ipfw with NAT and ARP

[Andrea E.]

Hi,

I am a newbie and my question is very easy perhaps. I work with FreeBSD 
5.2.1

I would like to configure a firewall with to interfaces (xl0 = LAN, xl1 
= External)

For NAT I have configured like discribed in the manualpage of natd:

ipfw -f flush
ipfw add divert natd all from any to any via xl1
ipfw add allow all from any to any

-> all is fine.

But, I wont so a simple firewall and for this reason, first I want to 
configure the ICMP-protocol:

ip_ext => External IP-Address

ipfw -f flush
ipfw add divert natd all from any to any via xl1
ipfw add allow icmp from $ip_ext to any icmptypes 8 out via xl1
ipfw add allow icmp from any to $ip_ext icmptypes 0  in via xl1

-> It's not ok. With "ethereal" no pakets are going out (test from an 
other system, connected with a HUP.)

When testing "ping" from external to external IP-Adress of my firewall, 
the ARP-request: to broadcast Who has xxx.xxx.xxx.xxx? Tell 
xxx.xxx.xxx.xxx fails

-> seems to have a problem to let ARP through the firewall.

Above -> "ipfw add allow all from any to any" let ARP through the 
firewall. So I think, thats the configuration of the rest of my computer 
(like kernel, rc.conf, etc. ist ok)

And there are no ARP-protocol in /etc/protocols, so I don't know, what I 
can do now.

There is a bug:
After restarting system with above configuration of icmp-protocol no 
ping-request is going out. After a flush of all rules and configuring of 
"ipfw add allow all from any to any" ping-request get an answer.
Very interesting is to flush all rules und to configure the firewall 
like the first configuring (to allow special rules for icmp-protocol -> 
all works very fine. ping-request get an answer. Whenn restarting system 
the ping-request get no answer again, I mean, the ping-request is not 
send out.

Can anybody help me? Hope to get an answer.

I hope you can understand me, my English isn't very well.

Greatings from Berlin,

	Andrea E.