DNS for Azure Point to Site (P2S) VPN - getting the internal IPs

I wanted to access all my Azure resources without making any of them visible to the Internet.  The easiest give my local machine access to everything on my Azure Virtual Network (VNET) was to connect to it over VPN. It turns out creating Azure VPN gateways and connecting to Azure VPN endpoints is easy.  There are some subtleties in getting DNS name resolution to work that can confuse when first starting out.

Setting the Stage

There are a few ways to get to Azure endpoints and resources that are blocked from the internet.  We can 

  1. Create a Point-to-Site connection from our local machines to Azure Network Gateways
  2. Create a Site-to-Site network connection from our local networks to Azure Network Gateways.
  3. Use Bastion Hosts
  4. Use Cloud Shell
  5. Leave everything open to the internet.
I chose a Point-to-Site (P2S) VPN connection that connects from my laptop to a VNet Gateway.  That joins my laptop to that VNet. Sample Infrastructure as Code templates and scripts are on GitHub.

Creating a Point-to-Site VPN in Azure

VPN services are tied to VNet Gateways.  This means you have to have a VNet to gateway into and a set of subnets. You will have subnets for your workloads and a subnet for the gateway itself. The gateway will present itself using Microsoft Azure public IP addresses.

Your local machine will open a connection to the VPN Gateway Public IP.  The gateway will give the local machine an IP on the VPN tunnel and create related IP in the gateway subnet. 

Click to Enlarge

Video

DNS over VPN

Azure resources can have some combination of public and private IP addresses.  The public addresses are returned by Azure public DNS.  The private addresses are returned by DNS internal to Azure.  This means that we want to use DNS internal to AZure when accessing resources over a VPN connection. The VPN tunnel is inside Azure on our VNet.

The VPN connection will add the IP addresses of any DNS servers that were configured into the Virtual Network Gateway's DNS server list.  We must add a DNS server to the VNet if we want DNS support for P2S or S2S connections. You must stand up your own DNS Server, actually a forwarder, and add it to the VNG DNS server list. Azure does not provide any DNS server that is addressable from the VPN connection.  

There is no pre-baked recipe for standing up a DNS server in Azure to tie to the VNG. Most people create a VM with BIND on it.  I use an Azure Container Instance in my environments because I am lazy and provisioning BIND in a container is a single call to the ACI.  I used this project with zero modification https://github.com/whiteducksoftware/az-dns-forwarder.
  1. Create a VNet
  2. Create necessary Subnets including a subnet for the Gateway
  3. Create a VNet Gateway
  4. Create a Point-to-Site VPN server in the VNet Gateway
  5. Deploy a DNS forwarder.  I created a subnet for infrastructure ACI deployments and deployed there.
  6. Add the DNS forwarder to the VNet
  7. Download the VPN configuration file and load it into your VPN
  8. Connect to Azure using client VPN software
  9. Verify name server lookups return the internal IP addresses.

Troubleshooting DNS

  1. Verify your VPN is connected.
  2. Check to see if your nameserver lookups return internal or external IPs.  
  3. Verify there is a DNS server on the VNET the gateway is bound to
  4. Verify the DNS server is listed in the generic VPN config file you downloaded.
  5. Verify the DNS server is in %homedir%\AppData\Roaming\Microsoft\Network\Connections\Pbk\rasphone.pbk
  6. Check to see if the priority of your VPN tunnel is higher than the network connection you connect to the internet with, usually ethernet or wi-fi.

Checking Network Metrics and priorities

The VPN tunnel should have a higher priority (lower index and Metric) than the NIC connection it rides on top of.  This means the PPP connection should come before your Ethernet or Wi-Fi connection.

Problem with Network Interface Priority

Azure internal names resolve to external IPs event while on the VPN.  External DNS is used when ethernet is connected because Ethernet Metric is the same as the VPN tunnel with a lower index than the VPN connection.  

You can see the Index/Metrics for each network interface and can see the DNS server used and the IP returned.


PS > netsh interface ipv4 show interfaces

Idx     Met         MTU          State                Name
---  ----------  ----------  ------------  ---------------------------
 60          25        1400  connected     FsiExample-VNET
  1          75  4294967295  connected     Loopback Pseudo-Interface 1
 23          70        1500  disconnected  Wi-Fi
  4          25        1500  connected     Ethernet
  5          25        1500  disconnected  Local Area Connection* 1
 12          65        1500  disconnected  Bluetooth Network Connection
 25          25        1500  disconnected  Local Area Connection* 2
 24          15        1500  connected     vEthernet (Default Switch)
 11          35        1500  connected     VMware Network Adapter VMnet1
 20          35        1500  connected     VMware Network Adapter VMnet8
 19          35        1500  connected     Azure Sphere
 56          15        1500  connected     vEthernet (WSL)

PS > nslookup   fsiexample0storage.blob.core.windows.net
Server:  Fios_Quantum_Gateway.fios-router.home
Address:  192.168.1.1

Non-authoritative answer:
Name:    blob.bn9prdstr05a.store.core.windows.net
Address:  52.239.174.132
Aliases:  fsiexample0storage.blob.core.windows.net
          fsiexample0storage.privatelink.blob.core.windows.net

Example Fix

Azure internal names resolve to external IPs when ethernet is connected because Ethernet Metric is the same as the VPN tunnel with a lower index than the VPN connection.

I unplugged my Ethernet cable which made Wi-Fi be my network connection.  The VPN connection now takes priority over the wi-fi connection.  DNS queries now return the internal IP

You can see the Index/Metrics for each network interface and can see the DNS server used and the IP returned.


PS C:\Users\joe> netsh interface ipv4 show interfaces

Idx     Met         MTU          State                Name
---  ----------  ----------  ------------  ---------------------------
 60          35        1400  connected     FsiExample-VNET
  1          75  4294967295  connected     Loopback Pseudo-Interface 1
 23          45        1500  connected     Wi-Fi
  4           5        1500  disconnected  Ethernet
  5          25        1500  disconnected  Local Area Connection* 1
 12          65        1500  disconnected  Bluetooth Network Connection
 25          25        1500  disconnected  Local Area Connection* 2
 24          15        1500  connected     vEthernet (Default Switch)
 11          35        1500  connected     VMware Network Adapter VMnet1
 20          35        1500  connected     VMware Network Adapter VMnet8
 19          35        1500  connected     Azure Sphere
 56          15        1500  connected     vEthernet (WSL)

PS C:\Users\joe> nslookup   fsiexample0storage.blob.core.windows.net
Server:  UnKnown
Address:  10.0.1.196

Non-authoritative answer:
Name:    fsiexample0storage.privatelink.blob.core.windows.net
Address:  10.0.1.4
Aliases:  fsiexample0storage.blob.core.windows.net


Sample GitHub project

https://github.com/freemansoft/vnet-p2s-vpn-bastion-azure 

Created 03/2022

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