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Network Working Group                                        P. Ferguson
Request for Comments: 2267                           Cisco Systems, Inc.
Category: Informational                                         D. Senie
                                                          BlazeNet, Inc.
                                                            January 1998

                       Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ
                       IP Source Address Spoofing

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   Recent occurrences of various Denial of Service (DoS) 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 method for using ingress traffic filtering to
   prohibit DoS attacks which use forged IP addresses to be propagated
   from 'behind' an Internet Service Provider's (ISP) aggregation point.

Table of Contents

    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  2
    2.  Background . . . . . . . . . . . . . . . . . . . . . . . .  2
    3.  Restricting forged traffic . . . . . . . . . . . . . . . .  5
    4.  Further capabilities for networking equipment. . . . . . .  6
    5.  Liabilities. . . . . . . . . . . . . . . . . . . . . . . .  6
    6.  Summary. . . . . . . . . . . . . . . . . . . . . . . . . .  7
    7.  Security Considerations. . . . . . . . . . . . . . . . . .  7
    8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . .  8
    9.  References . . . . . . . . . . . . . . . . . . . . . . . .  8
   10.  Authors' Addresses . . . . . . . . . . . . . . . . . . . .  9
   11.  Full Copyright Statement . . . . . . . . . . . . . . . . . 10

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RFC 2267               Network Ingress Filtering            January 1998

 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 new and innovative methods to mitigate these types of attacks.
   The difficulties in reaching this goal are numerous; some 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, however, has been an ongoing 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
   the system. By mentioning the method, he was concerned about
   encouraging it's use.

   While the filtering method discussed in this document does
   absolutely nothing to protect against flooding attacks which
   originate from valid prefixes (IP addresses), 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 prohibit
   attackers from  using forged source addresses which do not reside
   within a range of legitimately advertised prefixes.  In other words,
   if an ISP is aggregating routing announcements for multiple
   downstream networks, strict traffic filtering should be used to
   prohibit traffic which claims to have originated from outside of
   these aggregated announcements.

   An additional benefit of implementing this type of filtering is that
   it enables the originator to be easily traced to it's true source,
   since the attacker would have to use a valid, and legitimately
   reachable, source address.

 2. Background

   A simplified diagram of the TCP SYN flooding problem is depicted

    host <----- router <--- Internet <----- router <-- attacker


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RFC 2267               Network Ingress Filtering            January 1998

    no route
    no route
    no route



    o The "host" is the targeted machine.

    o The attacker resides within the "valid" prefix,

    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 generally present in the global
      Internet routing tables, and therefore, unreachable. However, any
      unreachable prefix could be used to perpetrate this attack

   Also worthy of mention is a case wherein the source address is forged
   to appear to have originated from within another legitimate network
   which appears in the global routing table(s). 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

   Further complicating matters, TCP SYN flood attacks will result in
   SYN-ACK packets being sent to one or many hosts which have no
   involvement in the attack, but which become secondary victims. This
   allows the attacker to abuse two or more systems at once.

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   Similar attacks have been attempted using UDP and ICMP flooding.
   The former attack (UDP flooding) uses forged packets to try and
   connect the chargen UDP service to the echo UDP service at another
   site.  Systems administrators should NEVER allow UDP packets destined
   for system diagnostic ports from outside of their administrative
   domain to reach their systems. The latter attack (ICMP flooding),
   uses an insidious feature in IP subnet broadcast replication
   mechanics. This attack relies on a router serving a large multi-
   access broadcast network to frame an IP broadcast address (such as
   one destined for into a Layer 2 broadcast frame (for
   ethernet, FF:FF:FF:FF:FF:FF). Ethernet NIC hardware (MAC-layer
   hardware, specifically) will only listen to a select number of
   addresses in normal operation.  The one MAC address that all devices
   share in common in normal operation is the media broadcast, or
   FF:FF:FF:FF:FF:FF.  In this case, a device will take the packet and
   send an interrupt for processing. Thus, a flood of these broadcast
   frames will consume all available resources on an end-system [9]. It
   is perhaps prudent that system administrators should consider
   ensuring that their border routers do not allow directed broadcast
   packets to be forwarded through their routers as a default.

