[client] Add non-root ICMP support to userspace firewall forwarder (#4792)

client/firewall/uspfilter/forwarder/endpoint.go

@@ -2,6 +2,7 @@ package forwarder
 
 import (
 	"fmt"
+	"sync/atomic"
 
 	wgdevice "golang.zx2c4.com/wireguard/device"
 	"gvisor.dev/gvisor/pkg/tcpip"
@@ -16,7 +17,7 @@ type endpoint struct {
 	logger     *nblog.Logger
 	dispatcher stack.NetworkDispatcher
 	device     *wgdevice.Device
-	mtu        uint32
+	mtu        atomic.Uint32
 }
 
 func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
@@ -28,7 +29,7 @@ func (e *endpoint) IsAttached() bool {
 }
 
 func (e *endpoint) MTU() uint32 {
-	return e.mtu
+	return e.mtu.Load()
 }
 
 func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
@@ -82,6 +83,22 @@ func (e *endpoint) ParseHeader(*stack.PacketBuffer) bool {
 	return true
 }
 
+func (e *endpoint) Close() {
+	// Endpoint cleanup - nothing to do as device is managed externally
+}
+
+func (e *endpoint) SetLinkAddress(tcpip.LinkAddress) {
+	// Link address is not used for this endpoint type
+}
+
+func (e *endpoint) SetMTU(mtu uint32) {
+	e.mtu.Store(mtu)
+}
+
+func (e *endpoint) SetOnCloseAction(func()) {
+	// No action needed on close
+}
+
 type epID stack.TransportEndpointID
 
 func (i epID) String() string {

client/firewall/uspfilter/forwarder/forwarder.go

@@ -7,6 +7,7 @@ import (
 	"net/netip"
 	"runtime"
 	"sync"
+	"time"
 
 	log "github.com/sirupsen/logrus"
 	"gvisor.dev/gvisor/pkg/buffer"
@@ -35,14 +36,16 @@ type Forwarder struct {
 	logger     *nblog.Logger
 	flowLogger nftypes.FlowLogger
 	// ruleIdMap is used to store the rule ID for a given connection
-	ruleIdMap    sync.Map
-	stack        *stack.Stack
-	endpoint     *endpoint
-	udpForwarder *udpForwarder
-	ctx          context.Context
-	cancel       context.CancelFunc
-	ip           tcpip.Address
-	netstack     bool
+	ruleIdMap        sync.Map
+	stack            *stack.Stack
+	endpoint         *endpoint
+	udpForwarder     *udpForwarder
+	ctx              context.Context
+	cancel           context.CancelFunc
+	ip               tcpip.Address
+	netstack         bool
+	hasRawICMPAccess bool
+	pingSemaphore    chan struct{}
 }
 
 func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.FlowLogger, netstack bool, mtu uint16) (*Forwarder, error) {
@@ -60,8 +63,8 @@ func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.Flow
 	endpoint := &endpoint{
 		logger: logger,
 		device: iface.GetWGDevice(),
-		mtu:    uint32(mtu),
 	}
+	endpoint.mtu.Store(uint32(mtu))
 
 	if err := s.CreateNIC(nicID, endpoint); err != nil {
 		return nil, fmt.Errorf("create NIC: %v", err)
@@ -103,15 +106,16 @@ func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.Flow
 
 	ctx, cancel := context.WithCancel(context.Background())
 	f := &Forwarder{
-		logger:       logger,
-		flowLogger:   flowLogger,
-		stack:        s,
-		endpoint:     endpoint,
-		udpForwarder: newUDPForwarder(mtu, logger, flowLogger),
-		ctx:          ctx,
-		cancel:       cancel,
-		netstack:     netstack,
-		ip:           tcpip.AddrFromSlice(iface.Address().IP.AsSlice()),
+		logger:        logger,
+		flowLogger:    flowLogger,
+		stack:         s,
+		endpoint:      endpoint,
+		udpForwarder:  newUDPForwarder(mtu, logger, flowLogger),
+		ctx:           ctx,
+		cancel:        cancel,
+		netstack:      netstack,
+		ip:            tcpip.AddrFromSlice(iface.Address().IP.AsSlice()),
+		pingSemaphore: make(chan struct{}, 3),
 	}
 
 	receiveWindow := defaultReceiveWindow
@@ -129,6 +133,8 @@ func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.Flow
 
 	s.SetTransportProtocolHandler(icmp.ProtocolNumber4, f.handleICMP)
 
