Three-Interface Firewall

Version 2.0.1

Setting up a Linux system as a firewall for a small network with DMZ is a fairly straight-forward task if you understand the basics and follow the documentation.

This guide doesn't attempt to acquaint you with all of the features of Shorewall. It rather focuses on what is required to configure Shorewall in one of its more popular configurations:

• Linux system used as a firewall/router for a small local network.
• Single public IP address.
• DMZ connected to a separate ethernet interface.
• Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up, ...

Here is a schematic of a typical installation.

Shorewall requires that you have the iproute/iproute2 package installed (on RedHat, the package is called iproute). You can tell if this package is installed by the presence of an ip program on your firewall system. As root, you can use the 'which' command to check for this program:

     [root@gateway root]# which ip
     /sbin/ip
     [root@gateway root]#

I recommend that you first read through the guide to familiarize yourself with what's involved then go back through it again making your configuration changes. Points at which configuration changes are recommended are flagged with . Configuration notes that are unique to LEAF/Bering are marked with 

    If you edit your configuration files on a Windows system, you must save them as Unix files if your editor supports that option or you must run them through dos2unix before trying to use them. Similarly, if you copy a configuration file from your Windows hard drive to a floppy disk, you must run dos2unix against the copy before using it with Shorewall.

• Windows Version of dos2unix
• Linux Version of dos2unix

Shorewall Concepts

    The configuration files for Shorewall are contained in the directory /etc/shorewall -- for simple setups, you will only need to deal with a few of these as described in this guide. After you have installed Shorewall, download the three-interface sample, un-tar it (tar -zxvf three-interfaces.tgz) and and copy the files to /etc/shorewall (the files will replace files with the same names that were placed in /etc/shorewall when Shorewall was installed).

As each file is introduced, I suggest that you look through the actual file on your system -- each file contains detailed configuration instructions and default entries.

Shorewall views the network where it is running as being composed of a set of zones. In the three-interface sample configuration, the following zone names are used:

Zone names are defined in /etc/shorewall/zones.

Shorewall also recognizes the firewall system as its own zone - by default, the firewall itself is known as fw.

Rules about what traffic to allow and what traffic to deny are expressed in terms of zones.

• You express your default policy for connections from one zone to another zone in the /etc/shorewall/policy file.
• You define exceptions to those default policies in the /etc/shorewall/rules file.

For each connection request entering the firewall, the request is first checked against the /etc/shorewall/rules file. If no rule in that file matches the connection request then the first policy in /etc/shorewall/policy that matches the request is applied. If that policy is REJECT or DROP  the request is first checked against the rules in /etc/shorewall/common (the samples provide that file for you).

The /etc/shorewall/policy file included with the three-interface sample has the following policies:

Source Zone	Destination Zone	Policy	Log Level	Limit:Burst
loc	net	ACCEPT
net	all	DROP	info
all	all	REJECT	info

In the three-interface sample, the line below is included but commented out. If you want your firewall system to have full access to servers on the internet, uncomment that line.

Source Zone	Destination Zone	Policy	Log Level	Limit:Burst
fw	net	ACCEPT

The above policy will:

1. allow all connection requests from your local network to the internet
2. drop (ignore) all connection requests from the internet to your firewall or local network
3. optionally accept all connection requests from the firewall to the internet (if you uncomment the additional policy)
4. reject all other connection requests.

    At this point, edit your /etc/shorewall/policy file and make any changes that you wish.

Network Interfaces

The firewall has three network interfaces. Where Internet connectivity is through a cable or DSL "Modem", the External Interface will be the ethernet adapter that is connected to that "Modem" (e.g., eth0)  unless you connect via Point-to-Point Protocol over Ethernet (PPPoE) or Point-to-Point Tunneling Protocol (PPTP) in which case the External Interface will be a ppp interface (e.g., ppp0). If you connect via a regular modem, your External Interface will also be ppp0. If you connect using ISDN, you external interface will be ippp0.

    If your external interface is ppp0 or ippp0 then you will want to set CLAMPMSS=yes in /etc/shorewall/shorewall.conf.

