tcpip/link/fdbased/endpoint.go - third_party/netstack - Git at Google

 // Copyright 2018 The gVisor Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 // +build linux

 // Package fdbased provides the implemention of data-link layer endpoints
 // backed by boundary-preserving file descriptors (e.g., TUN devices,
 // seqpacket/datagram sockets).
 //
 // FD based endpoints can be used in the networking stack by calling New() to
 // create a new endpoint, and then passing it as an argument to
 // Stack.CreateNIC().
 //
 // FD based endpoints can use more than one file descriptor to read incoming
 // packets. If there are more than one FDs specified and the underlying FD is an
 // AF_PACKET then the endpoint will enable FANOUT mode on the socket so that the
 // host kernel will consistently hash the packets to the sockets. This ensures
 // that packets for the same TCP streams are not reordered.
 //
 // Similarly if more than one FD's are specified where the underlying FD is not
 // AF_PACKET then it's the caller's responsibility to ensure that all inbound
 // packets on the descriptors are consistently 5 tuple hashed to one of the
 // descriptors to prevent TCP reordering.
 //
 // Since netstack today does not compute 5 tuple hashes for outgoing packets we
 // only use the first FD to write outbound packets. Once 5 tuple hashes for
 // all outbound packets are available we will make use of all underlying FD's to
 // write outbound packets.
 package fdbased

 import (
 	"fmt"
 	"syscall"

 	"github.com/google/netstack/tcpip"
 	"github.com/google/netstack/tcpip/buffer"
 	"github.com/google/netstack/tcpip/header"
 	"github.com/google/netstack/tcpip/link/rawfile"
 	"github.com/google/netstack/tcpip/stack"
 	"golang.org/x/sys/unix"
 )

 // linkDispatcher reads packets from the link FD and dispatches them to the
 // NetworkDispatcher.
 type linkDispatcher interface {
 	dispatch() (bool, *tcpip.Error)
 }

 // PacketDispatchMode are the various supported methods of receiving and
 // dispatching packets from the underlying FD.
 type PacketDispatchMode int

 const (
 	// Readv is the default dispatch mode and is the least performant of the
 	// dispatch options but the one that is supported by all underlying FD
 	// types.
 	Readv PacketDispatchMode = iota
 	// RecvMMsg enables use of recvmmsg() syscall instead of readv() to
 	// read inbound packets. This reduces # of syscalls needed to process
 	// packets.
 	//
 	// NOTE: recvmmsg() is only supported for sockets, so if the underlying
 	// FD is not a socket then the code will still fall back to the readv()
 	// path.
 	RecvMMsg
 	// PacketMMap enables use of PACKET_RX_RING to receive packets from the
 	// NIC. PacketMMap requires that the underlying FD be an AF_PACKET. The
 	// primary use-case for this is runsc which uses an AF_PACKET FD to
 	// receive packets from the veth device.
 	PacketMMap
 )

 type endpoint struct {
 	// fds is the set of file descriptors each identifying one inbound/outbound
 	// channel. The endpoint will dispatch from all inbound channels as well as
 	// hash outbound packets to specific channels based on the packet hash.
 	fds []int

 	// mtu (maximum transmission unit) is the maximum size of a packet.
 	mtu uint32

 	// hdrSize specifies the link-layer header size. If set to 0, no header
 	// is added/removed; otherwise an ethernet header is used.
 	hdrSize int

 	// addr is the address of the endpoint.
 	addr tcpip.LinkAddress

 	// caps holds the endpoint capabilities.
 	caps stack.LinkEndpointCapabilities

 	// closed is a function to be called when the FD's peer (if any) closes
 	// its end of the communication pipe.
 	closed func(*tcpip.Error)

 	inboundDispatchers []linkDispatcher
 	dispatcher         stack.NetworkDispatcher

 	// packetDispatchMode controls the packet dispatcher used by this
 	// endpoint.
 	packetDispatchMode PacketDispatchMode

 	// gsoMaxSize is the maximum GSO packet size. It is zero if GSO is
 	// disabled.
 	gsoMaxSize uint32
 }

