Internet Draft Paul Ferguson
November 1996 cisco Systems, Inc.
Expires in six months Daniel Senie
Proteon, Inc.
Network Ingress Filtering
Defending Against IP Source Address Spoofing
draft-ferguson-ingress-filtering-01.txt
Status of this Memo
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Abstract
Recent occurrences of various Denial of Service attacks which
have employed forged source addresses have proven to be a
troublesome issue for Internet Service Providers and the Internet
community overall. This paper discusses a simple, effective
and straightforward methods for using ingress traffic filtering
to deny attacks which use forged IP addresses.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Restricting forged traffic . . . . . . . . . . . . . . . . 4
4. Further capabilities for networking equipment. . . . . . . 5
5. Liabilities. . . . . . . . . . . . . . . . . . . . . . . . 5
6. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations. . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
A resurgence of Denial of Service Attacks [1] aimed at various
targets in the Internet have produced new challenges within the
Internet Service Provider (ISP) and network security communities to
find methods to mitigate these types of attacks. The difficulties
in reaching this goal are numerous; simple tools already exist to
limit the effectiveness and scope of these attacks, but they have
not been widely implemented.
This method of attack has been known for some time. Defending
against it has been a concern. Bill Cheswick is quoted in [2]
as saying that he pulled a chapter from his book, "Firewalls and
Internet Security" [3], at the last minute because there was no way
for an administrator of the system under attack to effectively defend
that system. By mentioning the method, he was concerned about
encouraging its use.
While the filtering method discussed in this document does
absolutely nothing to protect against flooding attacks which
originate from valid prefixes, it will prohibit an attacker within
the originating network from launching an attack of this nature using
forged source addresses that do not conform to ingress filtering
rules. All providers of Internet connectivity are urged to
implement filtering described in this document to disallow attackers
from using forged source addresses which do not reside within
legitimately advertised prefixes.
An additional benefit of implementing this type of filtering is that
it enables the originator to be easily traced, since the attacker
would have to use a valid, and reachable, source address.
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2. Background
A simplified diagram of the problem is depicted below:
9.0.0.0/8
host <----- router <--- Internet <----- router <-- attacker
TCP/SYN
<---------------------------------------------
Source: 192.168.0.4/32
SYN/ACK
no route
TCP/SYN
<---------------------------------------------
Source: 10.0.0.13/32
SYN/ACK
no route
TCP/SYN
<---------------------------------------------
Source: 172.16.0.2/32
SYN/ACK
no route
[etc.]
Assume:
o The host is the targeted machine.
o The attacker resides within the "valid" prefix 9.0.0.0/8
o The attacker launches the attack using randomly changing source
addresses; in this example, the source addresses are depicted
as from within [4], which are not present in the global Internet
routing tables, and therefore, unreachable. Any unreachable prefix
could be used to perpetrate this attack method.
Also worthy of mention is a case wherein the source address is
forged to appear to have originated from within another legitimate
network. For example, an attacker using a valid network address
could wreak havoc by making the attack appear to come from an
organization which did not, in fact, originate the attack and
was completely innocent. In such cases, the administrator of a
system under attack may be inclined to filter all traffic coming
from the apparent attack source. Adding such a filter would then
result in a denial of service to legitimate, non-hostile end-systems.
In this case, the administrator of the system under attack
unwittingly becomes an accomplice of the attacker.
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When an attack is launched using unreachable source address, the
target host attempts to reserve resources waiting for a response.
The attacker repeatedly changes the bogus source address on each
new packet sent, thus exhausting additional host resources.
Alternatively, if the attacker uses someone else's valid host address
as the source address, the system under attack will send a large
number of SYN/ACK packets to what it believes is the originator of
the connection establishment sequence. In this fashion, the attacker
does damage to two systems: the destination target system, as well as
the system which is actually using the spoofed address in the global
routing system.
The result of both attack methods is extremely degraded performance,
or worse, a system crash.
Responding to this threat, the operating system vendors have
modified their software to allow the targeted servers to sustain
attacks with very high connection attempt rates. This is a welcome
and necessary part of the solution to the problem. Ingress filtering
will take time to be implemented everywhere and be fully effective,
but the extensions to the operating systems can be implemented
quickly. This combination should prove effective against source
address spoofing. See [1] for vendor and platform software upgrade
information.
3. Restricting forged traffic
The problems encountered with this type of attack are numerous,
and involve shortcomings in host software implementations, routing
methodologies, and the TCP/IP protocols themselves. However, by
restricting transit traffic which originates from a downstream
network to known prefix(es), the problem of source address
spoofing can be virtually eliminated in the attack scenario.
11.0.0.0/8
/
router 1
/
/
/ 9.0.0.0/8
ISP <----- ISP <---- ISP <--- ISP <-- router <-- attacker
A B C D 2
/
/
/
router 3
/
12.0.0.0/8
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In the example above, the attacker resides within 9.0.0.0/8,
which is provided Internet connectivity by ISP D. An input
traffic filter on the ingress (input) link of "router 2", which
provides connectivity to the attacker's network, restricts traffic
to allow only traffic originating from source addresses within the
9.0.0.0/8 prefix, and prohibits an attacker from using "invalid"
source addresses which reside outside of this prefix range.
