Full Report
Rick Wisser // All right, you’ve taken all the precautions related to your network. You have lockout controls in place, you use awesome password policies (20 characters with uppercase, lowercase, […] The post Are you Snoopable?! appeared first on Black Hills Information Security, Inc..
Analysis Summary
# Best Practices: DNS Cache Snooping Prevention
## Overview
These practices focus on preventing the information disclosure vulnerability known as DNS Cache Snooping. Attackers exploit this by querying an internal or external DNS server in a way that forces it to reveal its local cache, thereby discovering an organization's browsing habits, sensitive external connections (e.g., financial institutions), and internal network topography. This information is then used to craft highly effective targeted attacks like spear-phishing.
## Key Recommendations
### Immediate Actions
1. **Test External DNS Exposure:** Conduct an immediate audit of all public-facing or accessible DNS servers (e.g., those within DMZs or accessible from the internet) using DNS cache snooping tools (like Nmap's `dns-cache-snoop.nse` script) against a curated list of organization-specific sensitive domains.
2. **Restrict Internal DNS Queries:** Ensure that internal, authoritative DNS servers are configured *not* to respond authoritatively to queries originating from external (untrusted) networks.
3. **Disable or Limit Recursion for External Sources:** Configure public-facing resolvers to either disable recursive lookup functionality entirely or explicitly only allow recursion for known, trusted internal IP ranges.
### Short-term Improvements (1-3 months)
1. **Implement Internal Testing:** Run DNS cache snooping checks from various points *inside* the network perimeter to identify which internal servers are vulnerable and what level of information disclosure is occurring within the LAN.
2. **Review Domain Blocking:** If using DNS filtering, verify that the implemented block lists are effective, and test specifically for unlisted sensitive domains (e.g., financial sites, known collaboration platforms) that may have been visited recently.
3. **Implement Transaction ID and Source Port Randomization (If Possible):** Where DNS server software allows, ensure high levels of entropy for transaction IDs and source ports to mitigate the chances of successful predictive attacks, hardening against related attacks like DNS Cache Poisoning.
### Long-term Strategy (3+ months)
1. **Consolidate and Harden DNS Infrastructure:** Audit the necessity of maintaining multiple internal DNS servers. Consolidate functionality where possible and apply strict hardening baselines (e.g., running the latest stable version, minimizing installed services).
2. **Adopt DNSSEC (DNS Security Extensions):** Plan and implement DNSSEC validation and signing across all authoritative domains to ensure data integrity and authenticity, mitigating related risks like cache poisoning impacting reputation.
3. **Establish Regular Auditing Schedule:** Formalize a recurring security hygiene schedule that includes quarterly penetration testing focused specifically on reconnaissance posturing, including DNS enumeration techniques.
## Implementation Guidance
### For Small Organizations
- Rely primarily on external, vetted DNS providers (e.g., enterprise-grade ISP recursive resolvers) rather than managing internal recursive servers, reducing local attack surface. If an internal server is required, ensure it only serves local records and disables all recursive queries.
- Use free or low-cost tools (like Nmap scripts) monthly to test external-facing DNS resolve endpoints.
### For Medium Organizations
- Implement separate, segregated DNS servers: one set for internal recursive resolution (tightly controlled access) and one set for authoritative zone hosting.
- Utilize firewall rules to explicitly deny UDP/TCP port 53 traffic initiated from the internet directly to internal recursive resolvers.
### For Large Enterprises
- Deploy split-horizon DNS services where internal clients receive internal IP information and external visibility only sees public-facing records.
- Implement dedicated, hardened DNS appliance infrastructure (or cloud-managed services) and enforce least privilege access to configuration panels across all resolvers.
- Benchmark DNS configurations against CIS Benchmarks for BIND or Microsoft DNS Server.
## Configuration Examples
*Note: Specific configuration syntax varies by DNS server (BIND, Windows DNS, etc.). The concept is to prevent information leakage.*
**Concept for BIND (Disabling Recursion for Untrusted Sources):**
text
options {
// Allow recursion only for specific internal subnets
allow-recursion { internal_nets; };
// Prevent external snooping attempts
allow-query { any; };
// Disable zone transfers unless needed for secondaries
allow-transfer { none; };
};
**Testing Methodology using Nmap:**
To test if an external DNS server (e.g., a public resolver) leaks cached data for your domain:
bash
nmap -sU -p 53 --script dns-cache-snoop --script-args='dns-cache-snoop.domains=mycompany.com,financialpartner.com' <Target_DNS_IP>
To test for timed results to see the *most recent* queries (if the server supports it):
bash
nmap -sU -p 53 --script dns-cache-snoop --script-args='dns-cache-snoop.domains=mycompany.com,anydomain.com;dns-cache-snoop.mode=timed' <Target_DNS_IP>
## Compliance Alignment
- **NIST SP 800-53 (AC-2):** Account Management (Ensuring configuration adheres to least-privilege principles for network services).
- **ISO/IEC 27002 (A.13.1.3):** Network Controls (Implementing controls to prevent unauthorized access to network services).
- **CIS Benchmarks:** Specific configurations for hardening DNS server software (e.g., BIND or Windows DNS).
## Common Pitfalls to Avoid
- **Assuming Internal Network Protection is Enough:** Attackers frequently target DNS resolvers accessible via the DMZ or misconfigured edge servers, treating DNS as a simple information retrieval service rather than a sensitive reconnaissance asset.
- **Ignoring Public Resolvers:** Failing to test public/third-party recursive resolvers that internal users might default to, as these can reveal browsing habits to an external party monitoring that resolver.
- **Over-reliance on Obscure Password Policies:** The article highlights that extreme application security is useless if an attacker can gain the same information (or more) simply by reading DNS logs.
## Resources
- **Nmap Documentation:** Official Nmap scripting engine documentation, specifically regarding the `dns-cache-snoop.nse` script.
- **CIS Benchmarks:** Current security configuration guides for DNS servers (e.g., for BIND or Microsoft DNS).
- **DNSSEC Documentation:** Resources on implementing DNS Security Extensions for data integrity.