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Network Working Group                                    M. Handley, Ed.Request for Comments: 4732                                           UCLCategory: Informational                                 E. Rescorla, Ed.                                                       Network Resonance                                             Internet Architecture Board                                                                     IAB                                                           November 2006Internet Denial-of-Service ConsiderationsStatus 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 IETF Trust (2006).Abstract   This document provides an overview of possible avenues for denial-   of-service (DoS) attack on Internet systems.  The aim is to encourage   protocol designers and network engineers towards designs that are   more robust.  We discuss partial solutions that reduce the   effectiveness of attacks, and how some solutions might inadvertently   open up alternative vulnerabilities.Handley, et al.              Informational                      [Page 1]

RFC 4732                   DoS Considerations              November 2006Table of Contents1. Introduction ....................................................32. An Overview of Denial-of-Service Threats ........................42.1. DoS Attacks on End-Systems .................................42.1.1. Exploiting Poor Software Quality ....................42.1.2. Application Resource Exhaustion .....................52.1.3. Operating System Resource Exhaustion ................62.1.4. Triggered Lockouts and Quota Exhaustion .............72.2. DoS Attacks on Routers .....................................82.2.1. Attacks on Routers through Routing Protocols ........82.2.2. IP Multicast-based DoS Attacks ......................92.2.3. Attacks on Router Forwarding Engines ...............102.3. Attacks on Ongoing Communications .........................112.4. Attacks Using the Victim's Own Resources ..................122.5. DoS Attacks on Local Hosts or Infrastructure ..............122.6. DoS Attacks on Sites through DNS ..........................152.7. DoS Attacks on Links ......................................162.8. DoS Attacks on Firewalls ..................................172.9. DoS Attacks on IDS Systems ................................182.10. DoS Attacks on or via NTP ................................182.11. Physical DoS .............................................182.12. Social Engineering DoS ...................................192.13. Legal DoS ................................................192.14. Spam and Black-Hole Lists ................................193. Attack Amplifiers ..............................................203.1. Methods of Attack Amplification ...........................203.2. Strategies to Mitigate Attack Amplification ...............224. DoS Mitigation Strategies ......................................224.1. Protocol Design ...........................................234.1.1. Don't Hold State for Unverified Hosts ..............234.1.2. Make It Hard to Simulate a Legitimate User .........234.1.3. Graceful Routing Degradation .......................244.1.4. Autoconfiguration and Authentication ...............244.2. Network Design and Configuration ..........................254.2.1. Redundancy and Distributed Service .................254.2.2. Authenticate Routing Adjacencies ...................254.2.3. Isolate Router-to-Router Traffic ...................264.3. Router Implementation Issues ..............................264.3.1. Checking Protocol Syntax and Semantics .............264.3.2. Consistency Checks .................................27           4.3.3. Enhance Router Robustness through                  Operational Adjustments ............................284.3.4. Proper Handling of Router Resource Exhaustion ......284.4. End-System Implementation Issues ..........................294.4.1. State Lookup Complexity ............................294.4.2. Operational Issues .................................305. Conclusions ....................................................30Handley, et al.              Informational                      [Page 2]

RFC 4732                   DoS Considerations              November 20066. Security Considerations ........................................317. Acknowledgements ...............................................318. Normative References ...........................................319. Informative References .........................................32Appendix A. IAB Members at the Time of This Writing ...............361.  Introduction   A Denial-of-Service (DoS) attack is an attack in which one or more   machines target a victim and attempt to prevent the victim from doing   useful work.  The victim can be a network server, client or router, a   network link or an entire network, an individual Internet user or a   company doing business using the Internet, an Internet Service   Provider (ISP), country, or any combination of or variant on these.   Denial-of-service attacks may involve gaining unauthorized access to   network or computing resources, but for the most part in this   document we focus on the cases where the denial-of-service attack   itself does not involve a compromise of the victim's computing   facilities.   Because of the closed context of the original ARPANET and NSFNet, no   consideration was given to denial-of-service attacks in the original   Internet Architecture.  As a result, almost all Internet services are   vulnerable to denial-of-service attacks of sufficient scale.  In most   cases, sufficient scale can be achieved by compromising enough end-   hosts (typically using a virus or worm) or routers, and using those   compromised hosts to perpetrate the attack.  Such an attack is known   as a Distributed Denial-of-Service (DDoS) attack.  However, there are   also many cases where a single well-connected end-system can   perpetrate a successful DoS attack.   This document is intended to serve several purposes:   o To highlight possible avenues for attack, and by so doing encourage     protocol designers and network engineers towards designs that are     more robust.   o To discuss partial solutions that reduce the effectiveness of     attacks.   o To highlight how some partial solutions can be taken advantage of     by attackers to perpetrate alternative attacks.   This last point appears to be a recurrent theme in DoS, and   highlights the lack of proper architectural solutions.  It is our   hope that this document will help initiate informed debate about   future architectural solutions that might be feasible and cost-   effective for deployment.Handley, et al.              Informational                      [Page 3]

RFC 4732                   DoS Considerations              November 2006   In addition, it is our hope that this document will spur discussion   leading to architectural solutions that reduce the susceptibility of   all Internet systems to denial-of-service attacks.   We note that in principle it is not possible to distinguish between a   sufficiently subtle DoS attack and a flash crowd (where unexpected   heavy but non-malicious traffic has the same effect as a DoS attack).   Whilst this is true, such malicious attacks are usually more   expensive to launch than many of the crude attacks that have been   seen to date.  Thus, defending against DoS is not about preventing   all possible attacks, but rather is largely a question of raising the   bar sufficiently high for malicious traffic.   However, it is also important to note that not all DoS problems are   malicious.  Failed links, flash crowds, misconfigured bots, and   numerous other causes can result in resource exhaustion problems, and   so the overall goal should be to be robust to all forms of overload.2.  An Overview of Denial-of-Service Threats   In this section, we will discuss a wide range of possible DoS   attacks.  This list cannot be exhaustive, but the intent is to   provide a good overview of the spectrum of possibilities that need to   be defended against.   We do not provide descriptions of any attacks that are not already   publicly well documented.2.1.  DoS Attacks on End-Systems   We first discuss attacks on end-systems.  An end-system in this   context is typically a PC or network server, but it can also include   any communication endpoint.  For example, a router also is an end-   system from the point of view of terminating TCP connections for BGP   [10] or ssh [46].2.1.1.  Exploiting Poor Software Quality   The simplest DoS attacks on end-systems exploit poor software quality   on the end-systems themselves, and cause that software to simply   crash.  For example, buffer-overflow attacks might be used to   compromise the end-system, but even if the buffer-overflow cannot be   used to gain access, it will usually be possible to overwrite memory   and cause the software to crash.  Such vulnerabilities can in   principle affect any software that uses data supplied from the   network.  Thus, not only might a web server be potentially   vulnerable, but it might also be possible to crash the back-end   software (such as a database) to which a web server provides data.Handley, et al.              Informational                      [Page 4]

RFC 4732                   DoS Considerations              November 2006   Software crashes due to poor coding affect not only application   software, but also the operating system kernel itself.  A classic   example is the so-called "ping of death", which became widely known   in 1996 [21].  This exploit caused many popular operating systems to   crash when sent a single fragmented ICMP echo request packet whose   fragments totaled more than the 65535 bytes allowed in an IPv4   packet.   While DoS attacks such as the ping of death are a significant   problem, they are not a significant architectural problem.  Once such   an attack is discovered, the relevant code can easily be patched, and   the problem goes away.  We should note though that as more and more   software becomes embedded, it is important not to lose the   possibility of upgrading the software in such systems.2.1.2.  Application Resource Exhaustion   Network applications exist in a context that has finite resources.   In processing network traffic, such an application uses these   resources to do its intended task.  However, an attacker may be able   to prevent the application from performing its intended task by   causing the application to exhaust the finite supply of a specific   resource.   The obvious resources that might be exhausted include:   o Available memory.   o The CPU cycles available.   o The disk space available to the application.   o The number of processes or threads or both that the application is     permitted to use.   o The configured maximum number of simultaneous connections the     application is permitted.   This list is clearly not exhaustive, but it illustrates a number of   points.   Some resources are self-renewing: CPU cycles fall in this category --   if the attack ceases, more CPU cycles become available.   Some resources such as disk space require an explicit action to free   up -- if the application cannot do this automatically then the   effects of the attack may be persistent after the attack has ceased.Handley, et al.              Informational                      [Page 5]

