DNS-based blacklistingEdit

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This example shows how to add a DNS-based blacklist to the sendmail configuration file. In this case the SBL-XBL list provided by Spamhaus, one of the most reputable and accurate blacklists, is used for the purposes of illustration.

FEATURE(`dnsbl',`sbl-xbl.spamhaus.org',`"554 Rejected " $&{client_addr} " - see http://www.spamhaus.org/ then contact " $&{postmaster_addr}')dnl

To minimize the damage done in the unlikely event of obtaining a false positive the error message that is returned to the client includes the following elements:

  • A 554 error code (as described in RFC 821/RFC 2821).
  • The connecting IP address found in the blacklist.
  • A link to the Spamhaus website where the sender can find out what the blacklisting means, why it occurred, and how to be removed from the blacklist.
  • An alternative contact address of the form postmaster@example.com that can be used to inform the server administrator, bypassing DNS-based blacklist checks.

This specification depends on a special ruleset that defines the postmaster_addr macro (see "Store postmaster address"), as well as the modification of the following line in the configuration file:

dnl FEATURE(delay_checks)dnl

Which should be changed to:

FEATURE(`delay_checks', `friend')dnl

After making these changes the Sendmail configuration should be rebuilt and the daemon restarted.

Appropriate entries should be added to the /etc/mail/access file to ensure that delivery to the postmaster and abuse addresses is not blocked by the blacklist:

Spam:abuse@	        →FRIEND
Spam:postmaster@	→FRIEND

To test that the blacklisting works correctly, messages can be sent to nelson-sbl-test@crynwr.com and nelson-xbl-test@crynwr.com.

To test that the postmaster address is not being subjected to DNS-based blacklisting (a requirement of RFC 2821) you can connect via telnet from the server to be tested to:

telnet://ns1.crynwr.com

The telnet-based test will try sending mail to "postmaster@example.com", and should the messages should be accepted for delivery.

Efficacy

During a test period of 24 hours in mid-July 2006 on the wincent.com mail server the DNS-based blacklist described here stopped 261 connection attempts.

For other efficacy statistics see combatting spam.

See also