Can I make this simple ipfw ruleset any more restrictive ?

[John-Mark Gurney]

John Case wrote this message on Tue, Sep 09, 2014 at 23:37 +0000:
> 
> I have a very simple firewall - it*blocks everything*, and the only 
> traffic that is allowed is for internal clients to make outbound 
> connections to tcp port 40.
> 
> Also, internal clients can ping/traceroute.
> 
> But that's it - no other connections in or out are allowed.  I have this 
> ruleset and it is working perfectly:
> 
> ipfw add 10 allow tcp from any to any established
> ipfw add 20 allow icmp from any to any icmptypes 0,3,8,11
> ipfw add 30 allow udp from any to any 33433-33499 in via fxp1
> ipfw add 40 allow tcp from any to any 40 in via fxp1
> 
> (fxp1 is the internal interface, and so I allow the port 40 connections 
> and the udp for traceroute only for requests that come in from the 
> internal network)
> 
> Is there anything I have screwed up here ?  Any unintentional traffic that 
> I am letting through ?
> 
> Is there any way to lock this down further, and make it even more strict ?

You could lock down the UDP ports to a single one and remember to use
-e with traceroute:
     -e      Firewall evasion mode.  Use fixed destination ports for UDP and
             TCP probes.  The destination port does NOT increment with each
             packet sent.


-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."