RFC 4732                   DoS Considerations              November 2006   This problem has been understood for many years, and it is common   practice for logs and incoming email to be stored in a separate disk   partition (/var on Unix systems) in order to limit the impact of   exhaustion.   Some resources are constrained by configuration: the maximum number   of processes and the maximum number of simultaneous connections are   not normally hard limits, but rather are configured limits.  The   purpose of such limits is clearly to allow the machine to perform   other tasks in the event the application misbehaves.  However, great   care needs to be taken to choose such limits appropriately.  For   example, if a machine's sole task is to be an FTP server, then   setting the maximum number of simultaneous connections to be   significantly less than the machine can service makes the attacker's   job easier.  But setting the limit too high may permit the attacker   to cause the machine to crash (due to poor OS design in handling   resource exhaustion) or permit livelock (see below), which are   generally even less desirable failure modes.2.1.3.  Operating System Resource Exhaustion   Conceptually, OS resource exhaustion and application resource   exhaustion are very similar.  However, in the case of application   resource exhaustion, the operating system may be able to protect   other tasks from being affected by the DoS attack.  In the case of   the operating system itself running out of resources, the problem may   be more catastrophic.   Perhaps the best-known DoS attack on an operating system is the TCP   SYN-flood [19], which is essentially a memory-exhaustion attack.  The   attacker sends a flood of TCP SYN packets to the victim, requesting   connection setup, but then does not complete the connection setup.   The victim instantiates state to handle the incoming connections.  If   the attacker can instantiate state faster than the victim times it   out, then the victim will run out of memory that it can use to hold   TCP state, and so it cannot service legitimate TCP connection setup   attempts.  This issue was exacerbated in some implementations by the   use of a small dedicated storage space for half-open connections,   which made the attack easier than it might otherwise have been.  In   the case of a poorly coded operating system, running out of resources   may also cause a system crash.   An alternative TCP DoS attack is the Ack-flood [23], which is   essentially a CPU exhaustion attack on the victim.  The attacker   floods the victim with TCP packets pretending to be from connections   that have never been established.  A busy server that has a large   number of outstanding connections needs to check which connection the   packet corresponds to.  Some TCP implementations implemented thisHandley, et al.              Informational                      [Page 6]

RFC 4732                   DoS Considerations              November 2006   search rather inefficiently, and so the attacker could use all the   victim's CPU resources servicing these packets rather than servicing   legitimate requests.   We note that strong authentication mechanisms do not necessarily   mitigate against such CPU exhaustion attacks.  In fact, poorly   designed authentication mechanisms using cryptographic methods can   exacerbate the problem.  If such an authentication mechanism allows   an attacker to present a packet to the victim that requires   relatively expensive cryptographic authentication before the packet   can be discarded, then this makes the attacker's CPU exhaustion   attack easier.   CPU exhaustion attacks can be also be exacerbated by poor OS handling   of incoming network traffic.  In the absence of malicious traffic, an   ideal OS should behave as follows:   o As incoming traffic increases, the useful work done by the OS     should increase until some resource (such as the CPU) is saturated.   o From this point on, as incoming traffic continues to increase the     useful work done should be constant.   However, this is often not the case.  Many systems suffer from   livelock [33] where, after saturation, increasing the load causes a   decrease in the useful work done.  One cause of this is that the   system spends an increasing amount of time processing network   interrupts for packets that will never be processed, and hence a   decreasing amount of time is available for the application for which   these packets were intended.2.1.4.  Triggered Lockouts and Quota Exhaustion   Many user-authentication mechanisms attempt to protect against   password guessing attacks by locking the user out after a small   number of failed authentications.  If an attacker can guess or   discover a user's ID, they may be able to trigger such a mechanism,   locking out the legitimate user.   Another way to deny service using protection mechanisms is to cause a   quota to be exhausted.  This is perhaps most common in the case of   small web servers being commercially hosted, where the server has a   contract with the hosting company allowing a fixed amount of traffic   per day.  An attacker may be able to rapidly exhaust this quota, and   cause service to be suspended.  Similar attacks may be possible   against other forms of quota.Handley, et al.              Informational                      [Page 7]

RFC 4732                   DoS Considerations              November 2006   In the absence of such quotas, if the victim is charged for their   network traffic, a financial denial-of-service may be possible.2.2.  DoS Attacks on Routers   Many of the denial-of-service attacks that can be launched against   end-systems can also be launched against the control processor of an   IP router, for example, by flooding the command and control access   ports.  In the case of a router, these attacks may cause the router   to stall, or may cause the router to cease processing routing   packets.  Even if the router does not stop servicing routing packets,   it may become sufficiently slow that routing protocols time out.  In   any of these circumstances, the consequence of routing failure is not   only that the router ceases to forward traffic, but also that it   causes routing protocol churn that may have further side effects.   An example of such a side effect is caused by BGP route flap damping   [11], which is intended to reduce global routing churn.  If an   attacker can cause BGP routing churn, route flap damping may then   cause the flapping routes to be suppressed [31].  This suppression   likely causes the networks served by those routes to become   unreachable.   A DoS attack on the router control processor might also prevent the   router from being managed effectively.  This may prevent actions   being taken that would mitigate the DoS attack, and it might prevent   diagnosis of the cause of the problem.2.2.1.  Attacks on Routers through Routing Protocols   In addition to their roles as end-systems, most routers run dynamic   routing protocols.  The routing protocols themselves can be used to   stage a DoS attack on a router or a network of routers.  This   requires the ability to send traffic from addresses that might   plausibly have generated the relevant routing messages, which is   somewhat difficult with interior routing protocols but fairly easy   with External Border Gateway Protocol (eBGP), for instance.   The simplest attack on a network of routers is to overload the   routing table with sufficiently many routes that the router runs out   of memory, or the router has insufficient CPU power to process the   routes [26].  We note that depending on the distribution and   capacities of various routers around the network, such an attack   might not overwhelm routers near to the attacking router, but might   cause problems to show up elsewhere in the network.   Some routing protocol implementations allow limits to be configured   on the maximum number of routes to be heard from a neighbor [27].Handley, et al.              Informational                      [Page 8]

RFC 4732                   DoS Considerations              November 2006   However, limits often make the problem worse rather than better, by   making it possible for the attacker to push out legitimate routes   with spoofed routes, thus creating an easy form of DoS attack.   An alternative attack is to overload the routers on the network by   creating sufficient routing table churn that routers are unable to   process the changes.  Many routing protocols allow damping factors to   be configured to avoid just such a problem.  However, as with table   size, such a threshold applied inconsistently may allow the spoofed   routes to merge with legitimate routes before the mechanism is   applied, causing legitimate routes to be damped.   The simplest routing attack on a specific destination is for an   attacker to announce a spoofed desirable route to that destination.   Such a route might be desirable because it has low metric, or because   it is a more specific route than the legitimate route.  In any event,   if the route is believed, it will cause traffic for the victim to be   drawn towards the attacking router, where it will typically be   discarded.   A more subtle denial-of-service attack might be launched against a   network rather than against a destination.  Under some circumstances,   the propagation of inconsistent routing information can cause traffic   to loop.  If an attacker can cause this to happen on a busy path, the   looping traffic might cause significant congestion, as well as fail   to reach the legitimate destination.   In the past, there have been cases where different generations of   routers interpreted a routing protocol specification differently.  In   particular, BGP specifies that in the case of an error, the BGP   peering should be dropped.  However, if some of the routers in a   network treat a particular route as valid and other routers treat the   route as invalid, then it may be possible to inject a BGP route at   one point in the Internet and cause peerings to be dropped at many   other places in the Internet.  Unlike many of the examples above,   while such an issue might be a serious short-term problem, this is   not a fundamental architectural problem.  Once the problem is   understood, deploying patched routing code can permanently solve the   issue.2.2.2.  IP Multicast-based DoS Attacks   There are essentially two forms of IP multicast: traditional Any-   Source Multicast (ASM), as specified inRFC 1112 [4] where multiple   sources can send to the same multicast group, and Source-Specific   Multicast (SSM) where the receiver must specify both the IP source   address and the group address.  The two forms of multicast provide   rather different DoS possibilities.Handley, et al.              Informational                      [Page 9]

