# Three-Interface Firewall

### Tom Eastep

Copyright © 2002-2006 Thomas M. Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2006-02-12

### Caution

This article applies to Shorewall 3.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release.

## Introduction

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.

  ### Note

  If you have more than one public IP address, this is not the guide you want -- see the Shorewall Setup Guide instead.

- DMZ connected to a separate ethernet interface. The purpose of a DMZ is to isolate those servers that are exposed to the Internet from your local systems so that if one of those servers is compromised there is still a firewall between the hacked server and your local systems.

- Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up, ...

Here is a schematic of a typical installation.

Figure 1. schematic of a typical installation

### Requirements

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]#

### Before you start

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.

### Caution

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

### Conventions

Points at which configuration changes are recommended are flagged with .

Configuration notes that are unique to LEAF/Bering are marked with .

## PPTP/ADSL

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.

## 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.

### Warning

Note to Debian Users

If you install using the .deb, you will find that your `/etc/shorewall` directory is empty. This is intentional. The released configuration file skeletons may be found on your system in the directory `/usr/share/doc/shorewall/default-config`. Simply copy the files you need from that directory to `/etc/shorewall` and modify the copies.

Note that you must copy `/usr/share/doc/shorewall/default-config/shorewall.conf` and /usr/share/doc/shorewall/default-config/modules to /etc/shorewall even if you do not modify those files.

After you have installed Shorewall, locate the three-interface Sample configuration:

1. If you installed using an RPM, the samples will be in the Samples/three-interfaces/ subdirectory of the Shorewall documentation directory. If you don't know where the Shorewall documentation directory is, you can find the samples using this command:

       ~# rpm -ql shorewall | fgrep three-interfaces
       /usr/share/doc/packages/shorewall/Samples/three-interfaces
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/interfaces
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/masq
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/policy
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/routestopped
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/rules
       /usr/share/doc/packages/shorewall/Samples/three-interfaces/zones
       ~#

2. If you installed using the tarball, the samples are in the Samples/three-interfaces directory in the tarball.

3. If you installed using the .deb, the samples are in /usr/share/doc/shorewall/examples/three-interfaces.

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

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

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

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

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

### Important

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    DEST        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.

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` file and make any changes that you wish.

## Network Interfaces

Figure 2. DMZ

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).

### Caution

Do NOT connect the internal and external interface to the same hub or switch except for testing. You can test using this kind of configuration if you specify the arp_filter option or the arp_ignore option in `/etc/shorewall/interfaces` for all interfaces connected to the common hub/switch. Using such a setup with a production firewall is strongly recommended against.

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:

### Tip

If your external interface is `ppp0` or `ippp0`, you can replace the “detect” in the second column with “-” (without the quotes).

### Tip

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

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`.

You will want to assign your local addresses from one sub-network or subnet and your DMZ addresses from another 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 “1” bits from the left of the subnet mask.

Table 1. Example sub-network

Range: | `10.10.10.0` - `10.10.10.255`
Subnet Address: | `10.10.10.0`
Broadcast Address: | `10.10.10.255`
CIDR Notation: | `10.10.10.0/24`

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 (Local Computers 1 & 2) should be configured with their default gateway set to the IP address of the firewall's internal interface and your DMZ computers (DMZ Computers 1 & 2) should be configured with their default gateway set to the IP address of the firewall's DMZ 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.

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

Figure 3. DMZ

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, in spite of all advice to the contrary, you are using this guide and want to use one-to-one NAT or Proxy ARP for your DMZ, remove the entry for eth2 from `/etc/shorewall/masq`.

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 is set correctly; if it is not, change it appropriately:

- `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    DEST                                          PROTO      DEST PORT(S)
    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>`.

### Important

Be sure to add your rules after the line that reads SECTON NEW.

Example 1. You run a Web Server on DMZ Computer 2 and you want to forward incoming TCP port 80 to that system

    #ACTION     SOURCE    DEST                PROTO      DEST PORT(S)
    Web/DNAT     net    dmz:10.10.11.2
    Web/ACCEPT   loc    dmz:10.10.11.2

- Entry 1 forwards port 80 from the Internet.

- Entry 2 allows connections from the local network.

Several 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    DEST                PROTO      DEST PORT(S)    SOURCE
      #                                                                  PORT(S)
      DNAT      net       dmz:10.10.11.2:80   tcp        80              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    DEST            PROTO  DEST PORT(S)  SOURCE   ORIGINAL
      #                                                        PORT(S)  DEST
      DNAT      loc       dmz:10.10.11.2  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    DEST             PROTO   DEST PORT(S)  SOURCE   ORIGINAL
         #                                                          PORT(S)  DEST
         DNAT      loc       dmz:10.10.11.2   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.

### Important

When testing DNAT rules like those shown above, you must test from a client OUTSIDE YOUR FIREWALL (in the 'net' zone). You cannot test these rules from inside the firewall!

