Add start/stop callbacks to packet proxy

This adds support for two additional functions in go plugins in the
`packet_proxy` module:

* `func OnStart() int`
* `func OnStop()`

These will be called when the packet proxy module is turned on and
off, respectively.

modules/packet_proxy/packet_proxy_linux_amd64.go

@@ -161,6 +161,18 @@ func (mod *PacketProxy) Configure() (err error) {
  return fmt.Errorf("Symbol OnPacket is not a valid callback function.")
  }
 
+ if sym, err = mod.plugin.Lookup("OnStart"); err == nil {
+ var onStartCb func() int
+ if onStartCb, ok = sym.(func() int); !ok {
+ return fmt.Errorf("OnStart signature does not match expected signature: 'func() int'")
+ } else {
+ var result int
+ if result = onStartCb(); result != 0 {
+ return fmt.Errorf("OnStart returned non-zero result. result=%d", result)
+ }
+ }
+ }
+
  mod.queue = new(nfqueue.Queue)
  if err = mod.queue.SetCallback(dummyCallback); err != nil {
  return
@@ -206,10 +218,22 @@ func (mod *PacketProxy) Start() error {
  })
 }
 
-func (mod *PacketProxy) Stop() error {
+func (mod *PacketProxy) Stop() (err error) {
  return mod.SetRunning(false, func() {
  mod.queue.StopLoop()
  mod.runRule(false)
+
+ var sym plugin.Symbol
+ if sym, err = mod.plugin.Lookup("OnStop"); err == nil {
+ var onStopCb func()
+ var ok bool
+ if onStopCb, ok = sym.(func()); !ok {
+ mod.Error("OnStop signature does not match expected signature: 'func()', unable to call OnStop.")
+ } else {
+ onStopCb()
+ }
+ }
+
  <-mod.done
  })
 }