DNS Policies Overview
Updated: April 28, 2016
Applies To: Windows Server Technical Preview
You can create DNS policies to control how a DNS Server handles queries based on different parameters. For instance, you may create a DNS policy to respond to a query asking for the IP address of a web server to respond with a different IP address based on the closest datacenter to the client.
DNS Policies can be used in different scenarios, including:
Application high availability. DNS clients are redirected to the healthiest endpoint for a given application.
Traffic Management. DNS clients are redirected to the closest datacenter.
Split Brain DNS. DNS records are split into different Zone Scopes, and DNS clients receive a response based on whether they are internal or external clients.
Filtering. DNS queries from a list of malicious IP addresses or FQDNs are blocked.
Forensics. Malicious DNS clients are redirected to a sink hole instead of the computer they are trying to reach.
Time of day based redirection. DNS clients can be redirected to datacenters based on the time of the day.
In order to create policies to support the scenarios listed above, it is necessary to be able to identify groups of records in a zone, groups of clients on a network, among other elements. These elements are represented by the following new DNS objects:
Client subnet: a client subnet object represents an IPv4 or IPv6 subnet from which queries are submitted to a DNS server. You can create subnets to later define policies to be applied based on what subnet the requests come from. For instance, in a split brain DNS scenario, the request for resolution for a name such as www.microsoft.com can be answered with an internal IP address to clients from internal subnets, and a different IP address to clients in external subnets.
Recursion scope: recursion scopes are unique instances of a group of settings that control recursion on a DNS server. A recursion scope contains a list of forwarders and specifies whether recursion is enabled. A DNS server can have many recursion scopes. DNS server recursion policies allow you to choose a recursion scope for a set of queries. If the DNS server is not authoritative for certain queries, DNS server recursion policies allow you to control how to resolve those queries. You can specify which forwarders to use and whether to use recursion.
Zone scopes: a DNS zone can have multiple zone scopes, with each zone scope containing their own set of DNS records. The same record can be present in multiple scopes, with different IP addresses. Also, zone transfers are done at the zone scope level. That means that records from a zone scope in a primary zone will be transferred to the same zone scope in a secondary zone.
Types of Policy
DNS Policies are divided by level and type. You can use Query Resolution Policies to define how queries are handled, and Zone Transfer Policies to define how zone transfers take place. Each policy type can be applied at the server level, or the zone level.
Query Resolution Policies
You can use DNS Query Resolution Policies to specify how incoming resolution queries are handled by a DNS server. Every DNS Query Resolution Policy contains the following elements:
|- Up to 256 characters
- Can contain any character valid for a file name
|- Enable (default)
|Once a query is classified by level and applies on, the server finds the first policy for which the query matches the criteria and applies it to query
|- Numeric value
- Unique value per policy containing the same level and applies on value
|Action to be performed by DNS server
|- Allow (default for zone level)
- Deny (default on server level)
|Policy condition (AND/OR) and list of criterion to be met for policy to be applied
|- Condition operator (AND/OR)
- List of criteria (see the criterion table below)
|List of zone scopes and weighted values per scope. Weighted values are used for load balancing distribution. For instance, if this list includes datacenter1 with a weight of 3 and datacenter2 with a weight of 5 the server will respond with a record from datacentre1 three times out of eight requests
|- List of zone scopes (by name) and weights
Server level policies can only have the values Deny or Ignore as an action.
The DNS policy criteria field is composed of two elements:
|Transport protocol used in the query. Possible entries are UDP and TCP
|- EQ,Spain,France - resolves to true if the subnet is identified as either Spain or France
- NE,Canada,Mexico – resolves to true if the client subnet is any subnet other than Canada and Mexico
|Transport protocol used in the query. Possible entries are UDP and TCP
|Network protocol used in the query. Possible entries are IPv4 and IPv6
|Server Interface IP address
|IP address for the incoming DNS server network interface
|FQDN of record in the query, with the possibility of using a wild card
|- EQ,www.contoso.com – resolves tot rue only the if the query is trying to resolve the www.contoso.com FQDN
- EQ,*.contoso.com,*.woodgrove.com – resolves to true if the query is for any record ending in contoso.comORwoodgrove.com
|Type of record being queried (A, SVR, TXT)
|- EQ,TXT,SRV – resolves tot rue if the query is requesting a TXT OR SRV record
- EQ,MX – resolves tot rue if the query is requesting an MX record
|Time of Day
|Time of day the query is received
|- EQ,10:00-12:00,22:00-23:00 – resolves tot rue if the query is received between 10 AM and noon, OR between 10PM and 11PM
Using the table above as a starting point, the table below could be used to define a criterion that is used to match queries for any type of records but SRV records in the contoso.com domain coming from a client in the 10.0.0.0/24 subnet via TCP between 8 and 10 PM through interface 10.0.0.3:
|Server Interface IP address
|Time of Day
You can create multiple query resolution policies of the same level, as long as they have a different value for the processing order. When multiple policies are available, the DNS server processes incoming queries in the following manner:
Recursion policies are a special type of server level policies. Recursion policies control how the DNS server performs recursion for a query. Recursion policies apply only when query processing reaches the recursion path. You can choose a value of DENY or IGNORE for recursion for a set of queries. Alternatively, you can choose a set of forwarders for a set of queries.
You can use recursion policies to implement a Split-brain DNS configuration. In this configuration, the DNS server performs recursion for a set of clients for a query, while the DNS server does not perform recursion for other clients for that query.
Recursion policies contains the same elements a regular DNS query resolution policy contains, along with the elements in the table below:
|Apply on recursion
|Specifies that this policy should only be used for recursion.
|Name of the recursion scope.
