Whenever you type a domain name, your Internet browser contacts a DNS server and makes a DNS Query.
Most Virtual Private Network providers fail to mention that while your connexion is encrypted using a VPN there is a high chance that a DNS leak will occur and your ISP will still be able to see what you are doing over the internet.
The problem occurs primarily when routers and computers are set to use automatic DHCP, this can force name lookups to bypass the name server supplied by the active VPN connection and instead use the one supplied by your ISP which allows them to see the websites you visit.
DNS leak test
If you want to check if you suffer from DNS leakage, connect to your usual VPN/proxy and visit DNSLeak
After you click on START you will see your DNS hostname and will be told if it leaks or not.
Solving DNS leakage
The easiest way I have found to stop DNS leakage is by not using the ISP name servers and choose a free public DNS provider instead.
List of free DNS providers:
Comodo public DNS
Google public DNS
OpenDNS public DNS
DNSAdvantadge public DNS
Instructions to change your computer DNS settings
Instructions to use Comodo public DNS
Instructions to use Google public DNS
Instructions to use OpenDNS public DNS
Instructions to use DNSAdvantadge public DNS
After you have done the change, carry out again the DNS leak test mentioned above. You should now see the DNS belonging to your new choosen provider.
Note: It looks as if Comodo Secure DNS and DNSAdvantadge are using the same network (UltraDNS).
DNS Nameserver speed test
Changing your DNS server can also increase or decrease the speed at which the websites are resolved, you can test your nameservers speed with the free utilities below:
NameBench (DNS benchmark utility)
GRC DNS Benchmark (No installation needed)