Your Firewall Setup
Last Modified: 26 March, 2000
You've just finished implementing
your new, shiny firewall. Or perhaps you've just inherited several
new firewalls with the company merger. Either way, you are probably
curious as to whether or not they are implemented properly. Will
your firewalls keep the barbarians out there at bay? Does it meet
your expectations? This paper will help you find out. Here
you will find a guide on how conduct a firewall audit and your firewall
rulebase. Examples provided here are based on Check Point FireWall-1,
but should apply to most firewalls.
Where to Start
This paper can help you in one of two situations. First, you have
certain expectations of what your firewall can or cannot do and you want
to validate those expectations. Second, you do not know what to expect,
so you need to audit your firewall to learn more. Either way, this
paper can hopefully help you out. We are not going to cover how to
audit or "hack" a network, that is a different subject. Also,
we are not going to discuss which firewall is better then others, each
firewall has its own advantages and disadvantages. What is going
to make or break you is not choosing the "best" firewall, but implementing
it correctly. That is the purpose of this paper, making sure
our firewall is correctly implemented and behaves as we expected it.
Our first step in auditing our firewall is defining what we expect.
What do we want our firewall to do? Most of you should have this
already defined in the form of a security policy. Make sure you have
an understanding of these expectations before you verify your firewall
setup. That way, when you are done with the process, you can compare
the results to your expectations. Some of you may be in the situation
where you don't know what to expect. Maybe you are new to the company
and need to assess the situation. Or perhaps your company has just
merged and you have assumed responsibility of several new networks.
Regardless, try to define some goals before you start, what would you like
to see happen.
There are two parts to auditing your firewall setup. First, you
want to test the firewall itself. As a critical system in your security
plan, you want to ensure this is secure. Second, you want to test
the rulebase, what traffic can pass through the firewall? The whole
purpose of the firewall is to control traffic, you want to verify it is
doing its job.
The Firewall. To audit your firewall you want to ensure
that it is secure, that no one from the outside or the inside can access
or modify your firewall. First, you want to ensure it is physically
secured with controlled access. Once someone has physical access,
game over. Next, the operating system itself should be fully armored.
I recommend you review an armoring checklist specifically designed for
your operating system. You need to ensure the operating system fully
complies with the armoring checklist. You can find more information
about armoring and checklists here for linux,
next step is to port scan your firewall, from both your internal network
and the Internet, scanning for icmp, udp and tcp. We want to identify,
what, if any ports are open on your firewall. On most properly
configured firewalls, you should find no open ports, you should not even
be able to ping it.
A properly armored firewall should have few services to start with.
Once the firewall is running, no ports should be exposed unless they absolutely
have to. Many of you CheckPoint FireWall-1 users will get a
nasty surprise when you find several ports open, such as 256, 257, and
258. These ports are for administration , open by default in the
properties. I highly recommend you disable them. ICMP is
also open by default, I highly recommend you disable this also. If
ICMP is open, your network can easily be mapped from the Internet.
If you need these ports or services to administer your firewall, then set
up a rule that limits what source IPs can connect to them. The idea
in securing your firewall is to deny access whenever possible. Every
rulebase should have a lockdown rule at the beginning that denies any traffic
to the firewall. That way your firewall is "sealed" from the world.
If you need access to the firewall, have the rule go before the lockdown
rule. All other rules should go after the lockdown rule.
Many people consider this a "ghosting" rule, thinking it hides the firewall,
it doesn't. What it does do is protect your firewall, ensuring that
whatever other rules you put in later, your firewall will still be protected.
For example, in the demo
rulebase, if you put rule #3 first, now everyone on your internal network
has full access to your firewall. The lockdown rule, when placed
first, protects you against that. That is the whole purpose of scanning
your firewall, ensuring that you have not accidentally exposed your firewall
to unauthorized users. To learn more about rulebase design, check out
Building Your Firewall
The Rulebase. Once you have audited your firewall, you
want to audit your rulebase. The goal is to ensure that the
firewall is enforcing what you expect it to. This is done by scanning
every network segment from every other network segment. You want
to validate that the firewall is accepting ONLY the traffic that you allow.
Many firewalls have several network segments, such as protected DMZs.
Make sure you validate your rulebase by scanning from every one of these
segment. I highly recommend you place a system on your DMZ and attempt
to penetrate you internal network, as your DMZ is highly vulnerable.
This simulates if one of your DMZ systems is compromised (such as a DNS
or webserver) and that your internal network is still protected by the
firewall. Remeber, your firewall rulebase should deny everything,
allowing only that which is specifically allowed. The fewer services you
accept and the fewer rule you have, the more secure your environment.
If during your audit you are not sure if a service should be blocked, block
it. If no one complains, then it was not needed.
Authentication / Encryption: There are several other features
you want to test, specifically authentication and encryption. Often
firewalls are expected to authenticate users to access a resource.
FW-1 has several different authentication options, be sure to test them.
For example, if you expect users to be authenticated before they access
your website, confirm this for yourself. Try accessing the website
without authenticating and see what happens. It is extremely easy
to make a mistake when you implement a rulebase. What you thought
was password protected may be wide open to the world. Apply the same
test for encryption. If you have resources that should only be accessed
while encrypted, test it out. Try accessing the resources without
encryption, can you get there? Also, run a sniffer such as snoop
or tcpdump during the test. Make sure your data is actually being
encrypted. You need to verify your expectations. Are your resources
protected in the manner you expected?
