How to remediate – DNS Server Spoofed Request Amplification DDoS

1. Introduction

The DNS Server Spoofed Request Amplification DDoS vulnerability allows attackers to use your DNS server in a distributed denial-of-service attack against other targets. This happens because the DNS server responds to all requests without restrictions, enabling ‘amplification’ of malicious traffic. Businesses can experience service outages and reputational damage if their DNS servers are compromised. Affected systems are typically publicly accessible DNS servers running standard configurations. Impact on confidentiality is low, integrity is low, and availability is high.

2. Technical Explanation

The root cause of this vulnerability is an open resolver configuration where the DNS server answers queries from any source without access controls. Attackers exploit this by sending spoofed requests to the DNS server, making it appear as if legitimate users are requesting information. The DNS server’s response is larger than the original request, amplifying the attack traffic directed towards a victim host. CVE-2006-0987 describes this issue.

• Root cause: Lack of access control on DNS queries.
• Exploit mechanism: An attacker sends spoofed DNS requests to amplify traffic against a target. For example, an attacker could query the root zone NS records with a spoofed source IP address belonging to the victim’s network.
• Scope: Publicly accessible DNS servers are affected.

3. Detection and Assessment

You can confirm if your system is vulnerable by checking its resolver configuration and monitoring for unusual query patterns. A quick check involves verifying if it responds to queries from external networks.

• Quick checks: Use the dig command with a public DNS server to see if it answers requests from an external IP address.
• Scanning: Nessus plugin ID 30894 can identify open resolvers. This is provided as an example only.
• Logs and evidence: Examine DNS server logs for excessive query traffic originating from unknown sources or unusual patterns of queries to root zone servers (‘.’). Look in /var/log/syslog or similar, depending on your distribution.

dig @your_dns_server example.com +trace

4. Solution / Remediation Steps

Fix the issue by restricting access to your DNS server from public networks or reconfiguring it to reject unauthorized queries.

4.1 Preparation

• Ensure you have a rollback plan in case of issues: revert configuration changes or restore from the snapshot. A change window may be needed to minimize disruption.

4.2 Implementation

1. Step 1: Edit the DNS server configuration file (e.g., /etc/named.conf on Linux).
2. Step 2: Add an ‘allow-recursion’ clause specifying trusted networks only.
3. Step 3: Restart the DNS service to apply the changes.

4.3 Config or Code Example

Before

options {
  recursion yes;
};

After

options {
  recursion yes;
  allow-recursion { 127.0.0.1; 192.168.1.0/24; }; // Replace with your trusted networks
};

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Least privilege: Restrict access to the DNS server only to authorized networks and users, reducing the potential impact if exploited.
• Safe defaults: Configure DNS servers with secure defaults that disable recursion for untrusted networks.

4.5 Automation (Optional)

Ansible can automate configuration changes.

---
- hosts: dns_servers
  become: true
  tasks:
    - lineinfile:
        path: /etc/named.conf
        regexp: '^options {'
        line: 'options {n  recursion yes;n  allow-recursion { 127.0.0.1; 192.168.1.0/24; };' # Replace with your trusted networks
    - service:
        name: named
        state: restarted

5. Verification / Validation

Confirm the fix by verifying that the DNS server no longer responds to queries from untrusted networks and monitoring for normal query patterns.

• Post-fix check: Run dig @your_dns_server example.com +trace from an external network; it should timeout or return a SERVFAIL error.
• Re-test: Repeat the initial detection method (using dig) to confirm that the server no longer answers queries from untrusted networks.
• Smoke test: Verify that internal DNS resolution continues to function normally.
• Monitoring: Monitor DNS server logs for excessive query traffic or unexpected patterns, and set up alerts if anomalies are detected.

dig @your_dns_server example.com +trace

6. Preventive Measures and Monitoring

Implement preventive measures to avoid similar issues in the future.

• Baselines: Update your security baseline or policy to include restrictions on DNS resolver access, such as requiring allow-recursion clauses.
• Asset and patch process: Review DNS server configurations regularly (e.g., quarterly) as part of your asset management process.

7. Risks, Side Effects, and Roll Back

Changes may disrupt legitimate DNS resolution if configured incorrectly.

• Roll back: Restore the original DNS server configuration file from your snapshot, and restart the service.

8. References and Resources

Links related to this specific vulnerability.

• Vendor advisory or bulletin: No vendor advisory available for this general issue.
• NVD or CVE entry: https://nvd.nist.gov/vuln/detail/CVE-2006-0987
• Product or platform documentation relevant to the fix: Consult your DNS server’s documentation for configuration details.