Fix minor bugs in a couple of interface IOCTLs.

gVisor incorrectly returns the wrong ARP type for SIOGIFHWADDR. This breaks
tcpdump as it tries to interpret the packets incorrectly.

Similarly, SIOCETHTOOL is used by tcpdump to query interface properties which
fails with an EINVAL since we don't implement it. For now change it to return
EOPNOTSUPP to indicate that we don't support the query rather than return
EINVAL.

NOTE: ARPHRD types for link endpoints are distinct from NIC capabilities
and NIC flags. In Linux all 3 exist eg. ARPHRD types are stored in dev->type
field while NIC capabilities are more like the device features which can be
queried using SIOCETHTOOL but not modified and NIC Flags are fields that can
be modified from user space. eg. NIC status (UP/DOWN/MULTICAST/BROADCAST) etc.

Updates #2746

PiperOrigin-RevId: 321436525

21 files changed, 153 insertions, 27 deletions

diff --git a/pkg/tcpip/header/arp.go b/pkg/tcpip/header/arp.go
index 718a4720a..83189676e 100644
--- a/pkg/tcpip/header/arp.go
+++ b/pkg/tcpip/header/arp.go
@@ -14,14 +14,33 @@
 
 package header
 
-import "gvisor.dev/gvisor/pkg/tcpip"
+import (
+	"encoding/binary"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
 
 const (
 	// ARPProtocolNumber is the ARP network protocol number.
 	ARPProtocolNumber tcpip.NetworkProtocolNumber = 0x0806
 
 	// ARPSize is the size of an IPv4-over-Ethernet ARP packet.
-	ARPSize = 2 + 2 + 1 + 1 + 2 + 2*6 + 2*4
+	ARPSize = 28
+)
+
+// ARPHardwareType is the hardware type for LinkEndpoint in an ARP header.
+type ARPHardwareType uint16
+
+// Typical ARP HardwareType values. Some of the constants have to be specific
+// values as they are egressed on the wire in the HTYPE field of an ARP header.
+const (
+	ARPHardwareNone ARPHardwareType = 0
+	// ARPHardwareEther specifically is the HTYPE for Ethernet as specified
+	// in the IANA list here:
+	//
+	// https://www.iana.org/assignments/arp-parameters/arp-parameters.xhtml#arp-parameters-2
+	ARPHardwareEther    ARPHardwareType = 1
+	ARPHardwareLoopback ARPHardwareType = 2
 )
 
 // ARPOp is an ARP opcode.
@@ -36,54 +55,64 @@ const (
 // ARP is an ARP packet stored in a byte array as described in RFC 826.
 type ARP []byte
 
-func (a ARP) hardwareAddressSpace() uint16 { return uint16(a[0])<<8 | uint16(a[1]) }
-func (a ARP) protocolAddressSpace() uint16 { return uint16(a[2])<<8 | uint16(a[3]) }
-func (a ARP) hardwareAddressSize() int     { return int(a[4]) }
-func (a ARP) protocolAddressSize() int     { return int(a[5]) }
+const (
+	hTypeOffset                 = 0
+	protocolOffset              = 2
+	haAddressSizeOffset         = 4
+	protoAddressSizeOffset      = 5
+	opCodeOffset                = 6
+	senderHAAddressOffset       = 8
+	senderProtocolAddressOffset = senderHAAddressOffset + EthernetAddressSize
+	targetHAAddressOffset       = senderProtocolAddressOffset + IPv4AddressSize
+	targetProtocolAddressOffset = targetHAAddressOffset + EthernetAddressSize
+)
+
+func (a ARP) hardwareAddressType() ARPHardwareType {
+	return ARPHardwareType(binary.BigEndian.Uint16(a[hTypeOffset:]))
+}
+
+func (a ARP) protocolAddressSpace() uint16 { return binary.BigEndian.Uint16(a[protocolOffset:]) }
+func (a ARP) hardwareAddressSize() int     { return int(a[haAddressSizeOffset]) }
+func (a ARP) protocolAddressSize() int     { return int(a[protoAddressSizeOffset]) }
 
 // Op is the ARP opcode.
-func (a ARP) Op() ARPOp { return ARPOp(a[6])<<8 | ARPOp(a[7]) }
+func (a ARP) Op() ARPOp { return ARPOp(binary.BigEndian.Uint16(a[opCodeOffset:])) }
 
