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# Firewalls FAQ

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URL: http://www.interhack.net/pubs/fwfaq/
Version: 10.0
Archive-name: firewalls-faq
Posting-Frequency: monthly

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                             Internet Firewalls:

Matt Curtin       Marcus J. Ranum

cmcurtin@interhack.net   mjr@nfr.com

Date: 2000/12/01 19:48:21
Revision: 10.0

Contents

* Contents
o 1.2 For Whom Is the FAQ Written?
o 1.3 Before Sending Mail
o 1.4 Where Can I find the Current Version of the FAQ?
o 1.5 Where Can I Find Non-English Versions of the FAQ?
o 1.6 Contributors
* 2 Background and Firewall Basics
o 2.1 What is a network firewall?
o 2.2 Why would I want a firewall?
o 2.3 What can a firewall protect against?
o 2.4 What can't a firewall protect against?
o 2.6 Will IPSEC make firewalls obsolete?
o 2.7 What are good sources of print information on firewalls?
o 2.8 Where can I get more information on firewalls on the Internet?
* 3 Design and Implementation Issues
o 3.1 What are some of the basic design decisions in a firewall?
o 3.2 What are the basic types of firewalls?
+ 3.2.1 Network layer firewalls
+ 3.2.2 Application layer firewalls
o 3.3 What are proxy servers and how do they work?
o 3.4 What are some cheap packet screening tools?
o 3.5 What are some reasonable filtering rules for a kernel-based
packet screen?
+ 3.5.1 Implementation
+ 3.5.2 Explanation
o 3.6 What are some reasonable filtering rules for a Cisco?
+ 3.6.1 Implementation
+ 3.6.2 Explanations
+ 3.6.3 Shortcomings
o 3.7 What are the critical resources in a firewall?
o 3.8 What is a DMZ, and why do I want one?
o 3.9 How might I increase the security and scalability of my DMZ?
o 3.10 What is a single point of failure', and how do I avoid
having one?
o 3.11 How can I block all of the bad stuff?
o 3.12 How can I restrict web access so users can't view sites
unrelated to work?
* 4 Various Attacks
o 4.1 What is source routed traffic and why is it a threat?
o 4.2 What are ICMP redirects and redirect bombs?
o 4.3 What about denial of service?
o 4.4 What are some common attacks, and how can I protect my system
against them?
+ 4.4.1 SMTP Server Hijacking (Unauthorized Relaying)
+ 4.4.2 Exploiting Bugs in Applications
+ 4.4.3 Bugs in Operating Systems
* 5 How Do I...
o 5.1 Do I really want to allow everything that my users ask for?
o 5.2 How do I make Web/HTTP work through my firewall?
o 5.3 How do I make SSL work through the firewall?
o 5.4 How do I make DNS work with a firewall?
o 5.5 How do I make FTP work through my firewall?
o 5.6 How do I make Telnet work through my firewall?
o 5.7 How do I make Finger and whois work through my firewall?
o 5.8 How do I make gopher, archie, and other services work through
my firewall?
o 5.9 What are the issues about X11 through a firewall?
o 5.10 How do I make RealAudio work through my firewall?
o 5.11 How do I make my web server act as a front-end for a database
that lives on my private network?
o 5.12 But my database has an integrated web server, and I want to
use that. Can't I just poke a hole in the firewall and tunnel that
port?
o 5.13 How Do I Make IP Multicast Work With My Firewall?
* A Some Commercial Products and Vendors
* B Glossary of Firewall-Related Terms
* C TCP and UDP Ports
o C.1 What is a port?
o C.2 How do I know which application uses what port?
o C.3 What are LISTENING ports?
o C.4 How do I determine what service the port is for?
o C.5 What ports are safe to pass through a firewall?
o C.6 The behavior of FTP
o C.7 What software uses what FTP mode?
o C.8 Is my firewall trying to connect outside?
o C.9 The anatomy of a TCP connection
* References

The Firewalls FAQ is currently undergoing revision. The maintainers welcome
input and comments on the contents of this FAQ. Comments related to the FAQ
should be addressed to firewalls-faq@interhack.net. Before you send us mail,
please be sure to see sections 1.2 and 1.3 to make sure this is the right
document for you to be reading.

1.2 For Whom Is the FAQ Written?

Firewalls have come a long way from the days when this FAQ started.
They've gone from being highly customized systems administered by their
implementors to a mainstream commodity. Firewalls are no longer solely in
the hands of those who design and implement security systems; even
security-conscious end-users have them at home.

We wrote this FAQ for computer systems developers and administrators. We
have tried to be fairly inclusive, making room for the newcomers, but we
still assume some basic technical background. If you find that you don't
understand this document, but think that you need to know more about
firewalls, it might well be that you actually need to get more background in
computer networking first. We provide references that have helped us;

1.3 Before Sending Mail

Note that this collection of frequently-asked questions is a result of
interacting with many people of different backgrounds in a wide variety of
public fora. The firewalls-faq address is not a help desk. If you're trying
to use an application that says that it's not working because of a firewall
and you think that you need to remove your firewall, please do not send us

If you want to know how to get rid of your firewall'' because you cannot
use some application, do not send us mail asking for help. We cannot help
you. Really.

Who can help you? Good question. That will depend on what exactly the
problem is, but here are several pointers. If none of these works, please
don't ask us for any more. We don't know.

* The provider of the software you're using.
* The provider of the network service you're using. That is, if you're on
AOL, ask them. If you're trying to use something on a corporate

1.4 Where Can I find the Current Version of the FAQ?

The FAQ can be found on the Web at

* http://www.interhack.net/pubs/fwfaq/.
* http://www.ranum.com/pubs/fwfaq/

It's also posted monthly to

* comp.security.firewalls,
* comp.security.unix,
* comp.security.misc,

Posted versions are archived in all the usual places. Unfortunately, the
version posted to Usenet and archived from that version lack the pretty
pictures and useful hyperlinks found in the web version.

1.5 Where Can I Find Non-English Versions of the FAQ?

Several translations are available. (If you've done a translation and it's
not listed here, please write us so we can update the master document.)

Norwegian
Translation by Jon Haugsand
http://helmersol.nr.no/haandbok/doc/brannmur/brannmur-faq.html

1.6 Contributors

Many people have written helpful suggestions and thoughtful commentary.
We're grateful to all contributors. We'd like to thank a few by name:
Keinanen Vesa, Allen Leibowitz, Brent Chapman, Brian Boyle, D. Clyde
Williamson, Paul D. Robertson, Richard Reiner, Humberto Ortiz Zuazaga, and
Theodore Hope.

remain intact. Translations of the complete text from the original English
to other languages are also explicitly allowed. Translators may add their
names to the Contributors'' section.

2 Background and Firewall Basics

Before being able to understand a complete discussion of firewalls, it's
important to understand the basic principles that make firewalls work.

2.1 What is a network firewall?

A firewall is a system or group of systems that enforces an access control
policy between two networks. The actual means by which this is accomplished
varies widely, but in principle, the firewall can be thought of as a pair of
mechanisms: one which exists to block traffic, and the other which exists to
permit traffic. Some firewalls place a greater emphasis on blocking traffic,
while others emphasize permitting traffic. Probably the most important thing
to recognize about a firewall is that it implements an access control
policy. If you don't have a good idea of what kind of access you want to
allow or to deny, a firewall really won't help you. It's also important to
recognize that the firewall's configuration, because it is a mechanism for
enforcing policy, imposes its policy on everything behind it. Administrators
for firewalls managing the connectivity for a large number of hosts
therefore have a heavy responsibility.

2.2 Why would I want a firewall?

The Internet, like any other society, is plagued with the kind of jerks
who enjoy the electronic equivalent of writing on other people's walls with
spraypaint, tearing their mailboxes off, or just sitting in the street
blowing their car horns. Some people try to get real work done over the
Internet, and others have sensitive or proprietary data they must protect.
Usually, a firewall's purpose is to keep the jerks out of your network while
still letting you get your job done.

Many traditional-style corporations and data centers have computing security
policies and practices that must be adhered to. In a case where a company's
policies dictate how data must be protected, a firewall is very important,
since it is the embodiment of the corporate policy. Frequently, the hardest
part of hooking to the Internet, if you're a large company, is not
justifying the expense or effort, but convincing management that it's safe
to do so. A firewall provides not only real security--it often plays an
important role as a security blanket for management.

Lastly, a firewall can act as your corporate ambassador'' to the Internet.
Many corporations use their firewall systems as a place to store public
bug-fixes, and so forth. Several of these systems have become important
parts of the Internet service structure (e.g.: UUnet.uu.net, whitehouse.gov,
gatekeeper.dec.com) and have reflected well on their organizational

2.3 What can a firewall protect against?

Some firewalls permit only email traffic through them, thereby protecting
the network against any attacks other than attacks against the email
service. Other firewalls provide less strict protections, and block services
that are known to be problems.

Generally, firewalls are configured to protect against unauthenticated
interactive logins from the outside'' world. This, more than anything,
helps prevent vandals from logging into machines on your network. More
elaborate firewalls block traffic from the outside to the inside, but permit
users on the inside to communicate freely with the outside. The firewall can
protect you against any type of network-borne attack if you unplug it.

Firewalls are also important since they can provide a single choke point''
where security and audit can be imposed. Unlike in a situation where a
computer system is being attacked by someone dialing in with a modem, the
firewall can act as an effective phone tap'' and tracing tool. Firewalls
provide an important logging and auditing function; often they provide
summaries to the administrator about what kinds and amount of traffic passed
through it, how many attempts there were to break into it, etc.

