# Standalone 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.

### Warning

The sample configuration files included with Shorewall 3.0.0 and 3.0.1 were broken. The first error produced was:

ERROR: No Firewall Zone Defined

There are corrected 'zones' and 'interfaces' files available at http://www.shorewall.net/pub/shorewall/3.0/shorewall-3.0.1/errata/one-interface/.

This problem was fixed in Shorewall 3.0.2.

## Introduction

Setting up Shorewall on a standalone Linux system is very easy 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 most common configurations:

- Linux system

- Single external IP address

- Connection through Cable Modem, DSL, ISDN, Frame Relay, dial-up... or connected to a LAN and you simply wish to protect your Linux system from other systems on that LAN.

### System 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 read through the guide first 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 .

## 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 only need to deal with a few of these as described in this guide. After you have installed Shorewall, you can find the Samples as follows:

1. If you installed using an RPM, the samples will be in the `Samples/one-interface` 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 one-interface
       /usr/share/doc/packages/shorewall/Samples/one-interface
       /usr/share/doc/packages/shorewall/Samples/one-interface/interfaces
       /usr/share/doc/packages/shorewall/Samples/one-interface/policy
       /usr/share/doc/packages/shorewall/Samples/one-interface/rules
       /usr/share/doc/packages/shorewall/Samples/one-interface/zones
       ~#

2. If you installed using the tarball, the samples are in the `Samples/one-interface` directory in the tarball.

3. If you installed using the .deb, the samples are in `/usr/share/doc/shorewall/examples/one-interface`.

### 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.

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 one-interface sample configuration, only two zones are defined:

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

Shorewall zones 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, the name of the firewall zone (“fw” in the above example) 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.

- 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 policy is applied.

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

    #SOURCE ZONE   DESTINATION ZONE   POLICY   LOG LEVEL   LIMIT:BURST
    $FW            net                ACCEPT
    net            all                DROP     info
    all            all                REJECT   info

The above policy will:

1. allow all connection requests from the firewall to the internet

2. drop (ignore) all connection requests from the internet to your firewall

3. reject all other connection requests (Shorewall requires this catchall policy).

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

## External Interface

The firewall has a single network interface. Where Internet connectivity is through a cable or DSL “Modem”, the External Interface will be the ethernet adapter (`eth0`) that is connected to that “Modem” 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, your external interface will be `ippp0`.

The Shorewall one-interface sample configuration assumes that the external interface is `eth0`. 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 interface. Some hints:

### Tip

If your external interface is `ppp0` or `ippp0`, you can replace the “detect” in the second column with “-” (minus 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 Internet Service Provider (ISP) will assign you a single IP address. That address can be assigned statically, by the Dynamic Host Configuration Protocol (DHCP), through the establishment of your dial-up connection, or during establishment of your other type of PPP (PPPoA, PPPoE, etc.) connection.

RFC-1918 reserves several Private IP address ranges for use in private networks:

    10.0.0.0    - 10.255.255.255
    172.16.0.0  - 172.31.255.255
    192.168.0.0 - 192.168.255.255

These addresses are sometimes referred to as non-routable because the Internet backbone routers will not forward a packet whose destination address is reserved by RFC-1918. In some cases though, ISPs are assigning these addresses then using Network Address Translation - NAT) to rewrite packet headers when forwarding to/from the internet.

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

## Enabling other Connections

Shorewall includes a collection of macros that can be used to quickly allow or deny services. You can find a list of the macros included in your version of Shorewall using the command ls `/usr/share/shorewall/macro.*`.

If you wish to enable connections from the internet to your firewall and you find an appropriate macro in `/etc/shorewall/macro.*`, the general format of a rule in `/etc/shorewall/rules` is:

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

### Important

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

Example 1. You want to run a Web Server and a IMAP Server on your firewall system:

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

You may also choose to code your rules directly without using the pre-defined macros. This will be necessary in the event that there is not a pre-defined macro that meets your requirements. In that case the general format of a rule in `/etc/shorewall/rules` is:

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

Example 2. You want to run a Web Server and a IMAP Server on your firewall system:

    #ACTION   SOURCE    DESTINATION     PROTO       DEST PORT(S)
    ACCEPT    net       $FW             tcp          80
    ACCEPT    net       $FW             tcp          143

If you don't know what port and protocol a particular application uses, see 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    DESTINATION     PROTO       DEST PORT(S)
    SSH/ACCEPT  net       $FW

At this point, edit `/etc/shorewall/rules` to add other connections as desired.

## 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 must edit /etc/shorewall/shorewall.conf and set STARTUP_ENABLED=Yes.

### Important

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

### Important

You must enable startup by editing `/etc/shorewall/shorewall.conf` and setting `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”.

### 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.

## A. Revision History

Revision History
Revision 2.0 | 2005-09-12 | TE
More 3.0 Updates
Revision 1.9 | 2005-09-02 | CR
Update for Shorewall 3.0
Revision 1.8 | 2005-07-12 | TE
Change reference to rfc1918 to bogons.
Revision 1.7 | 2004-02-16 | TE
Move /etc/shorewall/rfc1918 to /usr/share/shorewall.
Revision 1.6 | 2004-02-05 | TE
Update for Shorewall 2.0
Revision 1.5 | 2004-01-05 | TE
Standards Changes
Revision 1.4 | 2003-12-30 | TE
Add tip about /etc/shorewall/rfc1918 updates.
Revision 1.3 | 2003-11-15 | TE
Initial Docbook Conversion