 // Options specify the details about the fd-based endpoint to be created.
 type Options struct {
 	// FDs is a set of FDs used to read/write packets.
 	FDs []int

 	// MTU is the mtu to use for this endpoint.
 	MTU uint32

 	// EthernetHeader if true, indicates that the endpoint should read/write
 	// ethernet frames instead of IP packets.
 	EthernetHeader bool

 	// ClosedFunc is a function to be called when an endpoint's peer (if
 	// any) closes its end of the communication pipe.
 	ClosedFunc func(*tcpip.Error)

 	// Address is the link address for this endpoint. Only used if
 	// EthernetHeader is true.
 	Address tcpip.LinkAddress

 	// SaveRestore if true, indicates that this NIC capability set should
 	// include CapabilitySaveRestore
 	SaveRestore bool

 	// DisconnectOk if true, indicates that this NIC capability set should
 	// include CapabilityDisconnectOk.
 	DisconnectOk bool

 	// GSOMaxSize is the maximum GSO packet size. It is zero if GSO is
 	// disabled.
 	GSOMaxSize uint32

 	// PacketDispatchMode specifies the type of inbound dispatcher to be
 	// used for this endpoint.
 	PacketDispatchMode PacketDispatchMode

 	// TXChecksumOffload if true, indicates that this endpoints capability
 	// set should include CapabilityTXChecksumOffload.
 	TXChecksumOffload bool

 	// RXChecksumOffload if true, indicates that this endpoints capability
 	// set should include CapabilityRXChecksumOffload.
 	RXChecksumOffload bool
 }

 // New creates a new fd-based endpoint.
 //
 // Makes fd non-blocking, but does not take ownership of fd, which must remain
 // open for the lifetime of the returned endpoint.
 func New(opts *Options) (tcpip.LinkEndpointID, error) {
 	caps := stack.LinkEndpointCapabilities(0)
 	if opts.RXChecksumOffload {
 		caps |= stack.CapabilityRXChecksumOffload
 	}

 	if opts.TXChecksumOffload {
 		caps |= stack.CapabilityTXChecksumOffload
 	}

 	hdrSize := 0
 	if opts.EthernetHeader {
 		hdrSize = header.EthernetMinimumSize
 		caps |= stack.CapabilityResolutionRequired
 	}

 	if opts.SaveRestore {
 		caps |= stack.CapabilitySaveRestore
 	}

 	if opts.DisconnectOk {
 		caps |= stack.CapabilityDisconnectOk
 	}

 	if len(opts.FDs) == 0 {
 		return 0, fmt.Errorf("opts.FD is empty, at least one FD must be specified")
 	}

 	e := &endpoint{
 		fds:                opts.FDs,
 		mtu:                opts.MTU,
 		caps:               caps,
 		closed:             opts.ClosedFunc,
 		addr:               opts.Address,
 		hdrSize:            hdrSize,
 		packetDispatchMode: opts.PacketDispatchMode,
 	}

 	// Create per channel dispatchers.
 	for i := 0; i < len(e.fds); i++ {
 		fd := e.fds[i]
 		if err := syscall.SetNonblock(fd, true); err != nil {
 			return 0, fmt.Errorf("syscall.SetNonblock(%v) failed: %v", fd, err)
 		}

 		isSocket, err := isSocketFD(fd)
 		if err != nil {
 			return 0, err
 		}
 		if isSocket {
 			if opts.GSOMaxSize != 0 {
 				e.caps |= stack.CapabilityGSO
 				e.gsoMaxSize = opts.GSOMaxSize
 			}
 		}
 		inboundDispatcher, err := createInboundDispatcher(e, fd, isSocket)
 		if err != nil {
 			return 0, fmt.Errorf("createInboundDispatcher(...) = %v", err)
 		}
 		e.inboundDispatchers = append(e.inboundDispatchers, inboundDispatcher)
 	}