This is an important point: providing this choke point'' can serve the
same purpose on your network as a guarded gate can for your site's physical
premises. That means anytime you have a change in zones'' or levels of
sensitivity, such a checkpoint is appropriate. A company rarely has only an
outside gate and no receptionist or security staff to check badges on the
way in. If there are layers of security on your site, it's reasonable to
expect layers of security on your network.

2.4 What can't a firewall protect against?

Firewalls can't protect against attacks that don't go through the
firewall. Many corporations that connect to the Internet are very concerned
about proprietary data leaking out of the company through that route.
Unfortunately for those concerned, a magnetic tape can just as effectively
be used to export data. Many organizations that are terrified (at a
management level) of Internet connections have no coherent policy about how
dial-in access via modems should be protected. It's silly to build a 6-foot
thick steel door when you live in a wooden house, but there are a lot of
organizations out there buying expensive firewalls and neglecting the
numerous other back-doors into their network. For a firewall to work, it
must be a part of a consistent overall organizational security architecture.
Firewall policies must be realistic and reflect the level of security in the
entire network. For example, a site with top secret or classified data
doesn't need a firewall at all: they shouldn't be hooking up to the Internet
in the first place, or the systems with the really secret data should be
isolated from the rest of the corporate network.

Another thing a firewall can't really protect you against is traitors or
idiots inside your network. While an industrial spy might export information
through your firewall, he's just as likely to export it through a telephone,
FAX machine, or floppy disk. Floppy disks are a far more likely means for
information to leak from your organization than a firewall! Firewalls also
cannot protect you against stupidity. Users who reveal sensitive information
over the telephone are good targets for social engineering; an attacker may
be able to break into your network by completely bypassing your firewall, if
he can find a helpful'' employee inside who can be fooled into giving
access to a modem pool. Before deciding this isn't a problem in your
organization, ask yourself how much trouble a contractor has getting logged
into the network or how much difficulty a user who forgot his password has
getting it reset. If the people on the help desk believe that every call is
internal, you have a problem.

Lastly, firewalls can't protect against tunneling over most application
protocols to trojaned or poorly written clients. There are no magic bullets
and a firewall is not an excuse to not implement software controls on
internal networks or ignore host security on servers. Tunneling bad''
things over HTTP, SMTP, and other protocols is quite simple and trivially
demonstrated. Security isn't fire and forget''.

Firewalls can't protect very well against things like viruses. There are
too many ways of encoding binary files for transfer over networks, and too
many different architectures and viruses to try to search for them all. In
other words, a firewall cannot replace security-consciousness on the part of
your users. In general, a firewall cannot protect against a data-driven
attack--attacks in which something is mailed or copied to an internal host
where it is then executed. This form of attack has occurred in the past
against various versions of sendmail, ghostscript, and scripting mail user
agents like OutLook.

Organizations that are deeply concerned about viruses should implement
organization-wide virus control measures. Rather than trying to screen
viruses out at the firewall, make sure that every vulnerable desktop has
virus scanning software that is run when the machine is rebooted. Blanketing
your network with virus scanning software will protect against viruses that
come in via floppy disks, modems, and Internet. Trying to block viruses at
the firewall will only protect against viruses from the Internet--and the
vast majority of viruses are caught via floppy disks.

Nevertheless, an increasing number of firewall vendors are offering virus
detecting'' firewalls. They're probably only useful for naive users
exchanging Windows-on-Intel executable programs and malicious-macro-capable
application documents. There are many firewall-based approaches for dealing
with problems like the ILOVEYOU'' worm and related attacks, but these are
really oversimplified approaches that try to limit the damage of something
that is so stupid it never should have occurred in the first place. Do not
count on any protection from attackers with this feature.

A strong firewall is never a substitute for sensible software that
recognizes the nature of what it's handling--untrusted data from an
unauthenticated party--and behaves appropriately. Do not think that because
everyone'' is using that mailer or because the vendor is a gargantuan
multinational company, you're safe. In fact, it isn't true that everyone''
is using any mailer, and companies that specialize in turning technology
invented elsewhere into something that's easy to use'' without any
expertise are more likely to produce software that can be fooled.

2.6 Will IPSEC make firewalls obsolete?

Some have argued that this is the case. Before pronouncing such a sweeping
prediction, however, it's worthwhile to consider what IPSEC is and what it
does. Once we know this, we can consider whether IPSEC will solve the
problems that we're trying to solve with firewalls.

IPSEC (IP SECurity) refers to a set of standards developed by the Internet
Engineering Task Force (IETF). There are many documents that collectively
define what is known as IPSEC'' [4]. IPSEC solves two problems which have
plagued the IP protocol suite for years: host-to-host authentication (which
will let hosts know that they're talking to the hosts they think they are)
and encryption (which will prevent attackers from being able to watch the
traffic going between machines).

Note that neither of these problems is what firewalls were created to solve.
Although firewalls can help to mitigate some of the risks present on an
Internet without authentication or encryption, there are really two classes
of problems here: integrity and privacy of the information flowing between
hosts and the limits placed on what kinds of connectivity is allowed between
different networks. IPSEC addresses the former class and firewalls the
latter.

What this means is that one will not eliminate the need for the other, but
it does create some interesting possibilities when we look at combining
firewalls with IPSEC-enabled hosts. Namely, such things as
vendor-independent virtual private networks (VPNs), better packet filtering
(by filtering on whether packets have the IPSEC authentication header), and
application-layer firewalls will be able to have better means of host
just trusting'' the IP address presented.

2.7 What are good sources of print information on firewalls?

There are several books that touch on firewalls. The best known are:

* Building Internet Firewalls, 2d ed.
Authors
Elizabeth D. Zwicky, Simon Cooper, and D. Brent Chapman
Publisher
O'Reilly
Edition
2000
ISBN
1-56592-871-7
* Firewalls and Internet Security: Repelling the Wily Hacker
Authors
Bill Cheswick and Steve Bellovin
Publisher
Edition
1994
ISBN
0-201-63357-4
* Practical Internet & Unix Security
Authors
Simson Garfinkel and Gene Spafford
Publisher
O'Reilly
Edition
1996
ISBN
1-56592-148-8
Note
Discusses primarily host security.

Related references are:

* Internetworking with TCP/IP Vols I, II, and III
Authors
Douglas Comer and David Stevens
Publisher
Prentice-Hall
Edition
1991
ISBN
0-13-468505-9 (I), 0-13-472242-6 (II), 0-13-474222-2 (III)
Comment
A detailed discussion on the architecture and implementation of
the Internet and its protocols. Volume I (on principles, protocols
and architecture) is readable by everyone. Volume 2 (on design,
implementation and internals) is more technical. Volume 3 covers
client-server computing.
* Unix System Security--A Guide for Users and System Administrators
Author
David Curry
Publisher
Edition
1992
ISBN
0-201-56327-4

2.8 Where can I get more information on firewalls on the Internet?

Firewalls Mailing List
http://lists.gnac.net/firewalls/ The internet firewalls mailing list is
a forum for firewall administrators and implementors. To subscribe to
Firewalls, send subscribe firewalls in the body of a message (not in
the Subject:'' line) to majordomo@lists.gnac.net
Firewall-Wizards Mailing List
http://www.nfr.net/forum/firewall-wizards.html The Firewall Wizards
Mailing List is a moderated firewall and security related list that is
more like a journal than a public soapbox.
Firewall HOWTO
http://sunsite.unc.edu/LDP/HOWTO/Firewall-HOWTO.html Describes exactly
what is needed to build a firewall, particularly using Linux.
Firewall Toolkit (FWTK) and Firewall Papers
ftp://ftp.tis.com/pub/firewalls/
Marcus Ranum's firewall related publications
http://www.ranum.com/pubs/
Papers on firewalls and breakins
ftp://ftp.research.att.com/dist/internet_security/
Texas A&M University security tools
http://www.net.tamu.edu/ftp/security/TAMU/
COAST Project Internet Firewalls page
http://www.cs.purdue.edu/coast/firewalls/

3 Design and Implementation Issues

3.1 What are some of the basic design decisions in a firewall?

There are a number of basic design issues that should be addressed by the
lucky person who has been tasked with the responsibility of designing,
specifying, and implementing or overseeing the installation of a firewall.

The first and most important decision reflects the policy of how your
company or organization wants to operate the system: is the firewall in
place explicitly to deny all services except those critical to the mission
of connecting to the Net, or is the firewall in place to provide a metered
and audited method of queuing'' access in a non-threatening manner? There
are degrees of paranoia between these positions; the final stance of your
firewall might be more the result of a political than an engineering
decision.

The second is: what level of monitoring, redundancy, and control do you
want? Having established the acceptable risk level (e.g., how paranoid you
are) by resolving the first issue, you can form a checklist of what should
be monitored, permitted, and denied. In other words, you start by figuring
out your overall objectives, and then combine a needs analysis with a risk
assessment, and sort the almost always conflicting requirements out into a
laundry list that specifies what you plan to implement.

The third issue is financial. We can't address this one here in anything but
vague terms, but it's important to try to quantify any proposed solutions in
terms of how much it will cost either to buy or to implement. For example, a
complete firewall product may cost between $100,000 at the high end, and free at the low end. The free option, of doing some fancy configuring on a Cisco or similar router will cost nothing but staff time and a few cups of coffee. Implementing a high end firewall from scratch might cost several man-months, which may equate to$30,000 worth of staff salary and benefits.
The systems management overhead is also a consideration. Building a
home-brew is fine, but it's important to build it so that it doesn't require
constant (and expensive) attention. It's important, in other words, to
evaluate firewalls not only in terms of what they cost now, but continuing
costs such as support.

