TUCoPS :: General Information :: fwallfaq.txt

Firewalls FAQ

Firewalls FAQ (Rev 8, updated Sun Jan  7 23:13:54 1996)

Archive-name: firewalls-faq
Posting-Frequency: whenever updated
Last-modified: Sun Jan  7 23:13:54 1996
Version: 8
Internet Firewalls Frequently Asked Questions

FAQ Maintainer: Marcus J. Ranum

About the FAQ

This FAQ is not an advertisement or endorsement for any product, company, or
consultant. The maintainer welcomes input and comments on the contents of this
FAQ. Comments related to the FAQ should be addressed to Fwalls-FAQ@v-one.com.
The FAQ is also available via WWW from http://www.v-one.com. As of this
writing, the FAQ's primary format is HTML.


  1. What is a network firewall?

  2. Why would I want a firewall?

  3. What can a firewall protect against?

  4. What can't a firewall protect against?

  5. What about virusses?

  6. What are good sources of print information on firewalls?

  7.  Where can I get more information on firewalls on the network?

  8. What are some commercial products or consultants who sell/service

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

 10. What are some of the basic types of firewall?

 11. What are proxy servers and how do they work?

 12.  What are some cheap packet screening tools?

 13.  What are some reasonable filtering rules for a Cisco?

 14. How do I make Web/http work with a firewall?

 15. How do I make DNS work with a firewall?

 16. How do I make FTP work through my firewall?

 17. How do I make Telnet work through my firewall?

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

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

 20. What are the issues about X-Window through a firewall?

 21. What is source routed traffic and why is it a threat?

 22. What are ICMP redirects and redirect bombs?

 23. What about denial of service?

 24. Glossary of firewall related terms

 25. Contributors

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.

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.

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.

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.

What about virusses?

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

What are good sources of print information on firewalls?

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

   * Title: Firewalls and Internet Security: Repelling the Wily Hacker Authors:
     Bill Cheswick and Steve Bellovin Publisher: Addison Wesley Edition: 1994
     ISBN: 0-201-63357-4
   * Title: Building Internet Firewalls Authors: D. Brent Chapman and Elizabeth
     Zwicky Publisher: O'Reilly Edition: 1995 ISBN: 1-56592-124-0
   * Title: Practical Unix Security Authors: Simson Garfinkel and Gene Spafford
     Publisher: O'Reilly Edition: 1991 ISBN: 0-937175-72-2 (discusses primarily
     host security)

Related references are:

   * Titles: 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. Vol I (on principles, protocols and architecture) is
     readable by everyone, Vol 2 (on design, implementation and internals) is
     more technical, and Vol 3 (on client-server computing) is recently out.
   * Title: Unix System Security - A Guide for Users and System Administrators
     Author: David Curry Publisher: Addision Wesley Edition: 1992 ISBN:

Where can I get more information on firewalls on the network?

   * Ftp.greatcircle.com - Firewalls mailing list archives. Directory:
   * Firewall Howto - A how-to-build firewalls document.
   * Ftp.tis.com - Internet firewall toolkit and papers. Directory:
   * Research.att.com - Papers on firewalls and breakins. Directory:
   * Net.Tamu.edu - Texas AMU security tools. Directory: pub/security/TAMU
   * v-one.com - Internet attacks presentation, firewall standards

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 on the "Subject:" line) to "Majordomo@GreatCircle.COM".
Archives of past Firewalls postings are available for anonymous FTP from
ftp.greatcircle.com in pub/firewalls/archive

What are some commercial products or consultants who sell/service firewalls?

We feel this topic is too sensitive to address in a FAQ, however, an
independantly maintained list (no warrantee or recommendations are implied) can
be found at URL: http://www.access.digex.net/~bdboyle/firewall.vendor.html

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 is 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 plusses 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.

What are the basic types of firewalls?

Conceptually, there are two types of firewalls:

   * Network Level
   * 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.

Network level firewalls 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.


Example Network level firewall: In this example, 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.


Example Network level firewall: In this example, 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 sopftware 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.


Example Application level firewall: In this example, 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.

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 ftp.nec.com: /pub/security/socks.cstc Users are
encouraged to check the file "FILES" for a description of the directory's

What are some cheap packet screening tools?