 	return stack.RegisterLinkEndpoint(e), nil
 }

 func createInboundDispatcher(e *endpoint, fd int, isSocket bool) (linkDispatcher, error) {
 	// By default use the readv() dispatcher as it works with all kinds of
 	// FDs (tap/tun/unix domain sockets and af_packet).
 	inboundDispatcher, err := newReadVDispatcher(fd, e)
 	if err != nil {
 		return nil, fmt.Errorf("newReadVDispatcher(%d, %+v) = %v", fd, e, err)
 	}

 	if isSocket {
 		sa, err := unix.Getsockname(fd)
 		if err != nil {
 			return nil, fmt.Errorf("unix.Getsockname(%d) = %v", fd, err)
 		}
 		switch sa.(type) {
 		case *unix.SockaddrLinklayer:
 			// enable PACKET_FANOUT mode is the underlying socket is
 			// of type AF_PACKET.
 			const fanoutID = 1
 			const fanoutType = 0x8000 // PACKET_FANOUT_HASH | PACKET_FANOUT_FLAG_DEFRAG
 			fanoutArg := fanoutID | fanoutType<<16
 			if err := syscall.SetsockoptInt(fd, syscall.SOL_PACKET, unix.PACKET_FANOUT, fanoutArg); err != nil {
 				return nil, fmt.Errorf("failed to enable PACKET_FANOUT option: %v", err)
 			}
 		}

 		switch e.packetDispatchMode {
 		case PacketMMap:
 			inboundDispatcher, err = newPacketMMapDispatcher(fd, e)
 			if err != nil {
 				return nil, fmt.Errorf("newPacketMMapDispatcher(%d, %+v) = %v", fd, e, err)
 			}
 		case RecvMMsg:
 			// If the provided FD is a socket then we optimize
 			// packet reads by using recvmmsg() instead of read() to
 			// read packets in a batch.
 			inboundDispatcher, err = newRecvMMsgDispatcher(fd, e)
 			if err != nil {
 				return nil, fmt.Errorf("newRecvMMsgDispatcher(%d, %+v) = %v", fd, e, err)
 			}
 		}
 	}
 	return inboundDispatcher, nil
 }

 func isSocketFD(fd int) (bool, error) {
 	var stat syscall.Stat_t
 	if err := syscall.Fstat(fd, &stat); err != nil {
 		return false, fmt.Errorf("syscall.Fstat(%v,...) failed: %v", fd, err)
 	}
 	return (stat.Mode & syscall.S_IFSOCK) == syscall.S_IFSOCK, nil
 }

 // Attach launches the goroutine that reads packets from the file descriptor and
 // dispatches them via the provided dispatcher.
 func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	e.dispatcher = dispatcher
 	// Link endpoints are not savable. When transportation endpoints are
 	// saved, they stop sending outgoing packets and all incoming packets
 	// are rejected.
 	for i := range e.inboundDispatchers {
 		go e.dispatchLoop(e.inboundDispatchers[i])
 	}
 }

 // IsAttached implements stack.LinkEndpoint.IsAttached.
 func (e *endpoint) IsAttached() bool {
 	return e.dispatcher != nil
 }

 // MTU implements stack.LinkEndpoint.MTU. It returns the value initialized
 // during construction.
 func (e *endpoint) MTU() uint32 {
 	return e.mtu
 }

 // Capabilities implements stack.LinkEndpoint.Capabilities.
 func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
 	return e.caps
 }

 // MaxHeaderLength returns the maximum size of the link-layer header.
 func (e *endpoint) MaxHeaderLength() uint16 {
 	return uint16(e.hdrSize)
 }

 // LinkAddress returns the link address of this endpoint.
 func (e *endpoint) LinkAddress() tcpip.LinkAddress {
 	return e.addr
 }

 // virtioNetHdr is declared in linux/virtio_net.h.
 type virtioNetHdr struct {
 	flags      uint8
 	gsoType    uint8
 	hdrLen     uint16
 	gsoSize    uint16
 	csumStart  uint16
 	csumOffset uint16
 }

