Firewalls FAQ
Internet Firewalls:
Frequently Asked Questions
Matt Curtin Marcus J. Ranum
cmcurtin@interhack.net mjr@clark.net
Date: 1999/11/25 17:34:19
Revision: 9.4
Contents
* Contents
* 1 Administrativia
o 1.1 About the FAQ
o 1.2 Where Can I find the Current Version of the FAQ?
o 1.3 Contributors
o 1.4 Copyright and Usage
* 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 2.5 What about viruses?
o 2.6 What are good sources of print information on firewalls?
o 2.7 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 level 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?
o 3.6 Implementation
+ 3.6.1 Explanation
+ 3.6.2 What are some reasonable filtering rules for a Cisco?
+ 3.6.3 Implementation
+ 3.6.4 Explanations
+ 3.6.5 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 Session Hijacking
+ 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?
* A Some Commercial Products and Vendors
* B Glossary of Firewall-Related Terms
* References
1 Administrativia
1.1 About the FAQ
This FAQ is not an advertisement or endorsement for any product, company,
or consultant. 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 .
1.2 Where Can I find the Current Version of the FAQ?
The FAQ can be found on the Web at
* http://www.clark.net/pub/mjr/pubs/fwfaq/
* http://www.interhack.net/pubs/fwfaq/ .
It's also posted monthly to
* comp.security.firewalls ,
* comp.security.unix ,
* comp.security.misc ,
* comp.answers , and
* news.answers .
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.
1.3 Contributors
Cisco router configuration:
* Keinanen Vesa
* Allen Leibowitz, http://www.msen.com/~allen/
DNS hints:
* Brent Chapman, http://www.greatcircle.com/gca/staff/brent.html
Policy brief:
* Brian Boyle, http://users.iamdigex.net/bdboyle/
Kernel-based packet screen configuration:
* D. Clyde Williamson, http://www.interhack.net/people/dclydew/
Helpful commentary on firewall limitations, use of ICMP and TCP/UDP echo:
* Paul D. Robertson, proberts@clark.net
1.4 Copyright and Usage
Copyright ©1995-1996, 1998 Marcus J. Ranum. Copyright ©1998, 1999 Matt
Curtin. All rights reserved. This document may be used, reprinted, and
redistributed as is providing this copyright notice and all attributions
remain intact.
2 Background and Firewall Basics
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 what kind of access you want to permit
or deny, or you simply permit someone or some product to configure a
firewall based on what they or it think it should do, then they are making
policy for your organization as a whole.
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
information about corporate products and services, files to download,
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
sponsors.
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.
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.
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.
2.5 What about viruses?
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 and ghostscript, a freely-available
PostScript viewer.
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. Do not count on any protection from attackers with
this feature.
2.6 What are good sources of print information on firewalls?
There are several books that touch on firewalls. The best known are:
* Firewalls and Internet Security: Repelling the Wily Hacker
Authors
Bill Cheswick and Steve Bellovin
Publisher
Addison Wesley
Edition
1994
ISBN
0-201-63357-4
* Building Internet Firewalls
Authors
D. Brent Chapman and Elizabeth Zwicky
Publisher
O'Reilly
Edition
1995
ISBN
1-56592-124-0
* 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
Addison Wesley
Edition
1992
ISBN
0-201-56327-4
2.7 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.clark.net/pub/mjr/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 to explicitly 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 may 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 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 fiddling-with. 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 level
2. Application level
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.
3.2.1 Network level firewalls
These generally make their decisions based on the source, destination
addresses and ports in individual IP packets. A simple router is the
``traditional'' network level 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 level 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 level firewalls is that they route traffic directly though
them, so to use one you usually need to have a validly assigned IP address
block. Network level firewalls tend to be very fast and tend to be very
transparent to users.
Figure 1: Screened Host Firewall
In Figure 1, a network level 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 level. The single
host is a bastion host; a highly-defended and secured strong-point that
(hopefully) can resist attack.
Figure 2: Screened Subnet Firewall
Example Network level firewall : In figure 2, a network level 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 level. It is similar to a screened host,
except that it is, effectively, a network of screened hosts.
Application level firewalls 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 level 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 level firewalls such as those
built using the TIS firewall toolkit, are not particularly transparent to
end users and may require some training. Modern application level firewalls
are often fully transparent. Application level firewalls tend to provide
more detailed audit reports and tend to enforce more conservative security
models than network level firewalls.
Figure 3: Dual Homed Gateway
Example Application level firewall : In figure 3, an application level
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 level firewalls and
application level firewalls. It is likely that network level firewalls will
become increasingly ``aware'' of the information going through them, and
application level 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
data or passwords being sniffed.
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
protocol specific logging. For more information on SOCKS, see
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
ipfwadm also has masquerading (-M) capabilities. For more information on
switches and options, see the ipfwadm man page.
3.6 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 -f
ipfwadm -F -p deny
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
3.6.1 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
protocol)[protocol]-S (source)[subnet/mask] [originating ports]-D
(destination)[subnet/mask][port]
* 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.
3.6.2 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
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
to allow everybody access to Internet services, so all outgoing connections
are accepted. All incoming connections go through ``mailhost''. Mail and DNS
are only incoming services.
3.6.3 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 lists assumes that you are running Cisco IOS v. 10.3 or later.
no ip source-route
!
interface ethernet 0
ip address 195.55.55.1
!
interface serial 0
ip access-group 101 in
!
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 195.55.55.0 255.255.255.0
3.6.4 Explanations
* Drop all source-routed packets. Source routing can be used for address
spoofing.
* If incoming packet claims to be from local net, 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.
* Access-list 2 limits access to router itself (telnet & SNMP)
* All UDP traffic is blocked to protect RPC services
3.6.5 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
from scanning your systems.
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.
For example, a web server running on NT might be vulnerable to a number of
denial-of-service attacks against such services as NetBIOS and SMB. These
services are not required for the operation of a web server, so blocking TCP
connections to ports 135 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.
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
the bastion host and gain access to the internal network.
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
through your access router.
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
already made your job much easier. Instead of having to worry about every
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
your firewall.
* 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 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). They can be written as two 16-bit integers, or
one 24-bit and one 8-bit integer, or vice-versa. 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
department, not the firewall administrator.
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 Session Hijacking
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 [*] Transport Security Initiative [*]
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.
They just read the trade rags, and see advertisements, and they want to do
those things, too. It's important to look into what it is that they really
want to do, and 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'', and 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 [*] , Apache [*] , Netscape Proxy [*]
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 [*] .
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
non-standard addressing scheme on your internal network. In that case, you
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
want an interesting demonstration of this, ping the subnet broadcast address
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
headers, news articles, etc.
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.
An internal client asking about an internal host asks the internal server,
and gets an answer; an internal client asking about an external host asks
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
in your IN-ADDR.ARPA domains. These cause an an address-to-name lookup for
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'', RFC 1579 [*] .[1])
Other sites prefer to build client versions of the FTP program that are
linked against a SOCKS library.
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
version of finger. Controlling access to services and restricting them to
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
workstation's X display can monitor keystrokes that a user enters, download
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 [*] . 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 extract your entire proprietary database?
``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.
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. [*]
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-Level Firewall
A firewall system in which service is provided by processes that
maintain complete TCP connection state and sequencing. Application
level 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-Level Firewall
A firewall in which traffic is examined at the network protocol packet
level.
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,
to attempt to gain illicit access to systems.
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.