Updated: See https://securosis.com/wp-content/uploads/2007/11/ipfw-securosis.txt.

I need to completely thank and acknowledge windexh8er for suggesting this post in the comments on the Leopard firewall post, and providing the starting content. In his (or her) own words:

So how about everyone constantly complaining about the crap-tastic new implementation of the Leopard firewall we baseline a good IPFW config? Here’s for starters:

00100 allow ip from any to any via lo*
00110 deny ip from to any in
00120 deny ip from any to in
00500 check-state
00501 deny log ip from any to any frag
00502 deny log tcp from any to any established in
01500 allow udp from 5353 to any dst-port 1024-65535 in
01700 allow icmp from any to any icmptypes 3
01701 allow icmp from any to any icmptypes 4
01702 allow icmp from any to any icmptypes 8 out
01703 allow icmp from any to any icmptypes 0 in
01704 allow icmp from any to any icmptypes 11 in
65500 allow tcp from me to any keep-state
65501 allow udp from me to any keep-state
65534 deny log ip from any to any
65535 allow ip from any to any

this firewall configuration will do a number of things. First of all line 500 is key to checking the state table before we block any poser incoming connections. Line 502 blocks connections coming in that pretend they were established, but really weren’t. Line 501 is pretty self explanatory, blocking fragmented packets in. I know nothing I’m using is fragmenting, so YMMV.

Line 1500 is an example. Since Bonjour services cannot be tracked correctly in the state table we need to allow things back to 5353/UDP on the box (that is if you want to use it). But my example shows that I’m only allowing those services on my local network. Anytime I head to Panera or Starbucks I don’t have to worry about 5353 being ‘open’, unless of course those networks are using Most of the time they’re not. But if I noticed that I would disable that rule for the time being.

Next we get to ICMP. What do these let us do? ICMP type 3 let’s path MTU in and out (i.e. PMTU – Path MTU Discovery). Many people don’t realize the advantages of PMTU, because they think ICMP is inherently evil. Try doing some performance engineering and PMTU becomes a great resource. Anyway, type 3 is not evil. Next, type 8 is source quench. It will tell my upstream gateway to “slow down” if need be. Again, not evil for the most part. The pros outweigh the cons anyway. Types 8 and 0 rely on each other. 8 lets me ping out and 0 lets that back in. BUT – people will not be able to ping me. Sneaky sneaky The last one, type 11, will let me run traceroute.

So now 65500 and 65501 basically let my computer open any port out. In the essence of keeping this ruleset “set it and forget it” style this can be done better. Like specifying everything you need to let out and blocking everything else. But I can’t delve into that for ‘every’ user, so this makes it a little more convenient.

65534 is our deny. Notice all the denies I setup have logging statements. I always have a terminal running tailing my firewall log. Then again, for those who don’t know how to respond maybe just keep that on the down low – you might get sick if you saw all of the traffic hitting your box depending on the network you’re connected to.

Rich – you should start a thread for whittling down the best default ruleset for IPFW on Tiger/Leopard and let’s do a writeup on how to implement it

Ask and ye shall receive- I’ll be putting together some of my own suggestions, but this is a heck of a great start and I’m having trouble thinking of any good additions right now.

Let’s all pile on- once we get consensus I’ll do another post with the results.