Why is DNS a privacy concern?
The DNS is one of the most significant leaks of data about an individuals activity on the Internet.
Some of the issues in simple terms:
Almost every activity on the Internet starts with a DNS query (and often several).
- Those queries can reveal not only what websites an individual visits but also meta data about other services such as the domains of email contacts or chat services.
- Whilst the data in the DNS is public, individual transactions made by an end user should not be public.
- However DNS queries are sent in clear text (using UDP or TCP) which means passive eavesdroppers can observe all the DNS lookups performed.
- The DNS is a globally distributed system that crosses international boundaries and often uses servers in many different countries in order to provide resilience.
- It is well known that the NSA used the MORECOWBELL tool to perform mass surveillance of DNS traffic.
- Some ISPs embed user information (e.g. a user id or MAC address) within DNS queries that go to the ISPs resolver in order to provide services such as Parental Filtering. This allows for fingerprinting of individual users.
- Some CDNs embed user information (client subnets) in queries from resolvers to authoritative servers (to geo-locate end users). This allows for correlations of queries to particular subnets.
- Some ISPs log DNS queries at the resolver and share this information with third-parties in ways not known or obvious to end users.
For an expert review of this topic recommended reading is DNS Privacy Considerations.