--- name: hunting-for-dns-tunneling-with-zeek-c9067d9f8095 description: Detects DNS tunneling and covert-channel data exfiltration by analyzing Zeek dns.log for high-entropy subdomain queries, excessive query volume, abnormally long query lengths, and unusual DNS record types (TXT/NULL/CNAME). Use when hunting for DNS-based data exfiltration or C2 covert channels in network traffic, or when triaging suspicious DNS query volume/patterns surfaced by Zeek logs. domain: cybersecurity subdomain: threat-hunting tags: - threat-hunting - dns-tunneling - zeek - data-exfiltration - covert-channel - mitre-t1071-004 - network-monitoring version: '1.0' author: mahipal license: Apache-2.0 d3fend_techniques: - Application Protocol Command Analysis - Network Isolation - Network Traffic Analysis - Client-server Payload Profiling - DNS Traffic Analysis nist_csf: - DE.CM-01 - DE.AE-02 - DE.AE-07 - ID.RA-05 mitre_attack: - T1046 - T1057 - T1082 - T1083 - T1048 --- # Hunting for DNS Tunneling with Zeek ## When to Use - When hunting for data exfiltration over DNS covert channels - After threat intelligence indicates DNS-based C2 frameworks targeting your industry - When dns.log shows unusually high query volumes to specific domains - During investigation of suspected data theft where no HTTP/S exfiltration is found - When monitoring for tools like iodine, dnscat2, DNSExfiltrator, or DNS-over-HTTPS tunneling ## Prerequisites - Zeek deployed on network tap or SPAN port capturing DNS traffic - Zeek dns.log with full query and response fields - SIEM platform for dns.log analysis (Splunk, Elastic) - RITA (Real Intelligence Threat Analytics) for automated DNS analysis - Passive DNS data for historical domain resolution context ## Workflow 1. **Analyze Query Length Distribution**: DNS tunneling encodes data in subdomain labels, producing queries significantly longer than normal. Normal DNS queries average 20-30 characters; tunneling queries often exceed 50+ characters. Calculate mean and standard deviation of query lengths per domain. 2. **Calculate Subdomain Entropy**: Tunneling encodes data using Base32/Base64, producing high-entropy subdomain strings. Calculate Shannon entropy of subdomain labels -- values above 3.5 bits/character strongly suggest encoded data. 3. **Count Unique Subdomains Per Domain**: Legitimate domains have relatively few unique subdomains. DNS tunneling generates hundreds or thousands of unique subdomains under a single parent domain. 4. **Monitor DNS Record Type Distribution**: TXT, NULL, CNAME, and MX records can carry more data than A records. Excessive TXT queries to a single domain indicate data transfer via DNS. 5. **Detect High Query Volume**: Flag domains receiving more than 100 queries per hour from a single source, especially when combined with high subdomain uniqueness. 6. **Analyze Query Timing**: DNS tunneling tools produce regular query patterns (beaconing) or burst patterns (data transfer). Apply frequency analysis to DNS query timestamps. 7. **Cross-Reference with conn.log**: Correlate DNS queries with connection metadata to identify the process or endpoint generating suspicious queries. 