Setting up a network firewall

This is a setup or program, for Windows commercially sold by companies such as Symantec, of which they claim or pretend that it secures a home or desktop-pc with Internet access. This topic is highly relevant for users who do not know the methods their computers might be accessed via the Internet or how to disable them, especially if they are always online and connected via broadband links.

This is a box placed between the Internet and an intranet. To minimize the risk of compromising the firewall itself it should generally have only one role, that of protecting the intranet. Although not completely risk free, the tasks of doing the routing and eventually IP masquerading (rewriting IP-headers of the packets it routes from clients with private IP-addresses onto the Internet so that they seem to come from the firewall itself) are commonly considered harmless.

This is often an old box you may have retired and nearly forgotten, performing masquerading or routing functions, but offering a bunch of services, e.g., web-cache, mail, etc. This may be very commonly used for home networks, but can definitely not be considered as secure anymore because the combining of server and router on one machine raises the complexity of the setup.

This box performs masquerading or routing, but grants public access to some branch of your network which, because of public IP's and a physically separated structure, is neither considered to be part of the inter- nor intranet. These servers are those which must be easily accessible from both the inter- and intranet. The firewall protects them all.

Doing routing or masquerading, but permitting only selected services to be accessible, sometimes only by selected internal users or boxes; mostly used in highly secure business contexts, sometimes by distrusting employers. This was the common configuration of a firewall at the time of the Linux 2.2 kernel. It's still possible to configure a firewall this way, but it makes the rules quite complex and lengthy.

A Personal Firewall is supposed to let you access all the services offered on the Internet, but keep your box secure and your data private.

Below is a slightly modified version of Rusty Russell's recommendation from the Linux 2.4 Packet Filtering HOWTO:

cat > /etc/rc.d/init.d/firewall << "EOF"
#!/bin/sh

# Begin $rc_base/init.d/firewall

# Insert connection-tracking modules (not needed if built into the kernel).
modprobe ip_tables
modprobe iptable_filter
modprobe ip_conntrack
modprobe ip_conntrack_ftp
modprobe ipt_state
modprobe ipt_LOG

# allow local-only connections
iptables -A INPUT  -i lo -j ACCEPT
# free output on any interface to any ip for any service (equal to -P ACCEPT)
iptables -A OUTPUT -j ACCEPT

# permit answers on already established connections
# and permit new connections related to established ones (eg active-ftp)
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Log everything else:  What's Windows' latest exploitable vulnerability?
iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "

# set a sane policy:    everything not accepted > /dev/null
iptables -P INPUT    DROP
iptables -P FORWARD  DROP
iptables -P OUTPUT   DROP

# be verbose on dynamic ip-addresses     (not needed in case of static IP)
echo 2 > /proc/sys/net/ipv4/ip_dynaddr

# disable ExplicitCongestionNotification - too many routers are still ignorant
echo 0 > /proc/sys/net/ipv4/tcp_ecn

# End $rc_base/init.d/firewall
EOF

His script is quite simple, it drops all traffic coming in into your computer that wasn't initiated from your box, but as long as you are simply surfing the Internet you are unlikely to exceed its limits.

If you frequently encounter certain delays at accessing ftp-servers, please have a look at BusyBox - example no. 4.

Even if you have daemons or services running on your box, these should be inaccessible everywhere but from your box itself. If you want to allow access to services on your machine, such as ssh or pinging, take a look at BusyBox.

A true Firewall has two interfaces, one connected to an intranet, in this example, eth0, and one connected to the Internet, here, ppp0. To provide the maximum security against the box itself being broken into, make sure that there are no servers running on it, especially not X11 et al. And, as a general principle, the box itself should not access any untrusted service (Think of a name server giving answers that make your bind crash, or, even worse, that implement a worm via a buffer-overflow).

cat > /etc/rc.d/init.d/firewall << "EOF"
#!/bin/sh

# Begin $rc_base/init.d/firewall

echo
echo "You're using the example-config for a setup of a firewall"
echo "from the firewalling-hint written for LinuxFromScratch."
echo "This example is far from being complete, it is only meant"
echo "to be a reference."
echo "Firewall security is a complex issue, that exceeds the scope"
echo "of the quoted configuration rules."
echo "You can find some quite comprehensive information"
echo "about firewalls in Chapter 4 of the BLFS book."
echo "http://www.linuxfromscratch.org/blfs"
echo

# Insert iptables modules (not needed if built into the kernel).

modprobe ip_tables
modprobe iptable_filter
modprobe ip_conntrack
modprobe ip_conntrack_ftp
modprobe ipt_state
modprobe iptable_nat
modprobe ip_nat_ftp
modprobe ipt_MASQUERADE
modprobe ipt_LOG
modprobe ipt_REJECT