The Texas AMU security tools include software for implementing screening
routers (FTP net.tamu.edu, pub/security/TAMU). Karlbridge is a PC-based
screening router kit ftp://ftp.net.ohio-state.edu/pub/kbridge. A version of the
Digital Equipment Corporation "screend" kernel screening software is available
for BSD/386, NetBSD, and BSDI. There is a kernel-level packet screen called
ipfilter available for free, for BSD-based systems. Many commercial routers
support screening of various forms.

What are some reasonable filtering rules for a Cisco?

The following example 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.


In this example, a company has Class C network address 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.


   * 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.

  1. no ip source-route
  2. !
  3. interface ethernet 0
  4. ip address
  5. !
  6. interface serial 0
  7. ip access-group 101 in
  8. !
  9. access-list 101 deny ip
 10. access-list 101 permit tcp any any established
 11. !
 12. access-list 101 permit tcp any host eq smtp
 13. access-list 101 permit tcp any host eq dns
 14. access-list 101 permit udp any host eq dns
 15. !
 16. access-list 101 deny tcp any any range 6000 6003
 17. access-list 101 deny tcp any any range 2000 2003
 18. access-list 101 deny tcp any any eq 2049
 19. access-list 101 deny udp any any eq 204
 20. !
 21. access-list 101 permit tcp any 20 any gt 1024
 22. !
 23. access-list 101 permit icmp any any
 24. !
 25. snmp-server community FOOBAR RO 2
 26. line vty 0 4
 27. access-class 2 in
 28. access-list 2 permit


   * Drop all source-routed packets. Source routing can be used for address
   * 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 TCP connections in high port numbers.
     X-windows (port 6000+), OpenWindows (port 2000+) are few candidates. NFS
     (port 2049) runs usually over UDP, but it can be run over NFS, so you have
     better 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


   * 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 crackes install telnet backdoors on systems
     where they break in.
   * You can never be sure what services you have listening connections on high
     port numbers.
   * Checking source port on incoming FTP data connections is a weak security
     method. It also breaks access to some FTP sites. It makes users more
     difficult to use their backdoors, but doesn't prevent hackers to scan your

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

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 has an archive of examples for building firewalls using Cisco routers
(ftp://ftp.cisco.com/pub/acl-examples.tar.Z) Those examples are a bit
out-of-date, but there are some perl scripts which are pretty useful, once
adjusted for your network.

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

There are 3 ways to do it - Pick one:

   * Allow "established" connections out via a router, if you are using
     screening routers.
   * Use a Web client that supports SOCKS, and run SOCKS on your firewall.
   * Run some kind of proxy-capable Web server on the firewall. The TIS
     firewall toolkit includes a proxy called http-gw, which proxies Web,
     gopher/gopher+ and FTP. CERN httpd also has a proxy capability, which many
     sites use in combination with the server's ability to cache frequently
     accessed pages. Many Web clients have proxy server support (Netscape,
     Mosaic, Spry, Chameleon, etc) built directly into them.

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 authoritiative 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.

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 RFC1579 for more information)

Other sites prefer to build client versions of the FTP program that are linked
against a SOCKS library.

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.

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

Many firewall admings 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
accostomed to putting proprietary or sensitive information in their.plan files,
you have a more serious security problem than just a firewall can solve.

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

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.

What are the issues about X-Window through a firewall?

X Windows 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.

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 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.

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.

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.

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.
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.
     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.
     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
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,
     WNT, etc.) rather than a ROM-based or firmware operating system.
     An authentication technique whereby a server sends an unpredictable
     challenge to the user, who computes a response using some form of
     authentication token.
     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.
     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.
     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
Perimeter-based Security:
     The technique of securing a network by controlling access to all entry and
     exit points of the network.
     Organization-level rules governing acceptable use of computing resources,
     security practices, and operational procedures.
     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
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
     A self-replicating code segment. Viruses may or may not contain attack
     programs or trapdoors.


   * Primary Author: mjr@v-one.com - Marcus Ranum, V-ONE Corporation

   * Cisco Config (V2.0): vjk@relevantum.fi - Keinanen Vesa

   * Cisco Config (V1.0): allen@msen.com - Allen Leibowitz

   * DNS Hints: brent@greatcircle.com - Brent Chapman, Great Circle Associates

   * Policy Brief: bdboyle@erenj.com - Brian Boyle, Exxon Research

Copyright(C) 1995 Marcus J. Ranum. All rights reserved. This document may be
used, reprinted, and redistributed as is providing this copyright notice and
all attributions remain intact.

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