RFC 4732                   DoS Considerations              November 2006   ASM protocols such as PIM-SM [6], MSDP [32], and DVMRP [12] typically   cause some routers to instantiate routing state at the time a packet   is sent to a multicast group.  They do this to ensure that the   traffic goes to the group receivers and not to non-receivers.  Such   protocols are particularly vulnerable to DoS attacks, as an attacker   that sends to many multicast groups may cause both multicast routing   table explosion (and hence control processor memory exhaustion) and   multicast forwarding table exhaustion (and hence forwarding card   memory exhaustion or thrashing).   ASM also permits an attacker to send traffic to the same group as   legitimate traffic, potentially causing network congestion and   denying service to the legitimate group.   SSM does not permit senders to send to arbitrary groups unless a   receiver has requested the traffic.  Thus, sender-based attacks on   multicast routing state are not possible with SSM.  However, as with   ASM, a receiver can still join a large number of multicast groups   causing routers to hold a large amount of multicast routing state,   potentially causing memory exhaustion and hence denial-of-service to   legitimate traffic.   With IPv6, hosts are required to send ICMP Packet Too Big or   Parameter Problem messages under certain circumstances, even if the   destination address is a multicast address.  If the attacker can   place himself in the appropriate position in the multicast tree, a   packet with an unknown but mandatory Destination Option, for   instance, could generate a very large number of responses to the   claimed sender.   With IPv4, the same problem exists with multicast ICMP Echo Request   packets, but these are somewhat easier to filter.   The examples above should not be taken as exhaustive.  These are   actually specific cases of a general problem that can happen when a   multicast/broadcast request solicits a reply from a large number of   nodes.2.2.3.  Attacks on Router Forwarding Engines   Router vendors implement many different mechanisms for packet   forwarding, but broadly speaking they fall into two categories: ones   that use a forwarding cache, and ones that do not.  With a forwarding   cache, the forwarding engine does not hold the full routing table,   but rather holds just the currently active subset of the forwarding   table.Handley, et al.              Informational                     [Page 10]

RFC 4732                   DoS Considerations              November 2006   Many modern routers use a loosely coupled architecture, where one or   more control processors handle the routing protocols and communicate   over an internal network link to special-purpose forwarding engines,   which actually forward the data traffic.  In such architectures, it   may be possible for an attacker to overwhelm the communications link   between the control processor and the forwarding engine.  This is   possible because the forwarding engines support very high speed   links, and the control processor simply cannot handle a similar rate   of traffic.   There may be many ways in which an attacker can trigger communication   between the forwarding engines and the control processor.  The   simplest way is for the attacker to simply send to the router's IP   address, but this should in principle be relatively easy to prevent   using filtering on the forwarding engines.  Another way might be to   cause the router to forward data packets using the "slow path".  This   involves sending packets that require special attention from the   forwarding router; if the forwarding engine is not smart enough to   perform such forwarding, then it will typically pass the packet to   the control processor.  In a router using a forwarding cache, it may   be possible to overload the internal communications by thrashing the   forwarding cache.  Finally, any form of data-triggered communication   between the forwarding engine and the control processor might cause   such a problem.  Certain multicast routing protocols including PIM-SM   contain many such data triggered events that could potentially be   problematic.   The effects of overloading such internal communications are hard to   predict and are very implementation-dependent.  One possible effect   might be that the forwarding table in the forwarding engine gets out   of synchronization with the routing table in the control processor   that reflects what the routing protocols believe is happening.  This   might cause traffic to be dropped or to loop.   Finally, if an attacker can generate traffic that causes a router to   auto-install access control list (ACL) entries, perhaps by triggering   a response from an intrusion detection system, then it may be   possible to exhaust the ACL resources on the router.  This might   prevent future attacks from being filtered, or worse, cause ACL   processing to be handled by the route processor.2.3.  Attacks on Ongoing Communications   Instead of attacking the end-system itself, it is also possible for   an attacker to disrupt ongoing communications.  If an attacker can   observe a TCP connection, then it is relatively easy for them to   spoof packets to either reset that connection or to de-synchronize it   so that no further progress can be made [29].  Such attacks are notHandley, et al.              Informational                     [Page 11]

RFC 4732                   DoS Considerations              November 2006   prevented by transport or application-level security mechanisms such   as TLS [5] or ssh, because the authentication takes place after TCP   has finished processing the packets.   If an attacker cannot observe a TCP connection, but can infer that   such a connection exists, it is theoretically possible to reset or   de-synchronize that connection by spoofing packets into the   connection.  However, this might require an excessively large number   of spoofed packets to guess both the port of the active end of the   TCP connection (in most cases, the port of the passive end is   predictable) and the currently valid TCP sequence numbers.  However,   as some operating systems have poorly implemented predictable   algorithms for selecting either the dynamically selected port or the   TCP initial sequence number [41] [20], then such attacks have been   found to be feasible [34].  Advice as to how to reduce the   vulnerability in the specific case of TCP is available in [37].   An attacker might be able to significantly reduce the throughput of a   connection by sending spoofed ICMP source quench packets, although   most modern operating systems should ignore such packets.  However,   care should be taken in the design of future transport and signaling   protocols to avoid the introduction of similar mechanisms that could   be exploited.2.4.  Attacks Using the Victim's Own Resources   Instead of directly overloading the victim, it may be possible to   cause the victim or a machine on the same subnet as the victim to   overload itself.   An example of such an attack is documented in [18], where the   attacker spoofs the source address on a packet sent to the victim's   UDP echo port.  The source address is that of another machine that is   running a UDP chargen server (a chargen server sends a character   pattern back to the originating source).  The result is that the two   machines bounce packets back and forth as fast as they can,   overloading either the network between them or one of the end-systems   itself.2.5.  DoS Attacks on Local Hosts or Infrastructure   There are a number of attacks that might only be performed by a local   attacker.   An attacker with access to a subnet may be able to prevent other   local hosts from accessing the network at all by simply exhausting   the address pool allocated by a Dynamic Host Configuration Protocol   (DHCP) server.  This requires being able to spoof the MAC address ofHandley, et al.              Informational                     [Page 12]

RFC 4732                   DoS Considerations              November 2006   an ethernet or wireless card, but this is quite feasible with certain   hardware and operating systems.   An alternative DHCP-based attack is simply to respond faster than the   legitimate DHCP server, and to give out an address that is not useful   to the victim.   These sorts of bootstrapping attacks tend to be difficult to avoid   because most of the time trust relationships are established after IP   communication has already been established.   Similar attacks are possible through ARP spoofing [16]; an attacker   can respond to ARP requests before the victim and prevent traffic   from reaching the victim.  Some brands of ethernet switch allow an   even simpler attack: simply send from the victim's MAC address, and   the switch will redirect traffic destined for the victim to the   attacker's port.  This attack might also potentially be used to block   traffic from the victim by engaging screening or flap-dampening   algorithms in the switch, depending on the switch design.   It may be possible to cause broadcast storms [16] on a local LAN by   sending a stream of unicast IP packets to the broadcast MAC address.   Some hosts on the LAN may then attempt to forward the packets to the   correct MAC address, greatly amplifying the traffic on the LAN.   802.11 wireless networks provide many opportunities to deny service   to other users.  In some cases, the lack of defenses against DoS was   a deliberate choice--because 802.11 operates on unlicensed spectrum   it was assumed that there would be sources of interference and that   producing intentional radio-level jamming would be trivial.  Thus,   the amount of DoS protection possible at higher levels was minimal.   Nevertheless, some of the weaknesses of the protocols against more   sophisticated attacks are worth noting.  The most prominent of these   is that association is unprotected, thus allowing rogue access points   (APs) to solicit notifications that would otherwise have gone to   legitimate APs.   The SSID field provides effectively no defense against this kind of   attack.  Unless encryption is enabled, it is trivial to announce the   presence of a base station (or even of an ad-hoc mode host) with the   same network name (SSID) as the legitimate basestation.  Even adding   authentication and encryption a la 802.1X and 802.11i may not help   much in this respect.  The SSID space is unmanaged, so everyone is   free to put anything they want in the SSID field.  Most host stacks   don't deal gracefully with this.  Moreover, SSIDs are very often set   to the manufacturer's default, making them highly predictable.Handley, et al.              Informational                     [Page 13]

