14.5. Identification of Spam sources with DNS-based Blackhole Lists

Another means of combating spam is to use a DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL). DNSBLs are lists of IP addresses that the operator believes to be (potential) sources of spam. The lists are checked by DNS. If the IP of the sending email server is known to the DNS server, the message is rejected. The IP address is checked quickly and in a comparatively resource-friendly manner. The check is performed before the message is accepted. The extensive checking of the content with SpamAssassin and anti-virus software is only performed once it has been received. Postfix has integrated support for DNSBLs.

DNSBLs from various projects and companies are available on the internet. Please refer to the corresponding websites for further information on conditions and prices.

The Univention Configuration Registry Variable mail/postfix/smtpd/restrictions/recipient with a key-value pair SEQUENCE=RULE must be set to be able to use DNSBLs with Postfix: mail/postfix/smtpd/restrictions/recipient/SEQUENCE=RULE.

It can be used to configure recipient restrictions via the Postfix option smtpd_recipient_restrictions (see Postfix setting smtpd_recipient_restrictions). The sequential number is used to sort multiple rules alphanumerically, which can be used to influences the ordering.

Tip

Existing smtpd_recipient_restrictions regulations can be listed as follows:

$ ucr search --brief mail/postfix/smtpd/restrictions/recipient

In an unmodified Univention Corporate Server Postfix installation, the DNSBL should be added to the end of the smtpd_recipient_restrictions rules. For example:

$ ucr set mail/postfix/smtpd/restrictions/recipient/80="reject_rbl_client ix.dnsbl.manitu.net"