These steps cover writing and running your application:

1. From your Terminal or console application, create a new directory called ~/projects/go-programming-cookbook/chapter5/dns and navigate to this directory.
2. Run the following command:

$ go mod init github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter5/dns

You should see a file called go.mod that contains the following:

module github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter5/dns

3. Copy tests from ~/projects/go-programming-cookbook-original/chapter5/dns or use this as an exercise to write some of your own code!
4. Create a dns.go file with the following content:

package dns

import (
  "fmt"
  "net"

  "github.com/pkg/errors"
)

// Lookup holds the DNS information we care about
type Lookup struct {
  cname string
  hosts []string
}

// We can use this to print the lookup object
func (d *Lookup) String() string {
  result := ""
  for _, host := range d.hosts {
    result += fmt.Sprintf("%s IN A %s
", d.cname, host)
  }
  return result
}

// LookupAddress returns a DNSLookup consisting of a cname and host
// for a given address
func LookupAddress(address string) (*Lookup, error) {
  cname, err := net.LookupCNAME(address)
  if err != nil {
    return nil, errors.Wrap(err, "error looking up CNAME")
  }
  hosts, err := net.LookupHost(address)
  if err != nil {
    return nil, errors.Wrap(err, "error looking up HOST")
  }

  return &Lookup{cname: cname, hosts: hosts}, nil
}

5. Create a new directory named example and navigate to it.
6. Create a main.go file with the following content:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter5/dns"
)

func main() {
  if len(os.Args) < 2 {
    fmt.Printf("Usage: %s <address>
", os.Args[0])
    os.Exit(1)
  }
  address := os.Args[1]
  lookup, err := dns.LookupAddress(address)
  if err != nil {
    log.Panicf("failed to lookup: %s", err.Error())
  }
  fmt.Println(lookup)
}

7. Run the go run main.go golang.org command.
8. You may also run the following:

$ go build
$ ./example golang.org

You should see the following output:

$ go run main.go golang.org
golang.org. IN A 172.217.5.17
golang.org. IN A 2607:f8b0:4009:809::2011

9. The go.mod file may be updated and the go.sum file should now be present in the top-level recipe directory.
10. If you copied or wrote your own tests, go up one directory and run go test. Ensure that all the tests pass.