D. J. Bernstein
The walldns program
walldns is a reverse DNS wall.
It accepts iterative DNS queries for in-addr.arpa domains
from hosts around the Internet,
and supplies generic responses that avoid revealing local host information.
For example, walldns
provides a PTR record for 220.127.116.11.in-addr.arpa
as the name of IP address 18.104.22.168,
and a matching A record showing 22.214.171.124
as the IP address of 126.96.36.199.in-addr.arpa.
Normally walldns is set up by the
walldns runs chrooted in the directory
specified by the $ROOT environment variable,
under the uid and gid
specified by the $UID and $GID environment variables.
walldns listens for incoming UDP packets
addressed to port 53 of $IP.
It does not listen for TCP queries.
inverse queries, non-Internet-class queries, truncated packets,
packets that contain anything other than a single question,
queries for domains outside in-addr.arpa,
and request types other than A, PTR, and *.
walldns does not include NS or SOA records with its responses.
walldns uses TTLs slightly over one week.