Get DNS return code (Rcode) in golang

package main

import (
    "github.com/miekg/dns"
    "net"
    "os"
    "log"
    "fmt"
)

func main() {
    config, err := dns.ClientConfigFromFile("/etc/resolv.conf")
    if err != nil {
        log.Fatal(err)
    }
    
    c := new(dns.Client)

    m := new(dns.Msg)
    m.SetQuestion(dns.Fqdn(os.Args[1]), dns.TypeMX)
    m.RecursionDesired = true

    r, _, err := c.Exchange(m, net.JoinHostPort(config.Servers[0], config.Port))
    if r == nil {
        log.Fatalf("*** error: %s\n", err.Error())
    }

    fmt.Println(dns.RcodeToString[r.Rcode])
}