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Powerful Command-Line Applications in Go

Adding the Port Scanning Functionality

Explore how to develop port scanning features in a Go command-line interface tool. Learn to define custom types representing port states, implement scanning with network timeout controls, and handle multiple hosts with error detection. This lesson prepares you to build robust scanning functions integrated into your CLI applications.

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Our application is coming along nicely. We can manage hosts on which to execute a port scan. Let’s implement the port scanning functionality now. Let’s start by adding the functionality to the scan package. After that, we’ll implement the subcommand on the command-line tool.

Creating the scanHosts.go file

We create and edit the file scanHosts.go to hold the code related to the scan functionality. We add the package definition and the import list.

For this functionality, we’ll use:

  • The package fmt for formatted printing.
  • The package net for network-related functions.
  • The package time to define timeouts.
Go (1.6.2)
// Package scan provides types and functions to perform TCP port
// scans on a list of hosts
package scan
import (
  "fmt"
  "net"
  "time"
)
The import list

Next, we define a new custom type called PortState that represents the state for a single TCP port. This struct has two fields: Port of type int that corresponds to the TCP port and Open of type state that indicates whether the port is open or closed. We’ll define the type state shortly:

Go (1.6.2)
// PortState represents the state of a single TCP port
type PortState struct {
  Port int
  Open state
}
Portstate struct

We define the custom type state as a wrapper on the bool type. This type uses true or false to indicate ...