 // SetOp sets the ARP opcode.
 func (a ARP) SetOp(op ARPOp) {
-	a[6] = uint8(op >> 8)
-	a[7] = uint8(op)
+	binary.BigEndian.PutUint16(a[opCodeOffset:], uint16(op))
 }
 
 // SetIPv4OverEthernet configures the ARP packet for IPv4-over-Ethernet.
 func (a ARP) SetIPv4OverEthernet() {
-	a[0], a[1] = 0, 1       // htypeEthernet
-	a[2], a[3] = 0x08, 0x00 // IPv4ProtocolNumber
-	a[4] = 6                // macSize
-	a[5] = uint8(IPv4AddressSize)
+	binary.BigEndian.PutUint16(a[hTypeOffset:], uint16(ARPHardwareEther))
+	binary.BigEndian.PutUint16(a[protocolOffset:], uint16(IPv4ProtocolNumber))
+	a[haAddressSizeOffset] = EthernetAddressSize
+	a[protoAddressSizeOffset] = uint8(IPv4AddressSize)
 }
 
 // HardwareAddressSender is the link address of the sender.
 // It is a view on to the ARP packet so it can be used to set the value.
 func (a ARP) HardwareAddressSender() []byte {
-	const s = 8
-	return a[s : s+6]
+	return a[senderHAAddressOffset : senderHAAddressOffset+EthernetAddressSize]
 }
 
 // ProtocolAddressSender is the protocol address of the sender.
 // It is a view on to the ARP packet so it can be used to set the value.
 func (a ARP) ProtocolAddressSender() []byte {
-	const s = 8 + 6
-	return a[s : s+4]
+	return a[senderProtocolAddressOffset : senderProtocolAddressOffset+IPv4AddressSize]
 }
 
 // HardwareAddressTarget is the link address of the target.
 // It is a view on to the ARP packet so it can be used to set the value.
 func (a ARP) HardwareAddressTarget() []byte {
-	const s = 8 + 6 + 4
-	return a[s : s+6]
+	return a[targetHAAddressOffset : targetHAAddressOffset+EthernetAddressSize]
 }
 
 // ProtocolAddressTarget is the protocol address of the target.
 // It is a view on to the ARP packet so it can be used to set the value.
 func (a ARP) ProtocolAddressTarget() []byte {
-	const s = 8 + 6 + 4 + 6
-	return a[s : s+4]
+	return a[targetProtocolAddressOffset : targetProtocolAddressOffset+IPv4AddressSize]
 }
 
 // IsValid reports whether this is an ARP packet for IPv4 over Ethernet.
@@ -91,10 +120,8 @@ func (a ARP) IsValid() bool {
 	if len(a) < ARPSize {
 		return false
 	}
-	const htypeEthernet = 1
-	const macSize = 6
-	return a.hardwareAddressSpace() == htypeEthernet &&
+	return a.hardwareAddressType() == ARPHardwareEther &&
 		a.protocolAddressSpace() == uint16(IPv4ProtocolNumber) &&
-		a.hardwareAddressSize() == macSize &&
+		a.hardwareAddressSize() == EthernetAddressSize &&
 		a.protocolAddressSize() == IPv4AddressSize
 }
diff --git a/pkg/tcpip/link/channel/BUILD b/pkg/tcpip/link/channel/BUILD
index b8b93e78e..39ca774ef 100644
--- a/pkg/tcpip/link/channel/BUILD
+++ b/pkg/tcpip/link/channel/BUILD
@@ -10,6 +10,7 @@ go_library(
         "//pkg/sync",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
diff --git a/pkg/tcpip/link/channel/channel.go b/pkg/tcpip/link/channel/channel.go
index 20b183da0..a2bb773d4 100644
--- a/pkg/tcpip/link/channel/channel.go
+++ b/pkg/tcpip/link/channel/channel.go
@@ -23,6 +23,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -296,3 +297,8 @@ func (e *Endpoint) AddNotify(notify Notification) *NotificationHandle {
 func (e *Endpoint) RemoveNotify(handle *NotificationHandle) {
 	e.q.RemoveNotify(handle)
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*Endpoint) ARPHardwareType() header.ARPHardwareType {
+	return header.ARPHardwareNone
+}
diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index f34082e1a..32abe2a13 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -626,6 +626,14 @@ func (e *endpoint) GSOMaxSize() uint32 {
 	return e.gsoMaxSize
 }
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (e *endpoint) ARPHardwareType() header.ARPHardwareType {
+	if e.hdrSize > 0 {
+		return header.ARPHardwareEther
+	}
+	return header.ARPHardwareNone
+}
+
 // InjectableEndpoint is an injectable fd-based endpoint. The endpoint writes
 // to the FD, but does not read from it. All reads come from injected packets.
 type InjectableEndpoint struct {
diff --git a/pkg/tcpip/link/loopback/loopback.go b/pkg/tcpip/link/loopback/loopback.go
index 568c6874f..3b17d8c28 100644
--- a/pkg/tcpip/link/loopback/loopback.go
+++ b/pkg/tcpip/link/loopback/loopback.go
@@ -113,3 +113,8 @@ func (e *endpoint) WriteRawPacket(vv buffer.VectorisedView) *tcpip.Error {
 
