Get public IP address in Go #golang

addr.go

package main

import (
	"net"
	"sort"
	"strconv"
	"strings"
)

func rfc1918private(ip net.IP) bool {
	for _, cidr := range []string{"10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16"} {
		_, subnet, err := net.ParseCIDR(cidr)
		if err != nil {
			panic("failed to parse hardcoded rfc1918 cidr: " + err.Error())
		}
		if subnet.Contains(ip) {
			return true
		}
	}
	return false
}

func rfc4193private(ip net.IP) bool {
	_, subnet, err := net.ParseCIDR("fd00::/8")
	if err != nil {
		panic("failed to parse hardcoded rfc4193 cidr: " + err.Error())
	}
	return subnet.Contains(ip)
}

func isLoopback(ip net.IP) bool {
	for _, cidr := range []string{"127.0.0.0/8", "::1/128"} {
		_, subnet, err := net.ParseCIDR(cidr)
		if err != nil {
			panic("failed to parse hardcoded loopback cidr: " + err.Error())
		}
		if subnet.Contains(ip) {
			return true
		}
	}
	return false
}

func mightBePublic(ip net.IP) bool {
	return !rfc1918private(ip) && !rfc4193private(ip) && !isLoopback(ip)
}

type name_and_ip struct {
	string
	net.IP
}

func heuristic(ni name_and_ip) (ret int) {
	a := strings.ToLower(ni.string)
	ip := ni.IP
	if isLoopback(ip) {
		ret += 1000
	}
	if rfc1918private(ip) || rfc4193private(ip) {
		ret += 500
	}
	if strings.Contains(a, "dyn") {
		ret += 100
	}
	if strings.Contains(a, "dhcp") {
		ret += 99
	}
	for i := 0; i < len(ip); i++ {
		if strings.Contains(a, strconv.Itoa(int(ip[i]))) {
			ret += 5
		}
	}
	return ret
}

type nameAndIPByStabilityHeuristic []name_and_ip
func (nis nameAndIPByStabilityHeuristic) Len() int { return len(nis) }
func (nis nameAndIPByStabilityHeuristic) Swap(i, j int) { nis[i], nis[j] = nis[j], nis[i] }
func (nis nameAndIPByStabilityHeuristic) Less(i, j int) bool { return heuristic(nis[i]) < heuristic(nis[j]) }

func publicAddresses() ([]name_and_ip, error) {
	var ret []name_and_ip

	addrs, err := net.InterfaceAddrs()
	if err != nil {
		return nil, err
	}

	for _, addr := range addrs {
		ip, _, err := net.ParseCIDR(addr.String())
		if err != nil {
			return nil, err
		}
		// ignore unresolvable addresses
		names, err := net.LookupAddr(ip.String())
		if err != nil {
			continue
		}
		for _, name := range names {
			ret = append(ret, name_and_ip{name, ip})
		}
	}

	sort.Sort(nameAndIPByStabilityHeuristic(ret))
	return ret, nil
}

func main() {
	if names, err := publicAddresses(); err == nil {
		for _, ni := range names {
			println(ni.string, ni.IP.String(), heuristic(ni))
		}
	} else {
		panic(err)
	}

}

@andres-erbsen Hello, I have a very simple echo server app with a public domain dedication that prints the local IP and I'd like to use this to print the public IP as well. Would you be okay with me using this as part of the public domain project and mentioning your name in the README?

@andres-erbsen mightBePublic() is unused 🙃