[dns-operations] About open DNS resolvers

[esolve esolve]

Hi, all:

       I'm interested in issues on open DNS resolvers. In the following
page,
          http://dns.measurement-factory.com/surveys/openresolvers.html

      It discusses how to probe open resolvers, but I have some questions

      1 about the testing methodology, it needs to build a DNS server and
check whether it receives queries. Why can we just use "dig @target_ip
www.example.com" and see whether we can get a result?

      2 for testing whether a ip is open resolver, the page recommends to
use the following command line:
                               dig +short 2.2.2.4.dnsbl.openresolvers.org

         I test many IPs from the link:
http://www.petercooper.co.uk/list-of-public-dns-servers-you-can-use-40.html,
and I got null results, which means
 they are not open resolvers or they havn't been probed.  for example, I
tested 67.138.54.100 and got null result

                                             [usr at canard usr]$ dig +short
67.138.54.100.dnsbl.openresolvers.org
                                             [usr at canard usr]$

          But since I tested many ips and only got null results. Is my
testing wrong or not?

     3 I tried
                           dig @2.2.2.4 www.google.com
        and got no good results, but in the page, 2.2.2.4 is an open
resolver

     4 is there anybody who has a open resolver list?  if so, can you send
me a copy? I need them to do some tests, thanks!
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