Tuesday, June 17, 2008

OpenDNS - Domain filtering Cloud computing Service

I recently came across *opendns* service. Visit www.opendns.com to find out more details on this service. "opendns" name is a confusing name given the type of service they are providing. Initially, I thought it is some thing similar to dynamic DNS.

OpenDNS mainly provides domain blocking capability. Domains are arranged in multiple categories. It provides facility for users to configure the categories which are to be blocked. It also provides facility for users to create white list and block lists of domain names.

This service is using DNS protocol. It expects the user machines or routers to use their DNS Servers for domain name resolution. As part of DNS resolution, it appears to be extracting the domain name from DNS request packet, search in their local database, get the category and look in user preferences. If category is configured to be blocked or if domain is in the block list, then openDNS server seems to sending DNS response with its own IP address. Due to this, user browser session ends on this IP address. OpenDNS seems to be doing search on the domain name (Host field of HTTP request header) again to determine the category and it shows nice page indicating why it was blocked.

This service is good for residential users and even for business users. Residential users get benefited by blocking adult sites for kids and also stop while visiting phishing sites. Businesses also benefit as it stop users going to phishing sites. Having said that, this works fine only when CPE devices work in conjunction with openDNS service. Before going into the capabilities required in CPE devices, let me list down some limitations/issues in using opendns service.

  • Privacy issues: Some businesses find it difficult to trust opendns provider due to privacy issues where *openDNS* provider comes to know the sites business users visiting. Business may like to have facility for some users to bypass this service and for some mandate this service. Also, businesses like to have facility to bypass openDNS based Domain name resolutions for some specific domain names.
  • User or group based white list/block lists/category selection: There are different types of employees in businesses. Also, there are different types of home users - kids, parents, visitors, teens etc.. OpenDNS provides only one profile for all users. This may not be sufficient for many businesses and residential users.
  • Evasion: Kids can evade these filters if they use IP address in their browsers.
  • Updating Dynamic Public IP address with the opendns account
How can CPEs help?

User/Group based lists: User/Group based lists support is only possible if openDNS updates its functionality. One possible way is to have special DNS request with added information such as GroupID. OpenDNS Service can rovide facility in openDNS portal to create category selection/blocklist/whitelist onper group basis. Since one can't expect all PCs to support this special enhancement in the DNS protocol, this kind of support is possible with CPEs implementing DNS proxy to convert DNS requests to add Group ID.

Privacy: CPE devices can help in mitigating privacy issues by providing support to create 'skip' lists - Source skip list and Domain skip list. If the source IP address of the DNS request packet from internal PCs matches the entry in 'Source Skip' list, then it bypasses openDNS based resolution. It can do this by sending the DNS request to one of ISP Domain Name Servers. 'Domain Skip list' is checked for domain names inside DNS request sent by local machines. If there is a match, then it bypasses the openDNS resolution.

Evasion: CPE devices can monitor HTTP requests and check the 'Host' header line. If the 'Host' header line does not have domain name, but IP address, then we can certainly say that domain name is not used while browsing the site. CPE devices can provide configuration on type of action to take. It can provide options like 'Inform' and 'Deny'. 'Inform' action informs parent in case of RG environment or admin in case of business environment. 'Deny' action drops the connection and might even present local HTML error page to the user. Here too, we should 'skip' lists to help scenarios where some sites are only reachable via IP addresses - for example Intranet sites or partner sites etc..

There is another kind of evasion possible too. Local users using their own DNS Server or some public DNS Servers. CPE can check all DNS requests and ensure that only specified DNS Servers are used. It could even do Destination NAT to the required DNS Server IP address.

Dynamic IP address update: Today it is expected that special program is run in the PCs behind CPE routers. It does not work well if we have many machines or machines which do not run the software provided by OpenDNS folks. CPE device can help in those matters where it updates the dynamic public IP in openDNS Servers. CPE devices are already equipped with updating dynamic public IP addresses in DYNDNS servers. They could do additional job of upating in openDNS Server too.

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