On the technical side, there are a couple of decisions to make, based on the
fact that for all practical purposes what we are talking about is a static
traffic routing service placed between the network service provider's router
and your internal network. The traffic routing service may be implemented at
an IP level via something like screening rules in a router, or at an
application level via proxy gateways and services.

The decision to make is whether to place an exposed stripped-down machine on
the outside network to run proxy services for telnet, FTP, news, etc., or
whether to set up a screening router as a filter, permitting communication
with one or more internal machines. There are pluses and minuses to both
approaches, with the proxy machine providing a greater level of audit and
potentially security in return for increased cost in configuration and a
decrease in the level of service that may be provided (since a proxy needs
to be developed for each desired service). The old trade-off between
ease-of-use and security comes back to haunt us with a vengeance.

3.2 What are the basic types of firewalls?

Conceptually, there are two types of firewalls:

1.   Network layer
2.   Application layer

They are not as different as you might think, and latest technologies are
blurring the distinction to the point where it's no longer clear if either
one is better'' or worse.'' As always, you need to be careful to pick
the type that meets your needs.

Which is which depends on what mechanisms the firewall uses to pass traffic
from one security zone to another. The International Standards Organization
(ISO) Open Systems Interconnect (OSI) model for networking defines seven
layers, where each layer provides services that higher-level'' layers
depend on. In order from the bottom, these layers are physical, data link,
network, transport, session, presentation, application.

The important thing to recognize is that the lower-level the forwarding
mechanism, the less examination the firewall can perform. Generally
speaking, lower-level firewalls are faster, but are easier to fool into
doing the wrong thing.

3.2.1 Network layer firewalls

These generally make their decisions based on the source, destination
addresses and ports (see Appendix C for a more detailed discussion of ports)
in individual IP packets. A simple router is the traditional'' network
layer firewall, since it is not able to make particularly sophisticated
decisions about what a packet is actually talking to or where it actually
came from. Modern network layer firewalls have become increasingly
sophisticated, and now maintain internal information about the state of
connections passing through them, the contents of some of the data streams,
and so on. One thing that's an important distinction about many network
layer firewalls is that they route traffic directly though them, so to use
one you either need to have a validly assigned IP address block or to use a
private internet'' address block [3]. Network layer firewalls tend to be
very fast and tend to be very transparent to users.

Figure 1: Screened Host Firewall

[\begin{figure} \begin{center} \includegraphics {firewalls-faq1} \end{center}\end{figure}]

In Figure 1, a network layer firewall called a screened host firewall'' is
represented. In a screened host firewall, access to and from a single host
is controlled by means of a router operating at a network layer. The single
host is a bastion host; a highly-defended and secured strong-point that
(hopefully) can resist attack.

Figure 2: Screened Subnet Firewall

[\begin{figure} \begin{center} \includegraphics {firewalls-faq2} \end{center}\end{figure}]

Example Network layer firewall : In figure 2, a network layer firewall
called a screened subnet firewall'' is represented. In a screened subnet
firewall, access to and from a whole network is controlled by means of a
router operating at a network layer. It is similar to a screened host,
except that it is, effectively, a network of screened hosts.

3.2.2 Application layer firewalls

These generally are hosts running proxy servers, which permit no traffic
directly between networks, and which perform elaborate logging and auditing
of traffic passing through them. Since the proxy applications are software
components running on the firewall, it is a good place to do lots of logging
and access control. Application layer firewalls can be used as network
address translators, since traffic goes in one side'' and out the other,
after having passed through an application that effectively masks the origin
of the initiating connection. Having an application in the way in some cases
may impact performance and may make the firewall less transparent. Early
application layer firewalls such as those built using the TIS firewall
toolkit, are not particularly transparent to end users and may require some
training. Modern application layer firewalls are often fully transparent.
Application layer firewalls tend to provide more detailed audit reports and
tend to enforce more conservative security models than network layer
firewalls.

Figure 3: Dual Homed Gateway

[\begin{figure} \begin{center} \includegraphics {firewalls-faq3} \end{center}\end{figure}]

Example Application layer firewall : In figure 3, an application layer
firewall called a dual homed gateway'' is represented. A dual homed
gateway is a highly secured host that runs proxy software. It has two
network interfaces, one on each network, and blocks all traffic passing
through it.

The Future of firewalls lies someplace between network layer firewalls and
application layer firewalls. It is likely that network layer firewalls will
become increasingly aware'' of the information going through them, and
application layer firewalls will become increasingly low level'' and
transparent. The end result will be a fast packet-screening system that logs
and audits data as it passes through. Increasingly, firewalls (network and
application layer) incorporate encryption so that they may protect traffic
passing between them over the Internet. Firewalls with end-to-end encryption
can be used by organizations with multiple points of Internet connectivity
to use the Internet as a private backbone'' without worrying about their

3.3 What are proxy servers and how do they work?

A proxy server (sometimes referred to as an application gateway or
forwarder) is an application that mediates traffic between a protected
network and the Internet. Proxies are often used instead of router-based
traffic controls, to prevent traffic from passing directly between networks.
Many proxies contain extra logging or support for user authentication. Since
proxies must understand'' the application protocol being used, they can
also implement protocol specific security (e.g., an FTP proxy might be
configurable to permit incoming FTP and block outgoing FTP).

Proxy servers are application specific. In order to support a new protocol
via a proxy, a proxy must be developed for it. One popular set of proxy
servers is the TIS Internet Firewall Toolkit (FWTK'') which includes
proxies for Telnet, rlogin, FTP, X-Window, HTTP/Web, and NNTP/Usenet news.
SOCKS is a generic proxy system that can be compiled into a client-side
application to make it work through a firewall. Its advantage is that it's
easy to use, but it doesn't support the addition of authentication hooks or
http://www.socks.nec.com/.

3.4 What are some cheap packet screening tools?

The Texas AMU security tools include software for implementing screening
routers. Karlbridge is a PC-based screening router kit available from
ftp://ftp.net.ohio-state.edu/pub/kbridge/. A version of the Digital
Equipment Corporation screend'' kernel screening software is available for
BSD-derived operating systems. There are numerous kernel-level packet
screens, including ipf, ipfw, and ipfwadm. Typically, these are included in
various free Unix implementations, such as FreeBSD, OpenBSD, NetBSD, and
Linux. You might also find these tools available in your commercial Unix
implementation. If you're willing to get your hands a little dirty, it's
completely possible to build a secure and fully functional firewall for the
price of hardware and some of your time.

3.5 What are some reasonable filtering rules for a kernel-based packet
screen?

This example is written specifically for ipfwadm on Linux, but the
principles (and even much of the syntax) applies for other kernel interfaces
for packet screening on open source'' Unix systems.

There are four basic categories covered by the ipfwadm rules:

-A
Packet Accounting
-I
Input firewall
-O
Output firewall
-F
Forwarding firewall

switches and options, see the ipfwadm man page.

3.5.1 Implementation

Here, our organization is using a private (RFC 1918) Class C network
192.168.1.0. Our ISP has assigned us the address 201.123.102.32 for our
gateway's external interface and 201.123.102.33 for our external mail
server. Organizational policy says:

* Allow all outgoing TCP connections
* Allow incoming SMTP and DNS to external mail server
* Block all other traffic

The following block of commands can be placed in a system boot file (perhaps
rc.local on Unix systems).

ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 201.123.102.33 25
ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 201.123.102.33 53
ipfwadm -F -i m -b -P udp -S 0.0.0.0/0 1024:65535 -D 201.123.102.33 53
ipfwadm -F -a m -S 192.168.1.0/24 -D 0.0.0.0/0 -W eth0

/sbin/route add -host 201.123.102.33 gw 192.168.1.2

3.5.2 Explanation

* Line one flushes (-f) all forwarding (-F) rules.
* Line two sets the default policy (-p) to deny.
* Lines three through five are input rules (-i) in the following format:

ipfwadm -F (forward) -i (input) m (masq.) -b (bi-directional) -P
* Line six appends (-a) a rule that permits all internal IP addresses out
to all external addresses on all protocols, all ports.
* Line eight adds a route so that traffic going to 201.123.102.33 will be
directed to the internal address 192.168.1.2.

3.6 What are some reasonable filtering rules for a Cisco?

The example in figure 4 shows one possible configuration for using the
Cisco as filtering router. It is a sample that shows the implementation of
as specific policy. Your policy will undoubtedly vary.

Figure 4: Packet Filtering Router

[\begin{figure} \begin{center} \includegraphics {firewalls-faq4} \end{center}\end{figure}]

In this example, a company has Class C network address 195.55.55.0. Company
network is connected to Internet via IP Service Provider. Company policy is
are accepted. All incoming connections go through mailhost''. Mail and DNS
are only incoming services.

