Reverse IP in Go

reverse_ip.go

package main

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

const hexDigit = "0123456789abcdef"

func main() {
	reverse, err := reverseaddr(os.Args[1])
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Println(reverse)
}

func reverseaddr(addr string) (arpa string, err error) {
	ip := net.ParseIP(addr)
	if ip == nil {
		return "", &net.DNSError{Err: "unrecognized address", Name: addr}
	}
	if ip.To4() != nil {
		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
	}
	// Must be IPv6
	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
	// Add it, in reverse, to the buffer
	for i := len(ip) - 1; i >= 0; i-- {
		v := ip[i]
		buf = append(buf, hexDigit[v&0xF])
		buf = append(buf, '.')
		buf = append(buf, hexDigit[v>>4])
		buf = append(buf, '.')
	}
	// Append "ip6.arpa." and return (buf already has the final .)
	buf = append(buf, "ip6.arpa."...)
	return string(buf), nil
}

// Convert unsigned integer to decimal string.
func uitoa(val uint) string {
	if val == 0 { // avoid string allocation
		return "0"
	}
	var buf [20]byte // big enough for 64bit value base 10
	i := len(buf) - 1
	for val >= 10 {
		q := val / 10
		buf[i] = byte('0' + val - q*10)
		i--
		val = q
	}
	// val < 10
	buf[i] = byte('0' + val)
	return string(buf[i:])
}

Thank you, this is great 💯