Chapter 9. DNS and sendmail

DNS stands for Domain Name System. A domain is any logical or physical collection of related hosts or sites, such as example.gov or www.example.gov.

What’s New with V8.13

  • The dnsbl feature (7.2.1[3ed]) no longer uses the host database-map type to look up addresses. Instead, it uses the dns database-map type (Section 9.1.1 [V8.13]).

  • The DNSBL_MAP_OPT mc macro (Section 9.1.1 [V8.13]) has been added so that you may tune the database-map flags used with the dnsbl feature.

  • The check_relay rule set (7.1.1[3ed]) is now called with the value of ${client_name} macro (21.9.20[3ed]), allowing it to deal with bogus DNS entries (Section 9.1.2 [V8.13]).

Feature dnsbl Uses dns Database-Map

The dnsbl feature (7.2.1[3ed]) is used to enable the blocking of email from open relay sites, dial-up sites, or known spamming sites. It does so by invoking the RBL technique, which is discussed in 7.2[3ed].

Prior to V8.13, the dnsbl feature employed the host database-map type (23.7.9[3ed]) to look up addresses. Beginning with V8.13, this feature now uses the dns database-map type (23.7.6[3ed]).

The default declaration for the dns database-map for this feature looks like this:

Kdnsbl dns -R A -T<TMP>

If you wish to change the type of the lookup, you may redefine the dns -R A part of the expression:

define(`DNSBL_MAP´, `dns -R TXT´)
FEATURE(dnsbl, ...)

Here, the DNSBL_MAP redefines the lookup so that it performs TXT record lookups instead of A record lookups. Note that DNSBL_MAP must be defined ...

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