A configuração manual de um firewall pode ser complicada para o administrador novato (e muitas vezes para até mesmo o expert). No entanto, a comunidade de software livre tem criado um número de ferramentas que podem ser usadas para configurar facilmente um firewall local. Esteja avisado desde já que algumas destas ferramentas são orientadas somente para a proteção local (também chamadas de firewall pessoal) e algumas são mais versáteis e podem ser usadas para configurar regras complexas para proteger todas as redes.

Alguns softwares que podem ser usados para configurar regras de firewall em um sistema Debian são:

Lots of other iptables frontends come with Debian; an extensive list comparing the different packages in Debian is maintained at the http://wiki.debian.org/Firewalls.

Notice that some of the packages outlined previously will introduce firewalling scripts to be run when the system boots. Test them extensively before rebooting or you might find yourself locked from the box. If you mix different firewalling packages you can have undesired effects, usually, the firewalling script that runs last will be the one that configures the system (which might not be what you intend). Consult the package documentation and use either one of these setups.

As mentioned before, some programs, like firestarter, guarddog and knetfilter, are administration GUIs using either GNOME or KDE (last two). These applications are much more user-oriented (i.e. for home users) than some of the other packages in the list which might be more administrator-oriented. Some of the programs mentioned before (like bastille) are focused at setting up firewall rules to protect the host they run in but are not necessarily designed to setup firewall rules for firewall hosts that protect a network (like shorewall or fwbuilder).

There is yet another type of firewall application: application proxies. If you are looking into setting up an enterprise-level firewall that does packet filtering and provides a number of transparent proxies that can do fine-grain traffic analysis you should consider using zorp, which provides this in a single program. You can also manually setup this type of firewall host using the proxies available in Debian for different services like for DNS using bind (properly configured), dnsmasq, pdnsd or totd for FTP using frox or ftp-proxy, for X11 using xfwp, for IMAP using imapproxy, for mail using smtpd, or for POP3 using p3scan. For other protocols you can either use a generic TCP proxy like simpleproxy or a generic SOCKS proxy like dante-server, tsocks or socks4-server. Typically, you will also use a web caching system (like squid) and a web filtering system (like squidguard or dansguardian).

Another possibility is to manually configure your firewall rules through an init.d script that will run all the iptables commands. Take the following steps:

This is the sample firewall script:

#!/bin/sh
### BEGIN INIT INFO
# Provides:          myfirewall
# Required-Start:    $local_fs
# Required-Stop:     $local_fs
# Default-Start:     S
# Default-Stop:      0 6
# X-Start-Before:    $network
# X-Stop-After:      $network
# Short-Description: My custom firewall.
### END INIT INFO
#
# Simple example firewall configuration.
#
# Caveats:
# - This configuration applies to all network interfaces
#   if you want to restrict this to only a given interface use
#   '-i INTERFACE' in the iptables calls.
# - Remote access for TCP/UDP services is granted to any host, 
#   you probably will want to restrict this using '--source'.
#
# chkconfig: 2345 9 91
# description: Activates/Deactivates the firewall at boot time
#
# You can test this script before applying with the following shell
# snippet, if you do not type anything in 10 seconds the firewall
# rules will be cleared.
#---------------------------------------------------------------
#  while true; do test=""; read  -t 20 -p "OK? " test ; \
#  [ -z "$test" ] && /etc/init.d/myfirewall clear ; done
#---------------------------------------------------------------

PATH=/bin:/sbin:/usr/bin:/usr/sbin

# Services that the system will offer to the network
TCP_SERVICES="22" # SSH only
UDP_SERVICES=""
# Services the system will use from the network
REMOTE_TCP_SERVICES="80" # web browsing
REMOTE_UDP_SERVICES="53" # DNS
# Network that will be used for remote mgmt
# (if undefined, no rules will be setup)
# NETWORK_MGMT=192.168.0.0/24
# If you want to setup a management network (i.e. you've uncommented
# the above line) you will need to define the SSH port as well (i.e.
# uncomment the below line.) Remember to remove the SSH port from the
# TCP_SERVICES string.
# SSH_PORT="22"

if ! [ -x /sbin/iptables ]; then  
    exit 0
fi

fw_start () {

  # Input traffic:
  /sbin/iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
  # Services
  if [ -n "$TCP_SERVICES" ] ; then
  for PORT in $TCP_SERVICES; do
    /sbin/iptables -A INPUT -p tcp --dport ${PORT} -j ACCEPT
  done
  fi
  if [ -n "$UDP_SERVICES" ] ; then
  for PORT in $UDP_SERVICES; do
    /sbin/iptables -A INPUT -p udp --dport ${PORT} -j ACCEPT
  done
  fi
  # Remote management
  if [ -n "$NETWORK_MGMT" ] ; then
    /sbin/iptables -A INPUT -p tcp --src ${NETWORK_MGMT} --dport ${SSH_PORT} -j ACCEPT
  fi
  # Remote testing
  /sbin/iptables -A INPUT -p icmp -j ACCEPT
  /sbin/iptables -A INPUT -i lo -j ACCEPT
  /sbin/iptables -P INPUT DROP
  /sbin/iptables -A INPUT -j LOG

