Checkpoint

internal/netutil/arp/arp.go

@@ -0,0 +1,63 @@
+package arp
+
+import (
+	"net"
+	"sync"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+func init() {
+	go func() {
+		t := time.NewTicker(time.Second * 5)
+		for {
+			refresh()
+			<-t.C
+		}
+	}()
+}
+
+var table sync.Map
+
+func refresh() {
+	t, err := lookup()
+	if err != nil {
+		logrus.StandardLogger().WithError(err).Warn("arp cache refresh failed")
+	} else {
+		for k, v := range t {
+			logrus.StandardLogger().WithFields(logrus.Fields{
+				"mac": v,
+				"ip":  k,
+			}).Debug("Updating ARP cache")
+			table.Store(k, v)
+		}
+	}
+}
+
+func Get(addr net.Addr) net.HardwareAddr {
+	if addr == nil {
+		logrus.StandardLogger().Trace("No address found, can't lookup IP for MAC")
+		return nil
+	}
+
+	var ip net.IP
+	switch addr := addr.(type) {
+	case *net.TCPAddr:
+		ip = addr.IP
+	case *net.UDPAddr:
+		ip = addr.IP
+	}
+	if ip == nil {
+		logrus.StandardLogger().WithField("addr", addr.String()).Trace("No IP address found, can't lookup MAC")
+		return nil
+	}
+
+	if v, ok := table.Load(ip.String()); ok {
+		logrus.StandardLogger().WithField("ip", ip.String()).Tracef("%s is at %s", ip, v.(net.HardwareAddr).String())
+		return v.(net.HardwareAddr)
+	}
+
+	logrus.StandardLogger().WithField("ip", ip.String()).Trace("No MAC address found")
+	return nil
+}

internal/netutil/arp/arp_linux.go

@@ -0,0 +1,32 @@
+package arp
+
+import (
+	"bufio"
+	"net"
+	"os"
+	"strings"
+)
+
+func lookup() (map[string]net.HardwareAddr, error) {
+	f, err := os.Open("/proc/net/arp")
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = f.Close() }()
+
+	t := make(map[string]net.HardwareAddr)
+	s := bufio.NewScanner(f)
+	for i := 0; s.Scan(); i++ {
+		if i == 0 {
+			continue
+		}
+		line := strings.Fields(s.Text())
+		if len(line) < 4 {
+			continue
+		}
+		if mac, err := net.ParseMAC(line[3]); err == nil {
+			t[line[0]] = mac
+		}
+	}
+	return t, nil
+}

internal/netutil/arp/arp_unix.go

@@ -0,0 +1,37 @@
+//go:build !linux
+// +build !linux
+
+// ^ Linux isn't Unix anyway :P
+
+package arp
+
+import (
+	"net"
+	"os/exec"
+	"strings"
+)
+
+func lookup() (map[string]net.HardwareAddr, error) {
+	data, err := exec.Command("arp", "-an").Output()
+	if err != nil {
+		return nil, err
+	}
+
+	t := make(map[string]net.HardwareAddr)
+	for _, line := range strings.Split(string(data), "\n") {
+		fields := strings.Fields(line)
+		if len(fields) < 3 {
+			continue
+		}
+
+		// strip brackets around IP
+		ip := strings.ReplaceAll(fields[1], "(", "")
+		ip = strings.ReplaceAll(ip, ")", "")
+
+		if mac, err := net.ParseMAC(fields[3]); err == nil {
+			t[ip] = mac
+		}
+	}
+
+	return t, nil
+}