Standalone Firewall

Version 2.0.1

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:

This guide assumes 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 read through the guide first 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 .

    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.

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, download the one-interface sample, un-tar it (tar -zxvf one-interface.tgz) and and copy the files to /etc/shorewall (they will replace files with the same names that were placed in /etc/shorewall during Shorewall installation).

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 one zone is defined:

Name Description
net The Internet

Shorewall zones 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.

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 one-interface sample has the following policies:

SOURCE ZONE DESTINATION ZONE POLICY LOG LEVEL LIMIT:BURST
fw net ACCEPT    
net net DROP info  
all all REJECT info   
     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 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:

IP Addresses

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 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 one of the above ranges, you should remove the 'norfc1918' option from the entry in /etc/shorewall/interfaces.

Enabling other Connections

If you wish to enable connections from the internet to your firewall, the general format is:

ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw <protocol> <port>    

Example - You want to run a Web Server and a POP3 Server on your firewall system:

ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 80    
ACCEPT net fw tcp 110    

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 PROTOCOL PORT SOURCE PORT ORIGINAL ADDRESS
ACCEPT net fw tcp 22     
     ACCEPT	net	fw	tcp	22

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

Starting and Stopping Your Firewall

Arrow     The installation procedure configures your system to start Shorewall at system boot but beginning with Shorewall version 1.3.9 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 removing the file /etc/shorewall/startup_disabled.

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".

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.

Last updated 12/9/2002 - Tom Eastep

Copyright 2002 Thomas M. Eastep