3.6.1 Implementation

* Allow all outgoing TCP-connections
* Allow incoming SMTP and DNS to mailhost
* Allow incoming FTP data connections to high TCP port (>1024)
* Try to protect services that live on high port numbers

Only incoming packets from Internet are checked in this configuration. Rules
are tested in order and stop when the first match is found. There is an
implicit deny rule at the end of an access list that denies everything. This
IP access list assumes that you are running Cisco IOS v. 10.3 or later.

no ip source-route
!
interface ethernet 0
!
interface serial 0
ip access-group 101 in
!
access-list 101 deny ip 127.0.0.0 0.255.255.255 any
access-list 101 deny ip 10.0.0.0 0.255.255.255 any
access-list 101 deny ip 172.16.0.0 0.15.255.255 any
access-list 101 deny ip 192.168.0.0 0.0.255.255 any
access-list 101 deny ip any 0.0.0.255 255.255.255.0
access-list 101 deny ip any 0.0.0.0 255.255.255.0
!
access-list 101 deny ip 195.55.55.0 0.0.0.255
access-list 101 permit tcp any any established
!
access-list 101 permit tcp any host 195.55.55.10 eq smtp
access-list 101 permit tcp any host 195.55.55.10 eq dns
access-list 101 permit udp any host 192.55.55.10 eq dns
!
access-list 101 deny tcp any any range 6000 6003
access-list 101 deny tcp any any range 2000 2003
access-list 101 deny tcp any any eq 2049
access-list 101 deny udp any any eq 2049
!
access-list 101 permit tcp any 20 any gt 1024
!
access-list 101 permit icmp any any
!
snmp-server community FOOBAR RO 2
line vty 0 4
access-class 2 in
access-list 2 permit 195.55.55.0 0.0.0.255

3.6.2 Explanations

* Drop all source-routed packets. Source routing can be used for address
spoofing.
* Drop directed broadcasts, which are used in smurf attacks.
* If an incoming packet claims to be from a local net, loopback network,
or private network, drop it.
* All packets which are part of already established TCP-connections can
pass through without further checking.
* All connections to low port numbers are blocked except SMTP and DNS.
* Block all services that listen for TCP connections on high port
numbers. X-windows (port 6000+), OpenWindows (port 2000+) are a few
candidates. NFS (port 2049) runs usually over UDP, but it can be run
over TCP, so you should block it.
* Incoming connections from port 20 into high port numbers are supposed
to be FTP data connections.
* All UDP traffic is blocked to protect RPC services

3.6.3 Shortcomings

* You cannot enforce strong access policies with router access lists.
Users can easily install backdoors to their systems to get over no
incoming telnet'' or no X'' rules. Also crackers install telnet
backdoors on systems where they break in.
* You can never be sure what services you have listening for connections
on high port numbers.
* Checking the source port on incoming FTP data connections is a weak
security method. It also breaks access to some FTP sites. It makes use
of the service more difficult for users without preventing bad guys

Use at least Cisco version 9.21 so you can filter incoming packets and check
for address spoofing. It's still better to use 10.3, where you get some
extra features (like filtering on source port) and some improvements on
filter syntax.

You have still a few ways to make your setup stronger. Block all incoming
TCP-connections and tell users to use passive-FTP clients. You can also
block outgoing ICMP echo-reply and destination-unreachable messages to hide
your network and to prevent use of network scanners. Cisco.com use to have
an archive of examples for building firewalls using Cisco routers, but it
doesn't seem to be online anymore. There are some notes on Cisco access
control lists, at least, at
ftp://ftp.cisco.com/pub/mibs/app_notes/access-lists.

3.7 What are the critical resources in a firewall?

It's important to understand the critical resources of your firewall
architecture, so when you do capacity planning, performance optimizations,
etc., you know exactly what you need to do, and how much you need to do it
in order to get the desired result.

What exactly the firewall's critical resources are tends to vary from site
to site, depending on the sort of traffic that loads the system. Some people
think they'll automatically be able to increase the data throughput of their
firewall by putting in a box with a faster CPU, or another CPU, when this
isn't necessarily the case. Potentially, this could be a large waste of
money that doesn't do anything to solve the problem at hand or provide the
expected scalability.

On busy systems, memory is extremely important. You have to have enough RAM
to support every instance of every program necessary to service the load
placed on that machine. Otherwise, the swapping will start and the
productivity will stop. Light swapping isn't usually much of a problem, but
if a system's swap space begins to get busy, then it's usually time for more
RAM. A system that's heavily swapping is often relatively easy to push over
the edge in a denial-of-service attack, or simply fall behind in processing
the load placed on it. This is where long email delays start.

Beyond the system's requirement for memory, it's useful to understand that
different services use different system resources. So the configuration that
you have for your system should be indicative of the kind of load you plan
to service. A 700 MHz processor isn't going to do you much good if all
you're doing is netnews and mail, and are trying to do it on an IDE disk
with an ISA controller.

Table 1: Critical Resources for Firewall
Services

Service     Critical Resource

Email       Disk I/O

Netnews     Disk I/O

Web         Host OS Socket Performance

IP Routing  Host OS Socket Performance

Web Cache   Host OS Socket Performance, Disk I/O

3.8 What is a DMZ, and why do I want one?

DMZ'' is an abbreviation for demilitarized zone''. In the context of
firewalls, this refers to a part of the network that is neither part of the
internal network nor directly part of the Internet. Typically, this is the
area between your Internet access router and your bastion host, though it
can be between any two policy-enforcing components of your architecture.

A DMZ can be created by putting access control lists on your access router.
This minimizes the exposure of hosts on your external LAN by allowing only
recognized and managed services on those hosts to be accessible by hosts on
the Internet. Many commercial firewalls simply make a third interface off of
the bastion host and label it the DMZ. The point is that the network is
neither inside'' nor outside''.

For example, a web server running on NT might be vulnerable to a number of
denial-of-service attacks against such services as RPC, NetBIOS and SMB.
These services are not required for the operation of a web server, so
blocking TCP connections to ports 135, 137, 138, and 139 on that host will
reduce the exposure to a denial-of-service attack. In fact, if you block
everything but HTTP traffic to that host, an attacker will only have one
service to attack.

This illustrates an important principle: never offer attackers more to work
with than is absolutely necessary to support the services you want to offer
the public.

3.9 How might I increase the security and scalability of my DMZ?

A common approach for an attacker is to break into a host that's
vulnerable to attack, and exploit trust relationships between the vulnerable
host and more interesting targets.

If you are running a number of services that have different levels of
security, you might want to consider breaking your DMZ into several
security zones''. This can be done by having a number of different
networks within the DMZ. For example, the access router could feed two
ethernets, both protected by ACLs, and therefore in the DMZ.

On one of the ethernets, you might have hosts whose purpose is to service
your organization's need for Internet connectivity. These will likely relay
mail, news, and host DNS. On the other ethernet could be your web server(s)
and other hosts that provide services for the benefit of Internet users.

In many organizations, services for Internet users tend to be less carefully
guarded and are more likely to be doing insecure things. (For example, in
the case of a web server, unauthenticated and untrusted users might be
running CGI or other executable programs. This might be reasonable for your
web server, but brings with it a certain set of risks that need to be
managed. It is likely these services are too risky for an organization to
run them on a bastion host, where a slip-up can result in the complete
failure of the security mechanisms.)

By putting hosts with similar levels of risk on networks together in the
DMZ, you can help minimize the effect of a breakin at your site. If someone
breaks into your web server by exploiting some bug in your web server,
they'll not be able to use it as a launching point to break into your
private network if the web servers are on a separate LAN from the bastion
hosts, and you don't have any trust relationships between the web server and
bastion host.

Now, keep in mind that we're running ethernet here. If someone breaks into
your web server, and your bastion host is on the same ethernet, an attacker
can install a sniffer on your web server, and watch the traffic to and from
your bastion host. This might reveal things that can be used to break into

Splitting services up not only by host, but by network, and limiting the
level of trust between hosts on those networks, you can greatly reduce the
likelihood of a breakin on one host being used to break into the other.
Succinctly stated: breaking into the web server in this case won't make it
any easier to break into the bastion host.

You can also increase the scalability of your architecture by placing hosts
on different networks. The fewer machines that there are to share the
available bandwidth, the more bandwidth that each will get.

3.10 What is a single point of failure', and how do I avoid having one?

An architecture whose security hinges upon one mechanism has a single
point of failure. Software that runs bastion hosts has bugs. Applications
have bugs. Software that controls routers has bugs. It makes sense to use
all of these components to build a securely designed network, and to use
them in redundant ways.

If your firewall architecture is a screened subnet, you have two packet
filtering routers and a bastion host. (See question 3.2 from this section.)
Your Internet access router will not permit traffic from the Internet to get
all the way into your private network. However, if you don't enforce that
rule with any other mechanisms on the bastion host and/or choke router, only
one component of your architecture needs to fail or be compromised in order
to get inside. On the other hand, if you have a redundant rule on the
bastion host, and again on the choke router, an attacker will need to defeat
three mechanisms.

Further, if the bastion host or the choke router needs to invoke its rule to
block outside access to the internal network, you might want to have it
trigger an alarm of some sort, since you know that someone has gotten

3.11 How can I block all of the bad stuff?

For firewalls where the emphasis is on security instead of connectivity,
you should consider blocking everything by default, and only specifically
allowing what services you need on a case-by-case basis.

If you block everything, except a specific set of services, then you've
security problem with everything product and service around, you only need
to worry about every security problem with a specific set of services and
products. :-)

Before turning on a service, you should consider a couple of questions:

* Is the protocol for this product a well-known, published protocol?
* Is the application to service this protocol available for public
inspection of its implementation?
* How well known is the service and product?
* How does allowing this service change the firewall architecture? Will
an attacker see things differently? Could it be exploited to get at my
internal network, or to change things on hosts in my DMZ?

When considering the above questions, keep the following in mind:

* Security through obscurity'' is no security at all. Unpublished
protocols have been examined by bad guys and defeated.
* Despite what the marketing representatives say, not every protocol or
service is designed with security in mind. In fact, the number that are
is very few.
* Even in cases where security is a consideration, not all organizations
have competent security staff. Among those who don't, not all are
willing to bring a competent consultant into the project. The end
result is that otherwise-competent, well-intended developers can design
insecure systems.
* The less that a vendor is willing to tell you about how their system
really works, the more likely it is that security (or other) problems
exist. Only vendors with something to hide have a reason to hide their
designs and implementations.

