Now it is time to create your own file with custom firewall rules, in order to secure the inside network. There will be some complication in doing this because not all of the firewall functionalities are available on bridged packets. Furthermore, there is a difference between the packets that are in the process of being forwarded and packets that are being received by the local machine. In general, incoming packets are run through the firewall only once, not twice as is normally the case; in fact they are filtered only upon receipt, so rules that use out or xmit will never match. Personally, I use in via which is an older syntax, but one that has a sense when you read it. Another limitation is that you are restricted to use only pass or drop commands for packets filtered by a bridge. Sophisticated things like divert, forward or reject are not available. Such options can still be used, but only on traffic to or from the bridge machine itself (if it has an IP address).

New in FreeBSD 4.0, is the concept of stateful filtering. This is a big improvement for UDP traffic, which typically is a request going out, followed shortly thereafter by a response with the exact same set of IP addresses and port numbers (but with source and destination reversed, of course). For firewalls that have no statekeeping, there is almost no way to deal with this sort of traffic as a single session. But with a firewall that can “remember” an outgoing UDP packet and, for the next few minutes, allow a response, handling UDP services is trivial. The following example shows how to do it. It is possible to do the same thing with TCP packets. This allows you to avoid some denial of service attacks and other nasty tricks, but it also typically makes your state table grow quickly in size.

Let's look at an example setup. Note first that at the top of /etc/rc.firewall there are already standard rules for the loopback interface lo0, so we should not have to care for them anymore. Custom rules should be put in a separate file (say /etc/rc.firewall.local) and loaded at system startup, by modifying the row of /etc/rc.conf where we defined the open firewall:

firewall_type="/etc/rc.firewall.local"

For our example imagine to have the fxp0 interface connected towards the outside (Internet) and the xl0 towards the inside (LAN). The bridge machine has the IP 1.2.3.4 (it is not possible that your ISP can give you an address quite like this, but for our example it is good).

# Things that we have kept state on before get to go through in a hurry
add check-state

# Throw away RFC 1918 networks
add drop all from 10.0.0.0/8 to any in via fxp0
add drop all from 172.16.0.0/12 to any in via fxp0
add drop all from 192.168.0.0/16 to any in via fxp0

# Allow the bridge machine to say anything it wants
# (if the machine is IP-less do not include these rows)
add pass tcp from 1.2.3.4 to any setup keep-state
add pass udp from 1.2.3.4 to any keep-state
add pass ip from 1.2.3.4 to any

# Allow the inside hosts to say anything they want
add pass tcp from any to any in via xl0 setup keep-state
add pass udp from any to any in via xl0 keep-state
add pass ip from any to any in via xl0

# TCP section
# Allow SSH
add pass tcp from any to any 22 in via fxp0 setup keep-state
# Allow SMTP only towards the mail server
add pass tcp from any to relay 25 in via fxp0 setup keep-state
# Allow zone transfers only by the slave name server [dns2.nic.it]
add pass tcp from 193.205.245.8 to ns 53 in via fxp0 setup keep-state
# Pass ident probes.  It is better than waiting for them to timeout
add pass tcp from any to any 113 in via fxp0 setup keep-state
# Pass the "quarantine" range
add pass tcp from any to any 49152-65535 in via fxp0 setup keep-state

# UDP section
# Allow DNS only towards the name server
add pass udp from any to ns 53 in via fxp0 keep-state
# Pass the "quarantine" range
add pass udp from any to any 49152-65535 in via fxp0 keep-state

# ICMP section
# Pass 'ping'
add pass icmp from any to any icmptypes 8 keep-state
# Pass error messages generated by 'traceroute'
add pass icmp from any to any icmptypes 3
add pass icmp from any to any icmptypes 11

# Everything else is suspect
add drop log all from any to any

Those of you who have set up firewalls before may notice some things missing. In particular, there are no anti-spoofing rules, in fact we did not add:

add deny all from 1.2.3.4/8 to any in via fxp0

That is, drop packets that are coming in from the outside claiming to be from our network. This is something that you would commonly do to be sure that someone does not try to evade the packet filter, by generating nefarious packets that look like they are from the inside. The problem with that is that there is at least one host on the outside interface that you do not want to ignore: the router. But usually, the ISP anti-spoofs at their router, so we do not need to bother that much.

The last rule seems to be an exact duplicate of the default rule, that is, do not let anything pass that is not specifically allowed. But there is a difference: all suspected traffic will be logged.

There are two rules for passing SMTP and DNS traffic towards the mail server and the name server, if you have them. Obviously the whole rule set should be flavored to personal taste, this is only a specific example (rule format is described accurately in the ipfw(8) man page). Note that in order for “relay” and “ns” to work, name service lookups must work before the bridge is enabled. This is an example of making sure that you set the IP on the correct network card. Alternatively it is possible to specify the IP address instead of the host name (required if the machine is IP-less).

People that are used to setting up firewalls are probably also used to either having a reset or a forward rule for ident packets (TCP port 113). Unfortunately, this is not an applicable option with the bridge, so the best thing is to simply pass them to their destination. As long as that destination machine is not running an ident daemon, this is relatively harmless. The alternative is dropping connections on port 113, which creates some problems with services like IRC (the ident probe must timeout).

The only other thing that is a little weird that you may have noticed is that there is a rule to let the bridge machine speak, and another for internal hosts. Remember that this is because the two sets of traffic will take different paths through the kernel and into the packet filter. The inside net will go through the bridge, while the local machine will use the normal IP stack to speak. Thus the two rules to handle the different cases. The in via fxp0 rules work for both paths. In general, if you use in via rules throughout the filter, you will need to make an exception for locally generated packets, because they did not come in via any of our interfaces.