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 before this feature is declared for the feature to have any effect.

You may also list additional arguments for the dns database-map used with this feature. Those additional arguments will follow the -T<TMP> part in the declaration and are specified like this:

define(`DNSBL_MAP_OPT´, `-d1s´)
FEATURE(dnsbl, ...)

Here, the -d1s tells sendmail to reduce the res_search( ) _res.retry interval to one second from the default of five seconds. Note that DNSBL_MAP_OPT must be defined before this feature is declared for the feature to have any effect.

Prior to V8.13, the check_relay rule set (7.1.1^[3ed]) was called with a workspace that looked like this:

               host $| IPnumber

The host and IPnumber were separated by the $| operator. The host was the fully qualified canonical name of the connecting host. The IPnumber was the IP number of that host in dotted-quad form without surrounding square brackets, or the IPv6 number prefixed with a literal IPv6:. Because these values were looked up with DNS before the rule set was called, there was a possibility that those values could be falsely represented.

But, beginning with V8.13 sendmail, this problem is solved. The check_relay rule set is now called with a workspace that contains the value of the ${client_name} macro (21.9.20^[3ed]), which is assigned its value when a host connects to the listening daemon. This value is the canonical hostname of the connecting host, which is the same as the hostname stored in the $_ macro (21.9.1^[3ed]).

If you created your own Local_check_relay rule set, which relied on the old style workspace contents, you will need to redesign that rule set to use the new style workspace.