gvisor-lx/pkg/tcpip/link/ethernet/ethernet.go

// Copyright 2020 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.

// Package ethernet provides an implementation of an ethernet link endpoint that
// wraps an inner link endpoint.
package ethernet

import (
	"github.com/sagernet/gvisor/pkg/tcpip"
	"github.com/sagernet/gvisor/pkg/tcpip/header"
	"github.com/sagernet/gvisor/pkg/tcpip/link/nested"
	"github.com/sagernet/gvisor/pkg/tcpip/stack"
)

var (
	_ stack.NetworkDispatcher = (*Endpoint)(nil)
	_ stack.LinkEndpoint      = (*Endpoint)(nil)
)

// New returns an ethernet link endpoint that wraps an inner link endpoint.
func New(ep stack.LinkEndpoint) *Endpoint {
	var e Endpoint
	e.Endpoint.Init(ep, &e)
	return &e
}

// Endpoint is an ethernet endpoint.
//
// It adds an ethernet header to packets before sending them out through its
// inner link endpoint and consumes an ethernet header before sending the
// packet to the stack.
//
// +stateify savable
type Endpoint struct {
	nested.Endpoint
}

// LinkAddress implements stack.LinkEndpoint.
func (e *Endpoint) LinkAddress() tcpip.LinkAddress {
	if l := e.Endpoint.LinkAddress(); len(l) != 0 {
		return l
	}
	return header.UnspecifiedEthernetAddress
}

// MTU implements stack.LinkEndpoint.
func (e *Endpoint) MTU() uint32 {
	// It prevents upper-layers from sending larger than expected packets.
	if mtu := e.Endpoint.MTU(); mtu > header.EthernetMinimumSize {
		return mtu - header.EthernetMinimumSize
	}
	return 0
}

// DeliverNetworkPacket implements stack.NetworkDispatcher.
func (e *Endpoint) DeliverNetworkPacket(_ tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
	if !e.ParseHeader(pkt) {
		return
	}
	eth := header.Ethernet(pkt.LinkHeader().Slice())
	dst := eth.DestinationAddress()
	if dst == header.EthernetBroadcastAddress {
		pkt.PktType = tcpip.PacketBroadcast
	} else if header.IsMulticastEthernetAddress(dst) {
		pkt.PktType = tcpip.PacketMulticast
	} else if dst == e.LinkAddress() {
		pkt.PktType = tcpip.PacketHost
	} else {
		pkt.PktType = tcpip.PacketOtherHost
	}

	// Note, there is no need to check the destination link address here since
	// the ethernet hardware filters frames based on their destination addresses.
	e.Endpoint.DeliverNetworkPacket(eth.Type() /* protocol */, pkt)
}

// Capabilities implements stack.LinkEndpoint.
func (e *Endpoint) Capabilities() stack.LinkEndpointCapabilities {
	c := e.Endpoint.Capabilities()
	if c&stack.CapabilityLoopback == 0 {
		c |= stack.CapabilityResolutionRequired
	}
	return c
}

// MaxHeaderLength implements stack.LinkEndpoint.
func (e *Endpoint) MaxHeaderLength() uint16 {
	return header.EthernetMinimumSize + e.Endpoint.MaxHeaderLength()
}

// ARPHardwareType implements stack.LinkEndpoint.
func (e *Endpoint) ARPHardwareType() header.ARPHardwareType {
	if a := e.Endpoint.ARPHardwareType(); a != header.ARPHardwareNone {
		return a
	}
	return header.ARPHardwareEther
}

// AddHeader implements stack.LinkEndpoint.
func (*Endpoint) AddHeader(pkt *stack.PacketBuffer) {
	eth := header.Ethernet(pkt.LinkHeader().Push(header.EthernetMinimumSize))
	fields := header.EthernetFields{
		SrcAddr: pkt.EgressRoute.LocalLinkAddress,
		DstAddr: pkt.EgressRoute.RemoteLinkAddress,
		Type:    pkt.NetworkProtocolNumber,
	}
	eth.Encode(&fields)
}

// ParseHeader implements stack.LinkEndpoint.
func (*Endpoint) ParseHeader(pkt *stack.PacketBuffer) bool {
	_, ok := pkt.LinkHeader().Consume(header.EthernetMinimumSize)
	return ok
}