So what are the differences between the Split-Brain DNS policy and the split-Brain policy with selective recursion?

• An Open Resolver In the last example, recursion was enabled for both internal and external clients.

In other words, internally or externally, if the local DNS server didn’t have an answer for  either client, the DNS server could go out to the internet to query other DNS servers.  In effect, this made the DNS server an open resolver, and this could make the DNS server vulnerable to various malicious attacks.

• In this example with Selective Recursion enabled,only Internal clients can perform recursive queries  

• Recursion is blocked for all external clients

• With this possible security threat in mind, DNS administrators would like to block recursion for all external clients. And allow recursive queries for the internal clients. Making the DNS server more secure.  How Selective Recursion Works 
  

Our Scenario: 

• The recursion policies are evaluated by the DNS server.

If a query comes in for www.news.com on the internal Interface.  If that query matches the SplitBrainRecursionPolicy. This policy points to a recursion scope that will allow recursion. If the DNS server doesn’t know about www.news.com, it will perform recursion until it gets an answer, then caches that answer, and sends the result back to the internal client.  

If a query is received on the external interface, no policies match, and the default recursion setting, which is disabled, is applied.

This prevents the server from being an open resolver while acting as a caching resolver for the internal clients.

Terminology 

• Recursion Scopes
• Recursion Policies

So, what are DNS Recursion Scopes?  

• What are recursion scopes?

Recursion scopes are settings that control recursion. A recursion scope contains a list of forwarders and identifies whether recursion is enabled.

• So, what is the default recursion scope?  

The default recursion scope consists of server-level recursion and a list of forwarders. The default recursion scope cannot be deleted.

So, what is a Recursion Policy?

• Defines which clients are part of the recursion scope. In this example, we will create a Policy that will allow internal clients to perform recursive queries on the internet.
  

Configuring DNS Split-Brain Recursion Control consists of three steps. 

Step #1 We are going to disable external clients, and we are going to use the default recursion scope.

Step #2 We are going to enable internal clients. We are going to create a recursion scope.

Step #3 We are going to create a Recursion policy, which defines which clients are part of the policy.  

Step #1 This command disables Recursion for the default recursion scope (for all external clients) 

• Set-DnsServerRecursionScope -Name . -EnableRecursion $False 

This command disables recursion, the dot denotes the default recursion scope. The dollar sign is a variable. What this command is saying is that any queries coming in on the external interface will be disabled.

Step #2 This command creates a Recursion scope for the internal clients 

• Add-DnsServerRecursionScope -Name "InternalClients" -EnableRecursion $True  

What that is saying is that any queries coming on the internal interface will be enabled.

Step #3 This command creates a DNS Recursion Policies that allow recursion for all the internal clients. 

• Add-DnsServerQueryResolutionPolicy -Name "SplitBrainRecursionPolicy" -Action 

ALLOW -ApplyOnRecursion -RecursionScope "InternalClients" -ServerInterfaceIP "EQ,192.168.17.10“ 

             And this IP address is the interface on the internal side. So, what this policy is saying is that it has defined which clients are part of the recursion scope.

This command will allow us to view our query resolution policy.

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