This article is contributed. See the original author and article here.
Credit and thanks to Alexandru Jercaianu and Vladimir Cernov for implementation work
Over the last year, we have been improving the DNS over HTTPS (DoH) functionality in the Windows DNS client. Now we are pleased to introduce you to the different features now available through the Windows Insider program.
To start with, we want to note that the registry key controls documented in our original DoH testing blog post are no longer applicable. As stated there, those instructions were time limited to the initial DoH test rollout. If you did ever set that key, please delete it then reboot your machine before proceeding with the rest of this blog post.
Next, we will be reviewing the new configuration behavior, how Windows will know if a DNS server supports DoH, and what our next steps are in advancing encrypted DNS discovery.
The first control you should try out is the new UI fields in the Settings app, originally announced on the Insider blog. When Windows knows a given DNS server’s IP address has a corresponding DoH server, it will unlock a dropdown that lets you decide whether to require encryption always be used, use encryption but fall back to plain-text DNS when encryption fails, or not to use encryption (the default value).
For enterprise administrators, we have provided a new GPO for controlling DoH behavior. This will allow the use of DoH to be allowed, required, or prohibited system-wide.
- Allowed will defer the use of DoH to local settings available in the UI per network adapter.
- Required will prevent the use of configured DNS servers if they do not support DoH and will disable fallback to plain-text DNS.
- Prohibited will prevent any local DoH settings from taking effect, ensuring Windows functions as it did before the DoH client using plain-text DNS only.
The Name Resolution Policy Table (NRPT) allows administrators to specify rules for name resolution by namespace. For example, you can create an NRPT rule that specifies all queries for “*.microsoft.com” must be sent to a specific DNS server.
If Windows knows that a DNS server provided in an NRPT rule supports DoH (see the next section for how this works), then the traffic affected by the NRPT rule will inherit the benefits of using DoH. This allows admins who want to use DoH for some namespaces and not others to configure that behavior.
Knowing a server supports DoH
All these mechanisms rely on Windows already knowing a given DNS server IP address supports DoH. We ship a few definitions of known DoH servers in Windows:
Server IP addresses
Other definitions need to be added using the netsh command. To start with, you can check to see what DoH server definitions we already know by retrieving them:
netsh dns show encryption
Then you can add another server definition to the list and ensure it never falls back to plain-text DNS:
netsh dns add encryption server=<resolver-IP-address> dohtemplate=<resolver-DoH-template> autoupgrade=yes udpfallback=no
Add-DnsClientDohServerAddress -ServerAddress ‘<resolver-IP-address>’ -DohTemplate ‘<resolver-DoH-template>’ -AllowFallbackToUdp $False -AutoUpgrade $True
If you prefer to allow fallback so that when encryption fails you can still make DNS queries, you can run the same commands with the fallback flag toggled to add a new server:
netsh dns add encryption server=<resolver-IP-address> dohtemplate=<resolver-DoH-template> autoupgrade=yes udpfallback=yes
Add-DnsClientDohServerAddress -ServerAddress ‘<resolver-IP-address>’ -DohTemplate ‘<resolver-DoH-template>’ -AllowFallbackToUdp $True -AutoUpgrade $True
The `-AutoUpgrade` and `-AllowFallbackToUdp` flags together represent the values present in the Setting app per-server dropdown. If for some reason you want to add these DoH server definitions but leave them to use unencrypted DNS for now, you can set the `-AutoUpgrade` flag to false instead of true as in the examples above.
If you want to edit an existing list entry rather than adding a new one, you can use the `Set-DnsClientDohServerAddress` cmdlet in place of the `Add-DnsClientDohServerAddress` cmdlet.
It would be easier for users and administrators if we allowed a DoH server to have its IP address determined by resolving its domain name. However, we have chosen not to allow that. Supporting this would mean that before a DoH connection could we established, we would have to first send a plain-text DNS query to bootstrap it. This means a node on the network path could maliciously modify or block the DoH server name query. Right now, the only way we can avoid this is to have Windows know in advance the mapping between IP addresses and DoH templates.
Coming up next
Going forward, we want to be able to directly discover DoH server configuration from the DNS server. This would mean DoH servers could be used without having to include it in Windows or manually configure the IP address to DoH template mapping. We are currently contributing to two proposals in the IETF ADD WG to enable this: Discovery of Designated Resolvers (DDR) and Discovery of Network-designated Resolvers (DNR). We look forward to updating you with our first tests in supporting DoH discovery!
Brought to you by Dr. Ware, Microsoft Office 365 Silver Partner, Charleston SC.