DNS RPZ (Response Policy Zones) – Using DNS as a layer of defence – Part I

Update (06/08/2020)APNIC has published this post on their blog. Robbie Mitchell from APNIC was of great help in correcting a few things and polishing the article. You can read the Part 1 on the APNIC blog here

DNS(Domain Name System) is the crucial & ubiquitous fabric of the Internet.  While on the surface, users rely on accessing websites, apps, email etc underneath it’s the DNS database which provides the map for the Internet.

It’s fair to say that everything on the Internet begins with a DNS query. This means that the DNS is used for legitimate purposes and as well as abused by bad actors.

Adding a layer of security to a flat network

In the context of COVID-19, where most of us are working from home, security of the the devices & data being accessed from a hostile home network has become a major talking point over the last couple of months. The home network is atypical from an enterprise network from a security perspective and apart from its inherent flaws, it’s a flat network.

flat network is a computer network design approach that aims to reduce cost, maintenance and administration.[1] Flat networks are designed to reduce the number of routers and switches on a computer network by connecting the devices to a single switch instead of separate switches. Unlike a hierarchical network design, the network is not physically separated using different switches.

The topology of a flat network is not segmented or separated into different broadcast areas by using routers.


Here is a representation of a flat network design,

The constraints of a flat network are,

  • No segmentation of traffic – Single broadcast domain
  • Easy & rapid propagation of malicious traffic within the network

One of the layers of security that can be brought into a flat network at an economical cost is by leveraging DNS. Before we look into how that can be implemented, here is a DNS primer for what happens when a domain name is accessed in a network,

Shift of the recursive resolvers

In the above diagrammatic representation, the part which is doing the most heavy lifting is the Recursive DNS Server or Recursive resolver. At the very beginning of the Internet, users themselves ran recursive resolvers on the machines or in the network. This model slowly shifted to the network operators (ISP’s) offering this as a bundled, free of cost offering along with the service. And the model has moved DNS resolution further away from the user with the advent of the Cloud/Quad DNS providers. To name a notable few, Google Public DNS (,, Cloudflare (,, Quad9( etc.

While each of these open resolvers services promote faster dns resolution, in reality they are still further away from the user from a round trip metric. Even though all of these open resolver services use IP Anycast, the proximity to the user cannot compete with a local resolver. In obvious terms, the recursive resolver which is in the users network or even the resolver provided by the Internet Service Provider will always be closest.

The one definitive advantage that the cloud/quad DNS open resolvers provide is the availability of a large cache.

If you aren’t convinced yet on running your own DNS resolver instead of outsourcing it to the cloud/quad DNS providers, I would urge you to read Why should I run my own DNS resolver?

And most importantly, if you want to leverage DNS Response Policy Zones (DNS Firewall) to add a layer of security in your network, you need to run a recursive resolver.

What is DNS Response Policy Zones(RPZ) ?

  • It’s currently an Internet-draft and not a standard yet. The latest draft is available here
  • It’s a vendor neutral – BIND, Unbound, PowerDNS Recursor support it
  • Allows policy to be applied to DNS queries. Set a differentiated route for the bad domains
  • Economical solution – a RaspberryPi can act as recursive resolver with DNS RPZ for the entire network – especially useful & low cost solution for home networks, SOHO etc

Just like the functioning of a firewall, RPZ is made up of TRIGGERS & ACTIONS.

This is all good but without threat intelligence data, a DNS Firewall doesn’t add any value.

Threat intelligence RPZ feeds

While there are many threat intelligence providers which provide a DNS RPZ feed, below are some of the free/community ones,

Part II of this post will contain instructions for configuring a RPZ feed in ISC BIND9.

Junk to the root

DNS root servers are the heart of the DNS infrastructure. Although there are just 13 of them, the actual number comprises of 1084 instances in Anycast operated by 12 independent root server operators.

A recent study by ICANN OCTO on Analysis of the Effects of COVID-19-Related Lockdowns on IMRS Traffic shed some light on DNS traffic patterns before COVID-19 and during. While the study looked at the ICANN Managed Root Server Instance (IMRS) i.e a few instances of the L-Root Server ( l.root-servers.net), I wouldn’t be surprised if the pattern is similar for other root servers as well.

One stark observation in the study was the amount of DNS traffic for non-existent TLDs. As every DNS transaction begins with a query to the root server and goes down the delegation chain, queries for non-existent records are also sent to the root servers.

Topping the chart is browsers based on Chromium. Not surprising since Chromium based browsers send a 7-15 character three random strings on startup to check if the browser is sitting behind captive portal. Check my earlier blog post Chromium based browsers & DNS for more information on the topic.

So, I had sent in a question to the Ask Mr. DNS podcast asking if they knew if there was a formal specification/guidelines for consequences of excessively abusing the root servers. And guess what,

Oh, and the guys (or Matt, really) answer a really good question from Swapneel Patnekar about an ICANN paper on the effects of COVID-19 on the root name servers.

I would urge you to listen to the entire episode as it contains juicy bits by Kim Davies about the Root Key Signing Key Ceremony, but if you’re the impatient lot & !DNS Geek, skip to 31:48 to tune in for my few seconds of fame 😀

Chromium based browsers & DNS

While this is not something new, it perhaps has more significance because of the ever increasing market share of more than 60% of Chromium based browsers.

Chromium based browsers have a very uncanny method to check if the web browser is sitting behind a captive portal. And if you’re running a recursive resolver in your network with a large user base running Chromium based browsers (Google Chrome, Brave etc), it might even startle you if you observe the recursive resolver logs.

