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, 19 insertions, 4 deletions

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
 }