Golang – Check if IPv4 is valid and resolve FQDN

This recipe demonstrates if string is a valid IPv4, and also to resolve a FQDN to IP address.

This recipe uses os.Args to get the second argument as FQDN and call net.ResolveIPAddr() function to do a FQDN resolution to IP address.

package main

import (
	"fmt"
	"log"
	"net"
	"os"
)

func isIPv4Addr(ip string) bool {
	if net.ParseIP(ip) == nil {
		// invalid ipv4 address
		return false
	}
	return true
}

func resolve(fqdn string) string {
	// net.ResolveIPAddr() accepts two arguments: net is one of these "ip", "ip4", "ip6" and a fqdn
	i, e := net.ResolveIPAddr("ip", fqdn)
	if e != nil {
		log.Fatal(e)
	}
	// return the string of the ip address pointer.
	return i.String()
}

func main() {
	if len(os.Args) != 2 {
		// command line requires the program name and fqdn, which will be two indices in string slice.
		log.Fatalf("Usage example: %s www.google.com", os.Args[0])
	}
	fmt.Println(resolve(os.Args[1]))
}

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