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

1 files changed, 14 insertions, 16 deletions

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
 }