DNS Sinkhole

Why dis?

DNS can really be a pain in the ass for IR. It's hard to control the communication of systems due to its nature and it is rarely monitored.

No wonder your favourite threat actor loves DNS tunneling more than having a long walk at the beach or pizza.

To quickly get control over DNS traffic you can use AdGuard to

a) Whitelist traffic to prevent DNS Tunneling

b) Monitor the DNS traffic

• while you will be able to see what URLS have been requested, you should activate DNS Autiting on your Domaincontrollers and other DNS Servers too. Otherwise you will not be able to identify the origin host of the request. And you likely want to know that bad boy ;)

This is intended for quick deployment in incidents and not a recommendation for general integration in your production network!

Where to place the Sinkhole?

The sinkhole should be placed as close to the internet as possible. This will allow you to resolve internal services as usual while having control over internet communication.

Quick Setup

I suggest taking a Ubuntu. Install Docker and Docker Compose on it.

Disable DNSStubListener

create adguardhome.conf here:

/etc/systemd/resolved.conf.d/adguardhome.conf

with this content:

[Resolve]
DNS=127.0.0.1
DNSStubListener=no

save the corrent resolv.conf

mv /etc/resolv.conf /etc/resolv.conf.backup

activate the new one

ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf

restart resolved service

systemctl reload-or-restart systemd-resolved

set up AdGuard

get it running:

  git clone https://github.com/cgosec/DNS-Sinkhole
  cd DNS-Sinkhole
  docker-compose up -d
  docker-compose stop

if you don't want to use the prepared config you can leave the container running and just configure the AdGuard as needed

now copy the prepared config file

  cp AdGuardHome.yaml data/confdir/
  docker-compose start

the predefined credentials in the config are:

  admin  
  This$$is"aN1cePw00rNuut?

access the dashboard at http://your_ip:8083 if you activate the wieguard vpn in the docker-compose file, I sugges you out comment the port mapping to 8083 and 4443 and access the dashboard only from the vpn.

Change DNS Upstream Server:

edit the AdGuardHome.yaml

  nano data/confdir/AdGuardHome.yaml

add your DNS upstream server here (you can remove the others):

...
upstream_dns:
  - YOUR DESIRED DNS SERVER IP HERE
  - 8.8.4.4 
  - https://dns10.quad9.net/dns-query
...

Adding urls to the whitelist

edit the AdGuardHome.yaml

  nano data/confdir/AdGuardHome.yaml

add you whitelisted urls over the implicid block '/.*/' regex for a sub domain whitelist wildcard use '@@||YOUR_DOMAIN.com'

  ...
  user_rules: 
    - '@@||digicert.com'
    - '@@||lanner-lion.cloudsink.net'
    - '/.*/'
  ...

This is preconfigured to allow communication to CrowdStrike Falcon EDR - change as you need

you can check in the logs and dashboard for blocked DNS request to identify malicious traffic or troubleshooting for services you want to check