dns-reverse

A simple Go utility for performing reverse DNS (PTR) lookups, both in single and batch mode.

Features

• Single IP lookup: dns-reverse <ip> [dns-server]
• Batch mode: Process a file of IP addresses (plain text or CSV) and output results to CSV.
• Custom DNS server: Specify an alternative DNS server for lookups.
• Configurable timeout: Set the timeout for each DNS query.
• Parallel processing: Batch mode uses a worker pool for concurrent lookups (configurable number of workers).
• Verbose output: Optional logging for debugging.
• Version information: Print the version with -version.
• Robust input handling: Accepts plain text (one IP per line) or CSV (first column), ignores empty lines and comments (lines starting with #).

Installation

go install github.com/yourusername/dns-reverse@latest

Or build from source:

go build -o dns-reverse

Usage

Single Mode

dns-reverse 8.8.8.8
dns-reverse 8.8.8.8 8.8.4.4   # Use 8.8.4.4 as the DNS server

Batch Mode

dns-reverse -batch -i input.txt -o output.csv
dns-reverse -batch -i input.txt -o output.csv -s 1.1.1.1 -w 20 -t 30s

Input File Formats

The tool automatically detects the format based on file extension:

• .csv files are parsed as CSV (first column = IP)
• All other files are treated as plain text (one IP per line)

Plain text (examples/example_ips.txt):

8.8.8.8
8.8.4.4
1.1.1.1

CSV (examples/example_ips.csv):

IP,Description
8.8.8.8,Google Public DNS
8.8.4.4,Google Public DNS Secondary

Lines starting with # are treated as comments and ignored. Empty lines are also ignored.

Output CSV

The output CSV always has three columns:

• IP: The input IP address.
• Hostname: The PTR record(s) found, separated by ; if multiple.
• Status: OK for success, No hostname if no PTR record, or Error: <description> for failures.

Flags

Flag	Description	Default
-batch	Run in batch mode (requires -i and -o)	false
-i	Input file path (batch mode)	""
-o	Output file path (batch mode)	""
-s	Custom DNS server (IP address)	"" (system default)
-t	Timeout for each DNS query (e.g., 30s, 1m30s)	10s
-w	Number of concurrent workers (batch mode)	10
-v	Verbose output (logs progress and debug info)	false
-version	Print version and exit	false

Examples

Single lookup with custom DNS and timeout

dns-reverse -t 5s 9.9.9.9 8.8.8.8

Batch processing with 15 workers and verbose logging

dns-reverse -batch -i ips.csv -o results.csv -w 15 -v

Using example files

# Using plain text input
dns-reverse -batch -i examples/example_ips.txt -o examples/output.csv

# Using CSV input
dns-reverse -batch -i examples/example_ips.csv -o examples/output.csv

See the examples/ directory for sample input files:

• example_ips.txt - Plain text format
• example_ips.csv - CSV format
• example_output.csv - Expected output format

Testing

Run the unit tests:

go test ./...

License

MIT