What is Shorewall?

The Shoreline Firewall, more commonly known as "Shorewall", is a high-level tool for configuring Netfilter. You describe your firewall/gateway requirements using entries in a set of configuration files. Shorewall reads those configuration files and with the help of the iptables utility, Shorewall configures Netfilter to match your requirements. Shorewall can be used on a dedicated firewall system, a multi-function gateway/router/server or on a standalone GNU/Linux system. Shorewall does not use Netfilter's ipchains compatibility mode and can thus take advantage of Netfilter's connection state tracking capabilities.

First we need to login to the server via ssh
sentono@athena:/$

Install Shorewall

sentono@athena:/# sudo apt-get install shorewall
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
openbsd-inetd
Use 'apt-get autoremove' to remove them.
Suggested packages:
shorewall-doc
The following NEW packages will be installed:
shorewall
0 upgraded, 1 newly installed, 0 to remove and 26 not upgraded.
Need to get 0B/250kB of archives.
After unpacking 1241kB of additional disk space will be used.
Preconfiguring packages ...
Selecting previously deselected package shorewall.
(Reading database ... 16390 files and directories currently installed.)
Unpacking shorewall (from .../shorewall_3.2.6-2_all.deb) ...
Setting up shorewall (3.2.6-2) ...
#### WARNING ####
The firewall won't be started/stopped unless it is configured

Please read about Debian specific customization in
/usr/share/doc/shorewall/README.Debian.gz.
#################

Configure Shorewall Startup Service

sentono@athena:/#nano /etc/default/shorewall

#Now simply change the line below from 0 to 1

startup = 0
to
startup = 1

#save, and exit.

Shorewall configuration files are stored in two separate places
/etc/shorewall stores all the program configuration files.
/usr/share/shorewall stores supporting files and action files.

Configuring Shorewall

We need to copy all samples configuration file from /usr/share/doc/shorewall/default-config to /etc/shorewall

sentono@athena:/#cp /usr/share/doc/shorewall/default-config/* /etc/shorewall/

Now you have configuration files located at /etc/shorewall

Zones Configuration

First edit the zones file to specify the different network zones, these are just labels that you will use in the other files. Consider the Internet as one zone, and a private network as another zone. If you have this then the zones file would look like this:

sentono@athena:/$ nano /etc/shorewall/zones

# add 2 lines below into your zones file

net ipv4
loc ipv4

#save and exit

Interfaces Configuration

The next file to edit is the interfaces file to specify the interfaces on your machine. Here you will connect the zones that you defined in the previous step with an actual interface. The third field is the broadcast address for the network attached to the interface ("detect" will figure this out for you). Finally the last fields are options for the interface. The options listed below are a good starting point,

sentono@athena:/$ nano /etc/shorewall/interfaces

# add 2 lines below into interfaces file

net eth0 detect routefilter,norfc1918,logmartians,nosmurfs,tcpflags,blacklist
loc eth1 detect tcpflags

#save and exit

Policy Configuration

The next file defines your firewall default policy. The default policy is used if no other rules apply. Often you will set the default policy to REJECT or DROP as the default, and then configure specifically what ports/services are allowed in the next step, and any that you do not configure are by default rejected or dropped according to this policy. An example policy (based on the zones and interfaces we used above) would be:

sentono@athena:/$nano /etc/shorewall/policy

# add few lines below into policy files

fw net ACCEPT
fw loc ACCEPT
net all DROP info
# The FOLLOWING POLICY MUST BE LAST
all all REJECT info

# save and exit

This policy says: by default accept any traffic originating from the machine (fw) to the internet and to the local network. Anything that comes in from the internet destined to either the machine or the local network should be dropped and logged to the syslog level "info". The last line closes everything else off, and probably wont ever be touched. Note: DROP rules are dropped quietly, and REJECTs send something back letting the originator know they've been rejected.

Rules Configuration

The most important file is the rules. This is where you set what is allowed or not. Any new connection that comes into your firewall passes over these rules, if none of these apply, then the default policy will apply. Note: This is only for new connections, existing connections are automatically accepted. The comments in the file give you a good idea of how things work, but the following will provided an example that can give you a head-start:

sentono@athena:/$nano  /etc/shorewall/rules

# add few lines below into rules file

#ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL RATE USER/
# PORT PORT(S) DEST LIMIT GROUP
ACCEPT net fw icmp 8
ACCEPT fw net icmp
ACCEPT net fw tcp ssh,www,https,smtp,pop3,pop3s,imap2,imaps,submission
ACCEPT net fw udp https
# ACCEPT net:10.1.1.1 fw tcp ssh

# save and exit

This example can be written in long-hand as, "Accept any pings (icmp) from the internet to the machine, accept any tcp connections from the internet that are on any of the ports referenced in /etc/services for the services
ssh(22),www(80),https(443), etc. Also accept from the internet the udp connections to https(443). While you are at it, accept only tcp connections from the IP 10.1.1.1 coming from the internet to the ssh port (22).

Final step is start shorewall firewall

If there was a syntax error in your configuration you will get an error saying so and you should have a read of /var/log/shorewall-init.log to figure out why.

If everything does start up, you should make sure that you aren't blocking something that you don't mean to, you can do that by looking at your firewall logs.

Here is the result if someone try to attack the server

sentono@athena:/$tail -f /var/log/messages

Oct 9 15:52:06 athena kernel: [1274443.734684] Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:0c:29:61:de:33:00:d0:00:6b:54:00:08:00 src=218.232.95.60 DST=216.176.188.107 LEN=404 TOS=0x00 PREC=0x00 TTL=115 ID=43443 PROTO=UDP SPT=3664 DPT=1434 LEN=384
Oct 9 16:00:33 athena kernel: [1274950.625316] Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:0c:29:61:de:33:00:d0:00:6b:54:00:08:00 src=121.18.13.107 DST=216.176.188.107 LEN=40 TOS=0x00 PREC=0x00 TTL=113 ID=256 DF PROTO=TCP SPT=12200 DPT=7212 WINDOW=8192 RES=0x00 SYN URGP=0