Tom Eastep 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. This article applies to Shorewall 3.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release. Do not attempt to install Shorewall on a remote system. You are virtually assured to lock yourself out of that system.

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 detailed 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, you can find the Samples as follows: 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 | 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 ~# If you installed using the tarball, the samples are in the Samples/one-interface directory in the tarball. If you installed using the .deb, the samples are in /usr/share/doc/shorewall/examples/one-interface. 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 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. 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 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: 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 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. 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 - (minus the quotes). If your external interface is ppp0 or ippp0 or if you have a static IP address, you can remove dhcp from the option list.

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. Before starting Shorewall, you should look at the IP address of your external interface and if it is in one of the above ranges, you should remove the norfc1918 option from the entry in /etc/shorewall/interfaces.

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 Be sure to add your rules after the line that reads SECTION NEW. #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> #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. 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.

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. Users of the .deb package must edit /etc/default/shorewall and set STARTUP=1. 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.

Re-check each of the items flagged with a red arrow above. Check your log. Check the Troubleshooting Guide. Check the FAQ.

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.

2.0 2005-09-12 TE More 3.0 Updates 1.9 2005-09-02 CR Update for Shorewall 3.0 1.8 2005-07-12 TE Change reference to rfc1918 to bogons. 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