Before rebooting in order to load the new kernel or the required modules (according to
the previously chosen installation method), you have to make some changes to the /etc/rc.conf configuration file. The default rule of the firewall
is to reject all IP packets. Initially we will set up an open
firewall, in order to verify its operation without any issue related to packet filtering
(in case you are going to execute this procedure remotely, such configuration will avoid
you to remain isolated from the network). Put these lines in /etc/rc.conf:
firewall_enable="YES" firewall_type="open" firewall_quiet="YES" firewall_logging="YES"
The first row will enable the firewall (and will load the module ipfw.ko if it is not compiled in the kernel), the second one to set
up it in open mode (as explained in /etc/rc.firewall), the third one to not show rules loading and the
fourth one to enable logging support.
About the configuration of the network interfaces, the most used way is to assign an IP to only one of the network cards, but the bridge will work equally even if both interfaces or none has a configured IP. In the last case (IP-less) the bridge machine will be still more hidden, as inaccessible from the network: to configure it, you have to login from console or through a third network interface separated from the bridge. Sometimes, during the system startup, some programs require network access, say for domain resolution: in this case it is necessary to assign an IP to the external interface (the one connected to Internet, where DNS server resides), since the bridge will be activated at the end of the startup procedure. It means that the fxp0 interface (in our case) must be mentioned in the ifconfig section of the /etc/rc.conf file, while the xl0 is not. Assigning an IP to both the network cards does not make much sense, unless, during the start procedure, applications should access to services on both Ethernet segments.
There is another important thing to know. When running IP over Ethernet, there are actually two Ethernet protocols in use: one is IP, the other is ARP. ARP does the conversion of the IP address of a host into its Ethernet address (MAC layer). In order to allow the communication between two hosts separated by the bridge, it is necessary that the bridge will forward ARP packets. Such protocol is not included in the IP layer, since it exists only with IP over Ethernet. The FreeBSD firewall filters exclusively on the IP layer and therefore all non-IP packets (ARP included) will be forwarded without being filtered, even if the firewall is configured to not permit anything.
Now it is time to reboot the system and use it as before: there will be some new
messages about the bridge and the firewall, but the bridge will not be activated and the
firewall, being in open mode, will not avoid any
operations.
If there are any problems, you should sort them out now before proceeding.