DNS priority investigation

Author: jacob-pro

docs/DNS_PRIORITY.md

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+# Windows 10 DNS Priority
+
+The way DNS priority works 
+[changed in Windows 10](https://web.archive.org/web/20190106092511/https://blogs.technet.microsoft.com/networking/2015/08/14/adjusting-the-network-protocol-bindings-in-windows-10/)
+to use interface metric instead of binding order. However, it isn't entirely clear from the docs precisely how this
+priority works under certain scenarios, so I have done some experiments:
+
+## Testing Interface Metrics
+
+Ethernet 1 (index 9), DNS server: 10.1.1.254 \
+Ethernet 2 (index 24), DNS server: 10.2.2.254
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -InterfaceMetric 50
+Set-NetIPInterface -InterfaceAlias "Ethernet 2" -InterfaceMetric 100
+```
+
+Ethernet 1 wins:
+
+```powershell
+nslookup google.com
+Address:  10.1.1.254
+```
+
+Let's flip the metrics:
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -InterfaceMetric 100
+Set-NetIPInterface -InterfaceAlias "Ethernet 2" -InterfaceMetric 50
+```
+
+Now Ethernet 2 wins, this is expected:
+
+```powershell
+nslookup google.com
+Address:  10.2.2.254
+```
+
+## Does Route Priority Matter?
+
+Ethernet 1 (index 9), DNS server: 10.1.1.254 \
+Ethernet 2 (index 24), DNS server: 10.2.2.254
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -InterfaceMetric 50
+Set-NetIPInterface -InterfaceAlias "Ethernet 2" -InterfaceMetric 100
+Set-NetRoute -DestinationPrefix 0.0.0.0/0 -InterfaceAlias "Ethernet 1" -RouteMetric 256
+Set-NetRoute -DestinationPrefix 0.0.0.0/0 -InterfaceAlias "Ethernet 2" -RouteMetric 256
+```
+
+Initially Ethernet 1 wins for DNS:
+
+```powershell
+nslookup google.com
+Address:  10.1.1.254
+```
+
+And also routing (because 256 + 50 is less than 256 + 100):
+
+```powershell
+Find-NetRoute -RemoteIPAddress "8.8.8.8" | Select-Object InterfaceAlias
+Ethernet 1
+```
+
+But what if we change the route priority:
+
+```powershell
+Set-NetRoute -DestinationPrefix 0.0.0.0/0 -InterfaceAlias "Ethernet 1" -RouteMetric 9999
+Set-NetRoute -DestinationPrefix 0.0.0.0/0 -InterfaceAlias "Ethernet 2" -RouteMetric 5
+```
+
+Such that Ethernet 2 is the best route (5 + 100 is less than 9999 + 50)
+
+```powershell
+Find-NetRoute -RemoteIPAddress "8.8.8.8" | Select-Object InterfaceAlias
+Ethernet 1
+```
+
+The best DNS server is still on Ethernet 1:
+
+```powershell
+nslookup google.com
+Address:  10.1.1.254
+```
+
+So it would appear the route metrics are irrelevant.
+
+## How are IPv4 vs IPv6 interface metrics treated
+
+Note: Neither Ethernet adapters have an IPv6 address/route or DNS server.
+
+Initially 
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -AddressFamily IPv4 -InterfaceMetric 50
+Set-NetIPInterface -InterfaceAlias "Ethernet 2" -AddressFamily IPv4 -InterfaceMetric 100
+
+# Ethernet 2 has the best metric of any interface:
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -AddressFamily IPv6 -InterfaceMetric 50
+Set-NetIPInterface -InterfaceAlias "Ethernet 2" -AddressFamily IPv6 -InterfaceMetric 25
+```
+
+Ethernet 2 now has the highest priority DNS:
+
+```powershell
+nslookup google.com
+Address:  10.2.2.254
+```
+
+## DNS servers on the same interface
+
+In this scenario only one Ethernet adapter is enabled:
+
+IPv4 DNS server: 10.1.1.254 \
+IPv6 DNS server: ::1
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -InterfaceMetric 50
+```
+
+When both IPv4 and IPv6 metrics are equal the IPv6 DNS server is used first:
+
+```powershell
+nslookup google.com
+Address:  ::1
+```
+
+What about if the IPv4 metric is lower:
+
+```powershell
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -AddressFamily IPv4 -InterfaceMetric 50
+Set-NetIPInterface -InterfaceAlias "Ethernet 1" -AddressFamily IPv6 -InterfaceMetric 100
+```
+
+The IPv6 DNS server is still chosen first:
+
+```powershell
+nslookup google.com
+Address:  ::1
+```