Two ways to get the current local IP in Go #Golang

GetLocalIP.go

package main

import (
	"errors"
	"fmt"
	"net"
)

func main() {
	localIP, err := getLocalIPFromNetInterfaces()
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Printf("%-32s : %s\n", "Local IP from network interfaces", localIP)

	localIP, err = getLocalIPByUDPDialing()
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Printf("%-32s : %s\n", "Local IP from UDP dial", localIP)
}

func getLocalIPFromNetInterfaces() (string, error) {
	ifaces, err := net.Interfaces()
	if err != nil {
		return "", err
	}
	for _, iface := range ifaces {
		if iface.Flags&net.FlagUp == 0 {
			continue // interface down
		}
		if iface.Flags&net.FlagLoopback != 0 {
			continue // loopback interface
		}
		addrs, err := iface.Addrs()
		if err != nil {
			return "", err
		}
		for _, addr := range addrs {
			var ip net.IP
			switch v := addr.(type) {
			case *net.IPNet:
				ip = v.IP
			case *net.IPAddr:
				ip = v.IP
			}
			if ip == nil || ip.IsLoopback() {
				continue
			}
			ip = ip.To4()
			if ip == nil {
				ip = ip.To16()
				if ip == nil {
					continue
				}
			}
			return ip.String(), nil
		}
	}
	return "", errors.New("this machine is not connected to a network")
}

func getLocalIPByUDPDialing() (string, error) {
	// UDP connection => there is no handshake => also works if offline
	conn, err := net.Dial("udp", "1.2.3.4:1")
	if err != nil {
		return "", err
	}
	defer conn.Close()
	localAddr := conn.LocalAddr().(*net.UDPAddr)
	return localAddr.IP.String(), nil
}