 // These constants are declared in linux/virtio_net.h.
 const (
 	_VIRTIO_NET_HDR_F_NEEDS_CSUM = 1

 	_VIRTIO_NET_HDR_GSO_TCPV4 = 1
 	_VIRTIO_NET_HDR_GSO_TCPV6 = 4
 )

 // WritePacket writes outbound packets to the file descriptor. If it is not
 // currently writable, the packet is dropped.
 func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
 	if e.hdrSize > 0 {
 		// Add ethernet header if needed.
 		eth := header.Ethernet(hdr.Prepend(header.EthernetMinimumSize))
 		ethHdr := &header.EthernetFields{
 			DstAddr: r.RemoteLinkAddress,
 			Type:    protocol,
 		}

 		// Preserve the src address if it's set in the route.
 		if r.LocalLinkAddress != "" {
 			ethHdr.SrcAddr = r.LocalLinkAddress
 		} else {
 			ethHdr.SrcAddr = e.addr
 		}
 		eth.Encode(ethHdr)
 	}

 	if e.Capabilities()&stack.CapabilityGSO != 0 {
 		vnetHdr := virtioNetHdr{}
 		vnetHdrBuf := vnetHdrToByteSlice(&vnetHdr)
 		if gso != nil {
 			vnetHdr.hdrLen = uint16(hdr.UsedLength())
 			if gso.NeedsCsum {
 				vnetHdr.flags = _VIRTIO_NET_HDR_F_NEEDS_CSUM
 				vnetHdr.csumStart = header.EthernetMinimumSize + gso.L3HdrLen
 				vnetHdr.csumOffset = gso.CsumOffset
 			}
 			if gso.Type != stack.GSONone && uint16(payload.Size()) > gso.MSS {
 				switch gso.Type {
 				case stack.GSOTCPv4:
 					vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV4
 				case stack.GSOTCPv6:
 					vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV6
 				default:
 					panic(fmt.Sprintf("Unknown gso type: %v", gso.Type))
 				}
 				vnetHdr.gsoSize = gso.MSS
 			}
 		}

 		return rawfile.NonBlockingWrite3(e.fds[0], vnetHdrBuf, hdr.View(), payload.ToView())
 	}

 	if payload.Size() == 0 {
 		return rawfile.NonBlockingWrite(e.fds[0], hdr.View())
 	}

 	return rawfile.NonBlockingWrite3(e.fds[0], hdr.View(), payload.ToView(), nil)
 }

 // WriteRawPacket writes a raw packet directly to the file descriptor.
 func (e *endpoint) WriteRawPacket(dest tcpip.Address, packet []byte) *tcpip.Error {
 	return rawfile.NonBlockingWrite(e.fds[0], packet)
 }

 // dispatchLoop reads packets from the file descriptor in a loop and dispatches
 // them to the network stack.
 func (e *endpoint) dispatchLoop(inboundDispatcher linkDispatcher) *tcpip.Error {
 	for {
 		cont, err := inboundDispatcher.dispatch()
 		if err != nil || !cont {
 			if e.closed != nil {
 				e.closed(err)
 			}
 			return err
 		}
 	}
 }

 // GSOMaxSize returns the maximum GSO packet size.
 func (e *endpoint) GSOMaxSize() uint32 {
 	return e.gsoMaxSize
 }

 // InjectableEndpoint is an injectable fd-based endpoint. The endpoint writes
 // to the FD, but does not read from it. All reads come from injected packets.
 type InjectableEndpoint struct {
 	endpoint

 	dispatcher stack.NetworkDispatcher
 }

 // Attach saves the stack network-layer dispatcher for use later when packets
 // are injected.
 func (e *InjectableEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	e.dispatcher = dispatcher
 }

 // Inject injects an inbound packet.
 func (e *InjectableEndpoint) Inject(protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
 	e.dispatcher.DeliverNetworkPacket(e, "" /* remote */, "" /* local */, protocol, vv)
 }

