Basic Two-Interface Firewall

Setting up a Linux system as a firewall for a small network 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 its most common configuration:

Here is a schematic of a typical installation:

Figure 1. Common two interface firewall configuration

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

If you have an ADSL Modem and you use PPTP to communicate with a server in that modem, you must make the changes recommended here in addition to those detailed below. ADSL with PPTP is most commonly found in Europe, notably in Austria.

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.

If you are installing Shorewall version 3.4.0 or later then as each file is introduced, I suggest that you look at the actual file on your system and that you look at the man page for that file. For example, to look at the man page for the /etc/shorewall/zones file, type man shorewall-zones at a shell prompt.

If you are installing a Shorewall version earlier than 3.4.0, then 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 two-interface sample configuration, the following zone names are used:

#ZONE   TYPE     OPTIONS                 IN                      OUT
#                                        OPTIONS                 OPTIONS
fw      firewall
net     ipv4
loc     ipv4

Zones are defined in the /etc/shorewall/zones file.

Note that Shorewall recognizes the firewall system as its own zone - when the /etc/shorewall/zones file is processed, the name of the firewall zone is stored in the shell variable $FW which may be used to refer to the firewall zone throughout the Shorewall configuration.

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

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 there is a comon action defined for the policy in /etc/shorewall/actions or /usr/share/shorewall/actions.std then that action is peformed before the action is applied. The purpose of the common action is two-fold:

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

#SOURCE    DEST        POLICY      LOG LEVEL    LIMIT:BURST
loc        net         ACCEPT
net        all         DROP        info
all        all         REJECT      info

In the two-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    DEST        POLICY      LOG LEVEL    LIMIT:BURST
$FW        net         ACCEPT

The above policy will:

The word info in the LOG LEVEL column for the DROP and REJECT policies indicates that packets droped or rejected under those policies should be logged at that level.

It is important to note that Shorewall policies (and rules) refer to connections and not packet flow. With the policies defined in the /etc/shorewall/policy file shown above, connections are allowed from the loc zone to the net zone even though connections are not allowed from the loc zone to the firewall itself.

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

The firewall has two 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 via ISDN, your external interface will be ippp0.

root@lists:~# ip route ls
192.168.1.1 dev eth0  scope link 
192.168.2.2 dev tun0  proto kernel  scope link  src 192.168.2.1 
192.168.3.0/24 dev br0  proto kernel  scope link  src 192.168.3.254 
10.13.10.0/24 dev tun1  scope link 
192.168.2.0/24 via 192.168.2.2 dev tun0 
192.168.1.0/24 dev br0  proto kernel  scope link  src 192.168.1.254 
206.124.146.0/24 dev eth0  proto kernel  scope link  src 206.124.146.176 
10.10.10.0/24 dev tun1  scope link 
default via 206.124.146.254 dev eth0 
root@lists:~# 

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

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

The Shorewall two-interface sample configuration assumes that the external interface is eth0 and the internal interface is eth1. 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:

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. However your external 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 in your internal network (the Internal Interface 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

Before starting Shorewall, you should look at the IP address of your external interface and if it is one of the above ranges, you should remove the 'norfc1918' option from the external interface's entry in /etc/shorewall/interfaces.

root@lists:~# ip addr ls dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc htb qlen 1000
    link/ether 00:02:e3:08:48:4c brd ff:ff:ff:ff:ff:ff
    inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
    inet6 fe80::202:e3ff:fe08:484c/64 scope link 
       valid_lft forever preferred_lft forever
root@lists:~# 

You will want to assign your addresses from the same sub-network (subnet). For our purposes, we can consider a subnet to consists of a range of addresses x.y.z.0 - x.y.z.255. Such a subnet will have a Subnet Mask of 255.255.255.0. The address x.y.z.0 is reserved as the Subnet Address and x.y.z.255 is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is described using Classless InterDomain Routing (CIDR) notation with consists of the subnet address followed by /24. The “24” refers to the number of consecutive leading “1” bits from the left of the subnet mask.

It is conventional to assign the internal interface either the first usable address in the subnet (10.10.10.1 in the above example) or the last usable address (10.10.10.254).

One of the purposes of subnetting is to allow all computers in the subnet to understand which other computers can be communicated with directly. To communicate with systems outside of the subnetwork, systems send packets through a gateway (router).

Your local computers (computer 1 and computer 2 in the above diagram) should be configured with their default gateway to be the IP address of the firewall's internal interface.

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 (link).

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

The default gateway for computer's 1 & 2 would be 10.10.10.254.

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 computer 1 in the above diagram) 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 appear to the destination internet host 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 across the internet so the remote host can't address its response to computer 1). When the firewall receives a return packet, it rewrites the destination address back to 10.10.10.1 and forwards the packet on to computer 1.

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

In Shorewall, both Masquerading and SNAT are configured with entries in the /etc/shorewall/masq file. You will normally use Masquerading if your external IP is dynamic and SNAT if the IP is static.

If your external firewall interface is eth0, you do not need to modify the file provided with the sample. Otherwise, edit /etc/shorewall/masq and change the first column to the name of your external interface and the second column to the name of your internal interface.

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 (Masquerade). Entering your static IP in column 3 (SNAT) makes the processing of outgoing packets a little more efficient.

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

One of your goals may be to run one or more servers on your local 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 the 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    DEST                                          PROTO      DEST PORT(S)
DNAT      net       loc:<server local ip address>[:<server port>] <protocol> <port>

Shorewall has macros for many popular applications. Look at the output of shorewall show macros to see what is available in your release. Macros simplify creating DNAT rules by supplying the protocol and port(s) as shown in the following examples.