 	return nil
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*endpoint) ARPHardwareType() header.ARPHardwareType {
+	return header.ARPHardwareLoopback
+}
diff --git a/pkg/tcpip/link/muxed/BUILD b/pkg/tcpip/link/muxed/BUILD
index 82b441b79..e7493e5c5 100644
--- a/pkg/tcpip/link/muxed/BUILD
+++ b/pkg/tcpip/link/muxed/BUILD
@@ -9,6 +9,7 @@ go_library(
     deps = [
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
diff --git a/pkg/tcpip/link/muxed/injectable.go b/pkg/tcpip/link/muxed/injectable.go
index c69d6b7e9..c305d9e86 100644
--- a/pkg/tcpip/link/muxed/injectable.go
+++ b/pkg/tcpip/link/muxed/injectable.go
@@ -18,6 +18,7 @@ package muxed
 import (
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -129,6 +130,11 @@ func (m *InjectableEndpoint) Wait() {
 	}
 }
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*InjectableEndpoint) ARPHardwareType() header.ARPHardwareType {
+	panic("unsupported operation")
+}
+
 // NewInjectableEndpoint creates a new multi-endpoint injectable endpoint.
 func NewInjectableEndpoint(routes map[tcpip.Address]stack.InjectableLinkEndpoint) *InjectableEndpoint {
 	return &InjectableEndpoint{
diff --git a/pkg/tcpip/link/nested/BUILD b/pkg/tcpip/link/nested/BUILD
index bdd5276ad..2cdb23475 100644
--- a/pkg/tcpip/link/nested/BUILD
+++ b/pkg/tcpip/link/nested/BUILD
@@ -12,6 +12,7 @@ go_library(
         "//pkg/sync",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
diff --git a/pkg/tcpip/link/nested/nested.go b/pkg/tcpip/link/nested/nested.go
index 2998f9c4f..328bd048e 100644
--- a/pkg/tcpip/link/nested/nested.go
+++ b/pkg/tcpip/link/nested/nested.go
@@ -20,6 +20,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -129,3 +130,8 @@ func (e *Endpoint) GSOMaxSize() uint32 {
 	}
 	return 0
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType
+func (e *Endpoint) ARPHardwareType() header.ARPHardwareType {
+	return e.child.ARPHardwareType()
+}
diff --git a/pkg/tcpip/link/qdisc/fifo/BUILD b/pkg/tcpip/link/qdisc/fifo/BUILD
index 054c213bc..1d0079bd6 100644
--- a/pkg/tcpip/link/qdisc/fifo/BUILD
+++ b/pkg/tcpip/link/qdisc/fifo/BUILD
@@ -14,6 +14,7 @@ go_library(
         "//pkg/sync",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
diff --git a/pkg/tcpip/link/qdisc/fifo/endpoint.go b/pkg/tcpip/link/qdisc/fifo/endpoint.go
index b5dfb7850..c84fe1bb9 100644
--- a/pkg/tcpip/link/qdisc/fifo/endpoint.go
+++ b/pkg/tcpip/link/qdisc/fifo/endpoint.go
@@ -22,6 +22,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -207,3 +208,8 @@ func (e *endpoint) Wait() {
 
 	e.wg.Wait()
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType
+func (e *endpoint) ARPHardwareType() header.ARPHardwareType {
+	return e.lower.ARPHardwareType()
+}
diff --git a/pkg/tcpip/link/sharedmem/sharedmem.go b/pkg/tcpip/link/sharedmem/sharedmem.go
index 0374a2441..a36862c67 100644
--- a/pkg/tcpip/link/sharedmem/sharedmem.go
+++ b/pkg/tcpip/link/sharedmem/sharedmem.go
@@ -287,3 +287,8 @@ func (e *endpoint) dispatchLoop(d stack.NetworkDispatcher) {
 
