MaxMind DB Reader for Go

This is a Go reader for the MaxMind DB format. Although this can be used to read GeoLite2 and GeoIP2 databases, geoip2 provides a higher-level API for doing so.

This is not an official MaxMind API.

Installation

go get github.com/oschwald/maxminddb-golang/v2

Version 2.0 Features

Version 2.0 includes significant improvements:

• Modern API: Uses netip.Addr instead of net.IP for better performance
• Custom Unmarshaling: Implement Unmarshaler interface for zero-allocation decoding
• Network Iteration: Iterate over all networks in a database with Networks() and NetworksWithin()
• Enhanced Performance: Optimized data structures and decoding paths
• Go 1.24+ Support: Takes advantage of modern Go features including iterators
• Better Error Handling: More detailed error types and improved debugging
• Integrity Checks: Validate databases with Reader.Verify() and access metadata helpers such as Metadata.BuildTime()

See MIGRATION.md for guidance on updating existing v1 code.

Quick Start

package main

import (
	"fmt"
	"log"
	"net/netip"

	"github.com/oschwald/maxminddb-golang/v2"
)

func main() {
	db, err := maxminddb.Open("GeoLite2-City.mmdb")
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	ip, err := netip.ParseAddr("81.2.69.142")
	if err != nil {
		log.Fatal(err)
	}

	var record struct {
		Country struct {
			ISOCode string            `maxminddb:"iso_code"`
			Names   map[string]string `maxminddb:"names"`
		} `maxminddb:"country"`
		City struct {
			Names map[string]string `maxminddb:"names"`
		} `maxminddb:"city"`
	}

	err = db.Lookup(ip).Decode(&record)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("Country: %s (%s)\n", record.Country.Names["en"], record.Country.ISOCode)
	fmt.Printf("City: %s\n", record.City.Names["en"])
}

Usage Patterns

Basic Lookup

db, err := maxminddb.Open("GeoLite2-City.mmdb")
if err != nil {
	log.Fatal(err)
}
defer db.Close()

var record any
ip := netip.MustParseAddr("1.2.3.4")
err = db.Lookup(ip).Decode(&record)

Custom Struct Decoding

type City struct {
	Country struct {
		ISOCode string `maxminddb:"iso_code"`
		Names   struct {
			English string `maxminddb:"en"`
			German  string `maxminddb:"de"`
		} `maxminddb:"names"`
	} `maxminddb:"country"`
}

var city City
err = db.Lookup(ip).Decode(&city)

High-Performance Custom Unmarshaling

type FastCity struct {
	CountryISO string
	CityName   string
}

func (c *FastCity) UnmarshalMaxMindDB(d *maxminddb.Decoder) error {
	mapIter, size, err := d.ReadMap()
	if err != nil {
		return err
	}
	// Pre-allocate with correct capacity for better performance
	_ = size // Use for pre-allocation if storing map data
	for key, err := range mapIter {
		if err != nil {
			return err
		}
		switch string(key) {
		case "country":
			countryIter, _, err := d.ReadMap()
			if err != nil {
				return err
			}
			for countryKey, countryErr := range countryIter {
				if countryErr != nil {
					return countryErr
				}
				if string(countryKey) == "iso_code" {
					c.CountryISO, err = d.ReadString()
					if err != nil {
						return err
					}
				} else {
					if err := d.SkipValue(); err != nil {
						return err
					}
				}
			}
		default:
			if err := d.SkipValue(); err != nil {
				return err
			}
		}
	}
	return nil
}

Network Iteration

// Iterate over all networks in the database
for result := range db.Networks() {
	var record struct {
		Country struct {
			ISOCode string `maxminddb:"iso_code"`
		} `maxminddb:"country"`
	}
	err := result.Decode(&record)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s: %s\n", result.Prefix(), record.Country.ISOCode)
}

// Iterate over networks within a specific prefix
prefix := netip.MustParsePrefix("192.168.0.0/16")
for result := range db.NetworksWithin(prefix) {
	// Process networks within 192.168.0.0/16
}

Path-Based Decoding

var countryCode string
err = db.Lookup(ip).DecodePath(&countryCode, "country", "iso_code")

var cityName string
err = db.Lookup(ip).DecodePath(&cityName, "city", "names", "en")

Supported Database Types

This library supports all MaxMind DB (.mmdb) format databases, including:

MaxMind Official Databases:

• GeoLite/GeoIP City: Comprehensive location data including city, country, subdivisions
• GeoLite/GeoIP Country: Country-level geolocation data
• GeoLite ASN: Autonomous System Number and organization data
• GeoIP Anonymous IP: Anonymous network and proxy detection
• GeoIP Enterprise: Enhanced City data with additional business fields
• GeoIP ISP: Internet service provider information
• GeoIP Domain: Second-level domain data
• GeoIP Connection Type: Connection type identification

Third-Party Databases:

• DB-IP databases: Compatible with DB-IP's .mmdb format databases
• IPinfo databases: Works with IPinfo's MaxMind DB format files
• Custom databases: Any database following the MaxMind DB file format specification

The library is format-agnostic and will work with any valid .mmdb file regardless of the data provider.

Performance Tips

1. Reuse Reader instances: The Reader is thread-safe and should be reused across goroutines
2. Use specific structs: Only decode the fields you need rather than using any
3. Implement Unmarshaler: For high-throughput applications, implement custom unmarshaling
4. Consider caching: Use Result.Offset() as a cache key for database records

Getting Database Files

Free GeoLite2 Databases

Download from MaxMind's GeoLite page.

Documentation

• Go Reference
• MaxMind DB File Format Specification

Requirements

• Go 1.24 or later
• MaxMind DB file in .mmdb format

Contributing

Contributions welcome! Please fork the repository and open a pull request with your changes.

License

This is free software, licensed under the ISC License.