Your Local Interface will be an ethernet adapter (eth0, eth1 or eth2) and will be connected to a hub or switch. Your local computers will be connected to the same switch (note: If you have only a single local system, you can connect the firewall directly to the computer using a cross-over cable).

Your DMZ Interface will also be an ethernet adapter (eth0, eth1 or eth2) and will be connected to a hub or switch. Your DMZ computers will be connected to the same switch (note: If you have only a single DMZ system, you can connect the firewall directly to the computer using a cross-over cable).

Do not connect more than one interface to the same hub or switch (even for testing). It won't work the way that you expect it to and you will end up confused and believing that Shorewall doesn't work at all.

    The Shorewall three-interface sample configuration assumes that the external interface is eth0, the local interface is eth1 and the DMZ interface is eth2. If your configuration is different, you will have to modify the sample /etc/shorewall/interfaces file accordingly. While you are there, you may wish to review the list of options that are specified for the interfaces. Some hints:

• If your external interface is ppp0 or ippp0, you can replace the "detect" in the second column with "-".

• If your external interface is ppp0 or ippp0 or if you have a static IP address, you can remove "dhcp" from the option list.

IP Addresses

Before going further, we should say a few words about Internet Protocol (IP) addresses. Normally, your ISP will assign you a single Public IP address. This address may be assigned via the Dynamic Host Configuration Protocol (DHCP) or as part of establishing your connection when you dial in (standard modem) or establish your PPP connection. In rare cases, your ISP may assign you a static IP address; that means that you configure your firewall's external interface to use that address permanently. Regardless of how the address is assigned, it will be shared by all of your systems when you access the Internet. You will have to assign your own addresses for your internal network (the local and DMZ Interfaces on your firewall plus your other computers). RFC 1918 reserves several Private IP address ranges for this purpose:

     10.0.0.0    - 10.255.255.255
     172.16.0.0  - 172.31.255.255
     192.168.0.0 - 192.168.255.255

The foregoing short discussion barely scratches the surface regarding subnetting and routing. If you are interested in learning more about IP addressing and routing, I highly recommend "IP Fundamentals: What Everyone Needs to Know about Addressing & Routing", Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.

The remainder of this quide will assume that you have configured your network as shown here:

The default gateway for the DMZ computers would be 10.10.11.254 and the default gateway for the Local computers would be 10.10.10.254.

    WARNING: Your ISP  might assign your external interface an RFC 1918 address. If that address is in the 10.10.10.0/24 subnet then you will need to select a DIFFERENT RFC 1918 subnet for your local network and if it is in the 10.10.11.0/24 subnet then you will need to select a different RFC 1918 subnet for your DMZ.

IP Masquerading (SNAT)

The addresses reserved by RFC 1918 are sometimes referred to as non-routable because the Internet backbone routers don't forward packets which have an RFC-1918 destination address. When one of your local systems (let's assume local computer 1) sends a connection request to an internet host, the firewall must perform Network Address Translation (NAT). The firewall rewrites the source address in the packet to be the address of the firewall's external interface; in other words, the firewall makes it look as if the firewall itself is initiating the connection.  This is necessary so that the destination host will be able to route return packets back to the firewall (remember that packets whose destination address is reserved by RFC 1918 can't be routed accross the internet). When the firewall receives a return packet, it rewrites the destination address back to 10.10.10.1 and forwards the packet on to local computer 1.

On Linux systems, the above process is often referred to as IP Masquerading and you will also see the term Source Network Address Translation (SNAT) used. Shorewall follows the convention used with Netfilter:

• Masquerade describes the case where you let your firewall system automatically detect the external interface address.

• SNAT refers to the case when you explicitly specify the source address that you want outbound packets from your local network to use.

In Shorewall, both Masquerading and SNAT are configured with entries in the /etc/shorewall/masq file.

    If your external firewall interface is eth0, your local interface eth1 and your DMZ interface is eth2 then you do not need to modify the file provided with the sample. Otherwise, edit /etc/shorewall/masq and change it to match your configuration.

    If your external IP is static, you can enter it in the third column in the /etc/shorewall/masq entry if you like although your firewall will work fine if you leave that column empty. Entering your static IP in column 3 makes
processing outgoing packets a little more efficient.

