github.com/flowerwrong/netstack@v0.0.0-20191009141956-e5848263af28/tcpip/link/loopback/loopback.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package loopback provides the implemention of loopback data-link layer
    16  // endpoints. Such endpoints just turn outbound packets into inbound ones.
    17  //
    18  // Loopback endpoints can be used in the networking stack by calling New() to
    19  // create a new endpoint, and then passing it as an argument to
    20  // Stack.CreateNIC().
    21  package loopback
    22  
    23  import (
    24  	"github.com/FlowerWrong/netstack/tcpip"
    25  	"github.com/FlowerWrong/netstack/tcpip/buffer"
    26  	"github.com/FlowerWrong/netstack/tcpip/stack"
    27  )
    28  
    29  type endpoint struct {
    30  	dispatcher stack.NetworkDispatcher
    31  }
    32  
    33  // New creates a new loopback endpoint. This link-layer endpoint just turns
    34  // outbound packets into inbound packets.
    35  func New() stack.LinkEndpoint {
    36  	return &endpoint{}
    37  }
    38  
    39  // Attach implements stack.LinkEndpoint.Attach. It just saves the stack network-
    40  // layer dispatcher for later use when packets need to be dispatched.
    41  func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
    42  	e.dispatcher = dispatcher
    43  }
    44  
    45  // IsAttached implements stack.LinkEndpoint.IsAttached.
    46  func (e *endpoint) IsAttached() bool {
    47  	return e.dispatcher != nil
    48  }
    49  
    50  // MTU implements stack.LinkEndpoint.MTU. It returns a constant that matches the
    51  // linux loopback interface.
    52  func (*endpoint) MTU() uint32 {
    53  	return 65536
    54  }
    55  
    56  // Capabilities implements stack.LinkEndpoint.Capabilities. Loopback advertises
    57  // itself as supporting checksum offload, but in reality it's just omitted.
    58  func (*endpoint) Capabilities() stack.LinkEndpointCapabilities {
    59  	return stack.CapabilityRXChecksumOffload | stack.CapabilityTXChecksumOffload | stack.CapabilitySaveRestore | stack.CapabilityLoopback
    60  }
    61  
    62  // MaxHeaderLength implements stack.LinkEndpoint.MaxHeaderLength. Given that the
    63  // loopback interface doesn't have a header, it just returns 0.
    64  func (*endpoint) MaxHeaderLength() uint16 {
    65  	return 0
    66  }
    67  
    68  // LinkAddress returns the link address of this endpoint.
    69  func (*endpoint) LinkAddress() tcpip.LinkAddress {
    70  	return ""
    71  }
    72  
    73  // WritePacket implements stack.LinkEndpoint.WritePacket. It delivers outbound
    74  // packets to the network-layer dispatcher.
    75  func (e *endpoint) WritePacket(_ *stack.Route, _ *stack.GSO, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
    76  	views := make([]buffer.View, 1, 1+len(payload.Views()))
    77  	views[0] = hdr.View()
    78  	views = append(views, payload.Views()...)
    79  	vv := buffer.NewVectorisedView(len(views[0])+payload.Size(), views)
    80  
    81  	// Because we're immediately turning around and writing the packet back to the
    82  	// rx path, we intentionally don't preserve the remote and local link
    83  	// addresses from the stack.Route we're passed.
    84  	e.dispatcher.DeliverNetworkPacket(e, "" /* remote */, "" /* local */, protocol, vv)
    85  
    86  	return nil
    87  }
    88  
    89  // Wait implements stack.LinkEndpoint.Wait.
    90  func (*endpoint) Wait() {}