DNSGen: Cloud-Aware Subdomain Permutation for Modern Infrastructure Reconnaissance

Hook

Traditional subdomain brute-forcing treats api.example.com and dashboard.example.com identically—just entries in a wordlist. DNSGen extracts 'api' and 'dashboard' as context, generating api-v2-prod-us-east-1.example.com because it understands how modern infrastructure actually works.

Context

Subdomain enumeration has been a cornerstone of reconnaissance since the early days of penetration testing. For years, security researchers relied on static wordlists combined with DNS brute-forcing: throw 10,000 common subdomain names at a domain and see what resolves. Tools like dnsrecon and fierce dominated this space, essentially performing dictionary attacks against DNS infrastructure.

But modern cloud architecture broke this model. When organizations deploy applications across AWS regions, implement blue-green deployments, or adopt microservice architectures, their subdomain naming patterns become highly structured yet unpredictable. A company might use api-gateway-prod-us-west-2.example.com while their competitor uses usw2-prod-api.example.com. Static wordlists can't capture these variations without becoming unwieldy. DNSGen emerged to solve this problem: instead of guessing blindly, it analyzes existing discovered subdomains, extracts meaningful components, and generates intelligent permutations based on how modern DevOps teams actually name things. It's the difference between a dictionary attack and a context-aware attack that learns from your target's naming conventions.

Technical Insight

DNSGen's architecture centers on a modular permutation engine that treats domain components as semantic units rather than strings. When you feed it a subdomain like staging-api.example.com, it doesn't just see characters—it identifies 'staging' and 'api' as meaningful tokens that can be recombined, prefixed, suffixed, and mixed with infrastructure patterns.

The tool's workflow operates in three phases: extraction, permutation, and output. During extraction, DNSGen parses input domains and splits them on delimiters (hyphens, dots, underscores), building a vocabulary of words specific to your target. The permutation phase then applies multiple strategies simultaneously. Here's where it gets interesting: instead of simple concatenation, DNSGen includes permutators specifically designed for cloud patterns.

Consider this basic usage example:

echo 'api.example.com' | dnsgen -

This generates variations like:

api-1.example.com
api-2.example.com
api-dev.example.com
api-staging.example.com
api-prod.example.com
dev-api.example.com
v1-api.example.com
api-v2.example.com
api-internal.example.com

But the real power emerges with the cloud-aware permutations. If DNSGen sees existing subdomains like us-west-2-api.example.com, it recognizes the AWS region pattern and generates permutations for other regions:

cat discovered_subdomains.txt | dnsgen - | massdns -r resolvers.txt -t A -o S

Behind the scenes, DNSGen maintains permutator classes for different strategies. The cloud permutator knows AWS regions (us-east-1, eu-central-1), Azure patterns (eastus, westeurope), and GCP zones (us-central1-a). The microservice permutator adds prefixes like internal-, public-, gateway-, mesh-. The number permutator intelligently increments versions (v1 → v2, api2 → api3) rather than just appending random digits.

You can also feed DNSGen a custom wordlist to combine with extracted tokens:

cat subdomains.txt | dnsgen -w /path/to/custom_words.txt -

The wordlist format supports comments (lines starting with #), making it easy to organize infrastructure-specific terminology:

# Environments
dev
staging
prod
uat

# Services
gateway
auth
payment

# Infrastructure
kubernetes
k8s
docker

For quick reconnaissance, the fast mode (-f or --fast) reduces the permutation depth, generating fewer but higher-probability candidates. This is useful during initial enumeration when you want quick wins before committing to exhaustive generation.

The architecture decision to make DNSGen a pure permutation generator—without built-in DNS resolution—is deliberate and reflects Unix philosophy. By piping output to specialized resolvers like massdns or puredns, you can leverage tools optimized for high-performance DNS queries with features like wildcard detection and rate limiting. DNSGen focuses on doing one thing well: generating intelligent permutations based on context and modern infrastructure patterns.

Gotcha

DNSGen's biggest limitation is permutation explosion. Feed it a complex subdomain structure with multiple words, and the output can balloon to hundreds of thousands of candidates. For example, if you input api-gateway-v2-prod-us-east-1.example.com, DNSGen extracts six meaningful components and applies multiple permutation strategies to each, resulting in a combinatorial explosion. Without careful filtering or understanding of your target's naming conventions, you'll generate massive wordlists that are impractical to resolve, especially if you're working within rate-limit constraints or trying to stay under the radar.

The lack of built-in DNS resolution also creates workflow friction. You must pipe DNSGen's output to an external resolver, which means managing another tool's installation, configuration, and quirks. More critically, there's a time-of-check-time-of-use gap: subdomains might change between generation and resolution, and if your resolver fails or you need to re-run queries, you must regenerate permutations or save intermediate results. Tools like puredns or subfinder offer integrated resolution with wildcard filtering, making them simpler for practitioners who want fewer moving parts. DNSGen's modularity is powerful for experts who want granular control, but it adds complexity that not every reconnaissance workflow needs.

Verdict

Use DNSGen if you're conducting deep reconnaissance against organizations using modern cloud infrastructure, microservices, or complex deployment patterns where context-aware permutations will uncover subdomains that static wordlists miss. It's ideal for bug bounty hunters and red teamers who already have DNS enumeration pipelines with massdns or similar resolvers, and who need the intelligence layer that generates region-specific, environment-aware, and version-incremented variations. The tool shines when you've already discovered some subdomains through passive reconnaissance and want to expand laterally using the target's own naming conventions as a template. Skip DNSGen if you need an all-in-one solution with built-in resolution and wildcard handling—puredns or subfinder are better choices for simplified workflows. Also avoid it if you're targeting simple infrastructure without cloud deployment patterns, where traditional wordlist-based brute-forcing is sufficient and faster. Finally, skip it if you're uncomfortable managing permutation explosion or lack experience tuning output volumes—you'll generate unwieldy lists that waste resolution resources without careful input filtering or fast mode usage.