DNS Sinkhole

One-liner: A defensive technique that redirects DNS queries for malicious domains to a controlled IP address, preventing connections to attacker infrastructure.

🎯 What Is It?

DNS Sinkhole is a security mechanism that intercepts and redirects DNS requests for known malicious domains to a safe IP address (typically 0.0.0.0, 127.0.0.1, or a dedicated sinkhole server). When a user or infected system tries to resolve a malicious domain, the DNS server returns the sinkhole IP instead of the actual malicious server's address, effectively blocking the connection.

🤔 Why It Matters

• Prevents Command and Control (C2) — Stops malware from communicating with attacker servers
• Blocks malware downloads — Prevents initial infection or additional payload retrieval
• Protects against Phishing — Blocks access to phishing websites
• Enables detection — Sinkhole queries provide IOCs for investigation
• Zero-day protection — Works even against unknown malware using known bad domains

🔬 How It Works

Basic Flow

1. User/Malware queries: malicious-c2.evil.com
2. Internal DNS server checks blocklist
3. Domain matches blocklist entry
4. DNS returns: 0.0.0.0 (sinkhole IP)
5. Connection attempt fails or goes to sinkhole server
6. Event logged for investigation

Common Sinkhole IPs

Implementation Methods

1. DNS Server Configuration

• Bind/Unbound: RPZ (Response Policy Zones)
• Windows DNS: Conditional forwarding or zone redirect
• Pi-hole: Domain blocklist

2. DNS Firewall / RPZ

; Response Policy Zone example
evil-c2.com CNAME .  ; Null response
malware.net A 0.0.0.0  ; Sinkhole IP

3. Enterprise DNS Security

• Cisco Umbrella
• Infoblox DNS Firewall
• Akamai ETP

🛡️ Detection & Prevention

How to Detect Sinkholed Connections

Using SIEM or log analysis:

# Kibana/Elastic Query
dns.resolved_ip: "0.0.0.0" OR dns.resolved_ip: "127.0.0.1"

# Splunk Query
index=dns dns_answer="0.0.0.0" OR dns_answer="127.0.0.1"
| stats count by src_ip, query
| sort - count

Indicators to Monitor

• High volume of queries to sinkhole IP from single host → Infected system
• Repeated queries to same sinkholed domain → Persistent malware
• Queries to newly sinkholed domains → Recent compromise

Sigma Rule Example

title: DNS Query to Sinkholed Domain
logsource:
  category: dns
detection:
  selection:
    dns.resolved_ip:
      - '0.0.0.0'
      - '127.0.0.1'
  condition: selection
falsepositives:
  - Legitimate services using localhost
level: medium

⚔️ Offensive Evasion Techniques

Attackers try to bypass DNS sinkholing:

• Hard-coded IPs — Bypass DNS entirely
• DNS over HTTPS (DoH) — Encrypted DNS to public resolvers
• Domain Generation Algorithm (DGA) — Generate thousands of domains (only few need to work)
• Fast Flux — Rapidly change DNS responses
• Custom DNS resolvers — Use attacker-controlled DNS
• IP-based C2 — Direct IP connections

🎤 Interview Angles

Common Questions

• "What is a DNS sinkhole and how does it work?"
• "How would you detect a sinkholed connection in logs?"
• "What are the limitations of DNS sinkholing?"
• "How do attackers bypass DNS sinkholing?"

Key Talking Points

• Sinkholing is reactive — requires known bad domains
• Works best with threat intelligence feeds
• Combine with DNS logging for investigation
• DoH/DoT bypass traditional sinkholing (need TLS inspection)
• Effective first layer of defense, not complete solution

STAR Story

Situation: SIEM showed 115 hits to domains resolving to 0.0.0.0 from 12 unique hosts.
Task: Investigate potential malware infections indicated by DNS sinkhole.
Action: Queried DNS logs for all queries resolving to sinkhole IP. Identified 12 malicious domains and 7 infected hosts. Cross-referenced with threat intelligence feeds—domains linked to known Malware campaign. Isolated affected hosts, ran EDR scans, found trojan banking malware. Deployed updated IOCs to firewall.
Result: Contained infection to 7 hosts before data exfiltration. Blocked malware C2 for entire network via DNS sinkhole. Updated incident response playbook with sinkhole monitoring procedures.

✅ Best Practices

• Maintain up-to-date malicious domain feeds
• Use dedicated sinkhole server (not 0.0.0.0) for better logging
• Alert on sinkhole hits for immediate investigation
• Combine with Firewall IP blocking for defense-in-depth
• Monitor for DoH/DoT traffic bypassing internal DNS
• Log all DNS queries for forensics
• Regularly review and update blocklists

❌ Common Misconceptions

• "Sinkholing stops all malware" — Only works if malware uses DNS
• "Sinkhole = blocking" — Sinkholing redirects; blocking would be DNS filtering
• "0.0.0.0 is the only sinkhole IP" — Can use any IP; 0.0.0.0 is just common
• "Set and forget" — Requires continuous feed updates and monitoring

🔗 Related Concepts

• Domain Name System (DNS)
• DNS Tunneling
• Domain Generation Algorithm (DGA)
• Fast Flux
• Command and Control (C2)
• Cyber Threat Intelligence (CTI)
• Indicator of Compromise (IOC)
• Security Information and Event Management system (SIEM)

📚 References

• SANS: DNS Sinkhole as a Defensive Weapon
• MITRE ATT&CK: T1071.004 (Application Layer Protocol: DNS)
• Spamhaus Domain Block List (DBL)
• abuse.ch URLhaus feed