Here is a snippet from my unbound resolver as soon as I start Google Chrome on the machine(,

Jun  3 11:16:31 root unbound: [1283:0] info: pwpsfrn. A IN
Jun  3 11:16:31 root unbound: [1283:0] info: yeytluindg. A IN
Jun  3 11:16:31 root unbound: [1283:0] info: zkgtcrxrpfjcjxr. A IN

A research project at USC What’s In A Name? goes into some detail with the classification.

Here is the summary of the study,

Though the root server system handles this application-specific load sufficiently, it is clear that Chrome’s trick of using randomly generated names to discover whether it’s behind a captive portal contributes significantly to the traffic received at the root zone.

What’s in a name? – Wes Hardaker

33,384 open DNS resolvers in India

The Shadowserver Foundation releases and updates a scan report containing results for open resolvers on the Internet. Open resolvers basically respond to any a DNS queries from anyone on the Internet. Open resolvers are bad for the Internet primarily because they are a catalyst in a DNS amplification attack.

A Domain Name Server (DNS) Amplification attack is a popular form of Distributed Denial of Service (DDoS), in which attackers use publicly accessible open DNS servers to flood a target system with DNS response traffic. The primary technique consists of an attacker sending a DNS name lookup request to an open DNS server with the source address spoofed to be the target’s address. When the DNS server sends the DNS record response, it is sent instead to the target.


At the time of writing this, from an India perspective, there are 33,384 open resolvers. The number was 72,736 a couple of weeks ago.

Of the quantum, at that time,

ASNAS NameCount
AS9829BSNL-NIB National Internet Backbone77,736

So, what’s going on here ? Most likely, it’s a broken configuration in the CPE(Customer Premise Equipment) of AS9829 which is allowing DNS requests on the WAN IP address and performing recursion.

Most of the cheap CPE devices that get installed along with the connection run dnsmasq and the firmware never sees an update.

Interestingly, when I compare this with my own measurements, the number of IP addresses responding to port 53 in my results is much higher – 260,886. Though, I haven’t filtered the responses for IP addresses which are performing recursion. There could be IP addresses in the results which are configured as authoritative name servers and that’s perfectly valid.

For some reason, if you are running a DNS resolver on the Internet, strongly suggest that you restrict access by IP address/network.

A better approach is perhaps to configure the DNS resolver software on a RFC1918 private IP address & configure wireguard/openvpn. Using this approach, the resolver is never exposed to the Internet while at the same time, devices can send DNS queries via the wireguard/openvpn tunnel.

Educational & Research Institutions in India having their own ASN

A few months ago, Pranesh had asked if there are any universities in India that have their own ASN.

I think the answer warrants a few more details.

AS132785Shiv Nadar University
AS137282KIIT University
AS133552B.M.S College Of Engineering
AS38872Indian School of Business
AS137617Indian Institute Of Management, Ahmedabad
AS136304Institute Of Physics, Bhubaneswar
AS138231Indian Institute Of Information Technology, Allahabad
AS137956Indian Institute of Technology, Ropar
AS134901Indian Institute Of Science Education And Research
AS132749Indraprastha Institute of Information Technology, Delhi
AS2697ERNET (Education and Research Network) India (Also peers with AS55824 – NKN Core Network)

ASN’s part of NKN(National Knowledge Network) Core Network (AS55824)

AS59163GLA University
AS138155Jawaharlal Nehru University
AS55566Inter University Centre for Astronomy and Astrophysics
AS134023Aligarh Muslim University
AS132995South Asian University
AS58758Tata Institute of Fundamental Research (Also has AS4755 as IPv4 peer)
AS134934Institute For Stem Cell Biology And Regenerative Medicine (Also has AS45820 AS IPv4 peer)
AS134322Tata Institute of Fundamental Research (Also has AS9498 as IPv4 peer)
AS132524Tata Institute of Fundamental Research (Also has AS18101 as IPv4 peer)
AS23770Tata Institute of Fundamental Research (Also has AS45820 as IPv6 peer)
AS137136Indian Agricultural Statistics Research Institute
AS136005Raman Research Institute
AS135730Datta Meghe Institute Of Medical Sciences
AS133723Institute for Plasma Research
AS133313Saha Institute of Nuclear Physics
AS133273Tata Institute of Social Sciences
AS133002Indian Institute of Tropical Meteorology
AS132780Indian Institute of Technology, Delhi
AS131226Indian Institute Of Technology, Roorkee

While the data on NKN’s website mentions about 1622 connected institutions, apart from the list above, the majority of them do not have an ASN.

I will visit this post every few months and update the data.

Host a RIPE Atlas software probe in your network

RIPE Atlas is a global network of devices, called probes and anchors, that actively measure Internet connectivity. RIPE Atlas users can also perform customised measurements to gain valuable data about their own networks. 

At the time of writing, there were 11,009 probes connected, the total number of probes connected is maybe higher than this as probes go offline due to Internet disconnections, power issues especially in under developed/developing countries.

All this while, the probes have been hardware devices. A few pictures of the probe version 4 below,

That changed sometime in February 2020, the RIPE NCC released a software version of the RIPE Atlas probe. This is super useful (apart from the fact that the hardware probe costs money to manufacture and ship it and most importantly Indian customs 😢 ) as you can run the software probe on RaspberryPi along with many other supported operating systems(CentOS7,CentOS8, Debian 9, Debian 10 and Docker). 

For more information about installing the software probe and registration, please click the following link

If anyone needs any help in installing/registering the probe, feel free to ping 🙂