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”.
2006-06-06
Table of Contents
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.
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.
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 |
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).
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 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.
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
file. If no
rule in that file matches the connection request then the first policy in
/etc/shorewall/rules
/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 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:
allow all connection requests from the firewall to the internet
drop (ignore) all connection requests from the internet to your firewall
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.
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.
If you specify nobogons for your external
interface, you will want to check the Shorewall
Errata periodically for updates to the
/usr/share/shorewall/bogons file
.
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
.
Shorewall 2.0.0 and later include a collection of actions that can
be used to quickly allow or deny services. You can find a list of the
actions included in your version of Shorewall in the file
/usr/share/shorewall/actions.std
.
Those actions that allow a connection begin with “Allow”.
If you wish to enable connections from the internet to your firewall
and you find an appropriate “Allow” action in
/usr/share/shorewall/actions.std
, the general format
of a rule in /etc/shorewall/rules
is:
#ACTION SOURCE DESTINATION PROTO DEST PORT(S)
<action> net fw
Example 1. You want to run a Web Server and a POP3 Server on your firewall system:
#ACTION SOURCE DESTINATION PROTO DEST PORT(S) AllowWeb net fw AllowPOP3 net fw
You may also choose to code your rules directly without using the
pre-defined actions. This will be necessary in the event that there is not
a pre-defined action 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 POP3 Server on your firewall system:
#ACTION SOURCE DESTINATION PROTO DEST PORT(S) 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.
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) AllowSSH net fw
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
.
Users of the .deb package must edit
/etc/default/shorewall
and set
“startup=1”.
If you are running Shorewall 2.1.3 or later, 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”.
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.
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.
Revision History | ||
---|---|---|
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 |