Detecting Exfiltration Over DNS With Zeek

What it does

When your agent uses it

• Investigating suspected DNS tunneling
• Covert C2 over DNS
• Data exfiltration hidden in DNS queries against network traffic captured by Zeek

Example prompts

• “/detecting-exfiltration-over-dns-with-zeek”

Requirements

• Python 3

Workflow steps

8 steps, taken from the first numbered list in SKILL.md.

1. 1Parse Zeek dns.log headers: Read the TSV file, extract the #fields header line to identify column positions for ts, id.orig_h, query…
2. 2Extract and decompose queries: For each DNS query, split the FQDN into subdomain labels and parent domain. Skip queries to known safe…
3. 3Compute Shannon entropy: Calculate the information entropy of each subdomain label. Legitimate subdomains typically have entropy below…
4. 4Detect long labels: Flag DNS labels exceeding 52 characters (approaching the 63-character maximum). Long labels are a strong indicator of…
5. 5Count unique subdomains per domain: Track how many distinct subdomains each parent domain receives. Domains with more than 50 unique…
6. 6Identify query volume anomalies: Calculate queries-per-minute per source IP per domain. Exfiltration tools generate sustained high-volume…
7. 7Score and rank domains: Combine entropy, label length, uniqueness count, and query volume into a composite risk score. Rank domains by…
8. 8Generate detection report: Produce a JSON report with flagged domains, their evidence indicators, originating source IPs, and recommended…

What it can do on your machine

Read from SKILL.md and the folder at commit 54a7988. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Ships 1 file in scripts/ (Python), which the agent can run.

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Detecting Exfiltration Over DNS With Zeek loads about 1k tokens when it runs, and up to ~1.7k if it reads all its reference files. Until then it costs about 88 tokens; SKILL.md has 408 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~88

When it runs · the whole SKILL.md, loaded when a task matches

~1k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~1.7k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); the scripts in this folder are not scanned.

SKILL.md

The full file from mukul975/Anthropic-Cybersecurity-Skills at commit 54a7988, republished under its Apache-2.0 licence (© mukul975). 408 words, ~1,021 tokens.

name

detecting-exfiltration-over-dns-with-zeek

description

Detect DNS-based data exfiltration by analyzing Zeek dns.log for high-entropy subdomains, oversized TXT/NULL records, and anomalous query volume or patterns. Use when investigating suspected DNS tunneling, covert C2 over DNS, or data exfiltration hidden in DNS queries against network traffic captured by Zeek.

domain

cybersecurity

subdomain

network-security

tags

dns-exfiltration, zeek, entropy-analysis, threat-hunting

version

1.0

author

mahipal

license

Apache-2.0

nist_csf

PR.IR-01, DE.CM-01, ID.AM-03, PR.DS-02

mitre_attack

T1046, T1040, T1557, T1071, T1048

{
  "analysis_summary": {
    "total_queries_analyzed": 145832,
    "unique_domains": 3421,
    "flagged_domains": 3,
    "entropy_threshold": 3.5
  },
  "flagged_domains": [
    {
      "domain": "data.evil-c2.com",
      "unique_subdomains": 892,
      "avg_entropy": 4.72,
      "max_label_length": 61,
      "source_ips": ["10.0.1.45"],
      "risk_score": 9.4,
      "indicators": ["high_entropy", "long_labels", "high_subdomain_count"]
    }
  ]
}

© mukul975, Apache-2.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 3 other files (scripts, references) in skills/detecting-exfiltration-over-dns-with-zeek of mukul975/Anthropic-Cybersecurity-Skills.

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

Open the folder on GitHubat commit 54a7988