    If you are using the Debian package, please check your shorewall.conf file to ensure that the following are set correctly; if they are not, change them appropriately:

• NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)
• IP_FORWARDING=On
  

Port Forwarding (DNAT)

One of your goals will be to run one or more servers on your DMZ computers. Because these computers have RFC-1918 addresses, it is not possible for clients on the internet to connect directly to them. It is rather necessary for those clients to address their connection requests to your firewall who rewrites the destination address to the address of your server and forwards the packet to that server. When your server responds, the firewall automatically performs SNAT to rewrite the source address in the response.

The above process is called Port Forwarding or Destination Network Address Translation (DNAT). You configure port forwarding using DNAT rules in the /etc/shorewall/rules file.

The general form of a simple port forwarding rule in /etc/shorewall/rules is:

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
DNAT	net	dmz:<server local ip address> [:<server port>]	<protocol>	<port>

If you don't specify the <server port>, it is assumed to be the same as <port>.

Example - you run a Web Server on DMZ 2 and you want to forward incoming TCP port 80 to that system:

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
DNAT	net	dmz:10.10.11.2	tcp	80	# Forward port 80	from the internet
ACCEPT	loc	dmz:10.10.11.2	tcp	80	#Allow connections	from the local network

A couple of important points to keep in mind:

• When you are connecting to your server from your local systems, you must use the server's internal IP address (10.10.11.2).
• Many ISPs block incoming connection requests to port 80. If you have problems connecting to your web server, try the following rule and try connecting to port 5000 (e.g., connect to http://w.x.y.z:5000 where w.x.y.z is your external IP).

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
DNAT	net	dmz:10.10.11.2:80	tcp	5000

If you want to be able to access your server from the local network using your external address, then if you have a static external IP you can replace the loc->dmz rule above with:

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
DNAT	net	dmz:10.10.11.2:80	tcp	80	-	<external IP>

If you have a dynamic ip then you must ensure that your external interface is up before starting Shorewall and you must take steps as follows (assume that your external interface is eth0):

1. Include the following in /etc/shorewall/params:
   
   ETH0_IP=`find_interface_address eth0`
    
2. Make your loc->dmz rule:

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
DNAT	loc	dmz:10.10.11.2:80	tcp	80	-	$ETH0_IP

If you want to access your server from the DMZ using your external IP address, see FAQ 2a.

    At this point, add the DNAT and ACCEPT rules for your servers.

Domain Name Server (DNS)

Normally, when you connect to your ISP, as part of getting an IP address your firewall's Domain Name Service (DNS) resolver will be automatically configured (e.g., the /etc/resolv.conf file will be written). Alternatively, your ISP may have given you the IP address of a pair of DNS name servers for you to manually configure as your primary and secondary name servers. It is your responsibility to configure the resolver in your internal systems. You can take one of two approaches:

• You can configure your internal systems to use your ISP's name servers. If you ISP gave you the addresses of their servers or if those addresses are available on their web site, you can configure your internal systems to use those addresses. If that information isn't available, look in /etc/resolv.conf on your firewall system -- the name servers are given in "nameserver" records in that file.

•     You can configure a Caching Name Server on your firewall or in your DMZ. Red Hat has an RPM for a caching name server (which also requires the 'bind' RPM) and for Bering users, there is dnscache.lrp. If you take this approach, you configure your internal systems to use the caching name server as their primary (and only) name server. You use the internal IP address of the firewall (10.10.10.254 in the example above) for the name server address if you choose to run the name server on your firewall. To allow your local systems to talk to your caching name server, you must open port 53 (both UDP and TCP) from the local network to the server; you do that by adding the rules in /etc/shorewall/rules.

If you run the name server on the firewall:

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIGINAL ADDRESS
ACCEPT	loc	fw	tcp	53
ACCEPT	loc	fw	udp	53
ACCEPT	dmz	fw	tcp	53
ACCEPT	dmz	fw	udp	53

Last updated 6/27/2003 - Tom Eastep

Copyright 2002, 2003 Thomas M. Eastep