Add app for static ND

apps.ipv6.nd_static fills in static src/dst MAC addresses in packets
transmitted on a L3 port.  Together with the nd_light and ns_responder
apps, it covers all cases of bidirectional dynamic ND, inbound
dynamic, outbound static and bidirectional static neighbor discovery.

The ns_responder app has been adapted to optionally fill in the
src/dst MAC addresses as well to allow for upper-layer protocols to be
agnostic of MAC addresses no matter which ND discipline is being used
on the interface.

src/apps/ipv6/nd_static.lua

@@ -0,0 +1,42 @@
+-- Use of this source code is governed by the Apache 2.0 license; see COPYING.
+
+-- This app relays packets between its "nort" and "south" links and
+-- copies fixed src and dst MAC addresses to the Ethernet header of
+-- all packets coming in from "north".
+
+module(..., package.seeall)
+local ffi = require("ffi")
+local link = require("core.link")
+local ethernet = require("lib.protocol.ethernet")
+
+nd_static = subClass(nil)
+nd_static._name = "static ND"
+
+function nd_static:new (config)
+   assert(config and config.local_mac and config.remote_mac)
+   local o = nd_static:superClass().new(self)
+   o._eth = ethernet:new({ src = config.local_mac,
+                           dst = config.remote_mac })
+   o._header = o._eth:header()
+   return o
+end
+
+local empty, receive, transmit = link.empty, link.receive, link.transmit
+function nd_static:push()
+   local l_in = self.input.north
+   local l_out = self.output.south
+   if l_in and l_out then
+      while not empty(l_in) do
+         -- Insert the static MAC addresses
+         local p = receive(l_in)
+         ffi.copy(p.data, self._header, 12)
+         transmit(l_out, p)
+      end
+   end
+   l_in = self.input.south
+   l_out = self.output.north
+   while not empty(l_in) do
+      -- Pass all packets unchanged souh -> north 
+      transmit(l_out, receive(l_in))
+   end
+end

src/apps/ipv6/ns_responder.lua

@@ -32,6 +32,11 @@ function ns_responder:new(config)
    o._filter = filter
    o._dgram = datagram:new()
    packet.free(o._dgram:packet())
+   if config.remote_mac then
+      o._eth = ethernet:new({ src = config.local_mac,
+                              dst = config.remote_mac })
+      o._header = o._eth:header()
+   end
    return o
 end
 
@@ -89,8 +94,12 @@ function ns_responder:push()
    local l_out = self.output.south
    if l_in and l_out then
       while not link.empty(l_in) and not link.full(l_out) do
-         -- Pass everything on north -> south
-         link.transmit(l_out, link.receive(l_in))
+         local p = link.receive(l_in)
+         if self._header then
+            -- Insert the static MAC header
+            ffi.copy(p.data, self._header, 12)
+         end
+         link.transmit(l_out, p)
       end
    end
    l_in = self.input.south