RFC 4732                   DoS Considerations              November 2006   Some 802.11 basestations have limited memory for the number of   associations they can support.  If this is exceeded, they may drop   all associations.  In an attempt to forestall this problem, some APs   advertise their load so as to enable stations to choose APs that are   less loaded.  However, crude implementations of these algorithms can   result in instability.   Finally, as the authentication in 802.11 takes place at a   comparatively high level in the stack, it is possible to simply   deauthenticate or disassociate the victim from the basestation, even   if Wired Equivalent Privacy (WEP) is in use [30].  Bellardo and   Savage [15] describe some simple remedies that reduce the   effectiveness of such attacks.  While IEEE 802.11w will protect   Deauthenticate or Disassociate frames, this attack is still possible   via forging of Association frames.   What all these attacks have in common is that they exploit   vulnerabilities in the link auto-configuration mechanisms.  In a   wireless network, it is necessary for a station to detect the   presence of APs in order to choose which one to connect to.  In   802.11, this is handled via the Beacon and Probe Request/Response   mechanisms.   Beacons cannot easily be encrypted, because the station needs to   utilize them prior to authentication in order to discover which APs   it may wish to communicate with.  Since authentication can only occur   after interpreting the Beacon, an encrypted Beacon would present a   chicken-egg problem: you can't obtain a key to decrypt the Beacon   until completing authentication, and you may not be able to figure   out which AP to authenticate with prior to decrypting the Beacon.   Note that in principle you could encrypt Beacons with a shared   (per-AP) key but this would require each station to trial-decrypt   beacons until it finds one that matches up to whatever shared   authentication secret it had.  This is not particularly convenient.   As a result, discussions of Beacon frame security have largely   focused on authentication of Beacon frames, not encryption.  Even   here, solutions are difficult.  While it may be possible for a   station to validate a Beacon *after* authentication (either by   checking a Message Integrity Check (MIC) computed with the group key   provided by the AP or verifying the Beacon parameters during the   4-way handshake), doing so *before* authentication may require   synchronization of keys between APs within an SSID.Handley, et al.              Informational                     [Page 14]

RFC 4732                   DoS Considerations              November 20062.6.  DoS Attacks on Sites through DNS   In today's Internet, DNS is of sufficient importance that if access   to a site's DNS servers is denied, the site is effectively   unreachable, even if there is no actual communication problem with   the site itself.   Many of the attacks on end-systems described above can be perpetrated   on DNS servers.  As servers go, DNS servers are not particularly   vulnerable to DoS.  So long as a DNS server has sufficient memory, a   modern host can usually respond very rapidly to DNS requests for   which it is authoritative.  This was demonstrated in October 2002   when the root nameservers were subjected to a very large DoS attack   [38].  A number of the root nameservers have since been replicated   using anycast [1] to further improve their resistance to DoS.   However, it is important for authoritative servers to have relaying   disabled, or it is possible for an attacker to force the DNS servers   to hold state [40].   Many of the routing attacks can also be used against DNS servers by   targeting the routing for the server.  If the DNS server is co-   located with the site for which is authoritative, then the fact that   the DNS server is also unavailable is of secondary importance.   However, if all the DNS servers are made unavailable, this may cause   email to that site to bounce rather than being stored while the mail   servers are unreachable, so distribution of DNS server locations is   important.   Causing network congestion on links to and from a DNS server can have   similar effects to end-system attacks or routing attacks, causing DNS   to fail to obtain an answer, and effectively denying access to the   site being served.   We note that if an attacker can deny external access to all the DNS   servers for a site, this will not only cause email to that site to be   dropped, but it will also cause email from that site to be dropped.   This is because recent versions of mail transfer agents such as   sendmail will drop email if the mail originates from a domain that   does not exist.  This is a classic example of unexpected   consequences.  Sendmail performs this check as an anti-spam measure,   and spam itself can be viewed as a form of DoS attack.  Thus,   defending against one DoS attack opens up the vulnerability that   allows another DoS attack.  If a receiving implementation is using a   black-hole list (seeSection 2.14) served by DNS, an attacker can   also mount a DoS attack by attacking the black-hole server.Handley, et al.              Informational                     [Page 15]

RFC 4732                   DoS Considerations              November 2006   Finally, a data corruption attack is possible if a site's nameserver   is permitted to relay requests from untrusted third parties [40].   The attacker issues a query for the data he wishes to corrupt, and   the victim's nameserver relays the request to the authoritative   nameserver.  The request contains a 16-bit ID that is used to match   up the response with the request.  If the attacker spoofs sufficient   response packets from the authoritative nameserver just before the   official response arrives, each containing a forged response and a   different DNS ID, then there is a reasonable chance that one of the   forged responses will have the correct DNS ID.  The incorrect data   will then be believed and cached by the victim's nameserver, so   giving the incorrect response to future queries.  The probability of   the attack can further be increased if the attacker issues many   different requests for the same data with different DNS IDs, because   many nameserver implementations will issue relayed requests with   different DNS IDs, and so the response only has to match any one of   these request IDs [17] [36].   The use of anycast for DNS services makes it even more vulnerable to   spoofing attacks.  An attacker who can convince the ISP to accept an   anycast route to his fake DNS server can arrange to receive requests   and generate fake responses.  Anycast DNS also makes DoS attacks on   DNS easier.  The idea is to disable one of the DNS servers while   maintaining the BGP route to that server.  This creates failures for   any client that is routed to the (now defunct) server.2.7.  DoS Attacks on Links   The simplest DoS attack is to simply send enough non-congestion-   controlled traffic such that a link becomes excessively congested,   and legitimate traffic suffers unacceptably high packet loss.   Under some circumstances, the effect of such a link DoS can be much   more extensive.  We have already discussed the effects of denying   access to a DNS server.  Congesting a link might also cause a routing   protocol to drop an adjacency if sufficient routing packets are lost,   potentially greatly amplifying the effects of the attack.  Good   router implementations will prioritize the transmission of routing   packets, but this is not a total panacea.  If routers are peered   across a shared medium such as ethernet, it may be possible to   congest the medium sufficiently that routing packets are still lost.   Even if a link DoS does not cause routing packets to be lost, it may   prevent remote access to a router using ssh or Simple Network   Management Protocol (SNMP) [48].  This might make the router   unmanageable, or prevent the attack from being correctly diagnosed.Handley, et al.              Informational                     [Page 16]

RFC 4732                   DoS Considerations              November 2006   The prioritization of routing packets can itself cause a DoS problem.   If the attacker can cause a large amount of routing flux, it may be   possible for a router to send routing packets at a high enough rate   that normal traffic is effectively excluded.  However, this is   unlikely except on low-bandwidth links.   Finally, it may be possible for an attacker to deny access to a link   by causing the router to generate sufficient monitoring or report   traffic that the link is filled.  SNMP traps are one possible vector   for such an attack, as they are not normally congestion controlled.   Attackers with physical access to multiple access links can easily   bring down the link.  This is particularly easy to mount and   difficult to counter with wireless networks.2.8.  DoS Attacks on Firewalls   Firewalls are intended to defend the systems behind them against   attack.  In that they restrict the traffic that can reach those   systems, they may also aid in defending against denial-of-service   attacks.  However, under some circumstances the firewall itself may   also be used as a weapon in a DoS attack.   There are many different types of firewall, but generally speaking   they fall into stateful and stateless classes.  The state here refers   to whether the firewall holds state for the active flows traversing   the firewall.  Stateless firewalls generally can only be attacked by   attempting to exhaust the processing resources of the firewall.   Stateful firewalls can be attacked by sending traffic that causes the   firewall to hold excessive state or state that has pathological   structure.   In the case of excessive state, the firewall simply runs out of   memory, and can no longer instantiate the state required to pass   legitimate flows.  Most firewalls will then fail disconnected,   causing denial-of-service to the systems behind the firewall.   In the case of pathological structure, the attacker sends traffic   that causes the firewall's data structures to exhibit worst-case   behaviour.  An example of this would be when the firewall uses hash   tables to look up forwarding state, and the attacker can predict the   hash function used.  The attacker may then be able to cause a large   amount of flow state to hash to the same bucket, which causes the   firewall's lookup performance to change from O(1) to O(n), where n is   the number of flows the attacker can instantiate [28].  Thus, the   attacker can cause forwarding performance to degrade to the point   where service is effectively denied to the legitimate traffic   traversing the firewall.Handley, et al.              Informational                     [Page 17]

