LAN scan

scan.go

package main

import (
	"encoding/binary"
	"fmt"
	"log"
	"net"
	"strconv"
	"strings"
	"sync"
	"time"
)

const portToScan = 1400

// this code scans for web servers on a port (in my case for finding Sonos speakers)
// in practice I've replaced it with UPnP / SSPD discovery 
func main() {
	localSubnet := getLocalRange()
	fmt.Println("Local Subnet:", localSubnet)

	hosts := createHostRange(localSubnet)

	var wg sync.WaitGroup

	parallelism := 32
	hostCh := make(chan string, parallelism)

	// Start worker goroutines
	for i := 0; i < parallelism; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			for host := range hostCh {
				scanPort(host, portToScan)
			}
		}()
	}

	// Enqueue hosts
	for _, h := range hosts {
		hostCh <- h
	}

	// Close the channel after all hosts have been enqueued
	close(hostCh)

	// Wait for all workers to finish
	wg.Wait()
}

// getLocalRange returns local ip range or defaults on error to most common
func getLocalRange() string {
	addrs, err := net.InterfaceAddrs()
	if err != nil {
		return "192.168.1.0/24"
	}
	for _, address := range addrs {
		// check the address type and if it is not a loopback the display it
		if ipnet, ok := address.(*net.IPNet); ok && !ipnet.IP.IsLoopback() {
			if ipnet.IP.To4() != nil {
				split := strings.Split(ipnet.IP.String(), ".")
				return split[0] + "." + split[1] + "." + split[2] + ".0/24"
			}
		}
	}
	return "192.168.1.0/24"
}

// createHostRange converts a input ip addr string to a slice of ips on the cidr
func createHostRange(netw string) []string {
	_, ipv4Net, err := net.ParseCIDR(netw)
	if err != nil {
		log.Fatal(err)
	}

	mask := binary.BigEndian.Uint32(ipv4Net.Mask)
	start := binary.BigEndian.Uint32(ipv4Net.IP)
	finish := (start & mask) | (mask ^ 0xffffffff)

	var hosts []string
	for i := start + 1; i <= finish-1; i++ {
		ip := make(net.IP, 4)
		binary.BigEndian.PutUint32(ip, i)
		hosts = append(hosts, ip.String())
	}

	return hosts
}

// scanPort scans a single ip port combo
func scanPort(hostname string, port int) {
	address := hostname + ":" + strconv.Itoa(port)
	conn, err := net.DialTimeout("tcp", address, 3*time.Second)
	if err != nil {
		return
	}

	defer conn.Close()
	fmt.Printf("Found %s\n", hostname)
}