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Sendmail. Anti-Spam Configuration Control

Based on the original article: http://www.sendmail.org/m4/anti-spam.html

Anti-Spam Configuration Control

The primary anti-spam features available in sendmail are:

  • Relaying is denied by default.
  • Better checking on sender information.
  • Access database.
  • Header checks.

Relaying (transmission of messages from a site outside your domain to another site outside your domain) is denied by default. Note that this changed in sendmail 8.9; previous versions allowed relaying by default. If you want to revert to the old behaviour, you will need to use FEATURE(promiscuous_relay). You can allow certain domains to relay through your server by adding their domain name or IP address to class 'R' ($=R) using RELAY_DOMAIN() and RELAY_DOMAIN_FILE() or via the access database (described below).

If you use

FEATURE(relay_entire_domain)

then any host in any of your local domains (that is, the $=m class) will be relayed.

You can also allow relaying based on the MX records of the host portion of an incoming recipient address by using

FEATURE(relay_based_on_MX)

For example, if your server receives a recipient of user@domain.com and domain.com lists your server in its MX records, the mail will be accepted. Note that this will stop spammers from using your host to relay spam but it will not stop outsiders from using your server as a relay for their site. Along the same lines,

FEATURE(relay_local_from)

will allow relaying if the sender specifies a return path (i.e. MAIL FROM: <user@domain>) domain which is a local domain. This a dangerous feature as it will allow spammers to spam using your mail server by simply specifying a return address of user@your.domain.com. It should not be used unless absolutely necessary. </user@domain>

If source routing is used in the recipient address (i.e. RCPT TO: <user%site.com@othersite.com>), sendmail will check user@site.com for relaying if othersite.com is an allowed relay host in either class 'R', class 'm' if FEATURE(relay_entire_domain) is used, or the access database if FEATURE(access_db) is used. To prevent the address from being stripped down, use: </user%site.com@othersite.com>

FEATURE(loose_relay_check)

If you think you need to use this feature, you probably do not. This should only be used for sites which have no control over the addresses that they provide a gateway for. Use this FEATURE with caution as it can allow spammers to relay through your server if not setup properly.

As of 8.9, sendmail will refuse mail if the MAIL FROM: parameter has an unresolvable domain (i.e., one that DNS, your local name service, or special case rules in ruleset 3 cannot locate). If you want to continue to accept such domains, e.g. because you are inside a firewall that has only a limited view of the Internet host name space (note that you will not be able to return mail to them unless you have some "smart host" forwarder), use

FEATURE(accept_unresolvable_domains)

sendmail will also refuse mail if the MAIL FROM: parameter is not fully qualified (i.e., contains a domain as well as a user). If you want to continue to accept such senders, use

FEATURE(accept_unqualified_senders)

An "access" database can be created to accept or reject mail from selected domains. For example, you may choose to reject all mail originating from known spammers. To enable such a database, use

FEATURE(access_db)

The FEATURE macro can accept a second parameter giving the key file definition for the database; for example

FEATURE(access_db, hash -o /etc/mail/access)

The table itself uses e-mail addresses, domain names, and network numbers as keys. For example, 

spammer@aol.com   REJECT
cyberspammer.com  REJECT
203.0.113       REJECT

would refuse mail from spammer@aol.com, any user from cyberspammer.com (or any host within the cyberspammer.com domain), and any host on the 203.0.113.* network.

The value part of the map can contain: 

OK      accept mail even if other rules in the running ruleset would reject it.

RELAY   Allow domain to relay through your SMTP server.
 
RELAY   also serves an implicit OK for the other checks.

REJECT  reject the sender/recipient with a general purpose message.

DISCARD discard the message completely using the $#discard mailer ### any text where ### is an RFC 821 compliant error code and "any text" is a message to return for the command.

For example: 

cyberspammer.com       550 We don't accept mail from spammers
okay.cyberspammer.com  OK
sendmail.org           OK
128.32                 RELAY

would accept mail from okay.cyberspammer.com, but would reject mail from all other hosts at cyberspammer.com with the indicated message. It would allow accept mail from any hosts in the sendmail.org domain, and allow relaying for the 128.32.*.* network. Note, UUCP users may need to add hostname.UUCP to the access database or class 'R' ($=R). If you also use:

FEATURE(relay_hosts_only)

then the above example will allow relaying for sendmail.org, but not hosts within the sendmail.org domain. Note that this will also require hosts listed in class 'R' ($=R) to be fully qualified host names.

You can also use the access database to block sender addresses based on the username portion of the address. For example: 

FREE.STEALTH.MAILER@      550 Spam not accepted

Note that you must include the @ after the username to signify that this database entry is for checking only the username portion of the sender address.

If you use:

FEATURE(blacklist_recipients)

then you can add entries to the map for local users, hosts in your domains, or addresses in your domain which should not receive mail: 

badlocaluser

550 Mailbox disabled for this username 
host.mydomain.com

550 That host does not accept mail
user@otherhost.mydomain.com

550 Mailbox disabled for this recipient

This would prevent a recipient of badlocaluser@mydomain.com, any user at host.mydomain.com, and the single address user@otherhost.mydomain.com from receiving mail.

There is also a "Realtime Blackhole List" run by the MAPS project at http://maps.vix.com. This is a database maintained in DNS of spammers. To use this database, use

FEATURE(rbl)

This will cause sendmail to reject mail from any site in the Realtime Blackhole List database. You can specify an alternative RBL name server to contact by specifying an argument to the FEATURE.

The features described above make use of the check_relay, check_mail, and check_rcpt rulesets. If you wish to include your own checks, you can put your checks in the rulesets Local_check_relay, Local_check_mail, and Local_check_rcpt. For example if you wanted to block senders with all numeric usernames (i.e. 2312343@bigisp.com), you would use Local_check_mail and the new regex map: 

LOCAL_CONFIG
Kallnumbers regex -a@MATCH ^[0-9]+$


LOCAL_RULESETS
SLocal_check_mail
# check address against various regex checks
R$*                             $: $>Parse0 $>3 $1
R$+ < @ bigisp.com. > $*        $: $(allnumbers $1 $)
R@MATCH                         $#error $: 553 Header Error

These rules are called with the original arguments of the corresponding check_* ruleset. If the local ruleset returns $#OK, no further checking is done by the features described above and the mail is accepted. If the local ruleset resolves to a mailer (such as $#error or $#discard), the appropriate action is taken. Otherwise, the results of the local rewriting are ignored.

You can also reject mail on the basis of the contents of headers. This is done by adding a ruleset call to the 'H' header definition command in sendmail.cf. For example, this can be used to check the validity of a Message-ID: header: 

LOCAL_RULESETS
HMessage-Id: $>CheckMessageId

SCheckMessageId
R< $+ @ $+ >            $@ OK
R$*                     $#error $: 553 Header Error