If you're going to use Shorewall just for being a firewall on machines with just one interface, it's a little bit of an overkill, in my opinion. Still, I do like to use Shorewall for this task anyway. Here's a short introduction that may help you to get started.
So, I'm assuming a single-interface IPv4-only configuration here. IPv4 and IPv6 versions of shorewall are independent but similar packages.
$ sudo apt-get install shorewall
After installing, everything is still disabled. First, let's copy over an example of a single interface configuration.
$ sudo cp /usr/share/doc/shorewall/examples/one-interface/* /etc/shorewall
Then let's configure the files one by one.
Tip: use the
*.annotated.gz files for more annotations in your configuration files if you fancy that.
zones - Leave as default. Configuring zones is mandatory, but since it's a single interface, it only contains one zone apart from the firewall itself (
interfaces - Configure your single interface there. Default configuration will probably be quite okay, and I only remove
dhcp as I'm not running DHCP on my servers.
policy - Defines policies for traffic between zones. As your number of zones is just one, this is easy too. By default, all traffic inbound is disallowed and all traffic outbound is allowed. Leave that default if you like that.
rules - This is where you configure exceptions to the policies defined. A simple example for allowing inbound TCP port 1234 from the whole outside IPv4 world would be this:
ACCEPT net $FW tcp 1234
Consider using some macros that help your config files more readable, like this one you probably want to set anyway:
SSH(ACCEPT) net:126.96.36.199 $FW
To allow SSH access from the IP address
shorewall.conf - get it by
gunzipping the shipped
shorewall.conf.gz file. Check that
IP_FORWARDING=Off if you don't do any forwarding and consider setting
DISABLE_IPV6=Yes if you are not planning on configuring IPv6 on the machine. Finally, set
Enable it for boot time too, in
Now check your current configuration by running
$ shorewall check
Please do this every time you changed some file and before you actually restart Shorewall. It prevents downtime in case of a syntax error as it compiles complains about wrong syntax.
If all is well, you may give it a try to actually start:
$ sudo service shorewall start
And feel free to inspect the result in
$ sudo iptables -L -n -v