Tom Eastep 2004-10-27 2002-2004 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.

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

Points at which configuration changes are recommended are flagged with .

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 /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 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 norfc1918 for your external interface, you will want to check the Shorewall Errata periodically for updates to the /usr/share/shorewall/rfc1918 file. Alternatively, you can copy /usr/share/shorewall/rfc1918 to /etc/shorewall/rfc1918 then strip down your /etc/shorewall/rfc1918 file as I do.

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 /etc/shorewall/actions.std, the general format of a rule in /etc/shorewall/rules is: #ACTION SOURCE DESTINATION PROTO DEST PORT(S) <action> net fw #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> #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.

1.7 2004-02-16 TE Move /etc/shorewall/rfc1918 to /usr/share/shorewall. 1.6 2004-02-05 TE Update for Shorewall 2.0 1.5 2004-01-05 TE Standards Changes 1.4 2003-12-30 TE Add tip about /etc/shorewall/rfc1918 updates. 1.3 2003-11-15 TE Initial Docbook Conversion