TUN-6856: Refactor to lay foundation for tracing ICMP

Remove send and return methods from Funnel interface. Users of Funnel can provide their own send and return methods without wrapper to comply with the interface.
Move packet router to ingress package to avoid circular dependency

ingress/packet_router.go

@@ -0,0 +1,134 @@
+package ingress
+
+import (
+	"context"
+	"fmt"
+	"net/netip"
+
+	"github.com/rs/zerolog"
+
+	"github.com/cloudflare/cloudflared/packet"
+	quicpogs "github.com/cloudflare/cloudflared/quic"
+)
+
+// Upstream of raw packets
+type muxer interface {
+	SendPacket(pk quicpogs.Packet) error
+	// ReceivePacket waits for the next raw packet from upstream
+	ReceivePacket(ctx context.Context) (quicpogs.Packet, error)
+}
+
+// PacketRouter routes packets between Upstream and ICMPRouter. Currently it rejects all other type of ICMP packets
+type PacketRouter struct {
+	globalConfig           *GlobalRouterConfig
+	muxer                  muxer
+	logger                 *zerolog.Logger
+	checkRouterEnabledFunc func() bool
+	icmpDecoder            *packet.ICMPDecoder
+	encoder                *packet.Encoder
+}
+
+// GlobalRouterConfig is the configuration shared by all instance of Router.
+type GlobalRouterConfig struct {
+	ICMPRouter *icmpRouter
+	IPv4Src    netip.Addr
+	IPv6Src    netip.Addr
+	Zone       string
+}
+
+// NewPacketRouter creates a PacketRouter that handles ICMP packets. Packets are read from muxer but dropped if globalConfig is nil.
+func NewPacketRouter(globalConfig *GlobalRouterConfig, muxer muxer, logger *zerolog.Logger, checkRouterEnabledFunc func() bool) *PacketRouter {
+	return &PacketRouter{
+		globalConfig:           globalConfig,
+		muxer:                  muxer,
+		logger:                 logger,
+		checkRouterEnabledFunc: checkRouterEnabledFunc,
+		icmpDecoder:            packet.NewICMPDecoder(),
+		encoder:                packet.NewEncoder(),
+	}
+}
+
+func (r *PacketRouter) Serve(ctx context.Context) error {
+	for {
+		rawPacket, responder, err := r.nextPacket(ctx)
+		if err != nil {
+			return err
+		}
+		r.handlePacket(ctx, rawPacket, responder)
+	}
+}
+
+func (r *PacketRouter) nextPacket(ctx context.Context) (packet.RawPacket, *packetResponder, error) {
+	pk, err := r.muxer.ReceivePacket(ctx)
+	if err != nil {
+		return packet.RawPacket{}, nil, err
+	}
+	responder := &packetResponder{
+		datagramMuxer: r.muxer,
+	}
+	switch pk.Type() {
+	case quicpogs.DatagramTypeIP:
+		return packet.RawPacket{Data: pk.Payload()}, responder, nil
+	case quicpogs.DatagramTypeIPWithTrace:
+		return packet.RawPacket{}, responder, fmt.Errorf("TODO: TUN-6604 Handle IP packet with trace")
+	default:
+		return packet.RawPacket{}, nil, fmt.Errorf("unexpected datagram type %d", pk.Type())
+	}
+}
+
+func (r *PacketRouter) handlePacket(ctx context.Context, rawPacket packet.RawPacket, responder *packetResponder) {
+	// ICMP Proxy feature is disabled, drop packets
+	if r.globalConfig == nil {
+		return
+	}
+
+	if enabled := r.checkRouterEnabledFunc(); !enabled {
+		return
+	}
+
+	icmpPacket, err := r.icmpDecoder.Decode(rawPacket)
+	if err != nil {
+		r.logger.Err(err).Msg("Failed to decode ICMP packet from quic datagram")
+		return
+	}
+
+	if icmpPacket.TTL <= 1 {
+		if err := r.sendTTLExceedMsg(ctx, icmpPacket, rawPacket, r.encoder); err != nil {
+			r.logger.Err(err).Msg("Failed to return ICMP TTL exceed error")
+		}
+		return
+	}
+	icmpPacket.TTL--
+
+	if err := r.globalConfig.ICMPRouter.Request(ctx, icmpPacket, responder); err != nil {
+		r.logger.Err(err).
+			Str("src", icmpPacket.Src.String()).
+			Str("dst", icmpPacket.Dst.String()).
+			Interface("type", icmpPacket.Type).
+			Msg("Failed to send ICMP packet")
+	}
+}
+
+func (r *PacketRouter) sendTTLExceedMsg(ctx context.Context, pk *packet.ICMP, rawPacket packet.RawPacket, encoder *packet.Encoder) error {
+	var srcIP netip.Addr
+	if pk.Dst.Is4() {
+		srcIP = r.globalConfig.IPv4Src
+	} else {
+		srcIP = r.globalConfig.IPv6Src
+	}
+	ttlExceedPacket := packet.NewICMPTTLExceedPacket(pk.IP, rawPacket, srcIP)
+
+	encodedTTLExceed, err := encoder.Encode(ttlExceedPacket)
+	if err != nil {
+		return err
+	}
+	return r.muxer.SendPacket(quicpogs.RawPacket(encodedTTLExceed))
+}
+
+type packetResponder struct {
+	datagramMuxer muxer
+}
+
+func (pr *packetResponder) returnPacket(rawPacket packet.RawPacket) error {
+	return pr.datagramMuxer.SendPacket(quicpogs.RawPacket(rawPacket))
+}