1 files changed, 74 insertions, 0 deletions
diff --git a/adapters/netscan/netscan.go b/adapters/netscan/netscan.go
new file mode 100644
index 0000000..5aaf415
--- /dev/null
+++ b/adapters/netscan/netscan.go
@@ -0,0 +1,74 @@
+// Package netscan nudges the kernel into ARP-resolving every host on a subnet,
+// so devices that never talk to us still show up in the neighbour table. It does
+// this by attempting a TCP connection to each host: the result is irrelevant,
+// the ARP exchange the kernel performs to send the SYN is the point.
+package netscan
+
+import (
+	"fmt"
+	"net"
+	"sync"
+	"time"
+)
+
+const (
+	probePort    = "9" // discard; closed almost everywhere, so a fast RST
+	maxHostBits  = 12  // refuse to sweep anything larger than a /20 (~4094 hosts)
+	sweepWorkers = 32
+)
+
+func Sweep(cidr string, timeout time.Duration) error {
+	hosts, err := hostIPs(cidr)
+	if err != nil {
+		return err
+	}
+
+	sem := make(chan struct{}, sweepWorkers)
+	var wg sync.WaitGroup
+	for _, ip := range hosts {
+		wg.Add(1)
+		sem <- struct{}{}
+		go func(ip string) {
+			defer wg.Done()
+			defer func() { <-sem }()
+			if conn, err := net.DialTimeout("tcp", net.JoinHostPort(ip, probePort), timeout); err == nil {
+				conn.Close()
+			}
+		}(ip)
+	}
+	wg.Wait()
+	return nil
+}
+
+func hostIPs(cidr string) ([]string, error) {
+	_, ipnet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return nil, err
+	}
+	if ones, bits := ipnet.Mask.Size(); bits-ones > maxHostBits {
+		return nil, fmt.Errorf("netscan: subnet /%d too large to sweep", ones)
+	}
+
+	ip := make(net.IP, len(ipnet.IP))
+	copy(ip, ipnet.IP)
+
+	ips := []string{}
+	for ; ipnet.Contains(ip); inc(ip) {
+		ips = append(ips, ip.String())
+	}
+
+	// drop the network and broadcast addresses
+	if len(ips) > 2 {
+		ips = ips[1 : len(ips)-1]
+	}
+	return ips, nil
+}
+
+func inc(ip net.IP) {
+	for i := len(ip) - 1; i >= 0; i-- {
+		ip[i]++
+		if ip[i] != 0 {
+			break
+		}
+	}
+}