2. DNS
• Domain Name System
• Translates domain
names into IP addresses
• Hierarchical distributed
System
• Nodes are called as
name servers
• What are resolvers?
3. Comparing DNS Resolvers in the Wild
• DNS has evolved a lot over the years
• From a naming service to commodity infrastructure
• Many third party DNS resolvers have been developed
• E.g. OpenDNS extends DNS features by adding
misspelling correction, phishing protection, and content
filtering
• Do these third party DNS resolvers really improve
performance?
4. Content Delivery Networks
• Aim is to provide content with higher availability and
performance
• Brings content closer to the user by using carefully
positioned global infrastructure
• This global infrastructure includes thousands of servers
spread world wide
• Users, when request certain content, are directed to
closest CDN server having that data
• Assumes that DNS resolver is close to the client
originating the request
5. DNS today : Issues
• CDN’s assumption that DNS resolver is close to the client
originating the request may degrade performance
• A third party DNS which is not in network on the client
may direct it to a CDN server out of it’s network
• CDN’s associate a short TTL on their DNS answers making
caching useless
• “NXDOMAIN catcher” can also hamper some
applications like email
6. DNS today : Issues -> NXDOMAIN catcher
• NXDOMAIN -> Non-
Existent Domain
• NXDOMAIN status code
is caught and the IP
address of a certain
search website is
returned
• Advertisements can be
inserted to get profit
7. Measurements
• We need to compare responsiveness of various DNS
resolvers
• Local Resolver
• OpenDNS
• Google
• Done by performing DNS queries to 10000 hosts.
• Following information collected during the measurements
• Vantage Point
• Resolver
• Host
8. Inside facts of the measurements…
• Two hosts in same area can query DNS back to
back
• Response time will be shorter for the second
host as the reply will be in cache
• Compares response times between first and
second query
• Need to inspect timestamps in traces to find
location
• Traces need to be selected that do not interact
9. Comparison between good ISP and bad ISP
• As can be seen in above diagram, second query has faster
response time due to caching
• For second query in case of bad ISP, local DNS resolver has
almost similar delay as others
10. DNS Answers
• Graph shows that local resolver is more likely to return IP
addresses that are in client’s AS than Google or OpenDNS.
• This locally available content covers akamaized set completely
that is local DNS resolvers works better for CDN content.
11. Summary
• End-host experiences a very small latency to the
resolvers maintained by the local ISP
• There does exist cases where GoogleDNS and
OpenDNS outperform
• Several ISPs and OpenDNS rely on a load balancing
setup without a shared cache, resulting in poor
caching efficiency.
• Third-party DNS resolvers do not manage to redirect
the users towards content available within the ISP,
contrary to the local DNS ones.
