CODEWITHSUNDEEP
go
Godzilla74
Godzilla74

Reputation: 2512

Convert net mask into CIDR in Go

I'm looking for a way to convert a netmask string into CIDR notation in Go.

For instance, "255.255.255.0" -> "/24"

I'm currently obtaining an IP address and the net mask string with the below logic, which may just be complicating things.

I've been perusing the net library trying to see if there is a different function to use to accomplish what I'd like, which is really just a IP address in CIDR notation:

192.168.1.2/24

var mgmtInterface *net.Interface
var err error

mgmtInterface, err = net.InterfaceByName("eth0")
if err != nil {
  log.Println("Unable to find interface eth0, trying en0")
  mgmtInterface, err = net.InterfaceByName("en0")
}

addrs, err := mgmtInterface.Addrs()
if err != nil {
  log.Println("interface has no address")
}

for _, addr := range addrs {
  var ip net.IP
  var mask net.IPMask
  switch v := addr.(type) {
  case *net.IPNet:
    ip = v.IP
    mask = v.Mask
  case *net.IPAddr:
    ip = v.IP
    mask = ip.DefaultMask()
  }

  if ip == nil {
    continue
  }

  ip = ip.To4()
  if ip == nil {
    continue
  }

  // create the netmask
  cleanMask := fmt.Sprintf("%d.%d.%d.%d", mask[0], mask[1], mask[2], mask[3])

}

Upvotes: 3

Views: 5699

Answers (3)

Sean F
Sean F

Reputation: 4615

This is straightforward using the IPAddress Go library. Note that this code works equally well with both IPv4 and IPv6. Disclaimer: I am the project manager.

import (
    "fmt"
    "github.com/seancfoley/ipaddress-go/ipaddr"
)

func main() {
    maskStr := "255.255.255.0"
    pref := ipaddr.NewIPAddressString(maskStr).GetAddress().
        GetBlockMaskPrefixLen(true)
    fmt.Printf("prefix length for %s is %d", maskStr, pref.Len())
}

Output:

prefix length for 255.255.255.0 is 24

Upvotes: 0

Burak Serdar
Burak Serdar

Reputation: 51632

This isn't very obvious at first, but:

addr := ip.To4()
sz, _ := net.IPV4Mask(addr[0], addr[1], addr[2], addr[3]).Size()

Upvotes: 6

Alex Yu
Alex Yu

Reputation: 3537

I'm not aware about existense of such function but it's easy to create one.

CIDR notation is just a count of set bits in netmask.

So, crude solution could be:

func cidr(netmask string) int {       
        var mask uint32
    for idx, dotpart := range strings.Split(netmask, ".") {
        part, _ := strconv.Atoi(dotpart)    
        mask = mask | uint32(part) << uint32(24-idx*8)
    }   
    return len(fmt.Sprintf("%b", mask))
}

Upvotes: 0

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