3.12 How can I restrict web access so users can't view sites unrelated to
work?

A few years ago, someone got the idea that it's a good idea to block
bad'' web sites, i.e., those that contain material that The Company views
inappropriate''. The idea has been increasing in popularity, but there are
several things to consider when thinking about implementing such controls in

* It is not possible to practically block everything that an employer
deems inappropriate''. The Internet is full of every sort of
material. Blocking one source will only redirect traffic to another
source of such material, or cause someone to figure a way around the
block.
* Most organizations do not have a standard for judging the
appropriateness of material that their employees bring to work, i.e.,
books, magazines, etc. Do you inspect everyone's briefcase for
inappropriate material'' every day? If you do not, then why would you
inspect every packet for inappropriate material''? Any decisions
along those lines in such an organization will be arbitrary. Attempting
to take disciplinary action against an employee where the only standard
is arbitrary typically isn't wise, for reasons well beyond the scope of
this document.
* Products that perform site-blocking, commercial and otherwise, are
typically easy to circumvent. Hostnames can be rewritten as IP
addresses. IP addresses can be written as a 32-bit integer value, or as
four 8-bit integers (the most common form). Other possibilities exist,
as well. Connections can be proxied. Web pages can be fetched via
email. You can't block them all. The effort that you'll spend trying to
implement and manage such controls will almost certainly far exceed any
level of damage control that you're hoping to have.

The rule-of-thumb to remember here is that you cannot solve social problems
with technical solutions. If there is a problem with someone going to an
inappropriate'' web site, that is because someone else saw it and was
offended by what he saw, or because that person's productivity is below
expectations. In either case, those are matters for the personnel

4 Various Attacks

4.1 What is source routed traffic and why is it a threat?

Normally, the route a packet takes from its source to its destination is
determined by the routers between the source and destination. The packet
itself only says where it wants to go (the destination address), and nothing
about how it expects to get there.

There is an optional way for the sender of a packet (the source) to include
information in the packet that tells the route the packet should take to get
to its destination; thus the name source routing''. For a firewall, source
routing is noteworthy, since an attacker can generate traffic claiming to be
from a system inside'' the firewall. In general, such traffic wouldn't
route to the firewall properly, but with the source routing option, all the
routers between the attacker's machine and the target will return traffic
along the reverse path of the source route. Implementing such an attack is
quite easy; so firewall builders should not discount it as unlikely to
happen.

In practice, source routing is very little used. In fact, generally the main
legitimate use is in debugging network problems or routing traffic over
specific links for congestion control for specialized situations. When
building a firewall, source routing should be blocked at some point. Most
commercial routers incorporate the ability to block source routing
specifically, and many versions of Unix that might be used to build firewall
bastion hosts have the ability to disable or ignore source routed traffic.

4.2 What are ICMP redirects and redirect bombs?

An ICMP Redirect tells the recipient system to over-ride something in its
routing table. It is legitimately used by routers to tell hosts that the
host is using a non-optimal or defunct route to a particular destination,
i.e. the host is sending it to the wrong router. The wrong router sends the
host back an ICMP Redirect packet that tells the host what the correct route
should be. If you can forge ICMP Redirect packets, and if your target host
pays attention to them, you can alter the routing tables on the host and
possibly subvert the security of the host by causing traffic to flow via a
path the network manager didn't intend. ICMP Redirects also may be employed
for denial of service attacks, where a host is sent a route that loses it
connectivity, or is sent an ICMP Network Unreachable packet telling it that
it can no longer access a particular network.

Many firewall builders screen ICMP traffic from their network, since it
limits the ability of outsiders to ping hosts, or modify their routing
tables.

Before you decide to completely block ICMP, you should be aware of how the
TCP protocol does Path MTU Discovery'', to make certain that you don't
break connectivity to other sites. If you can't safely block it everywhere,
you can consider allowing selected types of ICMP to selected routing
devices. If you don't block it, you should at least ensure that your routers
and hosts don't respond to broadcast ping packets.

4.3 What about denial of service?

Denial of service is when someone decides to make your network or firewall
useless by disrupting it, crashing it, jamming it, or flooding it. The
problem with denial of service on the Internet is that it is impossible to
prevent. The reason has to do with the distributed nature of the network:
every network node is connected via other networks which in turn connect to
other networks, etc. A firewall administrator or ISP only has control of a
few of the local elements within reach. An attacker can always disrupt a
connection upstream'' from where the victim controls it. In other words,
if someone wanted to take a network off the air, they could do it either by
taking the network off the air, or by taking the networks it connects to off
the air, ad infinitum. There are many, many, ways someone can deny service,
ranging from the complex to the brute-force. If you are considering using
Internet for a service which is absolutely time or mission critical, you
should consider your fall-back position in the event that the network is
down or damaged.

TCP/IP's UDP echo service is trivially abused to get two servers to flood a
network segment with echo packets. You should consider commenting out unused
entries in /etc/inetd.conf of Unix hosts, adding no ip small-servers to
Cisco routers, or the equivalent for your components.

4.4 What are some common attacks, and how can I protect my system against
them?

Each site is a little different from every other in terms of what attacks
are likely to be used against it. Some recurring themes do arise, though.

4.4.1 SMTP Server Hijacking (Unauthorized Relaying)

This is where a spammer will take many thousands of copies of a message and
send it to a huge list of email addresses. Because these lists are often so
bad, and in order to increase the speed of operation for the spammer, many
have resorted to simply sending all of their mail to an SMTP server that
will take care of actually delivering the mail.

Of course, all of the bounces, spam complaints, hate mail, and bad PR come
for the site that was used as a relay. There is a very real cost associated
with this, mostly in paying people to clean up the mess afterward.

The Mail Abuse Prevention System <URL:http://maps.vix.com/> Transport
Security Initiative <URL:http://maps.vix.com/tsi/> maintains a
complete description of the problem, and how to configure about every
mailer on the planet to protect against this attack.

4.4.2 Exploiting Bugs in Applications

Various versions of web servers, mail servers, and other Internet service
software contain bugs that allow remote (Internet) users to do things
ranging from gain control of the machine to making that application crash
and just about everything in between.

The exposure to this risk can be reduced by running only necessary services,
keeping up to date on patches, and using products that have been around a
while.

4.4.3 Bugs in Operating Systems

Again, these are typically initiated by users remotely. Operating systems
that are relatively new to IP networking tend to be more problematic, as
more mature operating systems have had time to find and eliminate their
bugs. An attacker can often make the target equipment continuously reboot,
crash, lose the ability to talk to the network, or replace files on the
machine.

Here, running as few operating system services as possible can help. Also,
having a packet filter in front of the operating system can reduce the
exposure to a large number of these types of attacks.

And, of course, chosing a stable operating system will help here as well.
When selecting an OS, don't be fooled into believing that the pricier, the
better''. Free operating systems are often much more robust than their
commercial counterparts

5 How Do I...

5.1 Do I really want to allow everything that my users ask for?

It's entirely possible that the answer is no''. Each site has its own
policies about what is and isn't needed, but it's important to remember that
a large part of the job of being an organization's gatekeeper is education.
Users want streaming video, real-time chat, and to be able to offer services
to external customers that require interaction with live databases on the
internal network.

That doesn't mean that any of these things can be done without presenting
more risk to the organization than the supposed value'' of heading down
that road is worth. Most users don't want to put their organization at risk.
things, too. It's important to look into what it is that they really want to
do, and to help them understand how they might be able to accomplish their
real objective in a more secure manner.

You won't always be popular, and you might even find yourself being given
direction to do something incredibly stupid, like just open up ports foo
through bar''. If that happens, don't worry about it. It would be wise to
keep all of your exchanges on such an event so that when a 12-year-old
script kiddie breaks in, you'll at least be able to separate yourself from
the whole mess.

5.2 How do I make Web/HTTP work through my firewall?

There are three ways to do it.

1.   Allow established'' connections out via a router, if you are using
screening routers.
2.   Use a web client that supports SOCKS, and run SOCKS on your bastion
host.
3.   Run some kind of proxy-capable web server on the bastion host. Some
options include Squid <URL:http://squid.nlanr.net/>, Apache
<URL:http://www.apache.org/docs/mod/mod_proxy.html>, Netscape
Proxy <URL:http://home.netscape.com/proxy/v3.5/index.html>, and
http-gw from the TIS firewall toolkit. Most of these can also proxy
other protocols (such as gopher and ftp), and can cache objects
fetched, which will also typically result in a performance boost for
the users, and more efficient use of your connection to the Internet.
Essentially all web clients (Mozilla, Internet Explorer, Lynx, etc.)
have proxy server support built directly into them.

5.3 How do I make SSL work through the firewall?

SSL is a protocol that allows secure connections across the Internet.
Typically, SSL is used to protect HTTP traffic. However, other protocols
(such as telnet) can run atop SSL.

Enabling SSL through your firewall can be done the same way that you would
allow HTTP traffic, if it's HTTP that you're using SSL to secure, which is
usually true. The only difference is that instead of using something that
will simply relay HTTP, you'll need something that can tunnel SSL. This is a
feature present on most web object caches.

You can find out more about SSL from Netscape
<URL:http://developer.netscape.com/docs/manuals/security/sslin/contents.htm>.

5.4 How do I make DNS work with a firewall?

Some organizations want to hide DNS names from the outside. Many experts
don't think hiding DNS names is worthwhile, but if site/corporate policy
mandates hiding domain names, this is one approach that is known to work.
Another reason you may have to hide domain names is if you have a
have no choice but to hide those addresses. Don't fool yourself into
thinking that if your DNS names are hidden that it will slow an attacker
down much if they break into your firewall. Information about what is on
your network is too easily gleaned from the networking layer itself. If you
on your LAN and then do an arp -a.'' Note also that hiding names in the
DNS doesn't address the problem of host names leaking'' out in mail

This approach is one of many, and is useful for organizations that wish to
hide their host names from the Internet. The success of this approach lies
on the fact that DNS clients on a machine don't have to talk to a DNS server
on that same machine. In other words, just because there's a DNS server on a
machine, there's nothing wrong with (and there are often advantages to)
redirecting that machine's DNS client activity to a DNS server on another
machine.