   When an TCP SYN 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.

   In response to this threat, most 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 pervasively 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

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RFC 2267               Network Ingress Filtering            January 1998

 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, and intentionally advertised, prefix(es), the
   problem of source address spoofing can be virtually eliminated in
   this attack scenario.

                               router 1
         ISP <----- ISP <---- ISP <--- ISP <-- router <-- attacker
          A          B         C        D         2
              router 3

   In the example above, the attacker resides within, 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 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
    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

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 4. Further possible capabilities for networking equipment

   Additional functions should be considered for future platform
   implementations. The following one is worth noting:

      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 the source address on the packets which he
        is originating. 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. We have received reports
        that some vendors and some ISPs are already starting to
        implement this  capability.

   We considered suggesting routers also validate the source IP address
   of the sender as suggested in [8], but that methodology will not
   operate well in the real networks out there today. The method
   suggested is to look up source addresses to see that the return path
   to that address would flow out the same interface as the packet
   arrived upon. With the number of asymmetric routes in the Internet,
   this would clearly be problematic.

 5. Liabilities

   Filtering of this nature has the potential to break some types of
   "special" services. It is in the best interest of the ISP offering
   these types of special services, however, to consider alternate
   methods of implementing these services to avoid being affected by
   ingress traffic filtering.

   Mobile IP, as defined in [6], is specifically affected by ingress
   traffic filtering. As specified, traffic to the mobile node is
   tunneled, but traffic from the mobile node is not tunneled. This
   results in packets from the mobile node(s) which have source
   addresses that do not match with the network where the station is
   attached.  The Mobile IP Working Group is addressing this problem by
   specifying "reverse tunnels" in [7].  This work in progress provides
   a method for the data transmitted from the mobile node to be tunneled
   to the home agent before transmission to the Internet.  There are
   additional benefits to the reverse tunneling scheme, including better
   handling of multicast traffic. Those implementing mobile IP systems
   are encouraged to implement this method of reverse tunneling.

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RFC 2267               Network Ingress Filtering            January 1998

   As mentioned previously, while ingress traffic 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 within the known
   prefixes that are being advertised. This simplifies tracking down the
   culprit, and at worst, the administrator can block a range of source
   addresses until the problem is resolved.

   If ingress filtering is 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 and a destination of are allowed to reach the relay agent in routers when
   appropriate.  The scope of directed broadcast replication  should be
   controlled, however, and not arbitrarily forwarded.

 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 could also, in practice, 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 of this nature.

 7. Security Considerations

   The primary intent of this document is to inherently increase
   security practices and awareness for the Internet community as a
   whole; as more Internet Providers and corporate network
   administrators implement ingress filtering, the opportunity for an
   attacker to use forged source addresses as an attack methodology will
   significantly lessen. Tracking the source of an attack is simplified

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RFC 2267               Network Ingress Filtering            January 1998

   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
   [Priori Networks] and Steve Bielagus [OpenROUTE Networks, Inc.] for
   their comments and contributions.

 9. References

   [1]  CERT Advisory CA-96.21; 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]  Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G., and
        E. Lear, "Address Allocation for Private Internets", RFC 1918,
        February 1996.

   [5]  The North American Network Operators Group;

   [6]  Perkins, C., "IP Mobility Support", RFC 2002, October 1996.

   [7]  Montenegro, G., "Reverse Tunneling Mobile IP",
        Work in Progress.

   [8]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
        June 1995.

   [9]  Thanks to: Craig Huegen;
        See: http://www.quadrunner.com/~chuegen/smurf.txt.

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RFC 2267               Network Ingress Filtering            January 1998

 10. Authors' Addresses

   Paul Ferguson
   cisco Systems, Inc.
    400 Herndon Parkway
   Herndon, VA  USA 20170

   EMail: ferguson@cisco.com

   Daniel Senie
   BlazeNet, Inc.
    4 Mechanic Street
   Natick, MA  USA 01760

   EMail: dts@senie.com

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RFC 2267               Network Ingress Filtering            January 1998

 11.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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