مشکل ارور The requested URL could not be retrieved برای چیه ؟

سلام
من بعضی سایتا رو که با فری گیت باز میکنم ارور زیر رو میدن علت چیه ؟ در حالت عادی و با پراکسی و اینا بدون مشکل باز میشن فقط با فری گیت این مشکل رو داره علت چیه دوستان ؟

The requested URL could not be retrieved

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While trying to retrieve the URL: adrese site/
The following error was encountered:

Unable to determine IP address from host name for adrese site

The dnsserver returned:

Server Failure: The name server was unable to process this query.

This means that:

The cache was not able to resolve the hostname presented in the URL. Check if the address is correct. Your cache administrator is webmaster.

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Generated Tue, 01 Jan 2013 15:24:49 GMT by google.com (squid/2.7.STABLE6)