First, you set up a DNS server on the bastion host that the outside world
can talk to. You set this server up so that it claims to be authoritative
for your domains. In fact, all this server knows is what you want the
outside world to know; the names and addresses of your gateways, your
wildcard MX records, and so forth. This is the public'' server.

Then, you set up a DNS server on an internal machine. This server also
claims to be authoritative for your domains; unlike the public server, this
one is telling the truth. This is your normal'' nameserver, into which you
put all your normal'' DNS stuff. You also set this server up to forward
queries that it can't resolve to the public server (using a forwarders''
line in /etc/named.boot on a Unix machine, for example).

Finally, you set up all your DNS clients (the /etc/resolv.conf file on a
Unix box, for instance), including the ones on the machine with the public
server, to use the internal server. This is the key.

the internal server, which asks the public server, which asks the Internet,
and the answer is relayed back. A client on the public server works just the
same way. An external client, however, asking about an internal host gets
back the restricted'' answer from the public server.

This approach assumes that there's a packet filtering firewall between these
two servers that will allow them to talk DNS to each other, but otherwise
restricts DNS between other hosts.

Another trick that's useful in this scheme is to employ wildcard PTR records
any of your non-public hosts to return something like
unknown.YOUR.DOMAIN'' rather than an error. This satisfies anonymous FTP
sites like ftp.uu.net that insist on having a name for the machines they
talk to. This may fail when talking to sites that do a DNS cross-check in
which the host name is matched against its address and vice versa.

5.5 How do I make FTP work through my firewall?

Generally, making FTP work through the firewall is done either using a
proxy server such as the firewall toolkit's ftp-gw or by permitting incoming
connections to the network at a restricted port range, and otherwise
restricting incoming connections using something like established''
screening rules. The FTP client is then modified to bind the data port to a
port within that range. This entails being able to modify the FTP client
application on internal hosts.

In some cases, if FTP downloads are all you wish to support, you might want
to consider declaring FTP a dead protocol'' and letting you users download
files via the Web instead. The user interface certainly is nicer, and it
gets around the ugly callback port problem. If you choose the FTP-via-Web
approach, your users will be unable to FTP files out, which, depending on
what you are trying to accomplish, may be a problem.

A different approach is to use the FTP PASV'' option to indicate that the
remote FTP server should permit the client to initiate connections. The PASV
approach assumes that the FTP server on the remote system supports that
operation. (See Firewall-Friendly FTP'' [1].)

Other sites prefer to build client versions of the FTP program that are

5.6 How do I make Telnet work through my firewall?

Telnet is generally supported either by using an application proxy such as
the firewall toolkit's tn-gw, or by simply configuring a router to permit
outgoing connections using something like the established'' screening
rules. Application proxies could be in the form of a standalone proxy
running on the bastion host, or in the form of a SOCKS server and a modified
client.

5.7 How do I make Finger and whois work through my firewall?

Many firewall admins permit connections to the finger port from only
trusted machines, which can issue finger requests in the form of: finger
user@host.domain@firewall. This approach only works with the standard Unix
specific machines is managed using either tcp_wrappers or netacl from the
firewall toolkit. This approach will not work on all systems, since some
finger servers do not permit user@host@host fingering.

Many sites block inbound finger requests for a variety of reasons, foremost
being past security bugs in the finger server (the Morris internet worm made
these bugs famous) and the risk of proprietary or sensitive information
being revealed in user's finger information. In general, however, if your
users are accustomed to putting proprietary or sensitive information in
their .plan files, you have a more serious security problem than just a
firewall can solve.

5.8 How do I make gopher, archie, and other services work through my
firewall?

The majority of firewall administrators choose to support gopher and
archie through web proxies, instead of directly. Proxies such as the
firewall toolkit's http-gw convert gopher/gopher+ queries into HTML and vice
versa. For supporting archie and other queries, many sites rely on
Internet-based Web-to-archie servers, such as ArchiePlex. The Web's tendency
to make everything on the Internet look like a web service is both a
blessing and a curse.

There are many new services constantly cropping up. Often they are
misdesigned or are not designed with security in mind, and their designers
will cheerfully tell you if you want to use them you need to let port xxx
through your router. Unfortunately, not everyone can do that, and so a
number of interesting new toys are difficult to use for people behind
firewalls. Things like RealAudio, which require direct UDP access, are
particularly egregious examples. The thing to bear in mind if you find
yourself faced with one of these problems is to find out as much as you can
about the security risks that the service may present, before you just allow
it through. It's quite possible the service has no security implications.
It's equally possible that it has undiscovered holes you could drive a truck
through.

5.9 What are the issues about X11 through a firewall?

The X Windows System is a very useful system, but unfortunately has some
major security flaws. Remote systems that can gain or spoof access to a
copies of the contents of their windows, etc.

While attempts have been made to overcome them (E.g., MIT Magic Cookie'')
it is still entirely too easy for an attacker to interfere with a user's X
display. Most firewalls block all X traffic. Some permit X traffic through
application proxies such as the DEC CRL X proxy (FTP crl.dec.com). The
firewall toolkit includes a proxy for X, called x-gw, which a user can
invoke via the Telnet proxy, to create a virtual X server on the firewall.
When requests are made for an X connection on the virtual X server, the user
is presented with a pop-up asking them if it is OK to allow the connection.
While this is a little unaesthetic, it's entirely in keeping with the rest
of X.

5.10 How do I make RealAudio work through my firewall?

RealNetworks maintains some information about how to get RealAudio
working through your firewall <URL:http://www.real.com/firewall/>.  It
would be unwise to make any changes to your firewall without
understanding what the changes will do, exactly, and knowing what
risks the new changes will bring with them.

5.11 How do I make my web server act as a front-end for a database that
lives on my private network?

The best way to do this is to allow very limited connectivity between your
web server and your database server via a specific protocol that only
supports the level of functionality you're going to use. Allowing raw SQL,
or anything else where custom extractions could be performed by an attacker
isn't generally a good idea.

Assume that an attacker is going to be able to break into your web server,
and make queries in the same way that the web server can. Is there a
mechanism for extracting sensitive information that the web server doesn't
need, like credit card information? Can an attacker issue an SQL select and

E-commerce'' applications, like everything else, are best designed with
security in mind from the ground up, instead of having security added'' as
an afterthought. Review your architecture critically, from the perspective
of an attacker. Assume that the attacker knows everything about your
architecture. Now ask yourself what needs to be done to steal your data, to
make unauthorized changes, or to do anything else that you don't want done.
You might find that you can significantly increase security without
decreasing functionality by making a few design and implementation
decisions.

Some ideas for how to handle this:

* Extract the data you need from the database on a regular basis so
you're not making queries against the full database, complete with
information that attackers will find interesting.
* Greatly restrict and audit what you do allow between the web server and
database.

5.12 But my database has an integrated web server, and I want to use that.
Can't I just poke a hole in the firewall and tunnel that port?

If your site firewall policy is sufficiently lax that you're willing to
manage the risk that someone will exploit a vulnerability in your web server
that will result in partial or complete exposure of your database, then
there isn't much preventing you from doing this.

However, in many organizations, the people who are responsible for tying the
web front end to the database back end simply do not have the authority to
take that responsibility. Further, if the information in the database is
about people, you might find yourself guilty of breaking a number of laws if
you haven't taken reasonable precautions to prevent the system from being
abused.

In general, this isn't a good idea. See question 5.11 for some ideas on
other ways to accomplish this objective.

5.13 How Do I Make IP Multicast Work With My Firewall?

IP multicast is a means of getting IP traffic from one host to a set of
hosts without using broadcasting; that is, instead of every host getting the
traffic, only those that want it will get it, without each having to
maintain a separate connection to the server. IP unicast is where one host
talks to another, multicast is where one host talks to a set of hosts, and
broadcast is where one host talks to all hosts.

The public Internet has a multicast backbone (MBone'') where users can
engage in multicast traffic exchange. Common uses for the MBone are streams
of IETF meetings and similar such interaction. Getting one's own network
connected to the MBone will require that the upstream provider route
network will have to support multicast routing.

The role of the firewall in multicast routing, conceptually, is no different
from its role in other traffic routing. That is, a policy that identifies
which multicast groups are and aren't allowed must be defined and then a
system of allowing that traffic according to policy must be devised. Great
detail on how exactly to do this is beyond the scope of this document.
Fortunately, RFC 2588 [2] discusses the subject in more detail. Unless your
firewall product supports some means of selective multicast forwarding or
you have the ability to put it in yourself, you might find forwarding
multicast traffic in a way consistent with your security policy to be a

A Some Commercial Products and Vendors

We feel this topic is too sensitive to address in a FAQ, however, an
independently maintained list (no warranty or recommendations are implied)
can be found online.  <URL:http://www.thegild.com/firewall/>

B Glossary of Firewall-Related Terms

Abuse of Privilege
When a user performs an action that they should not have, according to
organizational policy or law.

Access Control Lists
Rules for packet filters (typically routers) that define which packets
to pass and which to block.

Access Router
A router that connects your network to the external Internet.
Typically, this is your first line of defense against attackers from
the outside Internet. By enabling access control lists on this router,
you'll be able to provide a level of protection for all of the hosts
behind'' that router, effectively making that network a DMZ instead
of an unprotected external LAN.