# allow local-only connections
iptables -A INPUT  -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# allow forwarding
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m state --state NEW -i ! ppp+       -j ACCEPT

# do masquerading    (not needed if intranet is not using private ip-addresses)
iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE

# Log everything for debugging (last of all rules, but before DROP/REJECT)
iptables -A INPUT   -j LOG --log-prefix "FIREWALL:INPUT  "
iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
iptables -A OUTPUT  -j LOG --log-prefix "FIREWALL:OUTPUT "

# set a sane policy
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP

# be verbose on dynamic ip-addresses (not needed in case of static IP)
echo 2 > /proc/sys/net/ipv4/ip_dynaddr

# disable ExplicitCongestionNotification
echo 0 > /proc/sys/net/ipv4/tcp_ecn

# activate TCPsyncookies
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

# activate Route-Verification = IP-Spoofing_protection
for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
        echo 1 > $f
done

# activate IP-Forwarding 
echo 1 > /proc/sys/net/ipv4/ip_forward
EOF

With this script your intranet should be sufficiently secure against external attacks. No one should be able to setup a new connection to any internal service and, if it's masqueraded, it's even invisible. Furthermore, your firewall should be nearly immune because there are no services running that a cracker could attack.

Note: if the interface you're connecting to the Internet doesn't connect via ppp, you will need to change ppp+ to the name of the interface which you are using. If you are using the same interface type to connect to both your intranet and the Internet, you need to use the actual name of the interface such as eth0, on both interfaces.

If you need stronger security (e.g., against DOS, connection highjacking, spoofing, etc.), have a look at the list of Links for further reading at the end of this section.

This scenario isn't too different from (Masquerading Router), but in this case you want to offer some services to your intranet. Examples of this can be when you want to admin your box from another host on your intranet or use it as a proxy or a name server. Note: Outlining a true concept of how to protect a server that offers services on the Internet goes far beyond the scope of this document, see Disclaimer.

Be cautious. Every service you offer and have enabled makes your setup more complex and your box less secure. You induce the risks of misconfigured services or running a service with an exploitable bug. A firewall should generally not run any extra services. See the introduction to Masquerading Router for some more details.

If the services you'd like to offer do not need to access the Internet themselves, like internal-only samba- or name-servers, it's quite simple and should still be acceptable from a security standpoint. Just add the following lines before the logging-rules into the script.

iptables -A INPUT  -i ! ppp+  -j ACCEPT
iptables -A OUTPUT -o ! ppp+  -j ACCEPT

If your daemons have to access the web themselves, like squid would need to, you could open OUTPUT generally and restrict INPUT.

iptables -A INPUT -m state --state ESTABLISHED,RELATED   -j ACCEPT
iptables -A OUTPUT                                      -j ACCEPT

However, it is generally not advisable to leave OUTPUT unrestricted. You lose any control over trojans who'd like to "call home", and a bit of redundancy in case you've (mis-)configured a service so that it does broadcast its existence to the world.

If you prefer to have this protection, you may restrict INPUT and OUTPUT on all ports except those that it's absolutely necessary to have open. Which ports you have to open depends on your needs: mostly you will find them by looking for failed accesses in your log-files.

These are only examples to show you some of the capabilities of the new firewall code in Linux-Kernel 2.4. Have a look at the man page of iptables. There you will find more of them. The port-numbers you'll need for this can be found in /etc/services, in case you didn't find them by trial and error in your log file.

If you add any of your offered or accessed services such as the above, maybe even in FORWARD and for intranet-communication, and delete the general clauses, you get an old fashioned packet filter.

If you'd like to have a look at the chains your firewall consists of and the order in which the rules take effect:

cat > /etc/rc.d/init.d/firewall.status << "EOF"
#!/bin/sh

# Begin $rc_base/init.d/firewall.status

echo "iptables.mangling:"
iptables -t mangle  -v -L -n --line-numbers

echo
echo "iptables.nat:"
iptables -t nat     -v -L -n --line-numbers

echo
echo "iptables.filter:"
iptables            -v -L -n --line-numbers
EOF

If you need to turn the firewall off, this script will do it:

cat > /etc/rc.d/init.d/firewall.stop << "EOF"
#!/bin/sh

# Being $rc_base/init.d/firewall.stop

# deactivate IP-Forwarding 
echo 0 > /proc/sys/net/ipv4/ip_forward

iptables -Z
iptables -F
iptables -t nat         -F PREROUTING
iptables -t nat         -F OUTPUT
iptables -t nat         -F POSTROUTING
iptables -t mangle      -F PREROUTING
iptables -t mangle      -F OUTPUT
iptables -X
iptables -P INPUT       ACCEPT
iptables -P FORWARD     ACCEPT
iptables -P OUTPUT      ACCEPT
EOF