#ACTION   SOURCE    DEST             PROTO     DEST PORT(S)
Web/DNAT  net       loc:10.10.10.2

#ACTION    SOURCE    DEST            PROTO     DEST PORT(S)
FTP/DNAT   net       loc:10.10.10.1

A couple of important points to keep in mind:

At this point, modify /etc/shorewall/rules to add any DNAT rules that you require.

For information about DNAT when there are multiple external IP addresses, see the Shorewall Aliased Interface documentation and the Shorewall Setup Guide.

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. Regardless of how DNS gets configured on your firewall, it is your responsibility to configure the resolver in your internal systems. You can take one of two approaches:

The two-interface sample includes the following rules:

#ACTION     SOURCE    DEST               PROTO     DEST PORT(S)
DNS/ACCEPT  $FW       net

This rule allows DNS access from your firewall and may be removed if you uncommented the line in /etc/shorewall/policy allowing all connections from the firewall to the internet.

In the rule shown above, “DNS/ACCEPT” is an example of a macro invocation. Shorewall includes a number of macros (see /usr/share/shorewall/macro.*) and you can add your own.

You don't have to use defined macros when coding a rule in /etc/shorewall/rules; Shorewall will start slightly faster if you code your rules directly rather than using macros. The the rule shown above could also have been coded as follows:

#ACTION    SOURCE    DEST               PROTO     DEST PORT(S)
ACCEPT     $FW       net                udp       53
ACCEPT     $FW       net                tcp       53

In cases where Shorewall doesn't include a defined macro to meet your needs, you can either define the macro yourself or you can simply code the appropriate rules directly.

The sample also includes:

#ACTION      SOURCE    DEST               PROTO     DEST PORT(S)
SSH/ACCEPT   loc       $FW  

That rule allows you to run an SSH server on your firewall and connect to that server from your local systems.

If you wish to enable other connections from your firewall to other systems, the general format using a macro is:

#ACTION         SOURCE    DEST               PROTO      DEST PORT(S)
<macro>/ACCEPT  $FW       <destination zone>

The general format when not using defined actions is:

#ACTION    SOURCE    DEST               PROTO      DEST PORT(S)
ACCEPT     $FW       <destination zone> <protocol> <port>

#ACTION     SOURCE    DEST               PROTO     DEST PORT(S)
Web/ACCEPT  net       $FW
Web/ACCEPT  loc       $FW       

If you don't know what port and protocol a particular application uses, look here.

#ACTION      SOURCE    DEST               PROTO     DEST PORT(S)
SSH/ACCEPT   net       $FW

Bering users will want to add the following two rules to be compatible with Jacques's Shorewall configuration.

#ACTION    SOURCE    DEST    PROTO     DEST PORT(S)
ACCEPT     loc       $FW     udp       53          #Allow DNS Cache to work
ACCEPT     loc       $FW     tcp       80          #Allow Weblet to work

Now edit your /etc/shorewall/rules file to add or delete other connections as required.

Shorewall does not maintain a log itself but rather relies on your system's logging configuration. The following commands rely on knowing where Netfilter messages are logged:

It is important that these commands work properly because when you encounter connection problems when Shorewall is running, the first thing that you should do is to look at the Netfilter log; with the help of Shorewall FAQ 17, you can usually resolve the problem quickly.

Most commonly, Netfilter messages are logged to /var/log/messages. Recent SuSE/OpenSuSE™ releases come preconfigured with syslog-ng and log netfilter messages to /var/log/firewall.

If you are running a distribution that logs netfilter messages to a log other than /var/log/messages, then modify the LOGFILE setting in /etc/shorewall/shorewall.conf to specify the name of your log.

The installation procedure configures your system to start Shorewall at system boot but startup is disabled so that your system won't try to start Shorewall before configuration is complete. Once you have completed configuration of your firewall, you must edit /etc/shorewall/shorewall.conf and set STARTUP_ENABLED=Yes.

The firewall is started using the “shorewall start” command and stopped using “shorewall stop”. When the firewall is stopped, routing is enabled on those hosts that have an entry in /etc/shorewall/routestopped. A running firewall may be restarted using the “shorewall restart” command. If you want to totally remove any trace of Shorewall from your Netfilter configuration, use “shorewall clear”.

The two-interface sample assumes that you want to enable routing to/from eth1 (the local network) when Shorewall is stopped. If your local network isn't connected to eth1 or if you wish to enable access to/from other hosts, change /etc/shorewall/routestopped accordingly.

I highly recommend that you review the Common Configuration File Features page -- it contains helpful tips about Shorewall features than make administering your firewall easier.

Once you have the two-interface setup working, the next logical step is to add a Wireless Network. The first step involves adding an additional network card to your firewall, either a Wireless card or an ethernet card that is connected to a Wireless Access Point.

Your new network will look similar to what is shown in the following figure.

The first thing to note is that the computers in your wireless network will be in a different subnet from those on your wired local LAN. In the above example, we have chosen to use the network 10.10.11.0/24. Computers 3 and 4 would be configured with a default gateway IP address of 10.10.11.254.

Second, we have chosen to include the wireless network as part of the local zone. Since Shorewall allows intra-zone traffic by default, traffic may flow freely between the local wired network and the wireless network.

There are only two changes that need to be made to the Shorewall configuration:

One other thing to note. To get Microsoft™ networking working between the wireless and wired networks, you will need either a WINS server or a PDC. I personally use Samba configured as a WINS server running on my firewall. Running a WINS server on your firewall requires the rules listed in the Shorewall/Samba documentation.