[client] Fix DNS resolution with userspace WireGuard and kernel firewall (#5873)

Author: lixmal

client/firewall/create_linux.go

@@ -56,6 +56,13 @@ func NewFirewall(iface IFaceMapper, stateManager *statemanager.Manager, flowLogg
 		return createUserspaceFirewall(iface, nil, disableServerRoutes, flowLogger, mtu)
 	}
 
+	// Native firewall handles packet filtering, but the userspace WireGuard bind
+	// needs a device filter for DNS interception hooks. Install a minimal
+	// hooks-only filter that passes all traffic through to the kernel firewall.
+	if err := iface.SetFilter(&uspfilter.HooksFilter{}); err != nil {
+		log.Warnf("failed to set hooks filter, DNS via memory hooks will not work: %v", err)
+	}
+
 	return fm, nil
 }
 

client/firewall/uspfilter/common/hooks.go

@@ -0,0 +1,37 @@
+package common
+
+import (
+	"net/netip"
+	"sync/atomic"
+)
+
+// PacketHook stores a registered hook for a specific IP:port.
+type PacketHook struct {
+	IP   netip.Addr
+	Port uint16
+	Fn   func([]byte) bool
+}
+
+// HookMatches checks if a packet's destination matches the hook and invokes it.
+func HookMatches(h *PacketHook, dstIP netip.Addr, dport uint16, packetData []byte) bool {
+	if h == nil {
+		return false
+	}
+	if h.IP == dstIP && h.Port == dport {
+		return h.Fn(packetData)
+	}
+	return false
+}
+
+// SetHook atomically stores a hook, handling nil removal.
+func SetHook(ptr *atomic.Pointer[PacketHook], ip netip.Addr, dPort uint16, hook func([]byte) bool) {
+	if hook == nil {
+		ptr.Store(nil)
+		return
+	}
+	ptr.Store(&PacketHook{
+		IP:   ip,
+		Port: dPort,
+		Fn:   hook,
+	})
+}

client/firewall/uspfilter/filter.go

@@ -142,15 +142,8 @@ type Manager struct {
 	mssClampEnabled bool
 
 	// Only one hook per protocol is supported. Outbound direction only.
-	udpHookOut atomic.Pointer[packetHook]
-	tcpHookOut atomic.Pointer[packetHook]
-}
-
-// packetHook stores a registered hook for a specific IP:port.
-type packetHook struct {
-	ip   netip.Addr
-	port uint16
-	fn   func([]byte) bool
+	udpHookOut atomic.Pointer[common.PacketHook]
+	tcpHookOut atomic.Pointer[common.PacketHook]
 }
 
 // decoder for packages
@@ -912,21 +905,11 @@ func (m *Manager) trackInbound(d *decoder, srcIP, dstIP netip.Addr, ruleID []byt
 }
 
 func (m *Manager) udpHooksDrop(dport uint16, dstIP netip.Addr, packetData []byte) bool {
-	return hookMatches(m.udpHookOut.Load(), dstIP, dport, packetData)
+	return common.HookMatches(m.udpHookOut.Load(), dstIP, dport, packetData)
 }
 
 func (m *Manager) tcpHooksDrop(dport uint16, dstIP netip.Addr, packetData []byte) bool {
-	return hookMatches(m.tcpHookOut.Load(), dstIP, dport, packetData)
-}
-
-func hookMatches(h *packetHook, dstIP netip.Addr, dport uint16, packetData []byte) bool {
-	if h == nil {
-		return false
-	}
-	if h.ip == dstIP && h.port == dport {
-		return h.fn(packetData)
-	}
-	return false
+	return common.HookMatches(m.tcpHookOut.Load(), dstIP, dport, packetData)
 }
 
 // filterInbound implements filtering logic for incoming packets.
@@ -1337,28 +1320,12 @@ func (m *Manager) ruleMatches(rule *RouteRule, srcAddr, dstAddr netip.Addr, prot
 
 // SetUDPPacketHook sets the outbound UDP packet hook. Pass nil hook to remove.
 func (m *Manager) SetUDPPacketHook(ip netip.Addr, dPort uint16, hook func(packet []byte) bool) {
-	if hook == nil {
-		m.udpHookOut.Store(nil)
-		return
-	}
-	m.udpHookOut.Store(&packetHook{
-		ip:   ip,
-		port: dPort,
-		fn:   hook,
-	})
+	common.SetHook(&m.udpHookOut, ip, dPort, hook)
 }
 
 // SetTCPPacketHook sets the outbound TCP packet hook. Pass nil hook to remove.
 func (m *Manager) SetTCPPacketHook(ip netip.Addr, dPort uint16, hook func(packet []byte) bool) {
-	if hook == nil {
-		m.tcpHookOut.Store(nil)
-		return
-	}
-	m.tcpHookOut.Store(&packetHook{
-		ip:   ip,
-		port: dPort,
-		fn:   hook,
-	})
+	common.SetHook(&m.tcpHookOut, ip, dPort, hook)
 }
 
