Go function to return the first IPv4 address of the named interface

address.go

package net

import (
	"net"
	"os"
)

func InterfaceIPv4Address(name string) net.IP {
	intf, err := net.InterfaceByName(name)
	if err != nil {
		os.Stderr.WriteString("Oops: " + err.Error() + "\n")
		os.Exit(1)
	}

	addrs, _ := intf.Addrs()
	for _, a := range addrs {
		if ipnet, ok := a.(*net.IPNet); ok {
			if ipnet.IP.To4() != nil {
				return ipnet.IP
			}
		}
	}

	return nil
}

main.go

package main

import (
  "fmt"
  "./net"
)

func main() {
  // Print my WLAN IPv4 address
  fmt.Println(net.InterfaceIPv4Address("en1").String())
}