Using DNS communications to filter domain names

1. A system, comprising:

a processor configured to:

intercept a DNS (Domain Name System) request from a client on a private network, wherein the processor is part of a security appliance configured to intercept communications associated with the private network and wherein the DNS request is directed to a DNS server;

extract a domain name from the DNS request;

determine that the domain name is unknown;

determine that at least a prescribed number of DNS requests including the DNS request to resolve unknown domain names have been received from the client;

flag the unknown domain names as suspicious or malicious; and

respond to the DNS request from the client with a spoofed DNS response comprising a non-existent or unavailable IP (Internet Protocol) address;

wherein the processor blocks the DNS request from being transmitted to any DNS server; and

a memory coupled to the processor and configured to provide the processor with instructions.

2. The system as recited in claim 1 , wherein the processor is further configured to compare the domain name extracted from the DNS request with a list or database of domain names.

3. The system as recited in claim 1 , wherein the processor is further configured to determine a category or classification of the domain name.

4. The system as recited in claim 1 , wherein the processor is further configured to block client access to unknown domain names.

5. The system as recited in claim 1 , wherein the processor is further configured to block client access to unknown domain names according to a policy.

6. The system as recited in claim 1 , wherein to flag the unknown domain names as suspicious or malicious comprises to blacklist the unknown domain names.

7. The system as recited in claim 1 , wherein the processor is further configured to determine that the client is compromised.

8. The system as recited in claim 1 , wherein the processor is further configured to facilitate sanitizing the client.

9. A method, comprising:

using a processor to intercept a DNS (Domain Name System) request from a client on a private network, wherein the processor is part of a security appliance configured to intercept communications associated with the private network and wherein the DNS request is directed to a DNS server;

extracting a domain name from the DNS request;

determining that the domain name is unknown;

determining that at least a prescribed number of DNS requests including the DNS request to resolve unknown domain names have been received from the client;

flagging the unknown domain names as suspicious or malicious; and

responding to the DNS request from the client with a spoofed DNS response comprising a non-existent or unavailable IP (Internet Protocol) address;

wherein the processor blocks the DNS request from being transmitted to any DNS server.

10. The method as recited in claim 9 , further comprising blocking client access to unknown domain names.

11. The method as recited in claim 9 , further comprising blocking client access to unknown domain names according to a policy.

12. The method as recited in claim 9 , wherein flagging the unknown domain names as suspicious or malicious comprises blacklisting the unknown domain names.

13. The method as recited in claim 9 , further comprising determining that the client is compromised.

14. The method as recited in claim 9 , further comprising facilitating sanitizing the client.

15. The method as recited in claim 9 , further comprising comparing the domain name extracted from the DNS request with a list or database of domain names.

16. The method as recited in claim 9 , further comprising determining a category or classification of the domain name.

17. A computer program product stored on a non-transitory computer readable storage medium and comprising computer instructions for:

intercepting a DNS (Domain Name System) request from a client on a private network, wherein the computer program product is part of a security appliance configured to intercept communications associated with the private network and wherein the DNS request is directed to a DNS server;

extracting a domain name from the DNS request;

determining that the domain name is unknown;

determining that at least a prescribed number of DNS requests including the DNS request to resolve unknown domain names have been received from the client;

flagging the unknown domain names as suspicious or malicious; and

responding to the DNS request from the client with a spoofed DNS response comprising a non-existent or unavailable IP (Internet Protocol) address;

wherein the computer program product blocks the DNS request from being transmitted to any DNS server.

18. The computer program product as recited in claim 17 , further comprising computer instructions for blocking client access to unknown domain names.

19. The computer program product as recited in claim 17 , further comprising computer instructions for blocking client access to unknown domain names according to a policy.

20. The computer program product as recited in claim 17 , wherein flagging the unknown domain names as suspicious or malicious comprises blacklisting the unknown domain names.

21. The computer program product as recited in claim 17 , further comprising computer instructions for determining that the client is compromised.

22. The computer program product as recited in claim 17 , further comprising computer instructions for facilitating sanitizing the client.

23. The computer program product as recited in claim 17 , further comprising computer instructions for comparing the domain name extracted from the DNS request with a list or database of domain names.

24. The computer program product as recited in claim 17 , further comprising computer instructions for determining a category or classification of the domain name.