Control forwarding per NetworkEndpoint

...instead of per NetworkProtocol to better conform with linux
(https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt):

```
conf/interface/*

forwarding - BOOLEAN
	Enable IP forwarding on this interface.  This controls whether packets
	received _on_ this interface can be forwarded.
```

Fixes #5932.

PiperOrigin-RevId: 373888000

1 files changed, 29 insertions, 0 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 8d615500f..dbba2c79f 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1000,3 +1000,32 @@ func (n *nic) checkDuplicateAddress(protocol tcpip.NetworkProtocolNumber, addr t
 
 	return d.CheckDuplicateAddress(addr, h), nil
 }
+
+func (n *nic) setForwarding(protocol tcpip.NetworkProtocolNumber, enable bool) tcpip.Error {
+	ep := n.getNetworkEndpoint(protocol)
+	if ep == nil {
+		return &tcpip.ErrUnknownProtocol{}
+	}
+
+	forwardingEP, ok := ep.(ForwardingNetworkEndpoint)
+	if !ok {
+		return &tcpip.ErrNotSupported{}
+	}
+
+	forwardingEP.SetForwarding(enable)
+	return nil
+}
+
+func (n *nic) forwarding(protocol tcpip.NetworkProtocolNumber) (bool, tcpip.Error) {
+	ep := n.getNetworkEndpoint(protocol)
+	if ep == nil {
+		return false, &tcpip.ErrUnknownProtocol{}
+	}
+
+	forwardingEP, ok := ep.(ForwardingNetworkEndpoint)
+	if !ok {
+		return false, &tcpip.ErrNotSupported{}
+	}
+
+	return forwardingEP.Forwarding(), nil
+}