Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 2827.
|Last updated||2013-03-02 (Latest revision 1997-07-02)|
|RFC stream||Internet Engineering Task Force (IETF)|
|IESG||IESG state||RFC 2827 (Best Current Practice)|
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Internet Draft Paul Ferguson July 1997 cisco Systems, Inc. Expires in six months Daniel Senie OpenROUTE Networks, Inc. Network Ingress Filtering: Defeating IP Source Address Spoofing Denial of Service Attacks draft-ferguson-ingress-filtering-02.txt Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract 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 deny DoS attacks which use forged IP addresses to be propagated from "behind" an Internet Service Provider's (ISP) aggregation point. draft-ferguson-ingress-filtering-02.txt [Page 1] INTERNET-DRAFT Network Ingress Filtering July 1997 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. . . . . . . . . . . . . . . . . . 7 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . 7 10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 7 1. Introduction A resurgence of Denial of Service Attacks  aimed at various targets in the Internet have produced new challenges within the Internet Service Provider (ISP) and network security communities to new and innovative 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  as saying that he pulled a chapter from his book, "Firewalls and Internet Security" , 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 prohibit attackers from using forged source addresses which do not reside within 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 announcements. 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. draft-ferguson-ingress-filtering-02.txt [Page 2] INTERNET-DRAFT Network Ingress Filtering July 1997 2. Background A simplified diagram of the problem is depicted below: 126.96.36.199/8 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 188.8.131.52/8 o The attacker launches the attack using randomly changing source addresses; in this example, the source addresses are depicted as from within , 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, ie. one which does not appear in the global routing system. 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. draft-ferguson-ingress-filtering-02.txt [Page 3] INTERNET-DRAFT Network Ingress Filtering July 1997 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, 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  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, and intentionally advertised, prefix(es), the problem of source address spoofing can be virtually eliminated in this attack scenario. 184.108.40.206/8 / router 1 / / / 220.127.116.11/8 ISP <----- ISP <---- ISP <--- ISP <-- router <-- attacker A B C D 2 / / / router 3 / 18.104.22.168/8 draft-ferguson-ingress-filtering-02.txt [Page 4] INTERNET-DRAFT Network Ingress Filtering July 1997 In the example above, the attacker resides within 22.214.171.124/8, 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 126.96.36.199/8 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 188.8.131.52/8 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 Additional functions could 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 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. 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  is affected by ingress filtering. As specified, traffic to the mobile node is tunneled, but traffic from the mobile node are not tunneled. This results in packets from the draft-ferguson-ingress-filtering-02.txt [Page 5] INTERNET-DRAFT Network Ingress Filtering July 1997 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 . This draft 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 tunneling. 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 within the known prefixes that are being advertised. 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 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 0.0.0.0 and a destination of 255.255.255.255 are allowed to reach the relay agent in routers when appropriate. 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. draft-ferguson-ingress-filtering-02.txt [Page 6] INTERNET-DRAFT Network Ingress Filtering July 1997 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 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)  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  CERT Advisory CA.96-12; TCP SYN Flooding and IP Spoofing Attacks; September 24, 1996  B. Ziegler, "Hacker Tangles Panix Web Site", Wall Street Journal, 12 September 1996  "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  RFC-1918, "Address Allocation for Private Internets"; Y. Rekhter, R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear; February 1996  The North American Network Operators Group; http://www.nanog.org  RFC-2002, "IP Mobility Support"; C. Perkins; October 1996  draft-ietf-mobileip-tunnel-reverse-02.txt, "Reverse Tunneling for Mobile IP"; G. Montenegro; March 1997 10. Authors' addresses Paul Ferguson Daniel Senie cisco Systems, Inc. OpenROUTE Networks, Inc. 400 Herndon Parkway 9 Technology Drive Herndon, VA USA 20170 Westboro, MA USA 01581 Email: email@example.com Email: firstname.lastname@example.org draft-ferguson-ingress-filtering-02.txt [Page 7]