Most DNSBLs follow a convention of entries for IPs in
127.0.0.0/8 (127.0.0.0-127.0.0.255) to
provide online indication of whether the DNSBL is operational.
This still has the problem that 127.0.0.0/8 is not
127.0.0.0-127.0.0.255. 127.0.0.0/8 is 127.0.0.0-127.255.255.255;
127.0.0.0-127.0.0.255 is 127.0.0.0/24.
Many DNSBLs arrange to have a query
of 127.0.0.2 return an A record indicating that the IP is
listed, and
a query of 127.0.0.1 return no A record (NXDOMAIN). When both of
these indicators are present, this indicates that the DNSBL is
functioning normally. See [DNSBL-EMAIL].
There is a problem with the above. The reason for the "MUST NOT list
127.0.0.1" (elsewhere) is [...]. This is something that Vixie said
years ago. Yet, we're telling them to explicitly list it here.
The text you quote says to list 127.0.0.2 and to _not_ list 127.0.0.1.
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