 // SetLogLevel sets the log level for the firewall manager

client/firewall/uspfilter/filter_test.go

@@ -202,9 +202,9 @@ func TestSetUDPPacketHook(t *testing.T) {
 
 	h := manager.udpHookOut.Load()
 	require.NotNil(t, h)
-	assert.Equal(t, netip.MustParseAddr("10.168.0.1"), h.ip)
-	assert.Equal(t, uint16(8000), h.port)
-	assert.True(t, h.fn(nil))
+	assert.Equal(t, netip.MustParseAddr("10.168.0.1"), h.IP)
+	assert.Equal(t, uint16(8000), h.Port)
+	assert.True(t, h.Fn(nil))
 	assert.True(t, called)
 
 	manager.SetUDPPacketHook(netip.MustParseAddr("10.168.0.1"), 8000, nil)
@@ -226,9 +226,9 @@ func TestSetTCPPacketHook(t *testing.T) {
 
 	h := manager.tcpHookOut.Load()
 	require.NotNil(t, h)
-	assert.Equal(t, netip.MustParseAddr("10.168.0.1"), h.ip)
-	assert.Equal(t, uint16(53), h.port)
-	assert.True(t, h.fn(nil))
+	assert.Equal(t, netip.MustParseAddr("10.168.0.1"), h.IP)
+	assert.Equal(t, uint16(53), h.Port)
+	assert.True(t, h.Fn(nil))
 	assert.True(t, called)
 
 	manager.SetTCPPacketHook(netip.MustParseAddr("10.168.0.1"), 53, nil)

client/firewall/uspfilter/hooks_filter.go

@@ -0,0 +1,90 @@
+package uspfilter
+
+import (
+	"encoding/binary"
+	"net/netip"
+	"sync/atomic"
+
+	"github.com/netbirdio/netbird/client/firewall/uspfilter/common"
+	"github.com/netbirdio/netbird/client/iface/device"
+)
+
+const (
+	ipv4HeaderMinLen = 20
+	ipv4ProtoOffset  = 9
+	ipv4FlagsOffset  = 6
+	ipv4DstOffset    = 16
+	ipProtoUDP       = 17
+	ipProtoTCP       = 6
+	ipv4FragOffMask  = 0x1fff
+	// dstPortOffset is the offset of the destination port within a UDP or TCP header.
+	dstPortOffset = 2
+)
+
+// HooksFilter is a minimal packet filter that only handles outbound DNS hooks.
+// It is installed on the WireGuard interface when the userspace bind is active
+// but a full firewall filter (Manager) is not needed because a native kernel
+// firewall (nftables/iptables) handles packet filtering.
+type HooksFilter struct {
+	udpHook atomic.Pointer[common.PacketHook]
+	tcpHook atomic.Pointer[common.PacketHook]
+}
+
+var _ device.PacketFilter = (*HooksFilter)(nil)
+
+// FilterOutbound checks outbound packets for DNS hook matches.
+// Only IPv4 packets matching the registered hook IP:port are intercepted.
+// IPv6 and non-IP packets pass through unconditionally.
+func (f *HooksFilter) FilterOutbound(packetData []byte, _ int) bool {
+	if len(packetData) < ipv4HeaderMinLen {
+		return false
+	}
+
+	// Only process IPv4 packets, let everything else pass through.
+	if packetData[0]>>4 != 4 {
+		return false
+	}
+
+	ihl := int(packetData[0]&0x0f) * 4
+	if ihl < ipv4HeaderMinLen || len(packetData) < ihl+4 {
+		return false
+	}
+
+	// Skip non-first fragments: they don't carry L4 headers.
+	flagsAndOffset := binary.BigEndian.Uint16(packetData[ipv4FlagsOffset : ipv4FlagsOffset+2])
+	if flagsAndOffset&ipv4FragOffMask != 0 {
+		return false
+	}
+
+	dstIP, ok := netip.AddrFromSlice(packetData[ipv4DstOffset : ipv4DstOffset+4])
+	if !ok {
+		return false
+	}
+
+	proto := packetData[ipv4ProtoOffset]
+	dstPort := binary.BigEndian.Uint16(packetData[ihl+dstPortOffset : ihl+dstPortOffset+2])
+
+	switch proto {
+	case ipProtoUDP:
+		return common.HookMatches(f.udpHook.Load(), dstIP, dstPort, packetData)
+	case ipProtoTCP:
+		return common.HookMatches(f.tcpHook.Load(), dstIP, dstPort, packetData)
+	default:
+		return false
+	}
+}
+
+// FilterInbound allows all inbound packets (native firewall handles filtering).
+func (f *HooksFilter) FilterInbound([]byte, int) bool {
+	return false
+}
+
+// SetUDPPacketHook registers the UDP packet hook.
+func (f *HooksFilter) SetUDPPacketHook(ip netip.Addr, dPort uint16, hook func([]byte) bool) {
+	common.SetHook(&f.udpHook, ip, dPort, hook)
+}
+
+// SetTCPPacketHook registers the TCP packet hook.
+func (f *HooksFilter) SetTCPPacketHook(ip netip.Addr, dPort uint16, hook func([]byte) bool) {
+	common.SetHook(&f.tcpHook, ip, dPort, hook)
+}