August 2014 | Reading time: ~8 min
As enterprises continue to face a barrage of sophisticated malware campaigns and botnet-driven threats, DNS sinkholing emerges as a pragmatic and proactive defense mechanism. A DNS sinkhole intercepts malicious DNS queries and redirects them to a controlled system rather than allowing them to reach a malicious server. In this post, I walk through the implementation of DNS sinkholes in an enterprise environment, exploring their architecture, use cases, and deployment tips.
Understanding DNS Sinkholing
DNS sinkholing is a technique where requests to known malicious domains are resolved to a non-routable or dummy IP address. Rather than letting an infected host contact its command and control server (C2), the DNS resolver redirects the traffic to a controlled environment—effectively breaking the attacker’s feedback loop.
This is particularly useful in environments where network segmentation and egress filtering are limited or infeasible. It gives administrators visibility into infected endpoints without compromising operations.
Architecture Overview
DNS sinkholes are typically implemented at the recursive DNS layer, using one of the following models:
- Local DNS resolver with sinkhole zones: The internal DNS server hosts sinkhole zones for malicious domains. Requests are redirected to an internal IP.
- External threat feed + DNS policy engine: Services like Infoblox or Cisco Umbrella apply dynamic blacklists to identify and sinkhole malicious domains in real time.
- Custom BIND configuration: Using zone files to redirect requests for known-bad domains to internal honeypots.
Steps to Implement DNS Sinkholing
To implement DNS sinkholing effectively, the following steps should be taken:
1. Curate a Threat Feed
Obtain or subscribe to a curated threat feed of known C2 domains, malware distribution domains, and compromised hostnames. OpenDNS, Emerging Threats, and Malware Domains List were viable sources as of 2014.
2. Configure DNS Zones
In a BIND server, you might create a zone file like this:
zone "maliciousdomain.com" IN {
type master;
file "/etc/bind/db.sinkhole";
};
The db.sinkhole file might contain a simple A record redirecting the domain to a blackhole address like 127.0.0.1 or an internal web server for logging.
3. Deploy a Sinkhole Server
The IP address to which queries are redirected should ideally run a web server or logger that captures the source IP of the infected machine. This provides visibility into which endpoints are compromised.
4. Monitor and Alert
Configure alerting on sinkhole hits. A good practice is to send syslog events to a SIEM like Splunk or QRadar, tagging infected hosts for follow-up remediation.
5. Educate the Incident Response Team
Security staff must understand that sinkhole hits represent infected clients, not blocked attacks. This distinction ensures appropriate responses: host isolation, malware scans, and forensic investigations.
Operational Tips
- Use internal addressing like
10.10.10.10or192.168.1.99instead of127.0.0.1to avoid loopback complications. - Tag your sinkhole domain zones for easy updates and auditing.
- Rotate or timestamp your zone file updates and use version control to track changes.
- Integrate threat feeds into automation scripts or orchestration tools like Ansible.
Limitations and Considerations
DNS sinkholing is a reactive measure and depends heavily on the quality of the domain blocklist. It also only intercepts threats that rely on DNS—direct IP connections, hardcoded C2 addresses, or proxy-tunneled traffic bypass DNS altogether.
Despite this, it remains a low-cost, low-risk mechanism to gain visibility and exert some level of control over infected endpoints.