 // NewInjectable creates a new fd-based InjectableEndpoint.
 func NewInjectable(fd int, mtu uint32, capabilities stack.LinkEndpointCapabilities) (tcpip.LinkEndpointID, *InjectableEndpoint) {
 	syscall.SetNonblock(fd, true)

 	e := &InjectableEndpoint{endpoint: endpoint{
 		fds:  []int{fd},
 		mtu:  mtu,
 		caps: capabilities,
 	}}

 	return stack.RegisterLinkEndpoint(e), e
 }
	// Copyright 2018 The gVisor Authors.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	// +build linux

	// Package fdbased provides the implemention of data-link layer endpoints
	// backed by boundary-preserving file descriptors (e.g., TUN devices,
	// seqpacket/datagram sockets).
	//
	// FD based endpoints can be used in the networking stack by calling New() to
	// create a new endpoint, and then passing it as an argument to
	// Stack.CreateNIC().
	//
	// FD based endpoints can use more than one file descriptor to read incoming
	// packets. If there are more than one FDs specified and the underlying FD is an
	// AF_PACKET then the endpoint will enable FANOUT mode on the socket so that the
	// host kernel will consistently hash the packets to the sockets. This ensures
	// that packets for the same TCP streams are not reordered.
	//
	// Similarly if more than one FD's are specified where the underlying FD is not
	// AF_PACKET then it's the caller's responsibility to ensure that all inbound
	// packets on the descriptors are consistently 5 tuple hashed to one of the
	// descriptors to prevent TCP reordering.
	//
	// Since netstack today does not compute 5 tuple hashes for outgoing packets we
	// only use the first FD to write outbound packets. Once 5 tuple hashes for
	// all outbound packets are available we will make use of all underlying FD's to
	// write outbound packets.
	package fdbased

	import (
	"fmt"
	"syscall"

	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
	"github.com/google/netstack/tcpip/link/rawfile"
	"github.com/google/netstack/tcpip/stack"
	"golang.org/x/sys/unix"
	)

	// linkDispatcher reads packets from the link FD and dispatches them to the
	// NetworkDispatcher.
	type linkDispatcher interface {
	dispatch() (bool, *tcpip.Error)
	}

	// PacketDispatchMode are the various supported methods of receiving and
	// dispatching packets from the underlying FD.
	type PacketDispatchMode int

	const (
	// Readv is the default dispatch mode and is the least performant of the
	// dispatch options but the one that is supported by all underlying FD
	// types.
	Readv PacketDispatchMode = iota
	// RecvMMsg enables use of recvmmsg() syscall instead of readv() to
	// read inbound packets. This reduces # of syscalls needed to process
	// packets.
	//
	// NOTE: recvmmsg() is only supported for sockets, so if the underlying
	// FD is not a socket then the code will still fall back to the readv()
	// path.
	RecvMMsg
	// PacketMMap enables use of PACKET_RX_RING to receive packets from the
	// NIC. PacketMMap requires that the underlying FD be an AF_PACKET. The
	// primary use-case for this is runsc which uses an AF_PACKET FD to
	// receive packets from the veth device.
	PacketMMap
	)

	type endpoint struct {
	// fds is the set of file descriptors each identifying one inbound/outbound
	// channel. The endpoint will dispatch from all inbound channels as well as
	// hash outbound packets to specific channels based on the packet hash.
	fds []int

	// mtu (maximum transmission unit) is the maximum size of a packet.
	mtu uint32

	// hdrSize specifies the link-layer header size. If set to 0, no header
	// is added/removed; otherwise an ethernet header is used.
	hdrSize int

	// addr is the address of the endpoint.
	addr tcpip.LinkAddress

	// caps holds the endpoint capabilities.
	caps stack.LinkEndpointCapabilities

	// closed is a function to be called when the FD's peer (if any) closes
	// its end of the communication pipe.
	closed func(*tcpip.Error)

	inboundDispatchers []linkDispatcher
	dispatcher stack.NetworkDispatcher

	// packetDispatchMode controls the packet dispatcher used by this
	// endpoint.
	packetDispatchMode PacketDispatchMode

