Convert net mask into CIDR in Go

Question

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])

} 

Answer 1

This isn't very obvious at first, but:

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