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name-resolution-with-azure-firewall.md

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SOLUTION: Configure a DNS on the cloud, and be sure that all machines are reachable via FQDN also from on-premise (Azure Firewall Version)

read also this blog post for more information on this solution.

Pre-requisites

In order to apply this solution:

  1. deploy HUB playground
  2. deploy ONPREMISE-2 playground
  3. configure the DNS on the cloud as documented here
  4. configure a site-to-site VPN as documented here

Solution

The solution implemented is described in the following schema.

DNS

Configure Azure Firewall for DNS resolution and proxy

Create the following Firewall Policy:

  • Name: hub-fw-policy
  • DNS: Enabled
    • DNS Servers: Default (Azure Provided)
    • DNS Proxy: Enabled

Associate the policy hub-fw-policy to hub-lab-vnet.

On spoke-01 virtual network go to DNS Servers and set as DNS Server custom 10.12.3.4 (the internal firewall IP).

On spoke-02 virtual network go to DNS Servers and set as DNS Server custom 10.12.3.4 (the internal firewall IP).

On spoke-03 virtual network go to DNS Servers and set as DNS Server custom 10.12.3.4 (the internal firewall IP).

On hub-lab-net virtual network go to DNS Servers and set as DNS Server custom 10.12.3.4 (the internal firewall IP).

Configure a DNS machine on prem

Open ssh on lin-onprem machine and install Bind9:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install bind9

Edit /etc/bind/named.conf.options file (i.e. with sudo nano /etc/bind/named.conf.options) and fill it with the following:

options {
    directory "/var/cache/bind";

    allow-query { localhost; 10.20.1.0/24; };

    recursion yes;
    forwarders {
        10.12.3.4;
    };
    forward only;

    dnssec-validation no; # needed for private dns zones
    auth-nxdomain no;     # conform to RFC1035
    listen-on-v6 { any; };
};

in this configuration, on-prem DNS (10.20.1.4) forwards all request to Azure Firewall (10.12.3.4) that acts as DNS proxy for the Azure managed DNS accessible from the hub. allow-query parameter allows machines on-prem to use this DNS. dnssec validation off is required to manage the private DNS zone.

Restart Bind9 service to load the updated configuration.

sudo service bind9 restart

Configure Client machine on-prem-2

Open ssh on lin-onprem-2 and set Lin-onprem as DNS server. To do it type sudo nano /etc/resolv.conf and replace nameserver row with the following:

nameserver 10.20.1.4

this is lin-onprem machine IP

Test Solution

From ssh on Lin-onprem machine type:

nslookup spoke-01-vm.cloudasset.internal 10.12.3.4 (direct query to Azure Firewall)

or

nslookup spoke-01-vm.cloudasset.internal (query to lin-onprem that forward the request to Azure Firewall)

if the answer is

Server:10.12.3.4
Address:10.12.3.4#53

Non-authoritative answer:
Name:spoke-01-vm.cloudasset.interal
Address: 10.13.1.4

On premise machine Lin-onprem-2 is able to resolve FQDN names for cloud machines (*.clouadasset.internal), with on premise DNS server - Lin-onprem - that forwards DNS queries to Azure Firewall on the cloud. Azure Firewall forwards the request to the Azure managed DNS. Azure managed DNS is able to resolve cloudasset.internal domain because hub-lab-vnet has been linked with the corresponding private zone.