Standalone Firewall

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:

[root@gateway root]# which ip
/sbin/ip
[root@gateway root]#

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 are installing Shorewall version 3.4.0 or later then as each file is introduced, I suggest that you look at the actual file on your system and that you look at the man page for that file. For example, to look at the man page for the /etc/shorewall/zones file, type man shorewall-zones at a shell prompt.

If you are installing a Shorewall version earlier than 3.4.0, then 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.

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:

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:

The word info in the LOG LEVEL column for the last two policies indicates that packets droped or rejected under those policies should be logged at that level.

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.

root@lists:~# ip route ls
192.168.2.2 dev tun0  proto kernel  scope link  src 192.168.2.1 
10.13.10.0/24 dev tun1  scope link 
192.168.2.0/24 via 192.168.2.2 dev tun0 
206.124.146.0/24 dev eth0  proto kernel  scope link  src 206.124.146.176 
10.10.10.0/24 dev tun1  scope link 
default via 206.124.146.254 dev eth0 
root@lists:~# 

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:

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.

root@lists:~# ip addr ls dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc htb qlen 1000
    link/ether 00:02:e3:08:48:4c brd ff:ff:ff:ff:ff:ff
    inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
    inet6 fe80::202:e3ff:fe08:484c/64 scope link 
       valid_lft forever preferred_lft forever
root@lists:~# 

Shorewall does not maintain a log itself but rather relies on your system's logging configuration. The following commands rely on knowing where Netfilter messages are logged:

It is important that these commands work properly because when you encounter connection problems when Shorewall is running, the first thing that you should do is to look at the Netfilter log; with the help of Shorewall FAQ 17, you can usually resolve the problem quickly.

Most commonly, Netfilter messages are logged to /var/log/messages. Recent SuSE/OpenSuSE™ releases come preconfigured with syslog-ng and log netfilter messages to /var/log/firewall.

If you are running a distribution that logs netfilter messages to a log other than /var/log/messages, then modify the LOGFILE setting in /etc/shorewall/shorewall.conf to specify the name of your log.

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

#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.

#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.

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”.

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.