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Firewalls FAQ



Frequently Asked Questions about Internet Firewalls.

This FAQ about Internet Firewalls was compiled and written by Matt Curtin cmcurtin@interhack.net and Marcus J. Ranum mjr@nfr.com with numerous contributions by others.

-01 About the FAQ (Firewalls FAQ)
-02 For Whom Is the FAQ Written? (Firewalls FAQ)
Firewalls have come a long way from the days when this FAQ started....
-03 Before Sending Mail (Firewalls FAQ)
Note that this collection of frequently-asked questions is a result ...
-04 Where Can I find the Current Version of the FAQ? (Firewalls FAQ)
The FAQ can be found on the Web ...
-05 Where Can I Find Non-English Versions of the FAQ? (Firewalls FAQ)
Several translations are available. (If you've done a translation and it'...
-06 Contributors (Firewalls FAQ)
Many people have written helpful suggestions and thoughtful commentary....
-07 Copyright and Usage (Firewalls FAQ)
Copyright ©1995-1996, 1998 Marcus J. Ranum. Copyright ©1998-2000 ...
-08 What is a network firewall?
A firewall is a system or group of systems that enforces an access ...
-09 Why would I want a firewall?
The Internet, like any other society, is plagued with the kind of ...
-10 What can a firewall protect against?
Some firewalls permit only email traffic through them, thereby ...
-11 What can't a firewall protect against?
Firewalls can't protect against attacks that don't go through ...
-12 What about viruses? (Firewalls)
Firewalls can't protect very well against things like viruses. There ...
-13 Will IPSEC make firewalls obsolete?
Some have argued that this is the case. Before pronouncing such a ...
-14 What are good sources of print information on firewalls?
-15 Where can I get more information on firewalls on the Internet?
-16 What are some of the basic design decisions in a firewall?
There are a number of basic design issues that should be addressed by ...
-17 What are the basic types of firewalls?
Conceptually, there are two types of firewalls:...
-18 Network layer firewalls
These generally make their decisions based on the source, ...
-19 Application layer firewalls
These generally are hosts running proxy servers, which permit no ...
-20 What are proxy servers and how do they work?
A proxy server (sometimes referred to as an application gateway ...
-21 What are some cheap packet screening tools?
The Texas AMU security tools include software for implementing ...
-22 What are some reasonable filtering rules for a kernel-based packet screen?
This example is written specifically for ipfwadm on Linux, but ...
-23 Implementation (filtering rules for a kernel-based packet screen)
Here, our organization is using a private (RFC 1918) Class C ...
-24 Explanation (filtering rules for a kernel-based packet screen)
* Line one flushes (-f) all forwarding (-F) rules....
-25 What are some reasonable filtering rules for a Cisco?
The example in figure 4 shows one possible configuration for using ...
-26 Implementation (filtering rules for a Cisco)
* Allow all outgoing TCP-...
-27 Explanations (filtering rules for a Cisco)
* Drop all source-routed packets. Source routing can be used for ...
-28 Shortcomings (filtering rules for a Cisco)
* You cannot enforce strong access policies with router access lists....
-29 What are the critical resources in a firewall?
It's important to understand the critical resources of your ...
-30 What is a DMZ, and why do I want one?
``DMZ'' is an abbreviation for ``demilitarized zone''. In the context ...
-31 How might I increase the security and scalability of my DMZ?
A common approach for an attacker is to break into a host that'...
-32 What is a `single point of failure', and how do I avoid having one?
An architecture whose security hinges upon one mechanism has a ...
-33 How can I block all of the bad stuff? (Firewalls)
For firewalls where the emphasis is on security instead of connectivity,...
-34 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 ...
-35 What is source routed traffic and why is it a threat? (Various Attacks - Firewalls)
Normally, the route a packet takes from its source to its destination ...
-36 What are ICMP redirects and redirect bombs? (Various Attacks - Firewalls)
An ICMP Redirect tells the recipient system to over-ride something in ...
-37 What about denial of service? (Various Attacks - Firewalls)
Denial of service is when someone decides to make your network or ...
-38 SMTP Server Hijacking (Unauthorized Relaying) (Common Attacks - Firewalls)
Each site is a little different from every other in terms of what ...
-39 Exploiting Bugs in Applications (Common Attacks - Firewalls)
Various versions of web servers, mail servers, and other Internet ...
-40 Bugs in Operating Systems (Common Attacks - Firewalls)
Again, these are typically initiated by users remotely. Operating ...
-41 Do I really want to allow everything that my users ask for? (Firewalls)
It's entirely possible that the answer is ``no''. Each site has its ...
-42 How do I make Web/HTTP work through my firewall?
There are three ways to do it....
-43 How do I make SSL work through the firewall?
SSL is a protocol that allows secure connections across the Internet....
-44 How do I make DNS work with a firewall?
Some organizations want to hide DNS names from the outside. Many ...
-45 How do I make FTP work through my firewall?
Generally, making FTP work through the firewall is done either using ...
-46 How do I make Telnet work through my firewall?
Telnet is generally supported either by using an application proxy such ...
-47 How do I make Finger and whois work through my firewall?
Many firewall admins permit connections to the finger port from ...
-48 How do I make gopher, archie, and other services work through my firewall?
The majority of firewall administrators choose to support gopher ...
-49 What are the issues about X11 through a firewall?
The X Windows System is a very useful system, but unfortunately has ...
-50 How do I make RealAudio work through my firewall?
RealNetworks maintains some information about how to get ...
-51 How do I make my web server act as a front-end for a database thatlives on my private network?
The best way to do this is to allow very limited connectivity between ...
-52 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 ...
-53 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 ...
-54 Glossary of Firewall-Related Terms
-55 TCP and UDP Ports
-56 How do I know which application uses what port? (Firewalls - TCP and UDP Ports)
There are several lists outlining the ``reserved'' and ``well known''...
-57 What are LISTENING ports? (Firewalls - TCP and UDP Ports)
Suppose you did ``netstat -a'' on your machine and ports 1025 and ...
-58 How do I determine what service the port is for? (Firewalls - TCP and UDP Ports)
-59 What ports are safe to pass through a firewall? (Firewalls - TCP and UDP Ports)
ALL.
-60 The behavior of FTP (Firewalls - TCP and UDP Ports)
Or, ``Why do I have to open all ports above 1024 to my FTP server?''...
-61 What software uses what FTP mode? (Firewalls - TCP and UDP Ports)
It is up to the client to decide what mode to use; the default mode when ...
-62 Is my firewall trying to connect outside? (Firewalls - TCP and UDP Ports)
My firewall logs are telling me that my web server is trying to ...
-63 The anatomy of a TCP connection (Firewalls - TCP and UDP Ports)
TCP is equipped with 6 ``flags'', which may be ON or OFF. These flags are:...









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