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Internet Firewalls Frequently Asked Questions
FAQ Maintainer: Marcus J. Ranum
About the FAQ
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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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
There are several books that touch on firewalls. The best known are:
Authors: Bill Cheswick and Steve Bellovin Publisher: Addison Wesley Edition: 1994 ISBN:
Internet Firewalls Authors: D. Brent Chapman and Elizabeth Zwicky
Publisher: O'Reilly Edition: 1995 ISBN: 1-56592-124-0
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: 0-201-56327-4
- Firewalls mailing list archives. Directory: pub/firewalls
- Firewall Howto
- A how-to-build firewalls document.
- Ftp.tis.com -
Internet firewall toolkit and papers. Directory: pub/firewalls
- Papers on firewalls and breakins. Directory: dist/internet_security
- 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
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:
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.
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
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.
Conceptually, there are two types of
- 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
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.
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
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.
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.
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
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
/pub/security/socks.cstc Users are encouraged to check the file
"FILES" for a description of the directory's contents.
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
available for free, for BSD-based systems.
Many commercial routers support screening
of various forms.
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 18.104.22.168.
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.
- no ip source-route
- interface ethernet 0
- ip address 22.214.171.124
- interface serial 0
- ip access-group 101 in
- access-list 101 deny ip 126.96.36.199 0.0.0.255
- access-list 101 permit tcp any any established
- access-list 101 permit tcp any host 188.8.131.52 eq smtp
- access-list 101 permit tcp any host 184.108.40.206 eq dns
- access-list 101 permit udp any host 220.127.116.11 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 204
- 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 18.104.22.168 255.255.255.0
- 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
- 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 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 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.
3 ways to do it - Pick one:
- Allow "established" connections out via a router, if you are using
- Use a Web client that supports SOCKS, and run SOCKS on your
- 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.
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.
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.
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.
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.
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.
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.
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)
prefer to build client versions of the FTP program that are linked
against a SOCKS library.
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.
Many firewall admings permit connections to the finger port from
only trusted machines, which can issue finger requests in the form of:
finger firstname.lastname@example.org@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
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
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
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.
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
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.
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.
- 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
- 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, 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
- 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
- Network-Level Firewall:
- A firewall in which traffic is examined at the network protocol
- 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
- A self-replicating code segment. Viruses may or may not contain
attack programs or trapdoors.
Primary Author: email@example.com - Marcus Ranum, V-ONE Corporation
Cisco Config (V2.0): firstname.lastname@example.org - Keinanen Vesa
Cisco Config (V1.0): email@example.com - Allen Leibowitz
Policy Brief: firstname.lastname@example.org - 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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