Standalone Firewall

Tom 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”.

2009/06/16


Table of Contents

Introduction
System Requirements
Before you start
Conventions
PPTP/ADSL
Shorewall Concepts
External Interface
IP Addresses
Logging
Enabling other Connections
Starting and Stopping Your Firewall
If it Doesn't Work
Additional Recommended Reading

Caution

This article applies to Shorewall 4.4 and later. If you are running a version of Shorewall earlier than Shorewall 4.4.0 then please see the documentation for that release.

Caution

Do not attempt to install Shorewall on a remote system. You are virtually assured to lock yourself out of that system.

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.

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-common | 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 a Shorewall 3.x .deb, the samples are in /usr/share/doc/shorewall/examples/one-interface. You must install the shorewall-doc package.

  4. If you installed using a Shorewall 4.x .deb, the samples are in /usr/share/doc/shorewall-common/examples/one-interface.. You do not need the shorewall-doc package to have access to the samples.

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-common/default-config. Simply copy the files you need from that directory to /etc/shorewall and modify the copies.

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 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.

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

  • It silently drops or rejects harmless common traffic that would otherwise clutter up your log — Broadcasts for example.

  • If ensures that traffic critical to correct operation is allowed through the firewall — ICMP fragmentation-needed for example.

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).

The word info in the LOG LEVEL column for the last two policies indicates that packets dropped or rejected under those policies should be logged at that level.

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 DSLModem”, the External Interface will be the Ethernet adapter (eth0) that is connected to that “Modemunless 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.

Caution

Be sure you know which interface is your external interface. Many hours have been spent floundering by users who have configured the wrong interface. If you are unsure, then as root type ip route ls at the command line. The device listed in the last (default) route should be your external interface.

Example:

root@lists:~# ip route ls
192.168.2.2 dev tun0  proto kernel  scope link  src 192.168.2.1 
10.13.10.0/24 dev tun1  scope link 
192.168.2.0/24 via 192.168.2.2 dev tun0 
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:~# 

In that example, eth0 is the external interface.

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.

Logging

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:

  • shorewall show log (Displays the last 20 Netfilter log messages)

  • shorewall logwatch (Polls the log at a settable interval

  • shorewall dump (Produces an extensive report for inclusion in Shorewall problem reports)

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.

Important

The LOGFILE setting does not control where the Netfilter log is maintained -- it simply tells the /sbin/shorewall utility where to find the log.

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.

While you are editing shorewall.conf, it is a good idea to check the value of the SUBSYSLOCK option. You can find a description of this option by typing 'man shorewall.conf' at a shell prompt and searching for SUBSYSLOCK.

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

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. Also, Operating Shorewall and Shorewall Lite contains a lot of useful operational hints.