+	f.checkICMPCapability()
+
 	log.Debugf("forwarder: Initialization complete with NIC %d", nicID)
 	return f, nil
 }
@@ -198,3 +204,24 @@ func buildKey(srcIP, dstIP netip.Addr, srcPort, dstPort uint16) conntrack.ConnKe
 		DstPort: dstPort,
 	}
 }
+
+// checkICMPCapability tests whether we have raw ICMP socket access at startup.
+func (f *Forwarder) checkICMPCapability() {
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	lc := net.ListenConfig{}
+	conn, err := lc.ListenPacket(ctx, "ip4:icmp", "0.0.0.0")
+	if err != nil {
+		f.hasRawICMPAccess = false
+		f.logger.Debug("forwarder: No raw ICMP socket access, will use ping binary fallback")
+		return
+	}
+
+	if err := conn.Close(); err != nil {
+		f.logger.Debug1("forwarder: Failed to close ICMP capability test socket: %v", err)
+	}
+
+	f.hasRawICMPAccess = true
+	f.logger.Debug("forwarder: Raw ICMP socket access available")
+}

client/firewall/uspfilter/forwarder/icmp.go

@@ -2,8 +2,11 @@ package forwarder
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"net/netip"
+	"os/exec"
+	"runtime"
 	"time"
 
 	"github.com/google/uuid"
@@ -14,30 +17,95 @@ import (
 )
 
 // handleICMP handles ICMP packets from the network stack
-func (f *Forwarder) handleICMP(id stack.TransportEndpointID, pkt stack.PacketBufferPtr) bool {
+func (f *Forwarder) handleICMP(id stack.TransportEndpointID, pkt *stack.PacketBuffer) bool {
 	icmpHdr := header.ICMPv4(pkt.TransportHeader().View().AsSlice())
-	icmpType := uint8(icmpHdr.Type())
-	icmpCode := uint8(icmpHdr.Code())
-
-	if header.ICMPv4Type(icmpType) == header.ICMPv4EchoReply {
-		// dont process our own replies
-		return true
-	}
 
 	flowID := uuid.New()
-	f.sendICMPEvent(nftypes.TypeStart, flowID, id, icmpType, icmpCode, 0, 0)
+	f.sendICMPEvent(nftypes.TypeStart, flowID, id, uint8(icmpHdr.Type()), uint8(icmpHdr.Code()), 0, 0)
 
-	ctx, cancel := context.WithTimeout(f.ctx, 5*time.Second)
+	// For Echo Requests, send and wait for response
+	if icmpHdr.Type() == header.ICMPv4Echo {
+		return f.handleICMPEcho(flowID, id, pkt, uint8(icmpHdr.Type()), uint8(icmpHdr.Code()))
+	}
+
+	// For other ICMP types (Time Exceeded, Destination Unreachable, etc), forward without waiting
+	if !f.hasRawICMPAccess {
+		f.logger.Debug2("forwarder: Cannot handle ICMP type %v without raw socket access for %v", icmpHdr.Type(), epID(id))
+		return false
+	}
+
+	icmpData := stack.PayloadSince(pkt.TransportHeader()).AsSlice()
+	conn, err := f.forwardICMPPacket(id, icmpData, uint8(icmpHdr.Type()), uint8(icmpHdr.Code()), 100*time.Millisecond)
+	if err != nil {
+		f.logger.Error2("forwarder: Failed to forward ICMP packet for %v: %v", epID(id), err)
+		return true
+	}
+	if err := conn.Close(); err != nil {
+		f.logger.Debug1("forwarder: Failed to close ICMP socket: %v", err)
+	}
+
+	return true
+}
+
+// handleICMPEcho handles ICMP echo requests asynchronously with rate limiting.
+func (f *Forwarder) handleICMPEcho(flowID uuid.UUID, id stack.TransportEndpointID, pkt *stack.PacketBuffer, icmpType, icmpCode uint8) bool {
+	select {
+	case f.pingSemaphore <- struct{}{}:
+		icmpData := stack.PayloadSince(pkt.TransportHeader()).ToSlice()
+		rxBytes := pkt.Size()
+
+		go func() {
+			defer func() { <-f.pingSemaphore }()
+
+			if f.hasRawICMPAccess {
+				f.handleICMPViaSocket(flowID, id, icmpType, icmpCode, icmpData, rxBytes)
+			} else {
+				f.handleICMPViaPing(flowID, id, icmpType, icmpCode, icmpData, rxBytes)
+			}
+		}()
+	default:
+		f.logger.Debug3("forwarder: ICMP rate limit exceeded for %v type %v code %v",
+			epID(id), icmpType, icmpCode)
+	}
+	return true
+}
+
+// forwardICMPPacket creates a raw ICMP socket and sends the packet, returning the connection.
+// The caller is responsible for closing the returned connection.
+func (f *Forwarder) forwardICMPPacket(id stack.TransportEndpointID, payload []byte, icmpType, icmpCode uint8, timeout time.Duration) (net.PacketConn, error) {
+	ctx, cancel := context.WithTimeout(f.ctx, timeout)
 	defer cancel()
 
