Check your DNS Servers configuration!
More effort is needed to educate administrators about the security risks of Open DNS Resolvers that threaten the safety of the global Internet. The main reason of Open DNS Resolvers is the lack of awareness among administrators. According to the Polish national CERT (CERT Polska), there was an increase of 56% of DNS servers in Polish networks that are incorrectly configured as completely open. Open DNS resolvers can be easily used to amplify DoS attacks.
Read more here - http://www.cert.pl/news/7006/langswitch_lang/en