**Shorewall** is an open source firewall tool for Linux that builds upon the Netfilter (iptables/ipchains) system built into the Linux kernel, making it easier to manage more complex configuration schemes by providing a higher level of abstraction for describing rules using text files.
The configuration files for Shorewall are contained in the directory /etc/shorewall
=== zones — Shorewall zone declaration file
The /etc/shorewall/zones file declares your network zones. You specify the hosts in each zone through entries in /etc/shorewall/interfaces
./etc/shorewall/zones
[source,bash]
----
#ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS
fw firewall
net ipv4
loc ipv4
----
=== interfaces — Shorewall interfaces file
The interfaces file serves to define the firewall's network interfaces to Shorewall.
./etc/shorewall/interfaces
[source,bash]
----
#ZONE INTERFACE BROADCAST OPTIONS
net eth0 dhcp,routefilter
loc eth1 detect
----
=== policy — Shorewall policy file
This file defines the high-level policy for connections between zone
./etc/shorewall/policy
[source,bash]
----
#SOURCE DEST POLICY LOGLEVEL LIMIT
loc net ACCEPT
net all DROP info
all all REJECT info
----
=== rules — Shorewall rules file
Entries in this file govern connection establishment by defining exceptions to the policies
./etc/shorewall/rules
[source,bash]
----
#ACTION SOURCE DEST PROTO DPORT
ACCEPT $FW net udp 53
ACCEPT net $FW udp 53
ACCEPT $FW net tcp 80
ACCEPT net $FW tcp 80
----
=== Compile then Execute
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.
./sbin/shorewall
[source,bash]
----
/sbin/shorewall start
/sbin/shorewall stop
/sbin/shorewall clear
----
.NOTE
[NOTE]
====
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.
====
== Three-Interface Firewall
image::dmz1.png[Three-Interface Firewall]
=== zones
./etc/shorewall/zones
[source,bash]
----
#ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS
fw firewall
net ipv4
loc ipv4
dmz ipv4 #new line
----
=== interfaces
./etc/shorewall/interfaces
[source,bash]
----
#ZONE INTERFACE BROADCAST OPTIONS
net eth0 dhcp,routefilter
loc eth1 detect
dmz eth2 detect #new line
----
=== policy
./etc/shorewall/policy
[source,bash]
----
#SOURCE DEST POLICY LOGLEVEL LIMIT
loc net ACCEPT
dmz net DROP #new line
net all DROP info
all all REJECT info
----
=== rules
./etc/shorewall/rules
[source,bash]
----
#ACTION SOURCE DEST PROTO DPORT
ACCEPT $FW net udp 53
ACCEPT net $FW udp 53
ACCEPT $FW net tcp 80
ACCEPT net $FW tcp 80
#New lines
ACCEPT $FW dmz udp 53
ACCEPT dmz $FW udp 53
ACCEPT $FW dmz tcp 80
ACCEPT dmz $FW tcp 80
ACCEPT loc dmz tcp 80 # Add your rules for the zones you have defined.
/etc/shorewall/masq - directs the firewall where to use many-to-one (dynamic) Network Address Translation (a.k.a. Masquerading) and Source Network Address Translation (SNAT).
This file is used to define dynamic NAT (Masquerading) and to define Source NAT (SNAT). It superseded shorewall-masq(5) in Shorewall 5.0.14.
./etc/shorewall/masq
[source,bash]
----
#ACTION SOURCE DEST
MASQUERADE 192.168.0.0/24 eth0
MASQUERADE 192.168.1.0/24 eth0
----
- You have a simple masquerading setup where eth0 connects to internet and eth1 connects to your local network with subnet 192.168.0.0/24.
- You add a router to your local network to connect subnet 192.168.1.0/24 which you also want to masquerade. You then add a second entry for eth0 to this file
[NOTE]
====
Beginning with that release, the Shorewall compiler will automatically convert existing masq files to the equivalent snat file, and rename the masq file to masq.bak.