 	lc := net.ListenConfig{}
-	// TODO: support non-root
 	conn, err := lc.ListenPacket(ctx, "ip4:icmp", "0.0.0.0")
 	if err != nil {
-		f.logger.Error2("forwarder: Failed to create ICMP socket for %v: %v", epID(id), err)
+		return nil, fmt.Errorf("create ICMP socket: %w", err)
+	}
 
-		// This will make netstack reply on behalf of the original destination, that's ok for now
-		return false
+	dstIP := f.determineDialAddr(id.LocalAddress)
+	dst := &net.IPAddr{IP: dstIP}
+
+	if _, err = conn.WriteTo(payload, dst); err != nil {
+		if closeErr := conn.Close(); closeErr != nil {
+			f.logger.Debug1("forwarder: Failed to close ICMP socket: %v", closeErr)
+		}
+		return nil, fmt.Errorf("write ICMP packet: %w", err)
+	}
+
+	f.logger.Trace3("forwarder: Forwarded ICMP packet %v type %v code %v",
+		epID(id), icmpType, icmpCode)
+
+	return conn, nil
+}
+
+// handleICMPViaSocket handles ICMP echo requests using raw sockets.
+func (f *Forwarder) handleICMPViaSocket(flowID uuid.UUID, id stack.TransportEndpointID, icmpType, icmpCode uint8, icmpData []byte, rxBytes int) {
+	sendTime := time.Now()
+
+	conn, err := f.forwardICMPPacket(id, icmpData, icmpType, icmpCode, 5*time.Second)
+	if err != nil {
+		f.logger.Error2("forwarder: Failed to send ICMP packet for %v: %v", epID(id), err)
+		return
 	}
 	defer func() {
 		if err := conn.Close(); err != nil {
@@ -45,38 +113,22 @@ func (f *Forwarder) handleICMP(id stack.TransportEndpointID, pkt stack.PacketBuf
 		}
 	}()
 
-	dstIP := f.determineDialAddr(id.LocalAddress)
-	dst := &net.IPAddr{IP: dstIP}
+	txBytes := f.handleEchoResponse(conn, id)
+	rtt := time.Since(sendTime).Round(10 * time.Microsecond)
 
-	fullPacket := stack.PayloadSince(pkt.TransportHeader())
-	payload := fullPacket.AsSlice()
+	f.logger.Trace4("forwarder: Forwarded ICMP echo reply %v type %v code %v (rtt=%v, raw socket)",
+		epID(id), icmpType, icmpCode, rtt)
 
-	if _, err = conn.WriteTo(payload, dst); err != nil {
-		f.logger.Error2("forwarder: Failed to write ICMP packet for %v: %v", epID(id), err)
-		return true
-	}
-
-	f.logger.Trace3("forwarder: Forwarded ICMP packet %v type %v code %v",
-		epID(id), icmpHdr.Type(), icmpHdr.Code())
-
-	// For Echo Requests, send and handle response
-	if header.ICMPv4Type(icmpType) == header.ICMPv4Echo {
-		rxBytes := pkt.Size()
-		txBytes := f.handleEchoResponse(icmpHdr, conn, id)
-		f.sendICMPEvent(nftypes.TypeEnd, flowID, id, icmpType, icmpCode, uint64(rxBytes), uint64(txBytes))
-	}
-
-	// For other ICMP types (Time Exceeded, Destination Unreachable, etc) do nothing
-	return true
+	f.sendICMPEvent(nftypes.TypeEnd, flowID, id, icmpType, icmpCode, uint64(rxBytes), uint64(txBytes))
 }
 
