How a DNS filter works varies based on what type of DNS filtering service is being used. This article discusses each type and how to bypass the blocking when it is in place.
Domain Name System
A Domain Name System (DNS) is an Internet service that translates a domain name into an IP address. It is like a phone book for the internet. For example, when you go to www.google.com, your computer asks a DNS server what the IP address of “www” is so your browser can load the website.
Organizations wanting to censor access to certain websites will often block access by configuring their firewall to send requests for blocked URLs directly to the Null 0 address which does not exist or cannot be resolved. However, when used in this manner it is easy to avoid the blocking by downloading a software program that changes your computer’s DNS settings so that the firewall will not know where to send requests for blocked URLs. The most popular open-source program for unblocking access to websites banned via DNS filtering is called OpenDNS.
Deep Packet Inspection
Another way organizations can block access to certain websites is by using what is known as Deep Packet Inspection (DPI). DPI allows an organization to inspect packets moving across its network and selectively block or modify them before forwarding them on. For example, an organization could use this method to edit all traffic destined for Facebook.com, removing images before forwarding the packets onto their destination.
A DNS filter works by looking for certain keywords, phrases, and numbers in the header information of packets being transmitted across its network. It will then selectively modify or block packets containing those terms.
For example, when using Deep Packet Inspection an organization may look for Facebook.com, Twiiter.com, Instagram.com, or other websites that are banned by the state, and edit them out before sending the packets onto their destination server. By doing this a state ISP can stop all citizens from accessing Facebook or other social media websites, even if they have a VPN enabled.
DNS blocking
DNS blocking can be used to block access to certain websites anywhere in the world. Deep Packet Inspection is often used by an organization with an absolute rule as a way of censoring what content its citizens can view online. In these countries, ISPs may also use Proxy Servers as a way around their filters so that members of society can continue to use the internet freely.
To avoid a DNS filter all you have to do is download a program that changes your computer’s DNS settings, and then the firewall will not know where to send requests for blocked URLs. Popular open-source programs used for this purpose include OpenDNS and DNSCrypt.
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Conclusion
To block access to websites via Deep Packet Inspection, you must edit all traffic destined for Facebook.com, removing images before forwarding the packets onto their destination server. In countries with an absolute rule, this censorship may also be paired with the use of VPNs or Proxy Servers to continue allowing members of society continued access to the internet, despite censoring most online content.