 	e.completed.Done()
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType
+func (*endpoint) ARPHardwareType() header.ARPHardwareType {
+	return header.ARPHardwareEther
+}
diff --git a/pkg/tcpip/link/tun/device.go b/pkg/tcpip/link/tun/device.go
index 6bc9033d0..47446efec 100644
--- a/pkg/tcpip/link/tun/device.go
+++ b/pkg/tcpip/link/tun/device.go
@@ -139,6 +139,7 @@ func attachOrCreateNIC(s *stack.Stack, name, prefix string, linkCaps stack.LinkE
 			stack:    s,
 			nicID:    id,
 			name:     name,
+			isTap:    prefix == "tap",
 		}
 		endpoint.Endpoint.LinkEPCapabilities = linkCaps
 		if endpoint.name == "" {
@@ -348,6 +349,7 @@ type tunEndpoint struct {
 	stack *stack.Stack
 	nicID tcpip.NICID
 	name  string
+	isTap bool
 }
 
 // DecRef decrements refcount of e, removes NIC if refcount goes to 0.
@@ -356,3 +358,11 @@ func (e *tunEndpoint) DecRef() {
 		e.stack.RemoveNIC(e.nicID)
 	})
 }
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (e *tunEndpoint) ARPHardwareType() header.ARPHardwareType {
+	if e.isTap {
+		return header.ARPHardwareEther
+	}
+	return header.ARPHardwareNone
+}
diff --git a/pkg/tcpip/link/waitable/BUILD b/pkg/tcpip/link/waitable/BUILD
index 0956d2c65..ee84c3d96 100644
--- a/pkg/tcpip/link/waitable/BUILD
+++ b/pkg/tcpip/link/waitable/BUILD
@@ -12,6 +12,7 @@ go_library(
         "//pkg/gate",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
@@ -25,6 +26,7 @@ go_test(
     deps = [
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/stack",
     ],
 )
diff --git a/pkg/tcpip/link/waitable/waitable.go b/pkg/tcpip/link/waitable/waitable.go
index 949b3f2b2..24a8dc2eb 100644
--- a/pkg/tcpip/link/waitable/waitable.go
+++ b/pkg/tcpip/link/waitable/waitable.go
@@ -25,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/gate"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -147,3 +148,8 @@ func (e *Endpoint) WaitDispatch() {
 
 // Wait implements stack.LinkEndpoint.Wait.
 func (e *Endpoint) Wait() {}
+
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (e *Endpoint) ARPHardwareType() header.ARPHardwareType {
+	return e.lower.ARPHardwareType()
+}
diff --git a/pkg/tcpip/link/waitable/waitable_test.go b/pkg/tcpip/link/waitable/waitable_test.go
index 63bf40562..ffb2354be 100644
--- a/pkg/tcpip/link/waitable/waitable_test.go
+++ b/pkg/tcpip/link/waitable/waitable_test.go
@@ -19,6 +19,7 @@ import (
 
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -81,6 +82,11 @@ func (e *countedEndpoint) WriteRawPacket(buffer.VectorisedView) *tcpip.Error {
 	return nil
 }
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*countedEndpoint) ARPHardwareType() header.ARPHardwareType {
+	panic("unimplemented")
+}
+
 // Wait implements stack.LinkEndpoint.Wait.
 func (*countedEndpoint) Wait() {}
 
diff --git a/pkg/tcpip/network/ip_test.go b/pkg/tcpip/network/ip_test.go
index 7c8fb3e0a..a5b780ca2 100644
--- a/pkg/tcpip/network/ip_test.go
+++ b/pkg/tcpip/network/ip_test.go
@@ -172,14 +172,19 @@ func (t *testObject) WritePacket(_ *stack.Route, _ *stack.GSO, protocol tcpip.Ne
 }
 
 // WritePackets implements stack.LinkEndpoint.WritePackets.
-func (t *testObject) WritePackets(_ *stack.Route, _ *stack.GSO, pkt stack.PacketBufferList, protocol tcpip.NetworkProtocolNumber) (int, *tcpip.Error) {
+func (*testObject) WritePackets(_ *stack.Route, _ *stack.GSO, pkt stack.PacketBufferList, protocol tcpip.NetworkProtocolNumber) (int, *tcpip.Error) {
 	panic("not implemented")
 }
 