RFC 4732                   DoS Considerations              November 20062.9.  DoS Attacks on IDS Systems   Intrusion detection systems (IDSs) suffer from similar problems to   firewalls.  It may be possible for an attacker to cause the IDS to   exhaust its available processing power, to run out of memory, or to   instantiate state with pathological structure.  Unlike a firewall, an   IDS will normally fail open, which will not deny service to the   systems protected by the IDS.  However, it may mean that subsequent   attacks that the IDS would have detected will be missed.   Some IDSs are reactive; that is, on detection of a hostile event they   react to block subsequent traffic from the hostile system, or to   terminate an ongoing connection from that system.  It may be possible   for an attacker to spoof packets from a legitimate system, and hence   cause the IDS to believe that system is hostile.  The IDS will then   cause traffic from the legitimate system to be blocked, hence denying   service to it.  The effect can be particularly bad if the legitimate   system is a router, DNS server, or other system whose performance is   essential for the operation of a large number of other systems.2.10.  DoS Attacks on or via NTP   Network time servers are generally not considered security-critical   services, but under some circumstances NTP servers might be used to   perpetrate a DoS attack.   The most obvious such attack is to DoS the NTP servers themselves.   Many end-systems have rather poor clock accuracy and so, without   access to network time, their clock will naturally drift.  This can   cause problems with distributed systems that rely on good clocks.   For example, one commonly used revision control system can fail if it   perceives the modification timestamp to be in the future.   If the NTP servers relied on by a host can be subverted, either   through compromising or impersonating them, then the attacker may be   able to control the host's system clock.  This can cause many   unexpected consequences, including the premature expiry of dated   resources such as encryption or authentication keys.  This in turn   can prevent access to other more critical services.2.11.  Physical DoS   The discussion thus far has centered on denial-of-service attacks   perpetrated using the network.  However, computer systems are only as   resilient as the weakest link.  It may be easier to deny service by   causing a power failure, by cutting network cables, or by simply   switching a system off, and so physical security is at least as   important as network security.  Physical attacks can also serve asHandley, et al.              Informational                     [Page 18]

RFC 4732                   DoS Considerations              November 2006   entry points for non-physical DoS, for instance, by reducing the   resources available to deal with overcapacity.2.12.  Social Engineering DoS   The weakest link may also be human.  In defending against DoS, the   possibility of denial-of-service through social engineering should   not be neglected, such as convincing an employee to make a   configuration change that prevents normal operation.2.13.  Legal DoS   Computer systems cannot be considered in isolation from the social   and legal systems in which they operate.  This document focuses   primarily on the technical issues, but we note that "cease and   desist" letters, government censorship, and other legal mechanisms   also touch on denial-of-service issues.2.14.  Spam and Black-Hole Lists   Unsolicited commercial email, also known as "spam", can effectively   cause denial-of-service to email systems.  While the intent is not   denial-of-service, the large amount of unwanted mail can waste the   recipient's time or cause legitimate email to fail to be noticed   amongst all the background noise.  If spam filtering software is   used, some level of false positives is to be expected, and so these   messages are effectively denied service.   One mechanism to reduce spam is the use of black-hole lists.  The IP   addresses of dial-up ISPs or mail servers used to originate or relay   spam are added to black-hole lists.  The recipients of mail choose to   consult these lists and reject spam if it originates or is relayed by   systems on the list.  One significant problem with such lists is that   it may be possible for an attacker to cause a victim to be black-   hole-listed, even if the victim was not responsible for relaying   spam.  Thus, the black-hole list itself can be a mechanism for   effecting a DoS attack.  Note that every black-hole list has its own   policy regarding additions, and some are less susceptible to this DoS   attack than others.  Consumers of black-hole list technology are   advised to investigate these policies before they subscribe.  Similar   considerations apply to feeds of bad BGP bad route advertisements.Handley, et al.              Informational                     [Page 19]

RFC 4732                   DoS Considerations              November 20063.  Attack Amplifiers   Many of the attacks described above rely on sending sufficient   traffic to overwhelm the victim.  Such attacks are made much easier   by the existence of "attack amplifiers", where an attacker can send   traffic from the spoofed source address of the victim and cause   larger responses to be returned to the victim.  A detailed discussion   of such reflection attacks can be found in [35].3.1.  Methods of Attack Amplification   The simplest such attack was the "smurf" attack [22], where an ICMP   echo request packet with the spoofed source address of the victim is   sent to the subnet-broadcast address of a network to be used as an   amplifier.  Every system on that subnet then responds with an ICMP   echo response that returns to the victim.  Smurf attacks are no   longer such a serious problem, as these days routers usually drop   such packets and end-systems do not respond to them.   An alternative form of attack amplifier is typified by a DNS   reflection attack.  An attacker sends a DNS request to a DNS server   requesting resolution of a domain name.  Again the source address of   the request is the spoofed address of the victim.  The request is   carefully chosen so that the size of the response is significantly   greater than the size of the request, thereby providing the   amplification.  As an aside, it is interesting to note that the   largest DNS responses tend to be those incorporating DNSsec   authentication information.  This attack amplifier can only be used   by an attacker with the ability to spoof the source address of the   victim.  However, we note that if the victim's DNS server is   configured to relay requests from external clients, it may be   possible to cause it to congest its own incoming network link.   Another variant of attack amplifier involves amplification through   retransmission.  This is typified by a TCP amplification attack known   as "bang.c".  The attacker sends a spoofed TCP SYN with the source   address of the victim to an arbitrary TCP server.  The server will   respond with a SYN|ACK that is sent to the victim, and when no final   ACK is received to complete the handshake, the SYN|ACK will be   retransmitted a number of times.  Typically, this attack uses a very   large list of arbitrarily chosen servers as reflectors.  For the   attack to be successful, the reflector must not receive a RST from   the victim in response to the SYN|ACK.  However, if the attack   traffic sufficiently overwhelms the server or access link to the   server, then packet loss will ensure that many reflectors do not   receive a RST in response to their SYN|ACK, and so continue to   retransmit.  The attack can be exacerbated by firewalls that silently   drop the incoming SYN|ACK without sending a RST.Handley, et al.              Informational                     [Page 20]

RFC 4732                   DoS Considerations              November 2006   Care must also be taken with services that relay requests.  If an   attacker can send a request to a proxy, and that proxy now attempts   to connect to a victim whose address is chosen by the attacker, then,   if the proxy repeatedly resends the request when receiving no answer,   this can also serve as an attack amplifier.   Another variant of amplification occurs in protocols that include,   within the protocol payload, an IP address or name of host to which   subsequent messages should be sent.  An example of such a protocol is   the Session Initiation Protocol (SIP) [50], which carries a payload   defined by the Session Description Protocol (SDP) [51].  The SDP   payload of the SIP message conveys the IP address and port to which   media packets, typically encoded using the Real Time Transport   Protocol (RTP) [52], are sent.   To launch this attack, an attacker sends a protocol message, and sets   the IP address within the payload to point to the attack target.  The   recipient of the message will generate subsequent traffic to that IP   address.  Depending on the protocol, this attack can provide   substantial amplification properties.  In the specific case of SIP,   if a caller makes calls to high-bandwidth media sources (such as a   video server or streaming audio server), a single SIP INVITE packet,   typically a few hundred bytes, can result in a nearly continuous   stream of media packets at rates anywhere from a few kbits per second   up to megabits per second.  This particular attack is called the   "voice hammer".   Unlike the other techniques described above, this technique does not   require the attacker to modify packets or even spoof their source IP   address.  This makes it easier to launch.   This attack is prevented through careful protocol design.  Protocols   should, whenever possible, avoid including IP addresses or hostnames   within protocol payloads as addresses to which subsequent messaging   should be sent.  Rather, when possible, messages should be sent to   the source IP from which the protocol packet came.  If such a design   is not possible, the protocol should include a handshake whereby it   can be positively determined that the protocol entity at that IP   address or hostname does, in fact, wish to receive that subsequent   messaging.  That handshake itself needs to be lightweight (to avoid   being the source of another DoS attack), and secured against the   spoofing of the handshake response.   Finally, a somewhat similar attack is possible with some protocols   where one message leads to another message that is not sent as a   reply to the source address of the first message.  This can be anHandley, et al.              Informational                     [Page 21]

RFC 4732                   DoS Considerations              November 2006   issue with protocols to enable mobility, for example, and might   permit an attacker to avoid ingress filtering.  Such protocols are   notoriously difficult to get right.3.2.  Strategies to Mitigate Attack Amplification   In general, the architectural lessons to be learnt are simple:   o  As far as possible, perform ingress filtering [7] [39] to prevent      source address spoofing.   o  Avoid designing protocols or mechanisms that can return      significantly larger responses than the size of the request,      unless a handshake is performed to validate the client's source      address.  Such a handshake needs to incorporate an unpredictable      nonce that is secure enough to mitigate the amplification effects      of the protocol.   o  All retransmission during initial connection setup should be      performed by the client.   o  Proxies should not arbitrarily relay requests to destinations      chosen by a client.   o  Avoid signaling third-party connections.  Any unavoidable third-      party connections set up by a signaling protocol should      incorporate lightweight validation before sending significant      data.4.  DoS Mitigation Strategies   A general problem with DoS defense is that it is not in principle   possible to distinguish between a flash crowd and a DoS attack.   Indeed, having your site taken down by a flash crowd is probably a   more common experience than having it DoS-ed -- so common it has   acquired its own names: being Slashdotted or Farked, after the web   sites that are common sources of flash crowds.  Thus, the first line   of defense against DoS attacks must be to provision your service so   that it can handle a foreseeable legitimate peak load.   Underprovisioned sites are the easiest to take down.   Specific strategies for DoS defense fall into two broad categories:   1.  Avoiding allowing attacks that are better than generic resource       consumption.   2.  Minimizing the extent to which generic resource consumption       attacks crowd out legitimate users.Handley, et al.              Informational                     [Page 22]

