Add loopback interface as an ethernet-based device

...to match Linux behaviour.

We can see evidence of Linux representing loopback as an ethernet-based
device below:
```
# EUI-48 based MAC addresses.
$ ip link show lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

# tcpdump showing ethernet frames when sniffing loopback and logging the
# link-type as EN10MB (Ethernet).
$ sudo tcpdump -i lo -e -c 2 -n
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on lo, link-type EN10MB (Ethernet), snapshot length 262144 bytes
03:09:05.002034 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 66: 127.0.0.1.9557 > 127.0.0.1.36828: Flags [.], ack 3562800815, win 15342, options [nop,nop,TS val 843174495 ecr 843159493], length 0
03:09:05.002094 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 66: 127.0.0.1.36828 > 127.0.0.1.9557: Flags [.], ack 1, win 6160, options [nop,nop,TS val 843174496 ecr 843159493], length 0
2 packets captured
116 packets received by filter
0 packets dropped by kernel
```

Wireshark shows a similar result as the tcpdump example above.

Linux's loopback setup: https://github.com/torvalds/linux/blob/5bfc75d92efd494db37f5c4c173d3639d4772966/drivers/net/loopback.c#L162

PiperOrigin-RevId: 391836719

4 files changed, 38 insertions, 25 deletions

diff --git a/pkg/tcpip/header/eth.go b/pkg/tcpip/header/eth.go
index 95ade0e5c..1f18213e5 100644
--- a/pkg/tcpip/header/eth.go
+++ b/pkg/tcpip/header/eth.go
@@ -49,9 +49,9 @@ const (
 	// EthernetAddressSize is the size, in bytes, of an ethernet address.
 	EthernetAddressSize = 6
 
-	// unspecifiedEthernetAddress is the unspecified ethernet address
+	// UnspecifiedEthernetAddress is the unspecified ethernet address
 	// (all bits set to 0).
-	unspecifiedEthernetAddress = tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00")
+	UnspecifiedEthernetAddress = tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00")
 
 	// EthernetBroadcastAddress is an ethernet address that addresses every node
 	// on a local link.
@@ -134,7 +134,7 @@ func IsValidUnicastEthernetAddress(addr tcpip.LinkAddress) bool {
 		return false
 	}
 
-	if addr == unspecifiedEthernetAddress {
+	if addr == UnspecifiedEthernetAddress {
 		return false
 	}
 
diff --git a/pkg/tcpip/header/eth_test.go b/pkg/tcpip/header/eth_test.go
index bf9ccbf1a..adc04e855 100644
--- a/pkg/tcpip/header/eth_test.go
+++ b/pkg/tcpip/header/eth_test.go
@@ -44,7 +44,7 @@ func TestIsValidUnicastEthernetAddress(t *testing.T) {
 		},
 		{
 			"Unspecified",
-			unspecifiedEthernetAddress,
+			UnspecifiedEthernetAddress,
 			false,
 		},
 		{
@@ -91,7 +91,7 @@ func TestIsMulticastEthernetAddress(t *testing.T) {
 		},
 		{
 			"Unspecified",
-			unspecifiedEthernetAddress,
+			UnspecifiedEthernetAddress,
 			false,
 		},
 		{
diff --git a/pkg/tcpip/link/ethernet/ethernet.go b/pkg/tcpip/link/ethernet/ethernet.go
index b427c6170..b9db273d0 100644
--- a/pkg/tcpip/link/ethernet/ethernet.go
+++ b/pkg/tcpip/link/ethernet/ethernet.go
@@ -42,6 +42,14 @@ 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
+}
+
 // DeliverNetworkPacket implements stack.NetworkDispatcher.
 func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
 	hdr, ok := pkt.LinkHeader().Consume(header.EthernetMinimumSize)
@@ -57,18 +65,22 @@ func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkP
 
 // Capabilities implements stack.LinkEndpoint.
 func (e *Endpoint) Capabilities() stack.LinkEndpointCapabilities {
-	return stack.CapabilityResolutionRequired | e.Endpoint.Capabilities()
+	c := e.Endpoint.Capabilities()
+	if c&stack.CapabilityLoopback == 0 {
+		c |= stack.CapabilityResolutionRequired
+	}
+	return c
 }
 
 // WritePacket implements stack.LinkEndpoint.
 func (e *Endpoint) WritePacket(r stack.RouteInfo, proto tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) tcpip.Error {
-	e.AddHeader(e.Endpoint.LinkAddress(), r.RemoteLinkAddress, proto, pkt)
+	e.AddHeader(e.LinkAddress(), r.RemoteLinkAddress, proto, pkt)
 	return e.Endpoint.WritePacket(r, proto, pkt)
 }
 
 // WritePackets implements stack.LinkEndpoint.
 func (e *Endpoint) WritePackets(r stack.RouteInfo, pkts stack.PacketBufferList, proto tcpip.NetworkProtocolNumber) (int, tcpip.Error) {
-	linkAddr := e.Endpoint.LinkAddress()
+	linkAddr := e.LinkAddress()
 
 	for pkt := pkts.Front(); pkt != nil; pkt = pkt.Next() {
 		e.AddHeader(linkAddr, r.RemoteLinkAddress, proto, pkt)
@@ -83,7 +95,10 @@ func (e *Endpoint) MaxHeaderLength() uint16 {
 }
 
 // ARPHardwareType implements stack.LinkEndpoint.
-func (*Endpoint) ARPHardwareType() header.ARPHardwareType {
+func (e *Endpoint) ARPHardwareType() header.ARPHardwareType {
+	if a := e.Endpoint.ARPHardwareType(); a != header.ARPHardwareNone {
+		return a
+	}
 	return header.ARPHardwareEther
 }
 
diff --git a/pkg/tcpip/transport/raw/endpoint.go b/pkg/tcpip/transport/raw/endpoint.go
index b3d8951ff..55854ba59 100644
--- a/pkg/tcpip/transport/raw/endpoint.go
+++ b/pkg/tcpip/transport/raw/endpoint.go
@@ -321,28 +321,26 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, tcp
 	}
 	defer route.Release()
 
+	pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
+		ReserveHeaderBytes: int(route.MaxHeaderLength()),
+		Data:               buffer.View(payloadBytes).ToVectorisedView(),
+	})
+	pkt.Owner = owner
+
 	if e.ops.GetHeaderIncluded() {
-		pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
-			Data: buffer.View(payloadBytes).ToVectorisedView(),
-		})
 		if err := route.WriteHeaderIncludedPacket(pkt); err != nil {
 			return 0, err
 		}
-	} else {
-		pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
-			ReserveHeaderBytes: int(route.MaxHeaderLength()),
-			Data:               buffer.View(payloadBytes).ToVectorisedView(),
-		})
-		pkt.Owner = owner
-		if err := route.WritePacket(stack.NetworkHeaderParams{
-			Protocol: e.TransProto,
-			TTL:      route.DefaultTTL(),
-			TOS:      stack.DefaultTOS,
-		}, pkt); err != nil {
-			return 0, err
-		}
+		return int64(len(payloadBytes)), nil
 	}
 
+	if err := route.WritePacket(stack.NetworkHeaderParams{
+		Protocol: e.TransProto,
+		TTL:      route.DefaultTTL(),
+		TOS:      stack.DefaultTOS,
+	}, pkt); err != nil {
+		return 0, err
+	}
 	return int64(len(payloadBytes)), nil
 }