Returns the in.arpa address for a given inet address. Check https://en.wikipedia.org/wiki/Reverse_DNS_lookup for more info.

in_arpa.sql

create or replace function in_arpa(inet)
  returns text
  language plpgsql
as
$function$
declare
  p_return text;
begin
  case family($1)
  when 4 then
    select array_to_string(
             array[a[4],a[3],a[2],a[1]], '.'
           )||'.in-addr.arpa'
    into   p_return
    from   ( select string_to_array(
            (split_part($1::text, '/', 1)), '.') as a ) as x;
  when 6 then
    select trim( leading '.' from
             reverse(
               regexp_replace(
                 array_to_string(
                   string_to_array(
                     host($1)
                   , ':'
                   , ''
                   ) --string_to_array
                   , ''
                   , repeat('0', 32-length(replace(host($1), ':', '')))
                 ) --array_to_string
               , '', '.', 'g') --regexp_replace
             ) --reverse
           ) --trim
         || 'ip6.arpa'
    into p_return;
  end case;

  return p_return;
end;
$function$;
comment on in_arpa( inet) is 'Returns the in.arpa address for a given inet address. Check https://en.wikipedia.org/wiki/Reverse_DNS_lookup for more info.';

Check http://steviethecat.tumblr.com/post/134938969631/reverse-dns-in-postgresql for little more info.