Recursion policies can only be created at the server level.
Zone Transfer Policies
Zone transfer policies control whether a zone transfer is allowed or not by your DNS server. You can create policies for zone transfer at either the server level or the zone level. Server level policies apply on every zone transfer query that occurs on the DNS server. Zone level policies apply only on the queries on a zone hosted on the DNS server. The most common use for zone level policies is to implement blocked or safe lists.
Zone transfer policies can only use DENY or IGNORE as actions.
You can use the server level zone transfer policy below to deny a zone transfer for the contoso.com domain from a given subnet:
Add-DnsServerZoneTransferPolicy –Name DenyTransferOfCOnsotostoFabrikam –Zone contoso.com –Action DENY –ClientSubnet “EQ,192.168.1.0/24”
You can create multiple zone transfer policies of the same level, as long as they have a different value for the processing order. When multiple policies are available, the DNS server processes incoming queries in the following manner:
Managing DNS Policies
You can create and manage DNS Policies by using PowerShell. The examples below go through different sample scenarios that you can configure through DNS Policies:
You can direct traffic based on an FQDN to different servers depending on the location of the DNS client. The example below shows how to create traffic management policies to direct the customers from a certain subnet to a North American datacenter and from another subnet to a European datacenter.
Add-DnsServerClientSubnet -Name "NorthAmericaSubnet" -IPv4Subnet "172.21.33.0/24"
Add-DnsServerClientSubnet -Name "EuropeSubnet" -IPv4Subnet "172.17.44.0/24"
Add-DnsServerZoneScope -ZoneName "Contoso.com" -Name "NorthAmericaZoneScope"
Add-DnsServerZoneScope -ZoneName "Contoso.com" -Name "EuropeZoneScope"
Add-DnsServerResourceRecord -ZoneName "Contoso.com" -A -Name "www" -IPv4Address "172.17.97.97" -ZoneScope "EuropeZoneScope"
Add-DnsServerResourceRecord -ZoneName "Contoso.com" -A -Name "www" -IPv4Address "172.21.21.21" -ZoneScope "NorthAmericaZoneScope"
Add-DnsServerQueryResolutionPolicy -Name "NorthAmericaPolicy" -Action ALLOW -ClientSubnet "eq,NorthAmericaSubnet" -ZoneScope "NorthAmericaZoneScope,1" -ZoneName "Contoso.com"
Add-DnsServerQueryResolutionPolicy -Name "EuropePolicy" -Action ALLOW -ClientSubnet "eq,EuropeSubnet" -ZoneScope "EuropeZoneScope,1" -ZoneName contoso.com
The first two lines of the script create client subnet objects for North America and Europe. The two lines after that create a zone scope within the contoso.com domain, one for each region. The two lines after that create a record in each zone that associates ww.contoso.com to different IP address, one for Europe, another one for North America. Finally, the last lines of the script create two DNS Query Resolution Policies, one to be applied to the North America subnet, another to the Europe subnet.
Block queries for a domain
You can use a DNS Query Resolution Policy to block queries to a domain. The example below blocks all queries to treyresearch.net:
Add-DnsServerQueryResolutionPolicy -Name "BlackholePolicy" -Action IGNORE -FQDN "EQ,*.treyresearch.com"
Block queries from a subnet
You can also block queries coming from a specific subnet. The script below creates a subnet for 220.127.116.11/24 and then creates a policy to ignore all queries coming from that subnet:
Add-DnsServerClientSubnet -Name "MaliciousSubnet06" -IPv4Subnet 18.104.22.168/24
Add-DnsServerQueryResolutionPolicy -Name "BlackholePolicyMalicious06" -Action IGNORE -ClientSubnet "EQ,MaliciousSubnet06"
Allow recursion for internal clients
You can control recursion by using a DNS Query Resolution Policy. The sample below can be used to enable recursion for internal clients, while disabling it for external clients in a split brain scenario.
Set-DnsServerRecursionScope -Name . -EnableRecursion $False
Add-DnsServerRecursionScope -Name "InternalClients" -EnableRecursion $True
Add-DnsServerQueryResolutionPolicy -Name "SplitBrainPolicy" -Action ALLOW -ApplyOnRecursion -RecursionScope "InternalClients" -ServerInterfaceIP "EQ,10.0.0.34"
The first line in the script changes the default recursion scope, simply named as “.” (dot) to disable recursion. The second line creates a recursion scope named InternalClients with recursion enabled. And the third line creates a policy to apply the newly create recursion scope to any queries coming in through a server interface that has 10.0.0.34 as an IP address.
Create a server level zone transfer policy
You can control zone transfer in a more granular form by using DNS Zone Transfer policies. The sample script below can be used to allow zone transfers for any server on a given subnet:
Add-DnsServerClientSubnet -Name "AllowedSubnet" -IPv4Subnet 172.21.33.0/24
Add-DnsServerZoneTransferPolicy -Name "NorthAmericaPolicy" -Action IGNORE -ClientSubnet "ne,AllowedSubnet"
The first line in the script creates a subnet object named AllowedSubnet with the IP block 172.21.33.0/24. The second line creates a zone transfer policy to allow zone transfers to any DNS server on the subnet previously created.
Create a zone level zone transfer policy
You can also create zone level zone transfer policies. The example below ignores any request for a zone transfer for contoso.com coming in from a server interface that has an IP address of 10.0.0.33:
Add-DnsServerZoneTransferPolicy -Name "InternalTransfers" -Action IGNORE -ServerInterfaceIP "ne,10.0.0.33" -PassThru -ZoneName "contoso.com"