initial: ZeroTier-like P2P mesh VPN server with multi-tenant Web UI

internal/vl2/arp.go

@@ -0,0 +1,120 @@
+package vl2
+
+import (
+	"encoding/binary"
+	"log/slog"
+	"net"
+	"sync"
+	"time"
+)
+
+type ARPEntry struct {
+	MAC      net.HardwareAddr
+	LastSeen time.Time
+}
+
+type ARPProxy struct {
+	cache map[string]*ARPEntry
+	mu    sync.RWMutex
+	log   *slog.Logger
+}
+
+func NewARPProxy(log *slog.Logger) *ARPProxy {
+	return &ARPProxy{
+		cache: make(map[string]*ARPEntry),
+		log:   log.With("component", "arp"),
+	}
+}
+
+func (a *ARPProxy) Learn(ip net.IP, mac net.HardwareAddr) {
+	key := ip.String()
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.cache[key] = &ARPEntry{
+		MAC:      mac,
+		LastSeen: time.Now(),
+	}
+}
+
+func (a *ARPProxy) Lookup(ip net.IP) net.HardwareAddr {
+	key := ip.String()
+	a.mu.RLock()
+	defer a.mu.RUnlock()
+	entry, ok := a.cache[key]
+	if !ok {
+		return nil
+	}
+	return entry.MAC
+}
+
+func (a *ARPProxy) HandleARP(frame *EthernetFrame) []byte {
+	if len(frame.Payload) < 28 {
+		return nil
+	}
+	pl := frame.Payload
+
+	hrd := binary.BigEndian.Uint16(pl[0:2])
+	pro := binary.BigEndian.Uint16(pl[2:4])
+	hln := pl[4]
+	pln := pl[5]
+	op := binary.BigEndian.Uint16(pl[6:8])
+
+	if hrd != 1 || pro != EtherTypeIPv4 || hln != 6 || pln != 4 {
+		return nil
+	}
+
+	senderMAC := net.HardwareAddr(pl[8:14])
+	senderIP := net.IP(pl[14:18])
+	targetIP := net.IP(pl[24:28])
+
+	a.Learn(senderIP, senderMAC)
+
+	if op == 1 { // ARP request
+		if localMAC := a.Lookup(targetIP); localMAC != nil {
+			reply := make([]byte, 42)
+			copy(reply[0:6], senderMAC)
+			copy(reply[6:12], localMAC)
+			binary.BigEndian.PutUint16(reply[12:14], EtherTypeARP)
+
+			reply[14] = 0x00
+			reply[15] = 0x01
+			binary.BigEndian.PutUint16(reply[16:18], EtherTypeIPv4)
+			reply[18] = 6
+			reply[19] = 4
+			binary.BigEndian.PutUint16(reply[20:22], 2)
+
+			copy(reply[22:28], localMAC)
+			copy(reply[28:32], targetIP)
+			copy(reply[32:38], senderMAC)
+			copy(reply[38:42], senderIP)
+
+			return reply
+		}
+	}
+
+	return nil
+}
+
+func (a *ARPProxy) PeerFromARP(frame *EthernetFrame) (net.IP, net.HardwareAddr) {
+	if len(frame.Payload) < 28 {
+		return nil, nil
+	}
+	pl := frame.Payload
+	hrd := binary.BigEndian.Uint16(pl[0:2])
+	pro := binary.BigEndian.Uint16(pl[2:4])
+	if hrd != 1 || pro != EtherTypeIPv4 {
+		return nil, nil
+	}
+	return net.IP(pl[14:18]), net.HardwareAddr(pl[8:14])
+}
+
+func (a *ARPProxy) CleanExpired() {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	cutoff := time.Now().Add(-30 * time.Minute)
+	for k, v := range a.cache {
+		if v.LastSeen.Before(cutoff) {
+			delete(a.cache, k)
+		}
+	}
+}