Golang : Reverse IP address for reverse DNS lookup example



Tags : golang ip-address reverse-dns-lookup sort

Problem:

You need to reverse a given IPv4 address into a DNS reverse record form to perform reverse DNS lookup or simply want to read an IP address backwards. How to do that?

Solution:

Break the given IP address into a slice and reverse the octets sequence with a for loop.

Edit: The previous solution used the sort package. However, it will sort the IP during reverse sorting(which is working as intended) ... instead of just reversing.

10.106 to 106.10 and this will introduce hard to trace bug.

Before : 106.10.138.240

After : 240.138.106.10 <-- notice the position of 106 octet swapped with 10 octet

The solution below will just reverse the octets position and no sort.

Here you go!


 package main

 import (
 "fmt"
 "net"
 "strings"
 )

 func ReverseIPAddress(ip net.IP) string {

 if ip.To4() != nil {
 // split into slice by dot .
 addressSlice := strings.Split(ip.String(), ".")
 reverseSlice := []string{}

 for i := range addressSlice {
 octet := addressSlice[len(addressSlice)-1-i]
 reverseSlice = append(reverseSlice, octet)
 }

 // sanity check
 //fmt.Println(reverseSlice)

 return strings.Join(reverseSlice, ".")

 } else {
 panic("invalid IPv4 address")
 }
 }

 func main() {

 ipAddress := net.ParseIP("106.10.138.240")

 fmt.Println("Before : ", ipAddress.To4())

 reverseIpAddress := ReverseIPAddress(ipAddress)

 fmt.Println("After : ", reverseIpAddress)

 // convert to DNS reverse record form
 reverseIpAddress = reverseIpAddress + ".in-addr.arpa"

 fmt.Println("With in-addr.arpa : ", reverseIpAddress)

 }

Output:

Before : 106.10.138.240

After : 240.138.10.106

With in-addr.arpa : 240.138.10.106.in-addr.arpa

References:

https://golang.org/pkg/sort/#StringSlice

http://unix.stackexchange.com/questions/132779/how-to-read-an-ip-address-backwards

  See also : Golang : Check if IP address is version 4 or 6



Tags : golang ip-address reverse-dns-lookup sort

By Adam Ng

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