1 files changed, 74 insertions, 0 deletions
diff --git a/pkg/tcpip/link/loopback/loopback.go b/pkg/tcpip/link/loopback/loopback.go
new file mode 100644
index 000000000..1a9cd09d7
--- /dev/null
+++ b/pkg/tcpip/link/loopback/loopback.go
@@ -0,0 +1,74 @@
+// Copyright 2016 The Netstack Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package loopback provides the implemention of loopback data-link layer
+// endpoints. Such endpoints just turn outbound packets into inbound ones.
+//
+// Loopback 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().
+package loopback
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/tcpip"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
+)
+
+type endpoint struct {
+	dispatcher stack.NetworkDispatcher
+}
+
+// New creates a new loopback endpoint. This link-layer endpoint just turns
+// outbound packets into inbound packets.
+func New() tcpip.LinkEndpointID {
+	return stack.RegisterLinkEndpoint(&endpoint{})
+}
+
+// Attach implements stack.LinkEndpoint.Attach. It just saves the stack network-
+// layer dispatcher for later use when packets need to be dispatched.
+func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
+	e.dispatcher = dispatcher
+}
+
+// MTU implements stack.LinkEndpoint.MTU. It returns a constant that matches the
+// linux loopback interface.
+func (*endpoint) MTU() uint32 {
+	return 65536
+}
+
+// Capabilities implements stack.LinkEndpoint.Capabilities. Loopback advertises
+// itself as supporting checksum offload, but in reality it's just omitted.
+func (*endpoint) Capabilities() stack.LinkEndpointCapabilities {
+	return stack.CapabilityChecksumOffload
+}
+
+// MaxHeaderLength implements stack.LinkEndpoint.MaxHeaderLength. Given that the
+// loopback interface doesn't have a header, it just returns 0.
+func (*endpoint) MaxHeaderLength() uint16 {
+	return 0
+}
+
+// LinkAddress returns the link address of this endpoint.
+func (*endpoint) LinkAddress() tcpip.LinkAddress {
+	return ""
+}
+
+// WritePacket implements stack.LinkEndpoint.WritePacket. It delivers outbound
+// packets to the network-layer dispatcher.
+func (e *endpoint) WritePacket(_ *stack.Route, hdr *buffer.Prependable, payload buffer.View, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
+	if len(payload) == 0 {
+		// We don't have a payload, so just use the buffer from the
+		// header as the full packet.
+		v := hdr.View()
+		vv := v.ToVectorisedView([1]buffer.View{})
+		e.dispatcher.DeliverNetworkPacket(e, "", protocol, &vv)
+	} else {
+		views := []buffer.View{hdr.View(), payload}
+		vv := buffer.NewVectorisedView(len(views[0])+len(views[1]), views)
+		e.dispatcher.DeliverNetworkPacket(e, "", protocol, &vv)
+	}
+
+	return nil
+}