The other day Microsoft announced something it calls Zero Trust DNS. At a high level, it is leveraging clients’ underlying name resolution capabilities to establish and enforce security controls below the application.

In design, it is quite similar to what we did in Windows 2008 and Network Access Protection (NAP), a now deprecated solution that if released today would be considered some flavor of “Zero Trust” network access control.

NAP supported several different enforcement approaches for network policy. One of the less secure methods assessed a client’s posture; if found to be non-compliant with organizational requirements, the DHCP server would assign a restricted IP address configuration. The most secure approach relied on certificate-based IPSEC and IPv6. Healthy clients were provisioned with what we called a health certificate. All participating devices would then communicate only with IPSEC-authenticated traffic and drop the rest.

If ZeroTrust DNS had been released in 2008, it would have been another enforcement mode for Network Access Protection. It operates very similarly, essentially functioning on the basis that :

1. The enterprise controls the endpoint and, through group policy, dictates DNS client behavior and network egress policy,
2. Leverage mutual TLS and DNS over HTTPS (DoH) to authenticate clients, and
3. Transform the DNS server into a policy server for network access.

I was always disappointed to see NAP get deprecated, especially the 802.1X and IPSEC-based enforcement models. Don’t get me wrong, there were many things we should have done differently, but the value of this kind of enforcement and visibility in an enterprise is immense. This is why, years later, the patterns in NAP were reborn in solutions like BeyondCorp and the myriad of “Zero Trust” solutions we see today.

So why might this ZeroTust DNS be interesting? One of the more common concerns I have heard from large enterprises is that the mass adoption of encrypted communications has made it hard for them to manage the security posture of their environment. This is because many of the controls they have historically relied on for security were designed around monitoring clear text traffic.

This is how we ended up with abominations like Enterprise Transport Security which intentionally weakens the TLS 1.3 protocol to enable attackers—erm, enterprises—to continue decrypting traffic. 

One of the theses of this Zero Trust DNS solution appears to be that by turning DNS into what we used to call a Policy Enforcement Point for the network enterprises get some of that visibility back. While they do not get cleartext traffic, they do get to reliably control and audit what domain names you resolve. When you combine that with egress network filtering, it has the potential to create a closed loop where an enterprise can have some confidence about where traffic is going and when. While I would not want my ISP to do any of this, I think it’s quite reasonable for an enterprise to do so; it’s their machine, their data, and their traffic. It also has the potential to be used as a way to make lateral movement in a network, when a compromise takes place, harder and maybe, in some cases, even make exfiltration harder.

Like all solutions that try to deliver network isolation properties, the sticking point comes back to how do you create useful policies that reduce your risk but still let work happen as usual. Having the rules based on high-level concepts like a DNS name should make this better, than with, for example, IPSEC-based isolation models, but it still won’t be trivial to manage. It looks like this will have all of those challenges still but that is true of all network segmentation approaches.

What I appreciate most about this is its potential to reduce an organization’s perceived need to deploy MiTM solutions. From a security protocol design perspective, what is happening here is a trade-off between metadata leakage and confidentiality. The MiTM solutions in use today cause numerous problems; they hinder the adoption of more secure protocol variants and objectively reduce enterprise security in at least one dimension. They also make rolling out new versions of TLS and more secure cryptography that much harder. Therefore, in my opinion, this is likely a good trade-off for some organizations.

To be clear, I do not believe that host-based logging of endpoint communications will lead all enterprises to abandon these MiTM practices. For example, some MiTM use cases focus on the intractable problems of Data Leak Protection, network traffic optimization, or single sign-on and privilege access management through protocol-level manipulation. These solutions clearly require cleartext access, and name-based access control and logging won’t be enough to persuade enterprises that rely on these technologies to move away from MiTM. However, there are some use cases where it might.

So, is this a good change or a bad change? I would say on average it’s probably good, and with some more investment from Microsoft, it could be a reasonable pattern to adopt for more granular network segmentation while giving enterprises more visibility in this increasingly encrypted world without needing to break TLS and other encryption schemes.