Application-Layer Firewall
A firewall system in which service is provided by processes that
maintain complete TCP connection state and sequencing. Application
layer firewalls often re-address traffic so that outgoing traffic
appears to have originated from the firewall, rather than the internal
host.

Authentication
The process of determining the identity of a user that is attempting to
access a system.

Authentication Token
A portable device used for authenticating a user. Authentication tokens
operate by challenge/response, time-based code sequences, or other
techniques. This may include paper-based lists of one-time passwords.

Authorization
The process of determining what types of activities are permitted.
Usually, authorization is in the context of authentication: once you
have authenticated a user, they may be authorized different types of
access or activity.

Bastion Host
A system that has been hardened to resist attack, and which is
installed on a network in such a way that it is expected to potentially
come under attack. Bastion hosts are often components of firewalls, or
may be outside'' web servers or public access systems. Generally, a
bastion host is running some form of general purpose operating system
(e.g., Unix, VMS, NT, etc.) rather than a ROM-based or firmware
operating system.

Challenge/Response
An authentication technique whereby a server sends an unpredictable
challenge to the user, who computes a response using some form of
authentication token.

Chroot
A technique under Unix whereby a process is permanently restricted to
an isolated subset of the filesystem.

Cryptographic Checksum
A one-way function applied to a file to produce a unique
fingerprint'' of the file for later reference. Checksum systems are a
primary means of detecting filesystem tampering on Unix.

Data Driven Attack
A form of attack in which the attack is encoded in innocuous-seeming
data which is executed by a user or other software to implement an
attack. In the case of firewalls, a data driven attack is a concern
since it may get through the firewall in data form and launch an attack
against a system behind the firewall.

Defense in Depth
The security approach whereby each system on the network is secured to
the greatest possible degree. May be used in conjunction with
firewalls.

DNS spoofing
Assuming the DNS name of another system by either corrupting the name
service cache of a victim system, or by compromising a domain name
server for a valid domain.

Dual Homed Gateway
A dual homed gateway is a system that has two or more network
interfaces, each of which is connected to a different network. In
firewall configurations, a dual homed gateway usually acts to block or
filter some or all of the traffic trying to pass between the networks.

Encrypting Router
see Tunneling Router and Virtual Network Perimeter.

Firewall
A system or combination of systems that enforces a boundary between two
or more networks.

Host-based Security
The technique of securing an individual system from attack. Host based
security is operating system and version dependent.

Insider Attack
An attack originating from inside a protected network.

Intrusion Detection
Detection of break-ins or break-in attempts either manually or via
software expert systems that operate on logs or other information
available on the network.

IP Spoofing
An attack whereby a system attempts to illicitly impersonate another
system by using its IP network address.

IP Splicing / Hijacking
An attack whereby an active, established, session is intercepted and
co-opted by the attacker. IP Splicing attacks may occur after an
authentication has been made, permitting the attacker to assume the
role of an already authorized user. Primary protections against IP
Splicing rely on encryption at the session or network layer.

Least Privilege
Designing operational aspects of a system to operate with a minimum
amount of system privilege. This reduces the authorization level at
which various actions are performed and decreases the chance that a
process or user with high privileges may be caused to perform
unauthorized activity resulting in a security breach.

Logging
The process of storing information about events that occurred on the
firewall or network.

Log Retention
How long audit logs are retained and maintained.

Log Processing
How audit logs are processed, searched for key events, or summarized.

Network-Layer Firewall
A firewall in which traffic is examined at the network protocol packet
layer.

Perimeter-based Security
The technique of securing a network by controlling access to all entry
and exit points of the network.

Policy
Organization-level rules governing acceptable use of computing
resources, security practices, and operational procedures.

Proxy
A software agent that acts on behalf of a user. Typical proxies accept
a connection from a user, make a decision as to whether or not the user
or client IP address is permitted to use the proxy, perhaps does
additional authentication, and then completes a connection on behalf of
the user to a remote destination.

Screened Host
A host on a network behind a screening router. The degree to which a
screened host may be accessed depends on the screening rules in the
router.

Screened Subnet
A subnet behind a screening router. The degree to which the subnet may
be accessed depends on the screening rules in the router.

Screening Router
A router configured to permit or deny traffic based on a set of
permission rules installed by the administrator.

Session Stealing
See IP Splicing.

Trojan Horse
A software entity that appears to do something normal but which, in
fact, contains a trapdoor or attack program.

Tunneling Router
A router or system capable of routing traffic by encrypting it and
encapsulating it for transmission across an untrusted network, for
eventual de-encapsulation and decryption.

Social Engineering
An attack based on deceiving users or administrators at the target
site. Social engineering attacks are typically carried out by
telephoning users or operators and pretending to be an authorized user,

Virtual Network Perimeter
A network that appears to be a single protected network behind
firewalls, which actually encompasses encrypted virtual links over
untrusted networks.

Virus
A replicating code segment that attaches itself to a program or data
file. Viruses might or might not not contain attack programs or
trapdoors. Unfortunately, many have taken to calling any malicious code
a virus''. If you mean trojan horse'' or worm'', say trojan
horse'' or worm''.

Worm
A standalone program that, when run, copies itself from one host to
another, and then runs itself on each newly infected host. The widely
reported Internet Virus'' of 1988 was not a virus at all, but
actually a worm.

C TCP and UDP Ports

by Mikael Olsson

This appendix will begin at a fairly basic'' level, so even if the first
points seem childishly self-evident to you, you might still learn something
from skipping ahead to something later in the text.

C.1 What is a port?

A port'' is virtual slot'' in your TCP and UDP stack that is used to
map a connection between two hosts, and also between the TCP/UDP layer and
the actual applications running on the hosts.

They are numbered 0-65535, with the range 0-1023 being marked as
reserved'' or privlileged'', and the rest (1024-65535) as dynamic'' or
unprivileged''.

There are basically two uses for ports:

* Listening'' on a port.
This is used by server applications waiting for users to connect, to
get to some well known service'', for instance HTTP (TCP port 80),
Telnet (TCP port 21), DNS (UDP and sometimes TCP port 53).
* Opening a dynamic'' port.
Both sides of a TCP connection need to be identified by IP addresses
and port numbers. Hence, when you want to connect'' to a server
process, your end of the communications channel also needs a port''.
This is done by choosing a port above 1024 on your machine that is not
currently in use by another communications channel, and using it as the
sender'' in the new connection.

Dynamic ports may also be used as listening'' ports in some applications,
most notably FTP.

Ports in the range 0-1023 are almost always server ports. Ports in the range
1024-65535 are usually dynamic ports (i.e., opened dynamically when you
connect to a server port). However, any port may be used as a server port,
and any port may be used as an outgoing'' port.

So, to sum it up, here's what happens in a basic connection:

* At some point in time, a server application on host 1.2.3.4 decides to
listen'' at port 80 (HTTP) for new connections.
* You (5.6.7.8) want to surf to 1.2.3.4, port 80, and your browser issues
a connect call to it.
* The connect call, realising that it doesn't yet have local port number,
goes hunting for one. The local port number is necessary since when the
replies come back some time in the future, your TCP/IP stack will have
to know to what application to pass the reply. It does this by
remembering what application uses which local port number. (This is
grossly simplified, no flames from programmers, please.)
* Your TCP stack finds an unused dynamic port, usually somewhere above
1024. Let's assume that it finds 1029.
* Your first packet is then sent, from your local IP, 5.6.7.8, port 1029,
to 1.2.3.4, port 80.
* The server responds with a packet from 1.2.3.4, port 80, to you,
5.6.7.8, port 1029.
* This procedure is actually longer than this, read on for a more
in-depth explanation of TCP connect sequences.

C.2 How do I know which application uses what port?

There are several lists outlining the reserved'' and well known''
ports, as well as commonly used'' ports, and the best one is:
ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers. For those of you
still reading RFC 1700 to find out what port number does what, STOP DOING
IT. It is horribly out of date, and it won't be less so tomorrow. Now, as
for trusting this information: These lists do not, in any way, constitute
any kind of holy bible on which ports do what.

Wait, let me rephrase that: THERE IS NO WAY OF RELIABLY DETERMINING WHAT
PORT DOES WHAT SIMPLY BY LOOKING IN A LIST.

C.3 What are LISTENING ports?

Suppose you did netstat -a'' on your machine and ports 1025 and 1030
showed up as LISTENing. What do they do?

Right, let's take a look in the assigned port numbers list.

blackjack       1025/tcp   network blackjack

Wait, what's happening? Has my workstation stolen my VISA number and decided
to go play blackjack with some rogue server on the internet? And what's that
software that BBN has installed?

This is NOT where you start panicking and send mail to the firewalls list.
In fact, this question has been asked maybe a dozen times during the past
six months, and every time it's been answered. Not that THAT keeps people
from asking the same question again.

If you are asking this question, you are most likely using a windows box.
The ports you are seeing are (most likely) two listening ports that the RPC
subsystem opens when it starts up.

This is an example of where dynamicly assigned ports may be used by server
processes. Applications using RPC will later on connect to port 135 (the
netbios portmapper'') to query where to find some RPC service, and get an
answer back saying that that particular service may be contacted on port
1025.

Now, how do we know this, since there's no list'' describing these ports?
Simple: There's no substitute for experience. And using the mailing list
search engines also helps a hell of a lot.

C.4 How do I determine what service the port is for?

Since it is impossible to learn what port does what by looking in a list,
how do i do it?

The old hands-on way of doing it is by shutting down nearly every
service/daemon running on your machine, doing netstat -a and taking note of
what ports are open. There shouldn't be very many listening ones. Then you
start turning all the services on, one by one, and take note of what new
ports show up in your netstat output.

