Dialup firewalling with FreeBSD

1 Preface

This document outlines the steps required to set up firewalling with FreeBSD when an IP address is assigned dynamically by your ISP. While every effort has been made to make this document as informative and correct as possible, you are welcome to mail any corrections, comments or suggestions to the author at <[email protected]>.

2 Kernel Options

In order to use IPFW, support for it must be compiled into the kernel. For more information on how to recompile the kernel, please see the kernel configuration section in the Handbook. The following options must be added into your kernel configuration file for IPFW support:

3 Changing /etc/rc.conf to load the firewall

/etc/rc.conf needs to be slightly modified in order to tell the system about the firewall and to specify the location for our rules file. Add the following lines to /etc/rc.conf:

firewall_enable="YES"
firewall_script="/etc/firewall/fwrules"

For more information on the functions of these statements take a look at /etc/defaults/rc.conf and read rc.conf(5)

4 Enable PPP's network address translation

In order to allow clients on your network to connect via your gateway, you will need to enable PPP's network address translation (NAT). In order to use PPP's NAT functions, add the following lines to /etc/rc.conf:

ppp_enable="YES"
ppp_mode="auto"
ppp_nat="YES"
ppp_profile="your_profile"

5 The rule set for the firewall

This is the point where we define the firewall rules for your system. The ruleset that we are about to describe is a generic template for most dialup users. While it will not suit the exact needs of every user, it provides you with a basic idea of how IPFW works and should be fairly easy to customize.

First, let's start with the basics of closed firewalling. Closed firewalling is based on the idea that everything is denied by default. The system administrator may then explicitly add rules for traffic that he or she would like to allow. Rules should be in the order of allow first, and then deny. The premise is that you add the rules for everything you would like to allow, and then everything else is automatically denied.

Following that, let's create the directory where we will store our firewall rules. In this example, we will use /etc/firewall. Change into the directory and edit the file fwrules as we specified in rc.conf. Please note that you can change this filename to anything you wish. This guide merely gives an example of a filename you may want to use.

Now, let's look at a nicely commented sample firewall file.

# Define the firewall command (as in /etc/rc.firewall) for easy
# reference.  Helps to make it easier to read.
fwcmd="/sbin/ipfw"

# Define our outside interface.  With userland-ppp this
# defaults to tun0.
oif="tun0"

# Define our inside interface.  This is usually your network
# card.  Be sure to change this to match your own network 
# interface.
iif="fxp0"

# Force a flushing of the current rules before we reload.
$fwcmd -f flush

# Check the state of all packets.
$fwcmd add check-state

# Stop spoofing on the outside interface.
$fwcmd add deny ip from any to any in via $oif not verrevpath

# Allow all connections that we initiate, and keep their state.
# but deny established connections that don't have a dynamic rule.
$fwcmd add allow ip from me to any out via $oif keep-state
$fwcmd add deny tcp from any to any established in via $oif

# Allow all connections within our network.
$fwcmd add allow ip from any to any via $iif

# Allow all local traffic.
$fwcmd add allow all from any to any via lo0
$fwcmd add deny all from any to 127.0.0.0/8
$fwcmd add deny ip from 127.0.0.0/8 to any

# Allow internet users to connect to the port 22 and 80.
# This example specifically allows connections to the sshd and a
# webserver.
$fwcmd add allow tcp from any to me dst-port 22,80 in via $oif setup keep-state

# Allow ICMP packets: remove type 8 if you don't want your host
# to be pingable.
$fwcmd add allow icmp from any to any via $oif icmptypes 0,3,8,11,12

# Deny and log all the rest.
$fwcmd add deny log ip from any to any

You now have a fully functional firewall that only allows connections to ports 22 and 80 and will log any other connection attempts. You may now safely reboot and the firewall should be automatically started and the ruleset loaded. If you find this incorrect in any way or experience any problems, or have any suggestions to improve this page, please email me.

6 Questions

# ipfw resetlog

       % cd /etc/firewall
        /etc/firewall% su
        Password:
        /etc/firewall# mv fwrules fwrules_tun0
        /etc/firewall# cat fwrules_tun0 | sed s/tun0/ppp0/g > fwrules
     

       % ifconfig
        (skipped...)
        ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1524
                    inet xxx.xxx.xxx.xxx --> xxx.xxx.xxx.xxx netmask 0xff000000
        (skipped...)
       

       % ifconfig
        (skipped...)
        ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
        (skipped...)
        tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1524
                (IPv6 stuff skipped...)
                    inet xxx.xxx.xxx.xxx --> xxx.xxx.xxx.xxx netmask 0xffffff00
                    Opened by PID xxxxx
            (skipped...)