-func (f *Forwarder) handleEchoResponse(icmpHdr header.ICMPv4, conn net.PacketConn, id stack.TransportEndpointID) int {
+func (f *Forwarder) handleEchoResponse(conn net.PacketConn, id stack.TransportEndpointID) int {
 	if err := conn.SetReadDeadline(time.Now().Add(5 * time.Second)); err != nil {
 		f.logger.Error1("forwarder: Failed to set read deadline for ICMP response: %v", err)
 		return 0
 	}
 
-	response := make([]byte, f.endpoint.mtu)
+	response := make([]byte, f.endpoint.mtu.Load())
 	n, _, err := conn.ReadFrom(response)
 	if err != nil {
 		if !isTimeout(err) {
@@ -85,31 +137,7 @@ func (f *Forwarder) handleEchoResponse(icmpHdr header.ICMPv4, conn net.PacketCon
 		return 0
 	}
 
-	ipHdr := make([]byte, header.IPv4MinimumSize)
-	ip := header.IPv4(ipHdr)
-	ip.Encode(&header.IPv4Fields{
-		TotalLength: uint16(header.IPv4MinimumSize + n),
-		TTL:         64,
-		Protocol:    uint8(header.ICMPv4ProtocolNumber),
-		SrcAddr:     id.LocalAddress,
-		DstAddr:     id.RemoteAddress,
-	})
-	ip.SetChecksum(^ip.CalculateChecksum())
-
-	fullPacket := make([]byte, 0, len(ipHdr)+n)
-	fullPacket = append(fullPacket, ipHdr...)
-	fullPacket = append(fullPacket, response[:n]...)
-
-	if err := f.InjectIncomingPacket(fullPacket); err != nil {
-		f.logger.Error1("forwarder: Failed to inject ICMP response: %v", err)
-
-		return 0
-	}
-
-	f.logger.Trace3("forwarder: Forwarded ICMP echo reply for %v type %v code %v",
-		epID(id), icmpHdr.Type(), icmpHdr.Code())
-
-	return len(fullPacket)
+	return f.injectICMPReply(id, response[:n])
 }
 
 // sendICMPEvent stores flow events for ICMP packets
@@ -152,3 +180,95 @@ func (f *Forwarder) sendICMPEvent(typ nftypes.Type, flowID uuid.UUID, id stack.T
 
