The information in this document applies only to 4.3 and later releases of Shorewall.

The configuration files for Shorewall are contained in the directory /etc/shorewall -- for simple setups, you will only need to deal with a few of them.

Shorewall views the network where it is running as being composed of a set of zones. Zones are declared and given a type in the /etc/shorewall/zones file.Here is the /etc/shorewall/zones file from the three-interface sample:

#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS
fw      firewall
net     ipv4
loc     ipv4
dmz     ipv4

Note that Shorewall recognizes the firewall system as its own zone. The name of the zone designating the firewall itself (usually 'fw' as shown in the above file) is stored in the shell variable $FW which may be used throughout the Shorewall configuration to refer to the firewall zone.

The simplest way to define the hosts in a zone is to associate the zone with a network interface using the /etc/shorewall/interfaces file. In the three-interface sample, the three zones are defined using that file as follows:

#ZONE   INTERFACE       OPTIONS
net     NET_IF          tcpflags,dhcp,nosmurfs,routefilter,logmartians,sourceroute=0,physical=eth0
loc     LOC_IF          tcpflags,nosmurfs,routefilter,logmartians,physical=eth1
dmz     DMZ_IF          tcpflags,nosmurfs,routefilter,logmartians,physical=eth2

The above file defines the net zone as all IPv4 hosts interfacing to the firewall through eth0, the loc zone as all IPv4 hosts interfacing through eth1 and the dmz as all IPv4 hosts interfacing through eth2. The interface names shown in the INTERFACE column are logical names which are used throughout the configuration to refer to the individual interfaces. The actual interface names are specified using the physical option. It is important to note that the composition of a zone is defined in terms of a combination of addresses and interfaces. When using the /etc/shorewall/interfaces file to define a zone, all addresses are included; when you want to define a zone that contains a limited subset of the IPv4 address space, you use the /etc/shorewall/hosts file or you may use the nets= option in /etc/shorewall/interfaces:

#ZONE   INTERFACE       OPTIONS
net     NET_IF          tcpflags,dhcp,nosmurfs,routefilter,logmartians,sourceroute=0,physical=eth0
loc     LOC_IF          tcpflags,nosmurfs,routefilter,logmartians,physical=eth1,nets=172.20.1.0/24
dmz     DMZ_IF          tcpflags,nosmurfs,routefilter,logmartians,physical=eth2

The above file defines the net zone as all IPv4 hosts interfacing to the firewall through eth0 except for 192.168.0.0/23, the loc zone as IPv4 hosts 192.168.0.0/24 interfacing through eth1 and the dmz as IPv4 hosts 192.168.1.0/24 interfacing through eth2 (Note that 192.168.0.0/24 together with 192.168.1.0/24 comprises 192.168.0.0/23).

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 default action defined for the policy in /etc/shorewall/shorewall.conf then that action is invoked before the policy is enforced. In the standard Shorewall distribution, the DROP policy has a default action called Drop and the REJECT policy has a default action called Reject. Default actions are used primarily to discard certain packets silently so that they don't clutter up your log.

The /etc/shorewall/policy file included with the three-interface sample has the following policies:

#SOURCE    DEST        POLICY      LOGLEVEL    LIMIT
loc        net         ACCEPT
net        all         DROP        info
all        all         REJECT      info

In the three-interface sample, the line below is included but commented out. If you want your firewall system to have full access to servers on the Internet, uncomment that line.

#SOURCE    DEST        POLICY      LOGLEVEL    LIMIT
$FW        net         ACCEPT

The above policies will:

A word about Shorewall logging is in order. Shorewall does not have direct control over where its messages are logged; that is determined by the configuration of the logging daemon (syslog, rsyslog, syslog-ng, ulogd, etc.). The LOGFILE setting in /etc/shorewall/shorewall.conf tells Shorewall where to find the log; it doesn't determine where messages are logged. See the Shorewall logging article for more information.

To illustrate how rules provide exceptions to policies, suppose that you have the polices listed above but you want to be able to connect to your firewall from the Internet using Secure Shell (SSH). Recall that SSH connects using TCP port 22. You would add the following rule to /etc/shorewall/rules:

#ACTION    SOURCE        DEST      PROTO      DPORT
ACCEPT     net           $FW       tcp        22

So although you have a policy of ignoring all connection attempts from the net zone (from the Internet), the above exception to that policy allows you to connect to the SSH server running on your firewall.

Because Shorewall makes no assumptions about what traffic you want accepted, there are certain rules (exceptions) that need to be added to almost any configuration.

Shorewall uses a "compile" then "execute" approach. The Shorewall configuration compiler reads the configuration files and generates a shell script. Errors in the compilation step cause the script to be discarded and the command to be aborted. If the compilation step doesn't find any errors then the shell script is executed.

The 'compiled' scripts are placed by default in the directory /var/lib/shorewall and are named to correspond to the command being executed. For example, the command /sbin/shorewall start will generate a script named /var/lib/shorewall/.start and, if the compilation is error free, that script will then be executed. If the script executes successfully, it then copies itself to /var/lib/shorewall/firewall. When an /sbin/shorewall stop or /sbin/shorewall clear command is subsequently executed, /var/lib/shorewall/firewall is run to perform the requested operation.

The AUTOMAKE option in /etc/shorewall/shorewall.conf may be set to automatically generate a new script when one of the configuration files is changed. When no file has changed since the last compilation, the /sbin/shorewall start, /sbin/shorewall reload and /sbin/shorewall restart commands will simply execute the current /var/lib/shorewall/firewall script.

Shorewall 4.5 and later consists of six packages.

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