-func (t *testObject) WriteRawPacket(_ buffer.VectorisedView) *tcpip.Error {
+func (*testObject) WriteRawPacket(_ buffer.VectorisedView) *tcpip.Error {
 	return tcpip.ErrNotSupported
 }
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*testObject) ARPHardwareType() header.ARPHardwareType {
+	panic("not implemented")
+}
+
 func buildIPv4Route(local, remote tcpip.Address) (stack.Route, *tcpip.Error) {
 	s := stack.New(stack.Options{
 		NetworkProtocols:   []stack.NetworkProtocol{ipv4.NewProtocol()},
diff --git a/pkg/tcpip/stack/forwarder_test.go b/pkg/tcpip/stack/forwarder_test.go
index a6546cef0..eefb4b07f 100644
--- a/pkg/tcpip/stack/forwarder_test.go
+++ b/pkg/tcpip/stack/forwarder_test.go
@@ -22,6 +22,7 @@ import (
 
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
 
 const (
@@ -301,6 +302,11 @@ func (e *fwdTestLinkEndpoint) WriteRawPacket(vv buffer.VectorisedView) *tcpip.Er
 // Wait implements stack.LinkEndpoint.Wait.
 func (*fwdTestLinkEndpoint) Wait() {}
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*fwdTestLinkEndpoint) ARPHardwareType() header.ARPHardwareType {
+	panic("not implemented")
+}
+
 func fwdTestNetFactory(t *testing.T, proto *fwdTestNetworkProtocol) (ep1, ep2 *fwdTestLinkEndpoint) {
 	// Create a stack with the network protocol and two NICs.
 	s := New(Options{
diff --git a/pkg/tcpip/stack/nic_test.go b/pkg/tcpip/stack/nic_test.go
index 31f865260..3bc9fd831 100644
--- a/pkg/tcpip/stack/nic_test.go
+++ b/pkg/tcpip/stack/nic_test.go
@@ -84,6 +84,11 @@ func (e *testLinkEndpoint) WriteRawPacket(buffer.VectorisedView) *tcpip.Error {
 	return tcpip.ErrNotSupported
 }
 
+// ARPHardwareType implements stack.LinkEndpoint.ARPHardwareType.
+func (*testLinkEndpoint) ARPHardwareType() header.ARPHardwareType {
+	panic("not implemented")
+}
+
 var _ NetworkEndpoint = (*testIPv6Endpoint)(nil)
 
 // An IPv6 NetworkEndpoint that throws away outgoing packets.
diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index 5cbc946b6..f260eeb7f 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -18,6 +18,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sleep"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
@@ -436,6 +437,12 @@ type LinkEndpoint interface {
 	// Wait will not block if the endpoint hasn't started any goroutines
 	// yet, even if it might later.
 	Wait()
+
+	// ARPHardwareType returns the ARPHRD_TYPE of the link endpoint.
+	//
+	// See:
+	// https://github.com/torvalds/linux/blob/aa0c9086b40c17a7ad94425b3b70dd1fdd7497bf/include/uapi/linux/if_arp.h#L30
+	ARPHardwareType() header.ARPHardwareType
 }
 
 // InjectableLinkEndpoint is a LinkEndpoint where inbound packets are
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 0aa815447..2b7ece851 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -1095,6 +1095,11 @@ type NICInfo struct {
 	// Context is user-supplied data optionally supplied in CreateNICWithOptions.
 	// See type NICOptions for more details.
 	Context NICContext
+
+	// ARPHardwareType holds the ARP Hardware type of the NIC. This is the
+	// value sent in haType field of an ARP Request sent by this NIC and the
+	// value expected in the haType field of an ARP response.
+	ARPHardwareType header.ARPHardwareType
 }
 
 // HasNIC returns true if the NICID is defined in the stack.
@@ -1126,6 +1131,7 @@ func (s *Stack) NICInfo() map[tcpip.NICID]NICInfo {
 			MTU:               nic.linkEP.MTU(),
 			Stats:             nic.stats,
 			Context:           nic.context,
+			ARPHardwareType:   nic.linkEP.ARPHardwareType(),
 		}
 	}
 	return nics