Additional Services: Firewalls today, such as FW-1, can
work with third party software for additional services. For example,
virus scanning in email or web content filtering. If you are using
any of these 3rd party services with your firewall, you need to test them.
For example, for virus scanning, send an infected email through the firewall
to ensure your virus scanning is working. If it does not, you will
need to review your configuration and resolve the problem. Be sure
to re-test the configuration to ensure the fix works.
Digging Deeper: Once you have identified what resources
are available, you can begin to dig deeper. You've determined what
the firewall allows through, now what threat does that pose?
This is where things become fuzzy, where auditing your firewall setup can
become auditing your network. You are no longer auditing your firewall,
but auditing the resources behind the firewall. However, since this
information will be important to you, we will cover the basics .
The goal is to determine what potential vulnerabilities exist for the accessible
resources. I recommend reviewing each accessible resource and
identify what vulnerabilities exist. For example, you determine that
the firewall allows http access, in your case to several IIS webservers.
Now you have to determine what threats that poses (hint, there are alot!).
Or, you identify a system running ftpd, in your case wu-ftpd 2.4.2 VR17
(in this case, upgrade to the latest version). If a vulnerability
exists, you either have to fix the vulnerability, or decide if the risk
is worth the service. One of the best resources for identifying
vulnerabilities (both Unix and NT) is Bugtraq's vulnerability database
I highly recommend you review this database for every resource you have
accessible. There are also a variety of tools that will help you
identify what vulnerabilities exist. Find several tools you feel
the most comfortable with and use this.
Logging: After you have verified your firewall and rulebase,
review the firewall logs. Did the firewall detect all of your scans,
did it set off the expected alerts? What traffic did it log, and
how? If your firewall did not detect most of this activity, something
is wrong, you need to be able to see this information.
Also, by reviewing the rule base, you will have a better understanding
of what to look for in the future when auditing your logs. For FW-1,
I always recommend Track Long. If you are going to log the rule,
log it long so you get all the information. For more information
on logs and alerts with FW-1, check out Intrusion
Detection for FW-1.
Now comes the fun stuff, the tools. How do we accomplish what
we just discussed? One of the best tools for auditing your firewall
and firewall rulebase is a good port scanner. As you saw, the biggest
priority is identifying what resources are accessible. Personally,
I believe one of the best port scanners is Fyodor's
nmap. There are two reasons why I believe this. First,
no other open source port scanner has more options then nmap, including
OS detection, stealth scanning, rpc detection, etc. Second, and just
as importantly, this tool is a what many of the bad guys are using.
If the black-hat community is running this tool against your firewall,
so should you. If you run nmap, I highly recommend you try the Stealth
Scan option, such as "-sS" or "-sF" (NOTE: Test stealth scanning on a test remote host first. Some systems panic/crash from a stealth scan). Be sure to verify if your
firewall logs can detect the stealth scans (FW-1 ver 4.0 should detect
all nmap stealth scans). Another option I like is "-g", which lets
you set the source port. You can test for misconfigured rules that
allow packets based on source ports, such as ftp data (port 20), dns lookups
(port 53) or return http traffic (port 80). A new option is "-sA"
which is deisgned specificall to test firewall rulebases. One example
of how to run nmap is as follows:
mozart #nmap -v -sR -P0
-T Aggressive -o nmap.out
For more information on nmap options and usage, I highly recommend you
read the excellent
docs. For all your Linux users out there, it comes
in rpm format, so you just install and run. Now the bad news, it
only runs on unix.
For you NT users, don't despair, there are several Windows based options.
My personal favorite is WS
Ping ProPack. Not only does it include a port scanner, but a
variety of other great unix tools, such as whois, snmpwalk, etc.
Unfortunately, the port scanner is not as flexible as you would find in
most unix port scanners.
Once you have identified what resources can be accessed with your port
scanner, you can dig deeper. As discussed above, there are a variety
of methods and tools to digging deeper. All of these
tools are shareware/freeware, so no excuses. Here are several of
|Saint (runs on Unix)
||Updated version of SATAN, excellent all purpose vulnerability scanner.
|Nessus (runs on Unix, client can
run on 95/NT)
||Similar to SAINT, excellent vulnerability database.
(runs on anything that has PERL)
||Searches websites for vulnerabilities
||Deteremine what ports firewall allows through.
(runs on Unix)
||Build your own packets (TCP/UDP/ICMP).
|NetBIOS Auditing Tool (runs on Unix
||NetBIOS Scanning tool
(runs on 95/NT)
||Enumerates NetBIOS Shares, Users, Groups, and Services
(runs on 95/NT)
||From the guys at Rhino9, scans for smb shares
(runs on 95/NT)
||Similar to WS Ping ProPack, but with some different goodies
If you want to learn more about auditing tools, I recommend you check
tool database. Tool learn more about exploit tools, I recommend you
check out technotronic.com exploit
A firewall is only as good as it's implementation. In today's
dynamic world of Internet access, it is easy to make mistakes during the
implementation process. By auditing your firewall setup, you can
ensure that the firewall is enforcing what you expect it to, in a secure
Lance Spitzner enjoys learning by blowing up his Unix systems at
home. Before this, he was an Officer
in the Rapid Deployment Force, where he blew up things of a different
nature. You can reach him at firstname.lastname@example.org