	// gsoMaxSize is the maximum GSO packet size. It is zero if GSO is
	// disabled.
	gsoMaxSize uint32
	}

	// Options specify the details about the fd-based endpoint to be created.
	type Options struct {
	// FDs is a set of FDs used to read/write packets.
	FDs []int

	// MTU is the mtu to use for this endpoint.
	MTU uint32

	// EthernetHeader if true, indicates that the endpoint should read/write
	// ethernet frames instead of IP packets.
	EthernetHeader bool

	// ClosedFunc is a function to be called when an endpoint's peer (if
	// any) closes its end of the communication pipe.
	ClosedFunc func(*tcpip.Error)

	// Address is the link address for this endpoint. Only used if
	// EthernetHeader is true.
	Address tcpip.LinkAddress

	// SaveRestore if true, indicates that this NIC capability set should
	// include CapabilitySaveRestore
	SaveRestore bool

	// DisconnectOk if true, indicates that this NIC capability set should
	// include CapabilityDisconnectOk.
	DisconnectOk bool

	// GSOMaxSize is the maximum GSO packet size. It is zero if GSO is
	// disabled.
	GSOMaxSize uint32

	// PacketDispatchMode specifies the type of inbound dispatcher to be
	// used for this endpoint.
	PacketDispatchMode PacketDispatchMode

	// TXChecksumOffload if true, indicates that this endpoints capability
	// set should include CapabilityTXChecksumOffload.
	TXChecksumOffload bool

	// RXChecksumOffload if true, indicates that this endpoints capability
	// set should include CapabilityRXChecksumOffload.
	RXChecksumOffload bool
	}

	// New creates a new fd-based endpoint.
	//
	// Makes fd non-blocking, but does not take ownership of fd, which must remain
	// open for the lifetime of the returned endpoint.
	func New(opts *Options) (tcpip.LinkEndpointID, error) {
	caps := stack.LinkEndpointCapabilities(0)
	if opts.RXChecksumOffload {
	caps \|= stack.CapabilityRXChecksumOffload
	}

	if opts.TXChecksumOffload {
	caps \|= stack.CapabilityTXChecksumOffload
	}

	hdrSize := 0
	if opts.EthernetHeader {
	hdrSize = header.EthernetMinimumSize
	caps \|= stack.CapabilityResolutionRequired
	}

	if opts.SaveRestore {
	caps \|= stack.CapabilitySaveRestore
	}

	if opts.DisconnectOk {
	caps \|= stack.CapabilityDisconnectOk
	}

	if len(opts.FDs) == 0 {
	return 0, fmt.Errorf("opts.FD is empty, at least one FD must be specified")
	}

	e := &endpoint{
	fds: opts.FDs,
	mtu: opts.MTU,
	caps: caps,
	closed: opts.ClosedFunc,
	addr: opts.Address,
	hdrSize: hdrSize,
	packetDispatchMode: opts.PacketDispatchMode,
	}

	// Create per channel dispatchers.
	for i := 0; i < len(e.fds); i++ {
	fd := e.fds[i]
	if err := syscall.SetNonblock(fd, true); err != nil {
	return 0, fmt.Errorf("syscall.SetNonblock(%v) failed: %v", fd, err)
	}

	isSocket, err := isSocketFD(fd)
	if err != nil {
	return 0, err
	}
	if isSocket {
	if opts.GSOMaxSize != 0 {
	e.caps \|= stack.CapabilityGSO
	e.gsoMaxSize = opts.GSOMaxSize
	}
	}
	inboundDispatcher, err := createInboundDispatcher(e, fd, isSocket)
	if err != nil {
	return 0, fmt.Errorf("createInboundDispatcher(...) = %v", err)
	}
	e.inboundDispatchers = append(e.inboundDispatchers, inboundDispatcher)
	}

