.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:
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]#
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.
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 described in the steps 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
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 if that file exists; otherwise the rules
in /etc/shorewall/common.def are checked.
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:
At this point, edit your /etc/shorewall/policy and make any changes that you wish.
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:
If your external interface is ppp0 or ippp0, you can replace the "detect" in the second column with "-".
If your external interface is ppp0 or ippp0
or if you have a static IP address, you can remove "dhcp" from the
option list.
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.
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
At this point, edit
/etc/shorewall/rules to add other connections as desired.
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 11/15/2003 - Tom Eastep