 	f.flowLogger.StoreEvent(fields)
 }
+
+// handleICMPViaPing handles ICMP echo requests by executing the system ping binary.
+// This is used as a fallback when raw socket access is not available.
+func (f *Forwarder) handleICMPViaPing(flowID uuid.UUID, id stack.TransportEndpointID, icmpType, icmpCode uint8, icmpData []byte, rxBytes int) {
+	ctx, cancel := context.WithTimeout(f.ctx, 5*time.Second)
+	defer cancel()
+
+	dstIP := f.determineDialAddr(id.LocalAddress)
+	cmd := buildPingCommand(ctx, dstIP, 5*time.Second)
+
+	pingStart := time.Now()
+	if err := cmd.Run(); err != nil {
+		f.logger.Warn4("forwarder: Ping binary failed for %v type %v code %v: %v", epID(id),
+			icmpType, icmpCode, err)
+		return
+	}
+	rtt := time.Since(pingStart).Round(10 * time.Microsecond)
+
+	f.logger.Trace3("forwarder: Forwarded ICMP echo request %v type %v code %v",
+		epID(id), icmpType, icmpCode)
+
+	txBytes := f.synthesizeEchoReply(id, icmpData)
+
+	f.logger.Trace4("forwarder: Forwarded ICMP echo reply %v type %v code %v (rtt=%v, ping binary)",
+		epID(id), icmpType, icmpCode, rtt)
+
+	f.sendICMPEvent(nftypes.TypeEnd, flowID, id, icmpType, icmpCode, uint64(rxBytes), uint64(txBytes))
+}
+
+// buildPingCommand creates a platform-specific ping command.
+func buildPingCommand(ctx context.Context, target net.IP, timeout time.Duration) *exec.Cmd {
+	timeoutSec := int(timeout.Seconds())
+	if timeoutSec < 1 {
+		timeoutSec = 1
+	}
+
+	switch runtime.GOOS {
+	case "linux", "android":
+		return exec.CommandContext(ctx, "ping", "-c", "1", "-W", fmt.Sprintf("%d", timeoutSec), "-q", target.String())
+	case "darwin", "ios":
+		return exec.CommandContext(ctx, "ping", "-c", "1", "-t", fmt.Sprintf("%d", timeoutSec), "-q", target.String())
+	case "freebsd":
+		return exec.CommandContext(ctx, "ping", "-c", "1", "-t", fmt.Sprintf("%d", timeoutSec), target.String())
+	case "openbsd", "netbsd":
+		return exec.CommandContext(ctx, "ping", "-c", "1", "-w", fmt.Sprintf("%d", timeoutSec), target.String())
+	case "windows":
+		return exec.CommandContext(ctx, "ping", "-n", "1", "-w", fmt.Sprintf("%d", timeoutSec*1000), target.String())
+	default:
+		return exec.CommandContext(ctx, "ping", "-c", "1", target.String())
+	}
+}
+
+// synthesizeEchoReply creates an ICMP echo reply from raw ICMP data and injects it back into the network stack.
+// Returns the size of the injected packet.
+func (f *Forwarder) synthesizeEchoReply(id stack.TransportEndpointID, icmpData []byte) int {
+	replyICMP := make([]byte, len(icmpData))
+	copy(replyICMP, icmpData)
+
+	replyICMPHdr := header.ICMPv4(replyICMP)
+	replyICMPHdr.SetType(header.ICMPv4EchoReply)
+	replyICMPHdr.SetChecksum(0)
+	replyICMPHdr.SetChecksum(header.ICMPv4Checksum(replyICMPHdr, 0))
+
+	return f.injectICMPReply(id, replyICMP)
+}
+
+// injectICMPReply wraps an ICMP payload in an IP header and injects it into the network stack.
+// Returns the total size of the injected packet, or 0 if injection failed.
+func (f *Forwarder) injectICMPReply(id stack.TransportEndpointID, icmpPayload []byte) int {
+	ipHdr := make([]byte, header.IPv4MinimumSize)
+	ip := header.IPv4(ipHdr)
+	ip.Encode(&header.IPv4Fields{
+		TotalLength: uint16(header.IPv4MinimumSize + len(icmpPayload)),
+		TTL:         64,
+		Protocol:    uint8(header.ICMPv4ProtocolNumber),
+		SrcAddr:     id.LocalAddress,
+		DstAddr:     id.RemoteAddress,
+	})
+	ip.SetChecksum(^ip.CalculateChecksum())
+
+	fullPacket := make([]byte, 0, len(ipHdr)+len(icmpPayload))
+	fullPacket = append(fullPacket, ipHdr...)
+	fullPacket = append(fullPacket, icmpPayload...)
+
+	// Bypass netstack and send directly to peer to avoid looping through our ICMP handler
+	if err := f.endpoint.device.CreateOutboundPacket(fullPacket, id.RemoteAddress.AsSlice()); err != nil {
+		f.logger.Error1("forwarder: Failed to send ICMP reply to peer: %v", err)
+		return 0
+	}
+
+	return len(fullPacket)
+}

client/firewall/uspfilter/forwarder/udp.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/netip"
 	"sync"
@@ -131,10 +132,10 @@ func (f *udpForwarder) cleanup() {
 }
 
 // handleUDP is called by the UDP forwarder for new packets
-func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
+func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) bool {
 	if f.ctx.Err() != nil {
 		f.logger.Trace("forwarder: context done, dropping UDP packet")
-		return
+		return false
 	}
 
 	id := r.ID()
@@ -144,7 +145,7 @@ func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
 	f.udpForwarder.RUnlock()
 	if exists {
 		f.logger.Trace1("forwarder: existing UDP connection for %v", epID(id))
-		return
+		return true
 	}
 
 	flowID := uuid.New()
@@ -162,7 +163,7 @@ func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
 	if err != nil {
 		f.logger.Debug2("forwarder: UDP dial error for %v: %v", epID(id), err)
 		// TODO: Send ICMP error message
-		return
+		return false
 	}
 
