Split LLDP code into separate file and refactor interface tracking

lldp.go

@@ -0,0 +1,56 @@
+package tendrils
+
+import (
+	"context"
+	"log"
+	"net"
+	"time"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/google/gopacket/pcap"
+)
+
+func (t *Tendrils) listenLLDP(ctx context.Context, iface net.Interface) {
+	handle, err := pcap.OpenLive(iface.Name, 65536, true, 5*time.Second)
+	if err != nil {
+		log.Printf("[ERROR] failed to open interface %s: %v", iface.Name, err)
+		return
+	}
+	defer handle.Close()
+
+	if err := handle.SetBPFFilter("ether proto 0x88cc"); err != nil {
+		log.Printf("[ERROR] failed to set BPF filter on %s: %v", iface.Name, err)
+		return
+	}
+
+	packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
+	packets := packetSource.Packets()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case packet, ok := <-packets:
+			if !ok {
+				return
+			}
+			t.handleLLDPPacket(iface.Name, packet)
+		}
+	}
+}
+
+func (t *Tendrils) handleLLDPPacket(ifaceName string, packet gopacket.Packet) {
+	lldpLayer := packet.Layer(layers.LayerTypeLinkLayerDiscovery)
+	if lldpLayer == nil {
+		return
+	}
+
+	lldp, ok := lldpLayer.(*layers.LinkLayerDiscovery)
+	if !ok {
+		return
+	}
+
+	log.Printf("[%s] lldp packet received: ChassisID=%x PortID=%s TTL=%d",
+		ifaceName, lldp.ChassisID.ID, lldp.PortID.ID, lldp.TTL)
+}