dnsgen

Overview

dnsgen is a Python tool that generates new domain names from existing subdomains through permutation and combination techniques. It’s designed to complement DNS enumeration tools like massdns by creating candidate domain names that can then be resolved against DNS servers. This approach helps discover hidden subdomains and related domains that wouldn’t be found through traditional wordlist-based brute forcing alone.

The tool analyzes patterns in known subdomains and generates variations, making it effective for finding naming patterns and discovering previously unknown infrastructure.

Installation

Via pip

pip install dnsgen

From source

git clone https://github.com/AlexisAhmed/dnsgen.git
cd dnsgen
pip install -r requirements.txt
python dnsgen.py

Verify installation

dnsgen --version
dnsgen --help

Basic Usage

Command	Description
dnsgen input.txt	Generate combinations from domains in input.txt
dnsgen -	Read domains from stdin
dnsgen input.txt -o output.txt	Save generated domains to output.txt
dnsgen input.txt -l	List only valid/existing domain combinations

Common Patterns

Subdomain enumeration workflow

# 1. Get initial subdomains (using assetfinder, amass, etc.)
assetfinder example.com | sort -u > subdomains.txt

# 2. Generate combinations with dnsgen
dnsgen subdomains.txt > candidates.txt

# 3. Resolve candidates with massdns
massdns -r /path/to/resolvers.txt candidates.txt -t A -o S

Using with pipes

# Chain assetfinder to dnsgen
assetfinder example.com | dnsgen - > generated.txt

# Generate and immediately pipe to massdns
dnsgen subdomains.txt | massdns -r resolvers.txt - -t A

Generate from certificate transparency logs

# First get domains from CT logs
crt.sh -d example.com | dnsgen - > ct_generated.txt

# Or combine multiple sources
(assetfinder example.com && crt.sh -d example.com) | dnsgen -

Advanced Options

Flag	Usage	Description
-w, --wordlist	-w words.txt	Use custom wordlist for generation
-l, --limit	-l 100	Limit output to N results
-f, --fast	dnsgen -f input.txt	Fast mode, reduce permutations
-d, --domain	-d example.com	Specify target domain explicitly

Permutation Techniques

Mutation generation

dnsgen uses several generation methods:

# Given: api.prod.example.com, api.staging.example.com

# Inserts: adds prefix/suffix combinations
# - api.prod.staging.example.com
# - prod-staging.example.com

# Replacements: swaps environment names
# - api.dev.example.com
# - api.test.example.com

# Separators: tests delimiter variations
# - api_prod.example.com
# - api-prod.example.com

Filtering and Validation

Basic filtering

# Generate and filter for specific patterns
dnsgen input.txt | grep -E "^(api|admin|dev)" > filtered.txt

# Remove duplicates
dnsgen input.txt | sort -u > unique.txt

# Count generated domains
dnsgen input.txt | wc -l

Integration with resolution

# Generate, then validate with dig
dnsgen input.txt | while read domain; do
  dig +short "$domain" @8.8.8.8 && echo "$domain is valid"
done

# Or use nslookup
dnsgen input.txt | while read domain; do
  nslookup "$domain" 8.8.8.8 | grep -q "Name:" && echo "$domain"
done

Real-World Scenarios

Bug bounty reconnaissance

# Comprehensive subdomain enumeration
assetfinder example.com > subs.txt
amass enum -d example.com >> subs.txt
crt.sh -d example.com | awk -F',' '{print $NF}' >> subs.txt
sort -u subs.txt > unique_subs.txt

# Generate candidates
dnsgen unique_subs.txt > candidates.txt

# Resolve with public DNS
massdns -r /opt/massdns/lists/resolvers.txt candidates.txt -t A -o S > resolved.txt

Internal network enumeration

# Generate from known internal subdomains
dnsgen internal_subs.txt > internal_candidates.txt

# Resolve against internal DNS
massdns -r internal_resolvers.txt internal_candidates.txt -t A

Monitoring for new infrastructure

# Regularly generate candidates from known domains
dnsgen known_domains.txt > current_candidates.txt

# Compare to previous results
diff previous_candidates.txt current_candidates.txt | grep "^>" > new_candidates.txt

# Resolve new candidates
massdns -r resolvers.txt new_candidates.txt -t A

Output Examples

Sample output format

api.prod.example.com
api.staging.example.com
api.dev.example.com
api-prod.example.com
api_prod.example.com
prod.api.example.com
staging.api.example.com
dev.api.example.com
prod-api.example.com
prod_api.example.com

With validation (using -l flag)

dnsgen input.txt -l
# Only outputs domains that can be validated

Performance Tips

Limit permutations for large input sets

# Fast mode for large wordlists
dnsgen -f large_input.txt > output.txt

# Or pipe through head to limit
dnsgen input.txt | head -10000 > output.txt

Parallel processing with xargs

# Process subdomains in parallel
cat input.txt | xargs -I {} dnsgen {} >> combined.txt

Troubleshooting

Tool not found

# Verify installation location
which dnsgen

# Or run as module
python -m dnsgen input.txt

Empty output

# Check input file format (one domain per line)
cat input.txt

# Verify file isn't empty
wc -l input.txt

# Test with simple input
echo "example.com" | dnsgen -

Memory issues with large inputs

# Split input file
split -l 5000 large.txt chunk_

# Process chunks separately
for chunk in chunk_*; do
  dnsgen "$chunk" >> output.txt
done

Integration with Other Tools

With subfinder

subfinder -d example.com -o subs.txt
dnsgen subs.txt > candidates.txt
massdns -r resolvers.txt candidates.txt -t A

With httprobe for live host detection

dnsgen subdomains.txt | while read domain; do
  echo "$domain" | httprobe
done

With nuclei for vulnerability scanning

dnsgen subdomains.txt | massdns -r resolvers.txt - -t A | \
  cut -d' ' -f3 | nuclei -l - -t http-servers

Best Practices

• Use dnsgen as part of a multi-stage enumeration process
• Combine with other subdomain discovery tools for comprehensive results
• Regularly update input domain lists for better permutation patterns
• Use resolver lists with public, fast-responding nameservers
• Validate generated domains before bulk resolution to avoid false positives
• Document all discovered domains and their resolution status
• Respect rate limits when resolving large candidate lists

Resources

• GitHub: https://github.com/AlexisAhmed/dnsgen
• massdns: https://github.com/blechschmidt/massdns
• Subdomain enumeration guide: https://host.io/subdomain-enumeration