RFC 4732                   DoS Considerations              November 2006   In the remainder of this section, we consider specific applications   of these two approaches at a variety of levels of network system   architecture.4.1.  Protocol Design4.1.1.  Don't Hold State for Unverified Hosts   From an end-system server point of view, one simple aim is to avoid   instantiating state without having completed a handshake with the   client to validate their address, and as far as possible to push work   and stateholding to client.  There are a number of techniques that   might be used to do this, including SYN cookies [2] [14].  All   client-server protocols should probably be designed to allow such   techniques to be used, but the enabling of the mechanism should   normally be at the server's discretion to avoid unnecessary work   under normal circumstances.4.1.2.  Make It Hard to Simulate a Legitimate User   Other than having massive overcapacity, the only real defense against   resource consumption attacks is to preferentially discriminate   against attackers.  The general idea is to find something that   legitimate users can do but attackers can't.  The most commonly   proposed approaches include:   1.  Puzzles: force the attacker to do some computation that would not       be onerous for a single user but is too expensive to do en masse       [14].   2.  Reverse Turing tests: specialized puzzles that are hard for       machines to do but easy for humans, thus making automated attacks       hard [13].   3.  Reachability testing: force the proposed client to demonstrate       that it can receive traffic at a given IP address.  This makes it       easier to trace attackers.   All of these techniques have substantial limitations.  Puzzles tend   to discriminate against legitimate users with slow computers.  In   addition, the wide availability of remotely controlled compromised   machines ("bots") means that attackers have ample computing power at   their disposal.  There has been substantial work in attacking reverse   Turing tests automatically, thus making them of limited   applicability.  Finally, reachability testing is substantially   weakened by bots because the attacker does not need to hide his   source address.Handley, et al.              Informational                     [Page 23]

RFC 4732                   DoS Considerations              November 20064.1.3.  Graceful Routing Degradation   A goal with routing protocols is that of graceful degradation in   overload, and automatic recovery after the source of the overload has   been remedied.  Some routing protocols satisfy this goal more than   others.  Although RIP [53] doesn't scale well, if a router runs out   of memory when receiving a RIP route, it can just drop the route and   send an infinite metric to its peers.  The route will later be   refreshed, and if the original source of the problem has been   resolved, the router will now be able to process it correctly.   On the other hand, BGP is stateful in the sense that a peer assumes   you have processed or chosen to filter any route that it sent you.   There is no mechanism to refresh state in the base BGP spec, and even   the later route refresh option [3] is hard to use in the presence of   overload.  A BGP router that cannot store a route it received has two   choices: completely restart BGP or shut down one or more peerings   [26].  This means that the effects of a BGP overload are rather more   severe than they need to be, and so amplifies the effect of any   attack.   In general, few routing protocol designs actively consider the   possible behaviour of routers under overload conditions; this should   be an explicit part of future routing protocol designs.  Although   precise details should clearly be left to implementors, the protocol   design needs to give them the capability to do their job properly.4.1.4.  Autoconfiguration and Authentication   Autoconfiguration mechanisms greatly ease deployment, and are   increasingly necessary as the number of networked devices grows   beyond what can be managed manually.  However, it should be   recognised that unauthenticated autoconfiguration opens up many   avenues for attack.  There is a clear tension between ease of   configuration and security of configuration, especially because there   are environments in which it is desirable for units to operate with   effectively no authentication (e.g., airport hotspots).  Future   autoconfiguration protocols should consider the need to allow   different end-systems to operate at different points in this spectrum   within the same autoconfiguration framework.  However, this also   implies that the network elements should avoid acting for   unauthenticated hosts, instead just letting them access the network   more or less directly.Handley, et al.              Informational                     [Page 24]

RFC 4732                   DoS Considerations              November 20064.2.  Network Design and Configuration   In general, networks should be provisioned with private, out-of-band   access to console or control ports so that such control facilities   will be available in the face of a DoS attack launched against either   the control or data plane of the (in-band) network.  Typically, such   out-of-band networks are provisioned on a separate infrastructure for   exactly this purpose.  Out-of-band access is a crucial capability for   DoS mitigation, since many of the typical redundancy and capacity   management techniques (such as prioritizing routing or network   management traffic) fail during such attacks.  In addition, many   redundancy protocols such as VRRP [47] can fail during such attacks   as they may be unable to keep adjacencies alive.   There are several default configuration settings that can also be   exploited to generate several of the attacks outlined in this   document.  For example, some vendors may have features such as IP   redirect, directed broadcast, and proxy ARP enabled by default.   Similar defaults, such as publicly readable SNMP [48] communities   (e.g., "public") can be used to reveal otherwise confidential   information to a prospective attacker.  Finally, other   unauthenticated configuration management protocols such as TFTP [49]   should be avoided if possible; at the very least access to TFTP   configuration archives should be protected and TFTP should be   filtered at administrative boundaries.  Finally, since many of the   password encryption techniques used by router vendors are reversible,   keeping such passwords on a configuration archive (as part of a   configuration file), even in the encrypted form written by the   router, can lead to unauthorized access if the archive is   compromised.4.2.1.  Redundancy and Distributed Service   A basic principle of designing systems to handle failure is to have   redundant servers that can take over when one fails.  This is equally   true in the case of DoS attacks, which often focus on a given server   and/or link.  If service delivery points can be distributed across   the network, then it becomes much harder to attack the entire   service.  In particular, this makes attacks on a single network link   more difficult.4.2.2.  Authenticate Routing Adjacencies   In general, cryptographic authentication mechanisms are too costly to   form the main part in DoS prevention.  However, routing adjacencies   are too important to risk an attacker being able to inject bad   routing information, which can affect more than the router in   question.  Additional non-cryptographic mechanisms should then beHandley, et al.              Informational                     [Page 25]

RFC 4732                   DoS Considerations              November 2006   used to avoid arbitrary end-systems being able to cause the router to   spend CPU cycles on validating authentication data.   For BGP, at the very least, this implies the use of TCP MD5 [9] or   IPsec authentication, combined with the GTSM [8] to prevent eBGP   association with non-immediate neighbors.  In the future, this will   likely imply better authentication of the routing information itself.4.2.3.  Isolate Router-to-Router Traffic   As far as is feasible, router-to-router traffic should be isolated   from data traffic.  How this should be implemented depends on the   precise technologies available, both in the router and at the link   layer.  The goal should be that failure of the link for data traffic   should also cause failure for the routing traffic, but that an   attacker cannot directly send packets to the control processor of the   routers.   A downside of this is that some diagnostic techniques (such as   pinging consecutive routers to find the source of a delay) may no   longer be possible.  Ideally, alternative mechanisms (which do not   open up additional avenues for DoS) should be designed to replace   such lost techniques.4.3.  Router Implementation Issues   Because a router can be considered as an end-system, it can   potentially benefit from all the prevention mechanisms prescribed for   end-system implementation.  However, one basic distinction between a   router and a host is that the former implements routing protocols and   forwards data, which in turn lead to additional router-specific   implementation considerations.  The issues described below are meant   to be illustrative and not exhaustive.4.3.1.  Checking Protocol Syntax and Semantics   Protocol syntax defines the formation of the protocol messages and   the rules of exchanges.  The questions addressed by protocol syntax   checking includes, but is not limited to, the following:   1.  Who sent the message?   2.  Does the content conform to the protocol format?   3.  Was the message sent with correct timing?Handley, et al.              Informational                     [Page 26]

