After upgrading to 10.8.2 I have got a problem with adding a new entry into hosts file: the update was recognised only after system restart. But a more easy solution was just to flash DNS cache! So,
... flushing your DNS cache in Mac OS X is actually really easy, and there are two different commands to use, one for Leopard and for Tiger. Depending on your version of OS X, open your Terminal and follow the appropriate directions below:
Flushing DNS Cache in OS X Lion (10.7) and OS X Mountain Lion (10.8)
Launch Terminal and enter the following command, you will need to enter an administrative password:
sudo killall -HUP mDNSResponder
Note the dscacheutil still exists in 10.7 and 10.8, but the official method to clear out DNS caches is through killing mDNSResponder. You can also find that process running in Activity Monitor.
Flush DNS Cache in Mac OS X 10.5, Mac OS X 10.6
Launch Terminal and issue the following command:
All done, your DNS has been flushed. On a side note, the dscacheutil is interesting in general and worth taking a look at, try the -statistics flag instead for some stats.
Flush your DNS Cache in Mac OS X 10.4 Tiger
Type the following command in the Terminal:
That’s it - now your DNS settings should be as you intended them to be :-)