 	// Create wait queue for blocking syscalls
@@ -173,10 +174,10 @@ func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
 		if err := outConn.Close(); err != nil {
 			f.logger.Debug2("forwarder: UDP outConn close error for %v: %v", epID(id), err)
 		}
-		return
+		return false
 	}
 
-	inConn := gonet.NewUDPConn(f.stack, &wq, ep)
+	inConn := gonet.NewUDPConn(&wq, ep)
 	connCtx, connCancel := context.WithCancel(f.ctx)
 
 	pConn := &udpPacketConn{
@@ -199,7 +200,7 @@ func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
 		if err := outConn.Close(); err != nil {
 			f.logger.Debug2("forwarder: UDP outConn close error for %v: %v", epID(id), err)
 		}
-		return
+		return true
 	}
 	f.udpForwarder.conns[id] = pConn
 	f.udpForwarder.Unlock()
@@ -208,6 +209,7 @@ func (f *Forwarder) handleUDP(r *udp.ForwarderRequest) {
 	f.logger.Trace1("forwarder: established UDP connection %v", epID(id))
 
 	go f.proxyUDP(connCtx, pConn, id, ep)
+	return true
 }
 
 func (f *Forwarder) proxyUDP(ctx context.Context, pConn *udpPacketConn, id stack.TransportEndpointID, ep tcpip.Endpoint) {
@@ -348,7 +350,7 @@ func (c *udpPacketConn) copy(ctx context.Context, dst net.Conn, src net.Conn, bu
 }
 
 func isClosedError(err error) bool {
-	return errors.Is(err, net.ErrClosed) || errors.Is(err, context.Canceled)
+	return errors.Is(err, net.ErrClosed) || errors.Is(err, context.Canceled) || errors.Is(err, io.EOF)
 }
 
 func isTimeout(err error) bool {

client/firewall/uspfilter/log/log.go

@@ -168,6 +168,15 @@ func (l *Logger) Warn3(format string, arg1, arg2, arg3 any) {
 	}
 }
 
+func (l *Logger) Warn4(format string, arg1, arg2, arg3, arg4 any) {
+	if l.level.Load() >= uint32(LevelWarn) {
+		select {
+		case l.msgChannel <- logMessage{level: LevelWarn, format: format, arg1: arg1, arg2: arg2, arg3: arg3, arg4: arg4}:
+		default:
+		}
+	}
+}
+
 func (l *Logger) Debug1(format string, arg1 any) {
 	if l.level.Load() >= uint32(LevelDebug) {
 		select {

client/iface/bind/ice_bind.go

@@ -27,8 +27,23 @@ type receiverCreator struct {
 	iceBind *ICEBind
 }
 
-func (rc receiverCreator) CreateIPv4ReceiverFn(pc *ipv4.PacketConn, conn *net.UDPConn, rxOffload bool, msgPool *sync.Pool) wgConn.ReceiveFunc {
-	return rc.iceBind.createIPv4ReceiverFn(pc, conn, rxOffload, msgPool)
+func (rc receiverCreator) CreateReceiverFn(pc wgConn.BatchReader, conn *net.UDPConn, rxOffload bool, msgPool *sync.Pool) wgConn.ReceiveFunc {
+	if ipv4PC, ok := pc.(*ipv4.PacketConn); ok {
+		return rc.iceBind.createIPv4ReceiverFn(ipv4PC, conn, rxOffload, msgPool)
+	}
+	// IPv6 is currently not supported in the udpmux, this is a stub for compatibility with the
+	// wireguard-go ReceiverCreator interface which is called for both IPv4 and IPv6.
+	return func(bufs [][]byte, sizes []int, eps []wgConn.Endpoint) (n int, err error) {
+		buf := bufs[0]
+		size, ep, err := conn.ReadFromUDPAddrPort(buf)
+		if err != nil {
+			return 0, err
+		}
+		sizes[0] = size
+		stdEp := &wgConn.StdNetEndpoint{AddrPort: ep}
+		eps[0] = stdEp
+		return 1, nil
+	}
 }
 
 // ICEBind is a bind implementation with two main features: