TUN-6592: Decrement TTL and return ICMP time exceed if it's 0

packet/router.go

@@ -0,0 +1,126 @@
+package packet
+
+import (
+	"context"
+	"net"
+	"net/netip"
+
+	"github.com/rs/zerolog"
+)
+
+var (
+	// Source IP in documentation range to return ICMP error messages if we can't determine the IP of this machine
+	icmpv4ErrFallbackSrc = netip.MustParseAddr("192.0.2.30")
+	icmpv6ErrFallbackSrc = netip.MustParseAddr("2001:db8::")
+)
+
+// ICMPRouter sends ICMP messages and listens for their responses
+type ICMPRouter interface {
+	// Serve starts listening for responses to the requests until context is done
+	Serve(ctx context.Context) error
+	// Request sends an ICMP message
+	Request(pk *ICMP, responder FunnelUniPipe) error
+}
+
+// Upstream of raw packets
+type Upstream interface {
+	// ReceivePacket waits for the next raw packet from upstream
+	ReceivePacket(ctx context.Context) (RawPacket, error)
+}
+
+type Router struct {
+	upstream   Upstream
+	returnPipe FunnelUniPipe
+	icmpProxy  ICMPRouter
+	ipv4Src    netip.Addr
+	ipv6Src    netip.Addr
+	logger     *zerolog.Logger
+}
+
+func NewRouter(upstream Upstream, returnPipe FunnelUniPipe, icmpProxy ICMPRouter, logger *zerolog.Logger) *Router {
+	ipv4Src, err := findLocalAddr(net.ParseIP("1.1.1.1"), 53)
+	if err != nil {
+		logger.Warn().Err(err).Msgf("Failed to determine the IPv4 for this machine. It will use %s as source IP for error messages such as ICMP TTL exceed", icmpv4ErrFallbackSrc)
+		ipv4Src = icmpv4ErrFallbackSrc
+	}
+	ipv6Src, err := findLocalAddr(net.ParseIP("2606:4700:4700::1111"), 53)
+	if err != nil {
+		logger.Warn().Err(err).Msgf("Failed to determine the IPv6 for this machine. It will use %s as source IP for error messages such as ICMP TTL exceed", icmpv6ErrFallbackSrc)
+		ipv6Src = icmpv6ErrFallbackSrc
+	}
+	return &Router{
+		upstream:   upstream,
+		returnPipe: returnPipe,
+		icmpProxy:  icmpProxy,
+		ipv4Src:    ipv4Src,
+		ipv6Src:    ipv6Src,
+		logger:     logger,
+	}
+}
+
+func (r *Router) Serve(ctx context.Context) error {
+	icmpDecoder := NewICMPDecoder()
+	encoder := NewEncoder()
+	for {
+		rawPacket, err := r.upstream.ReceivePacket(ctx)
+		if err != nil {
+			return err
+		}
+		icmpPacket, err := icmpDecoder.Decode(rawPacket)
+		if err != nil {
+			r.logger.Err(err).Msg("Failed to decode ICMP packet from quic datagram")
+			continue
+		}
+
+		if icmpPacket.TTL <= 1 {
+			if err := r.sendTTLExceedMsg(icmpPacket, rawPacket, encoder); err != nil {
+				r.logger.Err(err).Msg("Failed to return ICMP TTL exceed error")
+			}
+			continue
+		}
+		icmpPacket.TTL--
+
+		if err := r.icmpProxy.Request(icmpPacket, r.returnPipe); 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")
+			continue
+		}
+	}
+}
+
+func (r *Router) sendTTLExceedMsg(pk *ICMP, rawPacket RawPacket, encoder *Encoder) error {
+	var srcIP netip.Addr
+	if pk.Dst.Is4() {
+		srcIP = r.ipv4Src
+	} else {
+		srcIP = r.ipv6Src
+	}
+	ttlExceedPacket := NewICMPTTLExceedPacket(pk.IP, rawPacket, srcIP)
+
+	encodedTTLExceed, err := encoder.Encode(ttlExceedPacket)
+	if err != nil {
+		return err
+	}
+	return r.returnPipe.SendPacket(pk.Src, encodedTTLExceed)
+}
+
+// findLocalAddr tries to dial UDP and returns the local address picked by the OS
+func findLocalAddr(dst net.IP, port int) (netip.Addr, error) {
+	udpConn, err := net.DialUDP("udp", nil, &net.UDPAddr{
+		IP:   dst,
+		Port: port,
+	})
+	if err != nil {
+		return netip.Addr{}, err
+	}
+	defer udpConn.Close()
+	localAddrPort, err := netip.ParseAddrPort(udpConn.LocalAddr().String())
+	if err != nil {
+		return netip.Addr{}, err
+	}
+	localAddr := localAddrPort.Addr()
+	return localAddr, nil
+}