$z/.forward')dnl                        

				define(`confMAX_HEADERS_LENGTH', `32768')dnl      

				FEATURE(`relay_entire_domain')         

				FEATURE(`redirect')dnl         

				FEATURE(`use_cw_file')dnl   

				EXPOSED_USER(`root')         

				MASQUERADE_AS(foobirds.org)  

				FEATURE(masquerade_envelope)  

				FEATURE(genericstable)

This file contains all of the same features described earlier in this tutorial, plus the relay_entire_domain feature. This additional feature permits you to use the M class as a way to identify those hosts whose mail the server should relay.

Warning:
Be careful that you don't weaken the configuration so much that you become a spam source! All of the features listed above weaken the barrier to mail relaying, but some are worse than others. promiscuous_relay should not be used because it turns the system into a potential spam relay. Avoid the relay_local_from feature because it is very easy for spammers to write anything they want in the MAIL FROM: header, including your local domain name. Additionally, accept_unresolvable_domains should not be used unless it is absolutely required. It is intended for when sendmail really can't resolve domain names, such as on a laptop Linux system that does not always have access to a DNS server.

Next: Using sendmail to Block Spam