	return stack.RegisterLinkEndpoint(e), nil
	}

	func createInboundDispatcher(e *endpoint, fd int, isSocket bool) (linkDispatcher, error) {
	// By default use the readv() dispatcher as it works with all kinds of
	// FDs (tap/tun/unix domain sockets and af_packet).
	inboundDispatcher, err := newReadVDispatcher(fd, e)
	if err != nil {
	return nil, fmt.Errorf("newReadVDispatcher(%d, %+v) = %v", fd, e, err)
	}

	if isSocket {
	sa, err := unix.Getsockname(fd)
	if err != nil {
	return nil, fmt.Errorf("unix.Getsockname(%d) = %v", fd, err)
	}
	switch sa.(type) {
	case *unix.SockaddrLinklayer:
	// enable PACKET_FANOUT mode is the underlying socket is
	// of type AF_PACKET.
	const fanoutID = 1
	const fanoutType = 0x8000 // PACKET_FANOUT_HASH \| PACKET_FANOUT_FLAG_DEFRAG
	fanoutArg := fanoutID \| fanoutType<<16
	if err := syscall.SetsockoptInt(fd, syscall.SOL_PACKET, unix.PACKET_FANOUT, fanoutArg); err != nil {
	return nil, fmt.Errorf("failed to enable PACKET_FANOUT option: %v", err)
	}
	}

	switch e.packetDispatchMode {
	case PacketMMap:
	inboundDispatcher, err = newPacketMMapDispatcher(fd, e)
	if err != nil {
	return nil, fmt.Errorf("newPacketMMapDispatcher(%d, %+v) = %v", fd, e, err)
	}
	case RecvMMsg:
	// If the provided FD is a socket then we optimize
	// packet reads by using recvmmsg() instead of read() to
	// read packets in a batch.
	inboundDispatcher, err = newRecvMMsgDispatcher(fd, e)
	if err != nil {
	return nil, fmt.Errorf("newRecvMMsgDispatcher(%d, %+v) = %v", fd, e, err)
	}
	}
	}
	return inboundDispatcher, nil
	}

	func isSocketFD(fd int) (bool, error) {
	var stat syscall.Stat_t
	if err := syscall.Fstat(fd, &stat); err != nil {
	return false, fmt.Errorf("syscall.Fstat(%v,...) failed: %v", fd, err)
	}
	return (stat.Mode & syscall.S_IFSOCK) == syscall.S_IFSOCK, nil
	}

	// Attach launches the goroutine that reads packets from the file descriptor and
	// dispatches them via the provided dispatcher.
	func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
	e.dispatcher = dispatcher
	// Link endpoints are not savable. When transportation endpoints are
	// saved, they stop sending outgoing packets and all incoming packets
	// are rejected.
	for i := range e.inboundDispatchers {
	go e.dispatchLoop(e.inboundDispatchers[i])
	}
	}

	// IsAttached implements stack.LinkEndpoint.IsAttached.
	func (e *endpoint) IsAttached() bool {
	return e.dispatcher != nil
	}

	// MTU implements stack.LinkEndpoint.MTU. It returns the value initialized
	// during construction.
	func (e *endpoint) MTU() uint32 {
	return e.mtu
	}

	// Capabilities implements stack.LinkEndpoint.Capabilities.
	func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
	return e.caps
	}

	// MaxHeaderLength returns the maximum size of the link-layer header.
	func (e *endpoint) MaxHeaderLength() uint16 {
	return uint16(e.hdrSize)
	}

	// LinkAddress returns the link address of this endpoint.
	func (e *endpoint) LinkAddress() tcpip.LinkAddress {
	return e.addr
	}

	// virtioNetHdr is declared in linux/virtio_net.h.
	type virtioNetHdr struct {
	flags uint8
	gsoType uint8
	hdrLen uint16
	gsoSize uint16
	csumStart uint16
	csumOffset uint16
	}

	// These constants are declared in linux/virtio_net.h.
	const (
	_VIRTIO_NET_HDR_F_NEEDS_CSUM = 1

	_VIRTIO_NET_HDR_GSO_TCPV4 = 1
	_VIRTIO_NET_HDR_GSO_TCPV6 = 4
	)

