3.5 Domain Name System

Site administrators often do not wish to accept mail from hosts that do not have a proper reverse delegation in the Domain Name System. In the previous section we introduced the library function hostname, that looks up in the DNS the name corresponding to the IP address specified as its argument. If there is no corresponding name, the function returns its argument unchanged. This can be used to test if the IP was resolved, as illustrated in the example below:

require 'dns'

prog envfrom
do
  if hostname($client_addr) = $client_addr
    reject
  fi
done

The #require dns statement loads the module dns.mfl, after which the definition of hostname becomes available.

A similar function, resolve, which resolves the symbolic name to the corresponding IP address is provided in the same dns.mfl module.