RFC 4732                   DoS Considerations              November 2006   The first step in protocol syntax verification is to ensure that an   incoming message was sent by a legitimate party.  There are multiple   ways to perform this check.  One can verify the source IP address and   even the MAC address of the message.  Utilizing the fact that eBGP   peers are normally directly connected, one can also check the TTL   value in a packet and discard any BGP updates packet whose TTL is   less than some maximum value (typically, max TTL - 1) [8].   Cryptographic authentication should also be used whenever available   to verify that an incoming message is indeed from an expected sender.   For BGP, at the very least, this implies the use of TCP MD5 [9] or   IPsec authentication.   In addition to the sender verification, it is also important to check   the syntax of a received routing message, as opposed to assuming that   all messages came in a correct format.  It happened in the past that   routers crashed upon receiving ill-formed routing messages.  Such   faults will be prevented by performing rigorous syntax checking.4.3.2.  Consistency Checks   Protocol semantics define the meaning of the message content, the   interpretation of the values, and the actions to be taken according   to the content.  Here is a simple example of using semantic checking.   When a link failure causes a router in Autonomous System (AS) A to   send a peer router B a withdrawal message for prefix P, B should make   sure that any alternative path it finds to reach P does not go   through A.  This simple check is shown to significantly improve BGP   convergence time in many cases [42].   Another example of using semantic checking against false routing   injection is described in [44].  The basic idea is to attach to the   route announcement for prefix P a list of the valid origin ASes.  Due   to the rich connectivity in today's Internet topology, a remote AS   will receive routing updates from multiple different paths and can   check to see whether each update carries the identical origin AS   list.  Although a false origin may announce reachability to P, or   alter the origin AS list, it would be difficult, if not impossible,   to block the correct updates from propagating out, and thus remote   ASes can detect the existence of false updates by observing the   inconsistency of the received origin AS lists for P.  Research   studies show that the "allowed origin list" test can effectively   detect the majority of falsely originated updates.   Generally speaking, verifying the validity of BGP routes can be   challenging because BGP is policy driven and policies of individual   ISPs are not known in most cases.  But assuming that policies do not   change in short time scale, in principle one could verify new updates   against observed routes from the recent past, which reflect theHandley, et al.              Informational                     [Page 27]

RFC 4732                   DoS Considerations              November 2006   routing policies in place.  Research work is needed to explore this   direction.   Note that while the above steps are all fairly simple and don't   really "bulletproof" the protocol, each adds some degree of   protection.  As such, the combination of the above techniques can   result in an effective reduction in the probability of undetected   faults.4.3.3.  Enhance Router Robustness through Operational Adjustments   There exist a number of configuration tunings that can enhance   robustness of BGP operations.  One example is to let BGP peers   coordinate the setting of a limit on the number of prefixes that one   BGP speaker will send to its peer [43].  Although such a check does   not validate the prefix owned by each peer, it can prevent false   announcements of large numbers of invalid routes.  Had all BGP   routers been configured with this simple checking earlier, several   large-scale routing outages in the past could have been prevented.   Note, however, that care must be taken with hard limits of this type   because they can be used to mount a DoS because implementations often   discard excess routes indiscriminately, thus potentially causing   black-holing of correct routes.   Another example of useful configuration tuning is to adjust the BGP's   KeepAlive and Hold Timer values to minimize BGP peering session   resets.  Previous measurements show that heavy traffic load, such as   those caused by worms, can cause BGP KeepAlive messages to be delayed   or dropped, which in turn cause BGP peering session breakdown.  Such   load-induced session breaks and re-establishments can lead to an   excessive amount of BGP updates during the periods when stable   routing is needed most.4.3.4.  Proper Handling of Router Resource Exhaustion   In addition to the resource exhaustion problems that are generally   apply to all end-systems, as described inSection 2, router   implementations must also take special care in handling resource   exhaustions when they occur in order to keep the router operating   despite the problem.  For example, under normal operations a router   does not require a large cache to hold outstanding ARP requests   because the replies are normally received within a few milliseconds.   However, certain conditions can lead to ARP cache exhaustion, for   example, during a virus attack where many packets are sent to non-   existing IP addresses, thus there are no ARP replies to the requests   for those addresses.  Such phenomena have happened in the past and   led to routers failing to forward packets.Handley, et al.              Informational                     [Page 28]

RFC 4732                   DoS Considerations              November 2006   Another example is queue management.  Many high-end routers are   designed so that most packets can be handled purely in specialized   processors (Application-Specific Integrated Circuit (ASICs), Field   Programmable Gate-Arrays (FPGAs), etc.).  However, exceptional   packets must be routed to a supporting general purpose CPU for   handling.  On some such systems, it may be possible mount a low-   effort DoS attack by saturating the queues between the specialized   hardware and the supporting processor.   So the attack vector on routers/network devices is a low packets-   per-second (PPS) queue saturation attack on the ASIC's raw queue   structure.  The countermeasure here is to have multiple such queues   designed in such a way that it's difficult for an attacker to arrange   to fill multiple queues [45].4.4.  End-System Implementation Issues4.4.1.  State Lookup Complexity   Any system that instantiates per-connection state should take great   care to implement state-lookup mechanisms in such a way that   performance cannot be controlled by the attacker.  One way to achieve   this is to use hash tables where the hash mechanism is keyed in such   a way that the attacker cannot instantiate a large number of flows in   the same hash bucket.4.4.1.1.  Avoid Livelock   Most operating systems use network interrupts to receive data from   the network, which is a good solution if the host spends only a small   amount of its time handling network traffic.  However, this leaves   the host open to livelock [33], where under heavy load the OS spends   all its time handling interrupts and no time doing the work needed to   handle the traffic at the application level.  Server operating   systems should consider using network polling at times of heavy load,   rather than being interrupt-driven, and should be carefully   architected so that as far as reasonably possible, traffic received   by the OS is processed to completion or very cheaply discarded.4.4.1.2.  Use Unpredictable Values for Session IDs   Most recent TCP implementations use fairly good random mechanisms for   allocating the TCP initial sequence numbers.  In general, any   dynamically allocated value used purely to identify a communication   session should be allocated using an unpredictable mechanism, as this   increases the search space for an attacker that wishes to disrupt   ongoing communications.  Thus, the dynamically allocated port of the   active end of a TCP connection might also be randomly allocated.Handley, et al.              Informational                     [Page 29]

RFC 4732                   DoS Considerations              November 2006   With DNS, the ID that is used to match responses with requests should   also be randomly generated.  However, as the ID field is only 16   bits, the protection is rather limited.4.4.2.  Operational Issues4.4.2.1.  Eliminate Bad Traffic Early   Many DoS attacks are generic bandwidth consumption attacks that   operate by clogging the link that connects the victim server to the   Internet.  Filtering these attacks at the server does no good because   the traffic has already traversed the link that is the scarce   resource.  Such flows need to be filtered at some point closer to the   attacker.  Where possible, operators should filter out obviously bad   traffic.  In particular, they should perform ingress filtering [7].4.4.2.2.  Establish a Monitoring Framework   Network operators are strongly encouraged to establish a monitoring   framework to detect and log abnormal network activity.  One cannot   defend against an attack that one doesn't detect or understand.  Such   monitoring tools can be used to set a baseline of "normal" traffic,   and can be used to detect aberrant flows and determine the type and   source of the aberrant flows.  This is extremely helpful when   responding to distributed DoS attacks or a flash crowd, and should be   in place prior to the event.5.  Conclusions   In this document, we have highlighted possible avenues for DoS   attacks on networks and networked systems, with the aim of   encouraging protocol designers and network engineers towards designs   that are more robust.  We have discussed partial solutions that   reduce the effectiveness of attacks, and highlighted how some partial   solutions can be taken advantage of by attackers to perpetrate   alternative attacks.   Our focus has primarily been on protocol and network architecture   issues, but there are many things that network and service operators   can do to lessen the threat.  Further advice and information for   network operators can be found in [24] [39] [25].   It is our hope that this document will spur discussion leading to   architectural solutions that reduce the succeptibility of all   Internet systems to denial-of-service attacks.Handley, et al.              Informational                     [Page 30]

RFC 4732                   DoS Considerations              November 20066.  Security Considerations   This entire document is about security.7.  Acknowledgements   We are very grateful to Vern Paxson, Paul Vixie, Rob Thomas, Dug   Song, George Jones, Jari Arkko, Geoff Huston, and Barry Greene for   their constructive comments on earlier versions of this document.8.  Normative References   [1]  J. Abley, "Hierarchical Anycast for Global Service        Distribution",http://www.isc.org/index.pl?/pubs/tn/index.pl?tn=isc-tn-2003-1.txt.   [2]  D.J. Bernstein, "SYN Cookies",http://cr.yp.to/syncookies.html.   [3]  Chen, E., "Route Refresh Capability for BGP-4",RFC 2918,        September 2000.   [4]  Deering, S., "Host extensions for IP multicasting", STD 5,RFC1112, August 1989.   [5]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)        Protocol Version 1.1",RFC 4346, April 2006.   [6]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,        "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol        Specification (Revised)",RFC 4601, August 2006.   [7]  Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating        Denial of Service Attacks which employ IP Source Address        Spoofing",BCP 38,RFC 2827, May 2000.   [8]  Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL        Security Mechanism (GTSM)",RFC 3682, February 2004.   [9]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5        Signature Option",RFC 2385, August 1998.   [10] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4        (BGP-4)",RFC 4271, January 2006.   [11] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route Flap        Damping",RFC 2439, November 1998.Handley, et al.              Informational                     [Page 31]

