Detect presence with arp and arping on Linux

.gitignore

@@ -13,3 +13,5 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+
+.vscode/

neighbors/arp_linux.go

@@ -0,0 +1,66 @@
+package neighbors
+
+import (
+	"context"
+	"encoding/json"
+	"net"
+	"os/exec"
+)
+
+type (
+	arp struct {
+		cmd    string
+		arping ARPing
+	}
+
+	arpEntry struct {
+		IPAddress  string `json:"dst"`
+		MACAddress string `json:"lladdr"`
+		Interface  string `json:"dev"`
+	}
+)
+
+func NewARP(count uint) (ARP, error) {
+	cmd, err := exec.LookPath("ip")
+	if err != nil {
+		return nil, err
+	}
+
+	arping, err := NewARPing(count)
+	if err != nil {
+		return nil, err
+	}
+
+	return &arp{cmd: cmd, arping: arping}, nil
+}
+
+func (a *arp) Present(ctx context.Context, ifs map[string]bool, hws map[string]bool) (ok bool, err error) {
+	cmd := exec.CommandContext(ctx, a.cmd, "-family", "inet", "-json", "neighbor", "show", "nud", "reachable")
+	b, err := cmd.Output()
+	if err != nil {
+		return
+	}
+
+	var es []arpEntry
+	err = json.Unmarshal(b, &es)
+
+	for _, e := range es {
+		if ifs[e.Interface] {
+			var hwa net.HardwareAddr
+			hwa, err = net.ParseMAC(e.MACAddress)
+			if err != nil {
+				return
+			}
+			hw := hwa.String()
+
+			if hws[hw] {
+				ok, err = a.arping.Ping(ctx, e.Interface, hw, e.IPAddress)
+				if ok || err != nil {
+					return
+				}
+			}
+		}
+	}
+
+	return
+}