What it does. 'Detects DNS tunneling by computing Shannon entropy of DNS query names, Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill performing-dns-tunneling-detection, or copy the skill folder into ~/.claude/skills/performing-dns-tunneling-detection/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/performing-dns-tunneling-detection/SKILL.md

SKILL.md (verbatim)

name: performing-dns-tunneling-detection
description: 'Detects DNS tunneling by computing Shannon entropy of DNS query names,
  analyzing query length distributions, inspecting TXT record payloads, and identifying
  high subdomain cardinality. Uses scapy for packet capture analysis and statistical
  methods to distinguish legitimate DNS from covert channels. Use when hunting for
  data exfiltration.

  '
domain: cybersecurity
subdomain: security-operations
tags:
- dns-tunneling
- exfiltration-detection
- shannon-entropy
- dns-analysis
- threat-detection
- security-operations
version: '1.0'
author: mahipal
license: Apache-2.0
nist_csf:
- DE.CM-01
- RS.MA-01
- GV.OV-01
- DE.AE-02
mitre_attack:
- T1078
- T1190
- T1059
- T1048
- T1041

Performing DNS Tunneling Detection

When to Use

• When conducting security assessments that involve performing dns tunneling detection
• When following incident response procedures for related security events
• When performing scheduled security testing or auditing activities
• When validating security controls through hands-on testing

Prerequisites

• Familiarity with security operations concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

Instructions

Analyze DNS traffic for indicators of DNS tunneling using entropy analysis and statistical methods on query name characteristics.

import math
from collections import Counter

def shannon_entropy(data):
    if not data:
        return 0
    counter = Counter(data)
    length = len(data)
    return -sum((c/length) * math.log2(c/length) for c in counter.values())

# Legitimate domain: low entropy (~3.0-3.5)
print(shannon_entropy("www.google.com"))
# DNS tunnel: high entropy (~4.0-5.0)
print(shannon_entropy("aGVsbG8gd29ybGQ.tunnel.example.com"))

Key detection indicators:

1. High Shannon entropy in query names (> 3.5 for subdomain labels)
2. Unusually long query names (> 50 characters)
3. High volume of TXT record requests to a single domain
4. High unique subdomain count per parent domain
5. Non-standard character distribution in labels

Examples

from scapy.all import rdpcap, DNS, DNSQR
packets = rdpcap("dns_traffic.pcap")
for pkt in packets:
    if pkt.haslayer(DNSQR):
        query = pkt[DNSQR].qname.decode()
        entropy = shannon_entropy(query)
        if entropy > 4.0:
            print(f"Suspicious: {query} (entropy={entropy:.2f})")

Other files in this skill

• LICENSE
• references/api-reference.md
• scripts/agent.py

references/api-reference.md (verbatim)

API Reference: Performing DNS Tunneling Detection

scapy (DNS Packet Analysis)

from scapy.all import rdpcap, DNS, DNSQR, DNSRR, sniff

# Read from PCAP
packets = rdpcap("traffic.pcap")
for pkt in packets:
    if pkt.haslayer(DNSQR):
        qname = pkt[DNSQR].qname.decode()
        qtype = pkt[DNSQR].qtype  # 1=A, 16=TXT, 28=AAAA

# Live capture
def dns_callback(pkt):
    if pkt.haslayer(DNSQR):
        print(pkt[DNSQR].qname)

sniff(filter="udp port 53", prn=dns_callback, count=100)

Shannon Entropy Calculation

import math
from collections import Counter

def shannon_entropy(data):
    counter = Counter(data)
    length = len(data)
    return -sum((c/length) * math.log2(c/length)
                for c in counter.values())

# Normal domain: ~2.5-3.5 bits
# DNS tunnel:    ~4.0-5.0 bits

DNS Tunneling Indicators

Common DNS Tunnel Tools

References

• scapy: https://scapy.readthedocs.io/en/latest/
• DNS tunneling detection: https://www.sans.org/white-papers/dns-tunneling/
• iodine: https://github.com/yarrick/iodine

Back to mukul975/Anthropic-Cybersecurity-Skills (817 security skills) or Agent skills.