	// WritePacket writes outbound packets to the file descriptor. If it is not
	// currently writable, the packet is dropped.
	func (e endpoint) WritePacket(r stack.Route, gso stack.GSO, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) tcpip.Error {
	if e.hdrSize > 0 {
	// Add ethernet header if needed.
	eth := header.Ethernet(hdr.Prepend(header.EthernetMinimumSize))
	ethHdr := &header.EthernetFields{
	DstAddr: r.RemoteLinkAddress,
	Type: protocol,
	}

	// Preserve the src address if it's set in the route.
	if r.LocalLinkAddress != "" {
	ethHdr.SrcAddr = r.LocalLinkAddress
	} else {
	ethHdr.SrcAddr = e.addr
	}
	eth.Encode(ethHdr)
	}

	if e.Capabilities()&stack.CapabilityGSO != 0 {
	vnetHdr := virtioNetHdr{}
	vnetHdrBuf := vnetHdrToByteSlice(&vnetHdr)
	if gso != nil {
	vnetHdr.hdrLen = uint16(hdr.UsedLength())
	if gso.NeedsCsum {
	vnetHdr.flags = _VIRTIO_NET_HDR_F_NEEDS_CSUM
	vnetHdr.csumStart = header.EthernetMinimumSize + gso.L3HdrLen
	vnetHdr.csumOffset = gso.CsumOffset
	}
	if gso.Type != stack.GSONone && uint16(payload.Size()) > gso.MSS {
	switch gso.Type {
	case stack.GSOTCPv4:
	vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV4
	case stack.GSOTCPv6:
	vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV6
	default:
	panic(fmt.Sprintf("Unknown gso type: %v", gso.Type))
	}
	vnetHdr.gsoSize = gso.MSS
	}
	}

	return rawfile.NonBlockingWrite3(e.fds[0], vnetHdrBuf, hdr.View(), payload.ToView())
	}

	if payload.Size() == 0 {
	return rawfile.NonBlockingWrite(e.fds[0], hdr.View())
	}

	return rawfile.NonBlockingWrite3(e.fds[0], hdr.View(), payload.ToView(), nil)
	}

	// WriteRawPacket writes a raw packet directly to the file descriptor.
	func (e endpoint) WriteRawPacket(dest tcpip.Address, packet []byte) tcpip.Error {
	return rawfile.NonBlockingWrite(e.fds[0], packet)
	}

	// dispatchLoop reads packets from the file descriptor in a loop and dispatches
	// them to the network stack.
	func (e endpoint) dispatchLoop(inboundDispatcher linkDispatcher) tcpip.Error {
	for {
	cont, err := inboundDispatcher.dispatch()
	if err != nil \|\| !cont {
	if e.closed != nil {
	e.closed(err)
	}
	return err
	}
	}
	}

	// GSOMaxSize returns the maximum GSO packet size.
	func (e *endpoint) GSOMaxSize() uint32 {
	return e.gsoMaxSize
	}

	// InjectableEndpoint is an injectable fd-based endpoint. The endpoint writes
	// to the FD, but does not read from it. All reads come from injected packets.
	type InjectableEndpoint struct {
	endpoint

	dispatcher stack.NetworkDispatcher
	}

	// Attach saves the stack network-layer dispatcher for use later when packets
	// are injected.
	func (e *InjectableEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
	e.dispatcher = dispatcher
	}

	// Inject injects an inbound packet.
	func (e *InjectableEndpoint) Inject(protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
	e.dispatcher.DeliverNetworkPacket(e, "" /* remote /, "" / local */, protocol, vv)
	}

	// NewInjectable creates a new fd-based InjectableEndpoint.
	func NewInjectable(fd int, mtu uint32, capabilities stack.LinkEndpointCapabilities) (tcpip.LinkEndpointID, *InjectableEndpoint) {
	syscall.SetNonblock(fd, true)

	e := &InjectableEndpoint{endpoint: endpoint{
	fds: []int{fd},
	mtu: mtu,
	caps: capabilities,
	}}

	return stack.RegisterLinkEndpoint(e), e
	}