add :closed? method to Meterpreter Channel

Implement a check for self.cid.nil? in Meterpreter's Channel class
in the :closed? method for compatibility with the Socket's :closed?

Touch up the Rex DNS server's stop method using this method on
pivot sockets.

Add SOL_SOCKET and SO_REUSEADDR options to the Rex UDP sockets
created by the DNS components - the server socket, as well as the
client abstraction socket.

lib/rex/post/meterpreter/channel.rb

@@ -250,6 +250,13 @@ class Channel
     written.nil? ? 0 : written.value
   end
 
+  #
+  # Wrapper around check for self.cid
+  #
+  def closed?
+    self.cid.nil?
+  end
+
   #
   # Wrapper around the low-level close.
   #

lib/rex/proto/dns/server.rb

@@ -190,6 +190,7 @@ class Server
 
     if self.serve_udp
       @udp_sock = Rex::Socket::Udp.create(self.sock_options)
+      udp_sock.setsockopt(::Socket::SOL_SOCKET, ::Socket::SO_REUSEADDR, 1)
       self.listener_thread = Rex::ThreadFactory.spawn("UDPDNSServerListener", false) {
         monitor_listener
       }
@@ -221,7 +222,7 @@ class Server
     ensure
       while csock = ensure_close.shift
         csock.stop if csock.respond_to?(:stop)
-        csock.close unless csock.closed?
+        csock.close csock.respond_to?(:close) and !csock.closed?
       end
     end
     self.cache.stop(flush_cache)
@@ -299,6 +300,7 @@ protected
           'LocalHost' => self.udp_sock.localhost,
           'LocalPort' => self.udp_sock.localport
         )
+        cli.setsockopt(::Socket::SOL_SOCKET, ::Socket::SO_REUSEADDR, 1)
         dispatch_request(cli, buf)
       end
     end