RFC 4732                   DoS Considerations              November 2006   [12] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector        Multicast Routing Protocol",RFC 1075, November 1988.   [13] L. von Ahn, M. Blum, N. Hopper, and J. Langford.  CAPTCHA: Using        hard AI problems for security.  In Proceedings of Eurocrypt,        2003.9.  Informative References   [14] T. Aura, P. Nikander, J. Leiwo, "DOS-resistant authentication        with client puzzles", In B. Christianson, B. Crispo, and M. Roe,        editors, Proceedings of the 8th International Workshop on        Security Protocols, Lecture Notes in Computer Science,        Cambridge, UK, April 2000.   [15] J. Bellardo, S. Savage, "802.11 Denial-of-Service Attacks: Real        Vulnerabilities and Practical Solutions", Proceedings of the        USENIX Security Symposium, Washington D.C., August 2003.   [16] S.M. Bellovin, "Security Problems in the TCP/IP Protocol Suite",        Computer Communication Review, Vol. 19, No. 2, pp. 32-48, April        1989.   [17] CCAIS/RNP Alertas do Cais ALR-19112002a, "Vulnerability in the        sending requests control of Bind versions 4 and 8 allows DNS        spoofing",http://www.rnp.br/cais/alertas/2002/cais-ALR-19112002a.html.   [18] CERT Advisory CA-1996-01, "UDP Port Denial-of-Service Attack",        Feb 1996.   [19] CERT Advisory CA-1996-21, "TCP SYN Flooding and IP Spoofing        Attacks", Sept 1996.   [20] CERT Advisory CA-2001-09, "Statistical Weaknesses in TCP/IP        Initial Sequence Numbers", May 2001.   [21] CERT Advisory CA-1996-26, "Denial-of-Service Attack via ping",        Dec 1996.   [22] CERT Advisory CA-1998-01, "Smurf IP Denial-of-Service Attacks",http://www.cert.org/advisories/CA-1998-01.html, Jan 1998.   [23] CERT Incident Note IN-2000-05, "'mstream' Distributed Denial of        Service Tool", May 2000.   [24] CERT/CC - "Managing the Threat of Denial of Service Attacks",http://www.cert.org/archive/pdf/Managing_DoS.pdf.Handley, et al.              Informational                     [Page 32]

RFC 4732                   DoS Considerations              November 2006   [25] CERT/CC - "Trends in Denial of Service Attack Technology",http://www.cert.org/archive/pdf/DoS_trends.pdf.   [26] D.F. Chang, R. Govindan, J. Heidemann, "An Empirical Study of        Router Response to Large Routing Table Load", Proceedings of the        2nd Internet Measurement Workshop (IMW 2002), 2002.   [27] Cisco Systems, "Configuring the BGP Maximum-Prefix Feature",        Cisco Document ID: 25160,http://www.cisco.com/warp/public/459/bgp-maximum-prefix.html.   [28] Scott A Crosby and Dan S Wallach, "Denial of Service via        Algorithmic Complexity Attacks", Proceedings of the USENIX        Security Symposium, Washington D.C., August 2003.   [29] Laurent Joncheray, "Simple Active Attack Against TCP", 5th        USENIX Security Symposium, 1995.   [30] M. Lough, "A Taxonomy of Computer Attacks with Applications to        Wireless", PhD thesis, Virginia Polytechnic Institute, April        2001.   [31] Z. Mao, R. Govindan, G. Varghese, R. Katz, "Route Flap Dampening        Exacerbates Internet Routing Convergence", Proceedings of ACM        SIGCOMM, 2002.   [32] Fenner, B., Ed., and D. Meyer, Ed., "Multicast Source Discovery        Protocol (MSDP)",RFC 3618, October 2003.   [33] J. Mogul, KK.  Ramakrishnan, "Eliminating Receive Livelock in an        Interrupt-driven Kernel", ACM Transactions on Computer Systems,        Vol 15, Number 3, pp. 217-252, 1997.   [34] Watson, P., "Slipping in the Window: TCP Reset attacks",        Presentation at 2004 CanSecWest,http://www.cansecwest.com/archives.html.   [35] V. Paxson, "An Analysis of Using Reflectors for Distributed        Denial-of-Service Attacks", Computer Communication Review 31(3),        July 2001.   [36] Joe Stewart, "DNS Cache Poisoning - The Next Generation", Jan 27        2003,http://www.lurhq.com/dnscache.pdf.   [37] Stewart, R., Ed., and M. Dalal, Ed., "Improving TCP's Robustness        to Blind In-Window Attacks", Work in Progress, June 2006.Handley, et al.              Informational                     [Page 33]

RFC 4732                   DoS Considerations              November 2006   [38] P. Vixie, G. Sneeringer, M. Schleifer, "Events of 21-Oct-2002",http://f.root-servers.org/october21.txt.   [39] P. Vixie, "Securing the Edge",http://www.icann.org/committees/security/sac004.txt.   [40] D. Wessels, "Running An Authoritative-Only BIND Nameserver",http://www.isc.org/index.pl?/pubs/tn/index.pl?tn=isc-tn-2002-2.txt.   [41] M. Zalewski, "Strange Attractors and TCP/IP Sequence Number        Analysis",http://www.bindview.com/Services/Razor/Papers/2001/tcpseq.cfm.   [42] D. Pei, X. Zhao, L. Wang, D. Massey, A. Mankin, F. S. Wu, and L.        Zhang.  Improving BGP Conver-gence Through Assertions Approach.        In Proc. of IEEE INFOCOM, June 2002.   [43] Chavali, S., Radoaca, V., Miri, M., Fang, L., and S. Hares,        "Peer Prefix Limits Exchange in BGP", Work in Progress, April        2004.   [44] X. Zhao, D. Massey, A. Mankin, S.F. Wu, D. Pei, L. Wang, L.        Zhang, "BGP Multiple Origin AS (MOAS) Conflicts",http://nanog.org/mtg-0110/lixia.html, 2001.   [45] Cisco Systems, "Building Security Into the Hardware",ftp://ftp-eng.cisco.com/cons/isp/security/CPN-Summit-2004/Paris-Sept-04/SE14-BUILDING-SECURITY-INTO-THE-HARDWARE-        c1_8_30_04.pdf, 2004.   [46] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol        Architecture",RFC 4251, January 2006.   [47] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",RFC3768, April 2004.   [48] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for        Describing Simple Network Management Protocol (SNMP) Management        Frameworks", STD 62,RFC 3411, December 2002.   [49] Malkin, G. and A. Harkin, "TFTP Timeout Interval and Transfer        Size Options",RFC 2349, May 1998.   [50] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:        Session Initiation Protocol",RFC 3261, June 2002.Handley, et al.              Informational                     [Page 34]

RFC 4732                   DoS Considerations              November 2006   [51] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session        Description Protocol",RFC 4566, July 2006.   [52] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,        "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.   [53] Hedrick, C., "Routing Information Protocol",RFC 1058, June        1988.Handley, et al.              Informational                     [Page 35]

RFC 4732                   DoS Considerations              November 2006Appendix A.  IAB Members at the Time of This Writing   o  Bernard Aboba   o  Loa Andersson   o  Brian Carpenter   o  Leslie Daigle   o  Elwyn Davies   o  Kevin Fall   o  Olaf Kolkman   o  Kurtis Lindvist   o  David Meyer   o  David Oran   o  Eric Rescorla   o  Dave Thaler   o  Lixia ZhangHandley, et al.              Informational                     [Page 36]

RFC 4732                   DoS Considerations              November 2006Authors' Addresses   Mark J. Handley, Ed.   UCL   Gower Street   London  WC1E 6BT   UK   EMail: M.Handley@cs.ucl.ac.uk   Eric Rescorla, Ed.   Network Resonance   2483 E. Bayshore #212   Palo Alto  94303   USA   EMail: ekr@networkresonance.com   Internet Architecture Board   IAB   EMail: iab@ietf.orgHandley, et al.              Informational                     [Page 37]

RFC 4732                   DoS Considerations              November 2006Full Copyright Statement   Copyright (C) The IETF Trust (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY   IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR   PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Handley, et al.              Informational                     [Page 38]