Change DNS Servers to OpenDNS on the Fritzbox 7240

After you enabled telnet, just log in, go to the /var/flash folder and open the ar7.cfg file:

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cd /var/flash
nvi ar7.cfg

You're now in vi editing your settings file. To enable OpenDNS, just earch for the "overwrite_dns"; parameters and change them accordingly:

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overwrite_dns1 = 208.67.222.222;
overwrite_dns2 = 208.67.220.220; 

after that, save (:wq!) and reboot the box