Another way, that needs more guess work, is simply telnetting to the ports
and see what comes out. If nothing comes out, try typing some gibberish and
slamming Enter a few times, and see if something turns up. If you get binary

However, this will only tell you what listening ports are used. It won't
tell you about dynamically opened ports that may be opened later on by these
applications.

There are a few applications that might help you track down the ports used.

On Unix systems, there's a nice utility called lsof that comes preinstalled
on many systems. It will show you all open port numbers and the names of the
applications that are using them. This means that it might show you a lot of
locally opened files aswell as TCP/IP sockets. Read the help text. :-)

On windows systems, nothing comes preinstalled to assist you in this task.
(What's new?) There's a utility called Inzider'' which installs itself
inside the windows sockets layer and dynamically remembers which process
opens which port. The drawback of this approach is that it can't tell you
what ports were opened before inzider started, but it's the best that you'll
get on windows (to my knowledge). http://ntsecurity.nu/toolbox/inzider/.

C.5 What ports are safe to pass through a firewall?

ALL.

No, wait, NONE.

No, wait, uuhhh... I've heard that all ports above 1024 are safe since
they're only dynamic??

No. Really. You CANNOT tell what ports are safe simply by looking at its
number, simply because that is really all it is. A number. You can't mount
an attack through a 16-bit number.

The security of a port'' depends on what application you'll reach through
that port.

A common misconception is that ports 25 (SMTP) and 80 (HTTP) are safe to
pass through a firewall. *meep* WRONG. Just because everyone is doing it
doesn't mean that it is safe.

Again, the security of a port depends on what application you'll reach
through that port.

If you're running a well-written web server, that is designed from the
ground up to be secure, you can probably feel reasonably assured that it's
safe to let outside people access it through port 80. Otherwise, you CAN'T.

The problem here is not in the network layer. It's in how the application
processes the data that it receives. This data may be received through port
80, port 666, a serial line, floppy or through singing telegram. If the
application is not safe, it does not matter how the data gets to it. The
application data is where the real danger lies.

If you are interested in the security of your application, go
subscribe to bugtraq <URL:http://www.securityfocus.com> or try
searching their archives.

This is more of an application security issue rather than a firewall
security issue. One could argue that a firewall should stop all possible
attacks, but with the number of new network protocols, NOT designed with
security in mind, and networked applications, neither designed with security
in mind, it becomes impossible for a firewall to protect against all
data-driven attacks.

C.6 The behavior of FTP

Or, Why do I have to open all ports above 1024 to my FTP server?''

FTP doesn't really look a whole lot like other applications from a
networking perspective.

It keeps one listening port, port 21, which users connect to. All it does is
let people log on, and establish ANOTHER connection to do actual data
transfers. This second connection is usually on some port above 1024.

There are two modes, active'' (normal) and passive'' mode. This word
describes the server's behaviour.

In active mode, the client (5.6.7.8) connects to port 21 on the server
(1.2.3.4) and logs on. When file transfers are due, the client allocates a
dynamic port above 1024, informs the server about which port it opened, and
then the server opens a new connection to that port. This is the active''
role of the server: it actively establishes new connections to the client.

In passive mode, the connection to port 21 is the same. When file transfers
are due, the SERVER allocates a dynamic port above 1024, informs the client
about which port it opened, and then the CLIENT opens a new connection to
that port. This is the passive'' role of the server: it waits for the
client to establish the second (data) connection.

If your firewall doesn't inspect the application data of the FTP command
connection, it won't know that it needs to dynamically open new ports above
1024.

On a side note: The traditional behaviour of FTP servers in active mode is
to establish the data session FROM port 20, and to the dynamic port on the
client. FTP servers are steering away from this behaviour somewhat due to
the need to run as root'' on unix systems in order to be able to allocate
ports below 1024. Running as root'' is not good for security, since if
there's a bug in the software, the attacker would be able to compromise the
entire machine. The same goes for running as Administrator'' or SYSTEM''
(LocalSystem'') on NT machines, although the low port problem does not
apply on NT.

To sum it up, if your firewall understands FTP, it'll be able to handle the
data connections by itself, and you won't have to worry about ports above
1024.

If it does NOT, there are four issues that you need to address:

* Firewalling an FTP server in active mode
You need to let your server open new connections to the outside world
on ports 1024 and above
* Firewalling an FTP server in passive mode
You need to let the outside world connect to ports 1024 and above on
your server. CAUTION!!!! There may be applications running on some of
these ports that you do NOT want outside people using. Disallow access
* Firewalling FTP clients in active mode
You need to let the outside world connect to ports 1024 and above on
your clients. CAUTION!!!! There may be applications running on some of
these ports that you do NOT want outside people using. Disallow access
* Firewalling FTP clients in passive mode
You need to let your clients open new connections to the outside world
on ports 1024 and above.

Again, if your firewall understands FTP, none of the four points above apply
to you. Let the firewall do the job for you.

C.7 What software uses what FTP mode?

It is up to the client to decide what mode to use; the default mode when a
new connection is opened is active mode''.

Most FTP clients come preconfigured to use active mode, but provide an
option to use passive'' (PASV'') mode. An exception is the windows
command line FTP client which only operates in active mode.

Web Browsers generally use passive mode when connecting via FTP, with a
weird exception: MSIE 5 will use active FTP when FTP:ing in File
Explorer'' mode and passive FTP when FTP:ing in Web Page'' mode. There is
no reason whatsoever for this behaviour; my guess is that someone in Redmond
with no knowledge of FTP decided that Of course we'll use active mode when
we're in file explorer mode, since that looks more active than a web page''.
Go figure.

C.8 Is my firewall trying to connect outside?

My firewall logs are telling me that my web server is trying to connect
from port 80 to ports above 1024 on the outside. What is this?!

If you are seeing dropped packets from port 80 on your web server (or from
port 25 on your mail server) to high ports on the outside, they usually DO
NOT mean that your web server is trying to connect somewhere.

They are the result of the firewall timing out a connection, and seeing the
server retransmitting old responses (or trying to close the connection) to
the client.

TCP connections always involve packets traveling in BOTH directions in the
connection.

If you are able to see the TCP flags in the dropped packets, you'll see that
the ACK flag is set but not the SYN flag, meaning that this is actually not
a new connection forming, but rather a response of a previously formed
connection.

Read point 8 below for an in-depth explanation of what happens when TCP
connections are formed (and closed)

C.9 The anatomy of a TCP connection

TCP is equipped with 6 flags'', which may be ON or OFF. These flags are:

FIN
Controlled'' connection close
SYN
Open new connection
RST
Immediate'' connection close
PSH
Instruct receiver host to push the data up to the application rather
than just queue it
ACK
Acknowledge'' a previous packet
URG
Urgent'' data which needs to be processed immediately

In this example, your client is 5.6.7.8, and the port assigned to you
dynamically is 1049. The server is 1.2.3.4, port 80.

You begin the connection attempt:

5.6.7.8:1049 -> 1.2.3.4:80 SYN=ON

The server receives this packet and understands that someone wants to form a
new connection. A response is sent:

1.2.3.4:80 -> 5.6.7.8:1049 SYN=ON ACK=ON

The client receives the response, and informs that the response is received

5.6.7.8:1049 -> 1.2.3.4:80 ACK=ON

Here, the connection is opened. This is called a three-way handshake. Its
purpose is to verify to BOTH hosts that they have a working connection
between them.

The internet being what it is, unreliable and flooded, there are provisions
to compensate for packet loss.

If the client sends out the initial SYN without receiving a SYN+ACK within a
few seconds, it'll resend the SYN.

If the server sends out the SYN+ACK without receiving an ACK in a few
seconds, it'll resend the SYN+ACK packet.

The latter is actually the reason that SYN flooding works so well. If you
send out SYN packets from lots of different ports, this will tie up a lot of
resources on the server. If you also refuse to respond to the returned
SYN+ACK packets, the server will KEEP these connections for a long time,
resending the SYN+ACK packets. Some servers will not accept new connections
while there are enough connections currently forming; this is why SYN
flooding works.

All packets transmitted in either direction after the three-way handshake
will have the ACK bit set. Stateless packet filters make use of this in the
so called established'' filters: They will only let packets through that
have the ACK bit set. This way, no packet may pass through in a certain
direction that could form a new connection. Typically, you don't allow
outside hosts to open new connections to inside hosts by requiring the ACK
bit set on these packets.

When the time has come to close the connection, there are two ways of doing
it: Using the FIN flag, or using the RST flag. Using FIN flags, both
implementations are required to send out FIN flags to indicate that they
want to close the connection, and then send out acknowledgements to these
FINs, indicating that they understood that the other end wants to close the
connection. When sending out RST's, the connection is closed forcefully, and
you don't really get an indication of whether the other end understood your
reset order, or that it has in fact received all data that you sent to it.

The FIN way of closing the connection also exposes you to a
denial-of-service situation, since the TCP stack needs to remember the
closed connection for a fairly long time, in case the other end hasn't
received one of the FIN packets.

If sufficiently many connections are opened and closed, you may end up
having closed'' connections in all your connection slots. This way, you
wouldn't be able to dynamically allocate more connections, seeing that
they're all used. Different OSes handle this situation differently.

References

1    Steven M. Bellovin.
Firewall-friendly FTP.
RFC 1579.

2    R. Finlayson.
Ip multicast and firewalls.
RFC 2588, May 1999.

3    Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear.
RFC 1918, February 1996.

4    R. Thayer, N. Doraswamy, and R. Glenn.
RFC 2411, November 1998.