Configuring DNS Filtering
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At some point you may need to block or filter out a Domain that’s been proven to be infected with a virus, or a domain that doesn’t meet the security requirements of your organization.
• Most Policies can be set at the server or the zone level. If you need more control at the zone level, you can modify the Add-DNSServerQueryResolutionPolicy command by adding the parameter -zonename to identify the DNS Zone that applies to this policy. The example in this lecture will be applied at the server level.
Identifying the Malicious Domain
- Here’s an example – We have identified the Domain virus.com as being a malicious domain.
- Use this PowerShell command
- Add-DnsServerQueryResolutionPolicy –Name ‘BlockListPolicy’ –Action Ignore –FQDN “EQ,*.virus.com”
Now lets break this command down so you’ll understand what’s happening
Understanding the Command
The Add-DnsServerQueryResolutionPolicy determines how DNS queries are handled by the criteria that you specify in the policy. Here are the parameters that we’ll use with the command. Parameters are always preceded by a minus sign –
- -Name – gives the policy a name (BlockListPolicy)
- -Action – Gives an action, in this case the action is to ignore any queries coming in or going out to the virus.com domain or its sub-domains.
- FQDN – Designates the fully qualified domain name
- EQ – Means equal to, in this case equal to virus.com, and the *. designates all subdomains of virus.com
- After you have run this command DNS clients will no longer be able to resolve names in the virus.com domain and its subdomains.
- If you have any clients that have been accessing the virus.com domain you will have to reboot their computer or run the ipconfig /flushdns command to clear the cache.
If you want to verify your policy information you can type the command.
Get-DnsServerQueryResolutionPolicy -Name ‘BlocklistPolicy’ | Format-List * And I’ll explain this command once we get into PowerShell.
Let’s go ahead and see what these commands look like in PowerShell.
And there is our command to create the blocklist policy
Add-DnsServerQueryResolutionPolicy –Name ‘BlockListPolicy’ –Action Ignore –FQDN “EQ,*.virus.com”
- Now let’s go ahead and get some information on our block list policy.
- Here is the command
Get-DnsServerQueryResolutionPolicy -Name ‘BlocklistPolicy’ | Format-List *
There is the pipe operator.
- So, what’s a pipe operator?
- In PowerShell this symbol is called a Pipe operator. What this does, it takes the output of the policy named blocklistpolicy and formats it in list form.
- BTW – The pipe operator symbol is located on most keyboards, right above the enter key.
Ok let’s take a look at our list:
- Action – Ignore – all queries coming in or going out of virus.com or it’s sub-domains.
- AppliesOn – Any queries we have already defined
- Is Enabled – Is set to True
- Level – Server level, as opposed to zone level
Name – BlocklistPolicy
- Let’s go ahead and remove the Policy we just created.
Remove-DnsServerQueryResolutionPolicy -Name ‘BlockListPolicy , And it says removing the server level policy
Lets go back and see what information is left after we removed the policy.
Type the command
Get-DnsServerQueryResolutionPolicy -Name ‘BlocklistPolicy’ | Format-List * Error – Failed to get policy, The Policy does not exist.
- So that command was successful
One of the challenges that you may encounter is blocking a malicious zone or a zone that does not meet the security standards of your organization. You can use filters to successfully block these zones.
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