For DNAT troubleshooting tips, see FAQs 1a and 1b.

## 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 your 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    DEST                PROTO      DEST PORT(S)
    DNS/ACCEPT  loc       $FW
    DNS/ACCEPT  dmz       $FW

Run name server on DMZ computer 1:

    #ACTION     SOURCE    DEST                PROTO      DEST PORT(S)
    DNS/ACCEPT  loc       dmz:10.10.11.1
    DNS/ACCEPT  $FW       dmz:10.10.11.1

In the rules shown above, “DNS/ACCEPT” is an example of a defined macro. Shorewall includes a number of defined macros and you can add your own. To see the list of macros included with your version of Shorewall, run the command ls `/usr/share/shorewall/macro.*`.

You don't have to use defined macros when coding a rule in `/etc/shorewall/rules`. The first example above (name server on the firewall) could also have been coded as follows:

    #ACTION   SOURCE    DEST                PROTO      DEST PORT(S)
    ACCEPT    loc       $FW                 tcp        53
    ACCEPT    loc       $FW                 udp        53
    ACCEPT    dmz       $FW                 tcp        53
    ACCEPT    dmz       $FW                 udp        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. This page can be of help if you don't know the protocol and port involved.

## Other Connections

The three-interface sample includes the following rule:

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

That rule allow DNS access from your firewall and may be removed if you commented out the line in `/etc/shorewall/policy` allowing all connections from the firewall to the Internet.

The sample also includes:

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

Those rules allow you to run an SSH server on your firewall and in each of your DMZ systems and to connect to those servers from your local systems.

If you wish to enable other connections between your systems, the general format for using a defined macro is:

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

The general format when not using a defined action is:

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

Example 2. You want to run a publicly-available DNS server on your firewall system

Using defined macros:

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

Not using defined macros:

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

Those rules would of course be in addition to the rules listed above under "If you run the name server on your firewall".

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

### Important

I don't recommend enabling telnet to/from the Internet because it uses clear text (even for login!). If you want shell access to your firewall from the Internet, use SSH:

    #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
    ACCEPT    net       $FW                 tcp        80

- Entry 1 allows the DNS Cache to be used.

- Entry 2 allows the “weblet” to work.

Now modify `/etc/shorewall/rules` to add or remove other connections as required.

## Some Things to Keep in Mind

- You cannot test your firewall from the inside. Just because you send requests to your firewall external IP address does not mean that the request will be associated with the external interface or the “net” zone. Any traffic that you generate from the local network will be associated with your local interface and will be treated as loc->fw traffic.

- IP addresses are properties of systems, not of interfaces. It is a mistake to believe that your firewall is able to forward packets just because you can ping the IP address of all of the firewall's interfaces from the local network. The only conclusion you can draw from such pinging success is that the link between the local system and the firewall works and that you probably have the local system's default gateway set correctly.

- All IP addresses configured on firewall interfaces are in the $FW (fw) zone. If 192.168.1.254 is the IP address of your internal interface then you can write “$FW:192.168.1.254” in a rule but you may not write “loc:192.168.1.254”. Similarly, it is nonsensical to add 192.168.1.254 to the loc zone using an entry in `/etc/shorewall/hosts`.

- Reply packets do NOT automatically follow the reverse path of the one taken by the original request. All packets are routed according to the routing table of the host at each step of the way. This issue commonly comes up when people install a Shorewall firewall parallel to an existing gateway and try to use DNAT through Shorewall without changing the default gateway of the system receiving the forwarded requests. Requests come in through the Shorewall firewall where the destination IP address gets rewritten but replies go out unmodified through the old gateway.

- Shorewall itself has no notion of inside or outside. These concepts are embodied in how Shorewall is configured.

## Starting and Stopping Your Firewall

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 can enable Shorewall startup by editing `/etc/shorewall/shorewall.conf` and setting STARTUP_ENABLED=Yes.

### Important

Users of the `.deb` package must edit `/etc/default/shorewall` and set `startup=1`.

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 three-interface sample assumes that you want to enable routing to/from `eth1` (your local network) and `eth2` (DMZ) when Shorewall is stopped. If these two interfaces don't connect to your local network and DMZ or if you want to enable a different set of hosts, modify `/etc/shorewall/routestopped` accordingly.

### Warning

If you are connected to your firewall from the Internet, do not issue a shorewall stop command unless you have added an entry for the IP address that you are connected from to `/etc/shorewall/routestopped`. Also, I don't recommend using shorewall restart; it is better to create an alternate configuration and test it using the shorewall try command.

## If it Doesn't Work

- Re-check each of the items flagged with a red arrow above.

- Check your log.

- Check the Troubleshooting Guide.

- Check the FAQ.

## Additional Recommended Reading

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.