SMP and networking under FreeBSD 5.3

Alex Zbyslaw xfb52 at
Thu Jun 16 22:57:59 GMT 2005

Joe wrote:

>Thanks Alex, 
>   Below are my rules.  I have removed the IP addresses and
>replaced with x.x.x.x in most cases.  Also some ports have been
>turned to y's instead of the actual port. 
I don't want to go into the details of your firewall; all I can offer is 
general advice for you to apply if you wish.  There are plenty resources 
out there from the various man pages to the handbook.  Firewalls can be 
trickier than they look and NAT makes them significantly more 
complicated to fathom correctly.  I don't claim to be any kind of expert 
and everything I know started life being written by someone else :-)  
Any mistakes are most likely my own!  I will say that it is worth making 
sure you understand your own firewall. 

At one point you suggested that you wanted to make your firewall script 
start later so that you had access to your IP address.  I think you are 
on to a loser there because there is not particular time when DHCP 
finally gets the IP address.  If your provider is down, it might take 
minutes, hours or even days.  You could keep polling in some way to see 
if you had an IP address and not running your rules script until you 
did, but it would seem better to just write rules which work even 
without the IP address.  Plus, that would also not work if you ever had 
a second external interface (e.g. an old-fashioned modem) which needed 
firewalling irrespective of the status of your ethernet interface.

Although a firewall often need to know the actual addresses of hosts 
other than itself there is, as far as I can figure out, no logical 
reason for it to need to know it's own IP address if you have the "me" 
construct.  (If, like my machine, your firewall is just another computer 
on a small network that is allowed to do exactly the same things as any 
other host on that network, then it needn't even use "me".  This makes 
life much easier because it interferes less with NAT).

If you have "me" then you can always distinguish between your firewall 
and the rest of your network.

Take the non-NAT case first:

allow all from me to any out xmit ext_if
allow all from any to me in recv ext_if

These rules could only be triggered by packets addressed directly to 
your firewall.  If you follow it with e.g.

deny all from any to any out via ext_if
deny all from any to any in via ext_if

then you close off your internal network.

NAT makes things more complicated, because before or after NATing 
(depending on the direction) packets from your network can look like 
they originate on your machine or are destined for it.


allow all from me to any out xmit ext_if

must come before the NAT rule because after NAT-ing all your internal 
packets are going out ext_if.


allow all from any to me in recv ext_if

must come after the NAT rule to be sure that it is actually your 
firewall which is the recipient.

If all you have is a small network, then there may be no reason to 
differentiate your firewall from any other machine.  In this case, it is 
perfectly sufficient to  write rules based on the ext_if alone.

So I have rules like:
    # Allow connections initiated from internal network
    ipfw add allow tcp from any to any out xmit ext_if setup

     # Allow TCP through if setup succeeded
    ipfw add pass tcp from any to any via ext_if established

The only IP addresses in my whole firewall are the limited number of 
hosts which can initiate some kind of connection into my network


ipfw add allow tcp from x.x.x.x to any ssh setup

(x.x.x.x not because I need to hide the IP but because I can't be 
bothered to find it in the firewall script :-))

NB that rule says any for recipient because it was written before me 
existed.  But since my network is NATed, it would always be a packet 
header for my firewall and could only get elsewhere if I explicitly 
forwarded it.  There's no mention of the interface because a prior rule 
has already allowed internal connections which would match.  Looking at 
it now, I might get picky and put an interface spec in there just to be 

It's often said that there is no security in obscurity, and while I 
don't always agree, I do think that if you actually have to hide the IPs 
in your firewall for it be secure, then it isn't secure.  Since my 
firewall never mentions my IP address, I can publish the whole thing and 
even if it has flaws it won't help since you don't know where I am :-)

A bit long-winded, but I hope it helps,


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