8. **Validate with Domain Intelligence**: Check suspicious domains against WHOIS data, certificate transparency, and threat intelligence feeds. ## Key Concepts | Concept | Description | |---------|-------------| | T1071.004 | Application Layer Protocol: DNS | | T1048.003 | Exfiltration Over Alternative Protocol: DNS | | T1572 | Protocol Tunneling | | Shannon Entropy | Measure of randomness in subdomain strings | | Zeek dns.log | DNS query/response metadata | | RITA | Automated DNS tunneling detection from Zeek logs | | iodine | IPv4-over-DNS tunneling tool | | dnscat2 | DNS-based command-and-control tool | | DNSExfiltrator | Data exfiltration tool using DNS requests | ## Detection Queries ### Zeek Script -- DNS Tunnel Detection ```zeek @load base/protocols/dns module DNSTunnel; export { redef enum Notice::Type += { DNSTunnel::Long_DNS_Query }; const query_length_threshold = 50 &redef; const query_count_threshold = 100 &redef; } event dns_request(c: connection, msg: dns_msg, query: string, qtype: count, qclass: count) { if ( |query| > query_length_threshold ) { NOTICE([$note=DNSTunnel::Long_DNS_Query, $msg=fmt("Long DNS query detected: %s (%d chars)", query, |query|), $conn=c]); } } ``` ### Splunk -- DNS Tunneling Indicators from Zeek ```spl index=zeek sourcetype=bro_dns | rex field=query "(?[^.]+)\.(?[^.]+\.[^.]+)$" | stats count dc(subdomain) as unique_subs avg(len(query)) as avg_len max(len(query)) as max_len by src basedomain | where count > 100 AND (unique_subs > 50 OR avg_len > 40) | sort -unique_subs ``` ### Splunk -- High Entropy Subdomain Detection ```spl index=zeek sourcetype=bro_dns | rex field=query "^(?[^.]+)" | where len(subdomain) > 20 | eval char_count=len(subdomain) | stats count dc(query) as unique_queries avg(char_count) as avg_sub_len by src query_type_name basedomain | where unique_queries > 30 AND avg_sub_len > 25 | sort -unique_queries ``` ### RITA Analysis ```bash rita import /path/to/zeek/logs dataset_name rita show-dns-fqdn-ips-long dataset_name rita show-exploded-dns dataset_name rita show-dns-tunneling dataset_name --csv > dns_tunnel_results.csv ``` ## Common Scenarios 1. **dnscat2 C2**: Encodes command-and-control traffic in DNS CNAME/TXT queries with Base64-encoded subdomain labels. Produces high query volumes with long, high-entropy subdomains. 2. **iodine IPv4 Tunnel**: Creates a virtual network interface tunneling all IP traffic through DNS. Generates massive DNS query volumes with NULL record types. 3. **Data Exfiltration via DNS**: Sensitive data encoded in subdomain labels (e.g., `aGVsbG8gd29ybGQ.exfil.attacker.com`), sent as A or TXT queries. Each query carries ~63 bytes of data. 4. **DNS-over-HTTPS Tunneling**: Bypasses traditional DNS monitoring by sending DNS queries over HTTPS to public resolvers (8.8.8.8, 1.1.1.1), requiring TLS inspection for detection. 5. **Cobalt Strike DNS Beacon**: Uses DNS A/TXT records for C2 communication with configurable subdomain encoding schemes. ## Output Format ``` Hunt ID: TH-DNSTUNNEL-[DATE]-[SEQ] Source IP: [Internal IP] Source Host: [Hostname] Target Domain: [Base domain] Query Count: [Total queries in window] Unique Subdomains: [Count] Avg Query Length: [Characters] Max Query Length: [Characters] Subdomain Entropy: [Bits per character] Primary Record Type: [A/TXT/CNAME/NULL] Data Volume Estimate: [Bytes exfiltrated] Risk Level: [Critical/High/Medium/Low] ``` --- Source attribution: hironow Original project: https://github.com/hironow/cyber/blob/20efb3a6a2a9e7ebbde38ed32325979866110540/.agents/skills/hunting-for-dns-tunneling-with-zeek/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: hironow Original project: https://github.com/hironow/cyber/blob/20efb3a6a2a9e7ebbde38ed32325979866110540/.agents/skills/hunting-for-dns-tunneling-with-zeek/SKILL.md Attribution basis: Source repository record Fields of work: Data & Analytics, Development & Engineering Roles: Data Analyst, Security Engineer Specialties: API, Testing Tools: Rita, Zeek Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: c7550cb3c2b9eeb1b4c81b9b41eb1e137992f4130935961faa7da082e5aa05df