wgengine/netstack: always respond to 4via6 echo requests (#5712)

As the comment in the code says, netstack should always respond to ICMP
echo requests to a 4via6 address, even if the netstack instance isn't
normally processing subnet traffic.

Follow-up to #5709

Change-Id: I504d0776c5824071b2a2e0e687bc33e24f6c4746
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

wgengine/netstack/netstack.go

@@ -716,33 +716,49 @@ func (ns *Impl) shouldHandlePing(p *packet.Parsed) (_ netip.Addr, ok bool) {
 	if !p.IsEchoRequest() {
 		return netip.Addr{}, false
 	}
+
+	destIP := p.Dst.Addr()
+
+	// We need to handle pings for all 4via6 addresses, even if this
+	// netstack instance normally isn't responsible for processing subnets.
+	//
+	// For example, on Linux, subnet router traffic could be handled via
+	// tun+iptables rules for most packets, but we still need to handle
+	// ICMP echo requests over 4via6 since the host networking stack
+	// doesn't know what to do with a 4via6 address.
+	//
+	// shouldProcessInbound returns 'true' to say that we should process
+	// all IPv6 packets with a destination address in the 'via' range, so
+	// check before we check the "ProcessSubnets" boolean below.
+	if viaRange.Contains(destIP) {
+		// The input echo request was to a 4via6 address, which we cannot
+		// simply ping as-is from this process. Translate the destination to an
+		// IPv4 address, so that our relayed ping (in userPing) is pinging the
+		// underlying destination IP.
+		//
+		// ICMPv4 and ICMPv6 are different protocols with different on-the-wire
+		// representations, so normally you can't send an ICMPv6 message over
+		// IPv4 and expect to get a useful result. However, in this specific
+		// case things are safe because the 'userPing' function doesn't make
+		// use of the input packet.
+		return tsaddr.UnmapVia(destIP), true
+	}
+
+	// If we get here, we don't do anything unless this netstack instance
+	// is responsible for processing subnet traffic.
 	if !ns.ProcessSubnets {
 		return netip.Addr{}, false
 	}
 
-	destIP := p.Dst.Addr()
-
 	// For non-4via6 addresses, we don't handle pings if they're destined
 	// for a Tailscale IP.
-	if !viaRange.Contains(destIP) {
-		if tsaddr.IsTailscaleIP(destIP) {
-			return netip.Addr{}, false
-		}
-
-		return destIP, true
+	if tsaddr.IsTailscaleIP(destIP) {
+		return netip.Addr{}, false
 	}
 
-	// The input echo request was to a 4via6 address, which we cannot
-	// simply ping as-is from this process. Translate the destination to an
-	// IPv4 address, so that our relayed ping (in userPing) is pinging the
-	// underlying destination IP.
-	//
-	// ICMPv4 and ICMPv6 are different protocols with different on-the-wire
-	// representations, so normally you can't send an ICMPv6 message over
-	// IPv4 and expect to get a useful result. However, in this specific
-	// case things are safe because the 'userPing' function doesn't make
-	// use of the input packet.
-	return tsaddr.UnmapVia(destIP), true
+	// This netstack instance is processing subnet traffic, so handle the
+	// ping ourselves.
+	return destIP, true
 }
 
 func netaddrIPFromNetstackIP(s tcpip.Address) netip.Addr {

wgengine/netstack/netstack_test.go

@@ -5,6 +5,7 @@
 package netstack
 
 import (
+	"fmt"
 	"net/netip"
 	"runtime"
 	"testing"
@@ -216,36 +217,38 @@ func TestShouldHandlePing(t *testing.T) {
 		}
 	})
 
-	t.Run("ICMP6-4via6", func(t *testing.T) {
-		// The 4via6 route 10.1.1.0/24 siteid 7, and then the IP
-		// 10.1.1.9 within that route.
-		dst := netip.MustParseAddr("fd7a:115c:a1e0:b1a:0:7:a01:109")
-		expectedPingDst := netip.MustParseAddr("10.1.1.9")
-		icmph := packet.ICMP6Header{
-			IP6Header: packet.IP6Header{
-				IPProto: ipproto.ICMPv6,
-				Src:     srcIP,
-				Dst:     dst,
-			},
-			Type: packet.ICMP6EchoRequest,
-			Code: packet.ICMP6NoCode,
-		}
-		_, payload := packet.ICMPEchoPayload(nil)
-		icmpPing := packet.Generate(icmph, payload)
-		pkt := &packet.Parsed{}
-		pkt.Decode(icmpPing)
+	// Handle pings for 4via6 addresses regardless of ProcessSubnets
+	for _, subnets := range []bool{true, false} {
+		t.Run("ICMP6-4via6-ProcessSubnets-"+fmt.Sprint(subnets), func(t *testing.T) {
+			// The 4via6 route 10.1.1.0/24 siteid 7, and then the IP
+			// 10.1.1.9 within that route.
+			dst := netip.MustParseAddr("fd7a:115c:a1e0:b1a:0:7:a01:109")
+			expectedPingDst := netip.MustParseAddr("10.1.1.9")
+			icmph := packet.ICMP6Header{
+				IP6Header: packet.IP6Header{
+					IPProto: ipproto.ICMPv6,
+					Src:     srcIP,
+					Dst:     dst,
+				},
+				Type: packet.ICMP6EchoRequest,
+				Code: packet.ICMP6NoCode,
+			}
+			_, payload := packet.ICMPEchoPayload(nil)
+			icmpPing := packet.Generate(icmph, payload)
+			pkt := &packet.Parsed{}
+			pkt.Decode(icmpPing)
 
-		impl := makeNetstack(t, func(impl *Impl) {
-			impl.ProcessSubnets = true
+			impl := makeNetstack(t, func(impl *Impl) {
+				impl.ProcessSubnets = subnets
+			})
+			pingDst, ok := impl.shouldHandlePing(pkt)
+
+			// Handled due to being 4via6
+			if !ok {
+				t.Errorf("expected shouldHandlePing==true")
+			} else if pingDst != expectedPingDst {
+				t.Errorf("got dst %s; want %s", pingDst, expectedPingDst)
+			}
 		})
-		pingDst, ok := impl.shouldHandlePing(pkt)
-
-		// Handled due to being 4via6
-		if !ok {
-			t.Errorf("expected shouldHandlePing==true")
-		}
-		if pingDst != expectedPingDst {
-			t.Errorf("got dst %s; want %s", pingDst, expectedPingDst)
-		}
-	})
+	}
 }