After you've designed an appropriate firewall configuration, it's important to validate that it does in fact do what you want it to do. One way to do this is to use a test host outside your network to attempt to pierce your firewall: this can be quite clumsy and slow, though, and is limited to testing only those addresses that you can actually use.
A faster and easier method is available with the Linux firewall implementation. It allows you to manually generate tests and run them through the firewall configuration just as if you were testing with actual datagrams. All varieties of the Linux kernel firewall software, ipfwadm, ipchains, and iptables, provide support for this style of testing. The implementation involves use of the relevant check command.
The general test procedure is as follows:
Design and configure your firewall using ipfwadm, ipchains, or iptables.
Design a series of tests that will determine whether your firewall is actually working as you intend. For these tests you may use any source or destination address, so choose some address combinations that should be accepted and some others that should be dropped. If you're allowing or disallowing only certain ranges of addresses, it is a good idea to test addresses on either side of the boundary of the range—one address just inside the boundary and one address just outside the boundary. This will help ensure that you have the correct boundaries configured, because it is sometimes easy to specify netmasks incorrectly in your configuration. If you're filtering by protocol and port number, your tests should also check all important combinations of these parameters. For example, if you intend to accept only TCP under certain circumstances, check that UDP datagrams are dropped.
Develop ipfwadm, ipchains, or iptables rules to implement each test. It is probably worthwhile to write all the rules into a script so you can test and re-test easily as you correct mistakes or change your design. Tests use almost the same syntax as rule specifications, but the arguments take on slightly differing meanings. For example, the source address argument in a rule specification specifies the source address that datagrams matching this rule should have. The source address argument in test syntax, in contrast, specifies the source address of the test datagram that will be generated. For ipfwadm, you must use the –c option to specify that this command is a test, while for ipchains and iptables, you must use the –C option. In all cases you must always specify the source address, destination address, protocol, and interface to be used for the test. Other arguments, such as port numbers or TOS bit settings, are optional.
Execute each test command and note the output. The output of each test will be a single word indicating the final target of the datagram after running it through the firewall configuration—that is, where the processing ended. For ipchains and iptables, user-specified chains will be tested in addition to the built-in ones.
Compare the output of each test against the desired result. If there are any discrepancies, you will need to analyse your ruleset to determine where you've made the error. If you've written your test commands into a script file, you can easily rerun the test after correcting any errors in your firewall configuration. It's a good practice to flush your rulesets completely and rebuild them from scratch, rather than to make changes dynamically. This helps ensure that the active configuration you are testing actually reflects the set of commands in your configuration script.
Let's take a quick look at what a manual test transcript would look like for our na´ve example with ipchains. You will remember that our local network in the example was 172.16.1.0 with a netmask of 255.255.255.0, and we were to allow TCP connections out to web servers on the net. Nothing else was to pass our forward chain. Start with a transmission that we know should work, a connection from a local host to a web server outside:
# ipchains -C forward -p tcp -s 172.16.1.0 1025 -d 18.104.22.168 80 -i eth0 accepted
Note the arguments had to be supplied and the way they've been used to describe a datagram. The output of the command indicates that that the datagram was accepted for forwarding, which is what we hoped for.
Now try another test, this time with a source address that doesn't belong to our network. This one should be denied:
# ipchains -C forward -p tcp -s 172.16.2.0 1025 -d 22.214.171.124 80 -i eth0 denied
Try some more tests, this time with the same details as the first test, but with different protocols. These should be denied, too:
# ipchains -C forward -p udp -s 172.16.1.0 1025 -d 126.96.36.199 80 -i eth0 denied # ipchains -C forward -p icmp -s 172.16.1.0 1025 -d 188.8.131.52 80 -i eth0 denied
Try another destination port, again expecting it to be denied:
# ipchains -C forward -p tcp -s 172.16.1.0 1025 -d 184.108.40.206 23 -i eth0 denied
You'll go a long way toward achieving peace of mind if you design a series of exhaustive tests. While this can sometimes be as difficult as designing the firewall configuration, it's also the best way of knowing that your design is providing the security you expect of it.