  # Output:
  /sbin/iptables -A OUTPUT -j ACCEPT -o lo 
  /sbin/iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
  # ICMP is permitted:
  /sbin/iptables -A OUTPUT -p icmp -j ACCEPT
  # So are security package updates:
  # Note: You can hardcode the IP address here to prevent DNS spoofing
  # and to setup the rules even if DNS does not work but then you 
  # will not "see" IP changes for this service:
  /sbin/iptables -A OUTPUT -p tcp -d security.debian.org --dport 80 -j ACCEPT 
  # As well as the services we have defined:
  if [ -n "$REMOTE_TCP_SERVICES" ] ; then
  for PORT in $REMOTE_TCP_SERVICES; do
    /sbin/iptables -A OUTPUT -p tcp --dport ${PORT} -j ACCEPT
  done
  fi
  if [ -n "$REMOTE_UDP_SERVICES" ] ; then
  for PORT in $REMOTE_UDP_SERVICES; do
    /sbin/iptables -A OUTPUT -p udp --dport ${PORT} -j ACCEPT
  done
  fi
  # All other connections are registered in syslog
  /sbin/iptables -A OUTPUT -j LOG
  /sbin/iptables -A OUTPUT -j REJECT 
  /sbin/iptables -P OUTPUT DROP
  # Other network protections
  # (some will only work with some kernel versions)
  echo 1 > /proc/sys/net/ipv4/tcp_syncookies
  echo 0 > /proc/sys/net/ipv4/ip_forward 
  echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts 
  echo 1 > /proc/sys/net/ipv4/conf/all/log_martians 
  echo 1 > /proc/sys/net/ipv4/ip_always_defrag
  echo 1 > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
  echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
  echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
  echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route

}

fw_stop () {
  /sbin/iptables -F
  /sbin/iptables -t nat -F
  /sbin/iptables -t mangle -F
  /sbin/iptables -P INPUT DROP
  /sbin/iptables -P FORWARD DROP
  /sbin/iptables -P OUTPUT ACCEPT
}

fw_clear () {
  /sbin/iptables -F
  /sbin/iptables -t nat -F
  /sbin/iptables -t mangle -F
  /sbin/iptables -P INPUT ACCEPT
  /sbin/iptables -P FORWARD ACCEPT
  /sbin/iptables -P OUTPUT ACCEPT
}


case "$1" in
  start|restart)
    echo -n "Starting firewall.."
    fw_stop 
    fw_start
    echo "done."
    ;;
  stop)
    echo -n "Stopping firewall.."
    fw_stop
    echo "done."
    ;;
  clear)
    echo -n "Clearing firewall rules.."
    fw_clear
    echo "done."
    ;;
  *)
    echo "Usage: $0 {start|stop|restart|clear}"
    exit 1
    ;;
  esac
exit 0

Instead of including all of the iptables rules in the init.d script you can use the iptables-restore program to restore the rules saved using iptables-save. In order to do this you need to setup your rules, save the ruleset under a static location (such as /etc/default/firewall)

You can use also the network configuration in /etc/network/interfaces to setup your firewall rules. For this you will need to:

You can optionally also setup a set of rules to be applied when the network interface is down creating a set of rules, saving it in /etc/iptables.down.rules and adding this directive to the interface configuration:

    post-down iptables-restore < /etc/iptables.down.rules

For more advanced firewall configuration scripts through ifupdown you can use the hooks available to each interface as in the *.d/ directories called with run-parts (see run-parts(8) manual page).

Testing your firewall configuration is as easy, and as dangerous, as just running your firewall script (or enabling the configuration you defined in your firewall configuration application). However, if you are not careful enough and you are configuring your firewall remotely (like through an SSH connection) you could lock yourself out.

There are several ways to prevent this. One is running a script in a separate terminal that will remove the firewall configuration if you don't feed it input. An example of this is:

$  while true; do test=""; read  -t 20 -p "OK? " test ; \
  [ -z "$test" ] && /etc/init.d/firewall clear ; done

Another one is to introduce a backdoor in your system through an alternate mechanism that allows you to either clear the firewall system or punch a hole in it if something goes awry. For this you can use knockd and configure it so that a certain port connection attempt sequence will clear the firewall (or add a temporary rule). Even though the packets will be dropped by the firewall, since knockd binds to the interface and sees you will be able to work around the problem.

Testing a firewall that is protecting an internal network is a different issue, you will want to look at some of the tools used for remote vulnerability assessment (see Seção 8.1, “Ferramentas de verificação remota de vulnerabilidades”) to probe the network from the outside in (or from any other direction) to test the effectiveness of the firewall configuation.