In other words, the ingress filter on "router 2" above would check:
IF packet's source address from within 9.0.0.0/8
THEN forward as appropriate
IF packet's source address is anything else
THEN deny packet
Network administrators should log information on packets which are
dropped. This then provides a basis for monitoring any suspicious
activity.
4. Further capabilities for networking equipment
Several additional functions could be considered for future
platform implementations. These include:
o Implementation of automatic filtering on remote access servers.
In most cases, a user dialing into an access server is an
individual user on a single PC. The ONLY valid source IP
address for packets originating from that PC is the one
assigned by the ISP (whether statically or dynamically
assigned). The remote access server could check every packet
on ingress to ensure the user is not spoofing addresses.
Obviously, provisions also need to be made for cases where the
customer legitimately is attaching a net or subnet via a remote
router, but this could certainly be implemented as an optional
parameter.
o Examination of forwarded packets for valid return path. Routers
could perform a look up on the source address of packets being
forwarded in their routing tables. If the routing table indicates
a different interface as the next hop than the interface on which
the packet arrived, then the packet would be discarded. This
could lead to problems when network administrators set up
multiple paths in such a way that traffic doesn't always flow
on the same path in both directions (asymmetric routing). Note
that this check may degrade router performance.
5. Liabilities
Filtering of this nature has the potential to break some types of
special services, such as some IP Mobility implementations. It is
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in the best interest of the ISP offering these types of special
services, however, to consider alternate methods of implementation,
such as point-to-point tunneling, or any other method which is not
affected by ingress traffic filtering.
While ingress filtering drastically reduces the success of source
address spoofing, it does not preclude an attacker using a forged
source address of another host within the permitted prefix filter
range. It does, however, ensure that when an attack of this nature
does indeed occur, a network administrator can be sure that the
attack is actually originating from where it advertises. This
simplifies tracking down of the culprit, and at worst, the
administrator can block a range of source addresses until the
problem is resolved.
If ingress filtering used in an environment where DHCP or BOOTP
is used, the network administrator would be well advised to ensure
that packets with a source address of 0.0.0.0 and a destination
of 255.255.255.255 are allowed to be forwarded to the appropriate
destination. Since the router is most likely performing as the BOOTP
or DHCP relay agent, the router will then be able to forward the
requests.
6. Summary
Ingress traffic filtering at the periphery of Internet connected
networks will reduce the effectiveness of source address spoofing
denial of service attacks. Network service providers and
administrators have already begun implementing this type of
filtering on periphery routers, and it is recommended that all
service providers do so as soon as possible. In addition to aiding
the Internet community as a whole to defeat this attack method, it
can also assist service providers in locating the source of the
attack if service providers can categorically demonstrate that their
network already has ingress filtering in place on customer links.
Corporate network administrators should implement filtering to
ensure their corporate networks are not the source of such
problems. Indeed, filtering could be used within an organization to
ensure users do not cause problems by improperly attaching systems
to the wrong networks. The filtering would also block a disgruntled
employee from anonymous attacks.
It is the responsibility of all network administrators to ensure
they do not become the unwitting source of an attack.
7. Security considerations
The primary consideration is to inherently increase security for the
Internet community as a whole; as more Internet Providers and
corporate network administrators implement ingress filtering, the
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opportunity for an attacker to use forged source addresses as an
attack methodology will lessen. Tracking the source of an attack is
simplified when the source is more likely to be "valid." By reducing
the number and frequency of attacks in the Internet as a whole,
there will be more resources for tracking the attacks which
ultimately do occur.
8. Acknowledgments
The North American Network Operators Group (NANOG) [5] group as a
whole deserves special credit for openly discussing these issues and
actively seeking possible solutions. Also, thanks to Justin Newton
[Erol's Internet, Inc.] and Steve Bielagus [Proteon, Inc.] for their
comments and contributions.
9. References
[1] CERT Advisory CA.96-12; TCP SYN Flooding and IP Spoofing
Attacks; September 24, 1996
[2] B. Ziegler, "Hacker Tangles Panix Web Site", Wall Street Journal,
12 September 1996
[3] "Firewalls and Internet Security: Repelling the Wily Hacker";
William R. Cheswick and Steven M. Bellovin, Addison-Wesley
Publishing Company, 1994; ISBN 0-201-63357-4
[4] RFC-1918, "Address Allocation for Private Internets"; Y. Rekhter,
R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear; February 1996
[5] The North American Network Operators Group;
http://www.nanog.org
10. Authors' addresses
Paul Ferguson Daniel Senie
cisco Systems, Inc. Proteon, Inc.
400 Herndon Parkway 9 Technology Drive
Herndon, VA USA 20170 Westboro, MA USA 01581
Email: pferguso@cisco.com Email: dts@proteon.com
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