Network Working Group                                    M. Handley, Ed.
Request for Comments: 4732                                           UCL
Category: Informational                                 E. Rescorla, Ed.
                                                       Network Resonance
                                             Internet Architecture Board
                                                           November 2006
Network Working Group                                    M. Handley, Ed.
Request for Comments: 4732                                           UCL
Category: Informational                                 E. Rescorla, Ed.
                                                       Network Resonance
                                             Internet Architecture Board
                                                           November 2006

Internet Denial-of-Service Considerations


Status of This Memo


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


Copyright Notice


Copyright (C) The IETF Trust (2006).




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.


Table of Contents


   1. Introduction ....................................................3
   2. An Overview of Denial-of-Service Threats ........................4
      2.1. DoS Attacks on End-Systems .................................4
           2.1.1. Exploiting Poor Software Quality ....................4
           2.1.2. Application Resource Exhaustion .....................5
           2.1.3. Operating System Resource Exhaustion ................6
           2.1.4. Triggered Lockouts and Quota Exhaustion .............7
      2.2. DoS Attacks on Routers .....................................8
           2.2.1. Attacks on Routers through Routing Protocols ........8
           2.2.2. IP Multicast-based DoS Attacks ......................9
           2.2.3. Attacks on Router Forwarding Engines ...............10
      2.3. Attacks on Ongoing Communications .........................11
      2.4. Attacks Using the Victim's Own Resources ..................12
      2.5. DoS Attacks on Local Hosts or Infrastructure ..............12
      2.6. DoS Attacks on Sites through DNS ..........................15
      2.7. DoS Attacks on Links ......................................16
      2.8. DoS Attacks on Firewalls ..................................17
      2.9. DoS Attacks on IDS Systems ................................18
      2.10. DoS Attacks on or via NTP ................................18
      2.11. Physical DoS .............................................18
      2.12. Social Engineering DoS ...................................19
      2.13. Legal DoS ................................................19
      2.14. Spam and Black-Hole Lists ................................19
   3. Attack Amplifiers ..............................................20
      3.1. Methods of Attack Amplification ...........................20
      3.2. Strategies to Mitigate Attack Amplification ...............22
   4. DoS Mitigation Strategies ......................................22
      4.1. Protocol Design ...........................................23
           4.1.1. Don't Hold State for Unverified Hosts ..............23
           4.1.2. Make It Hard to Simulate a Legitimate User .........23
           4.1.3. Graceful Routing Degradation .......................24
           4.1.4. Autoconfiguration and Authentication ...............24
      4.2. Network Design and Configuration ..........................25
           4.2.1. Redundancy and Distributed Service .................25
           4.2.2. Authenticate Routing Adjacencies ...................25
           4.2.3. Isolate Router-to-Router Traffic ...................26
      4.3. Router Implementation Issues ..............................26
           4.3.1. Checking Protocol Syntax and Semantics .............26
           4.3.2. Consistency Checks .................................27
           4.3.3. Enhance Router Robustness through
                  Operational Adjustments ............................28
           4.3.4. Proper Handling of Router Resource Exhaustion ......28
      4.4. End-System Implementation Issues ..........................29
           4.4.1. State Lookup Complexity ............................29
           4.4.2. Operational Issues .................................30
   5. Conclusions ....................................................30
   1. Introduction ....................................................3
   2. An Overview of Denial-of-Service Threats ........................4
      2.1. DoS Attacks on End-Systems .................................4
           2.1.1. Exploiting Poor Software Quality ....................4
           2.1.2. Application Resource Exhaustion .....................5
           2.1.3. Operating System Resource Exhaustion ................6
           2.1.4. Triggered Lockouts and Quota Exhaustion .............7
      2.2. DoS Attacks on Routers .....................................8
           2.2.1. Attacks on Routers through Routing Protocols ........8
           2.2.2. IP Multicast-based DoS Attacks ......................9
           2.2.3. Attacks on Router Forwarding Engines ...............10
      2.3. Attacks on Ongoing Communications .........................11
      2.4. Attacks Using the Victim's Own Resources ..................12
      2.5. DoS Attacks on Local Hosts or Infrastructure ..............12
      2.6. DoS Attacks on Sites through DNS ..........................15
      2.7. DoS Attacks on Links ......................................16
      2.8. DoS Attacks on Firewalls ..................................17
      2.9. DoS Attacks on IDS Systems ................................18
      2.10. DoS Attacks on or via NTP ................................18
      2.11. Physical DoS .............................................18
      2.12. Social Engineering DoS ...................................19
      2.13. Legal DoS ................................................19
      2.14. Spam and Black-Hole Lists ................................19
   3. Attack Amplifiers ..............................................20
      3.1. Methods of Attack Amplification ...........................20
      3.2. Strategies to Mitigate Attack Amplification ...............22
   4. DoS Mitigation Strategies ......................................22
      4.1. Protocol Design ...........................................23
           4.1.1. Don't Hold State for Unverified Hosts ..............23
           4.1.2. Make It Hard to Simulate a Legitimate User .........23
           4.1.3. Graceful Routing Degradation .......................24
           4.1.4. Autoconfiguration and Authentication ...............24
      4.2. Network Design and Configuration ..........................25
           4.2.1. Redundancy and Distributed Service .................25
           4.2.2. Authenticate Routing Adjacencies ...................25
           4.2.3. Isolate Router-to-Router Traffic ...................26
      4.3. Router Implementation Issues ..............................26
           4.3.1. Checking Protocol Syntax and Semantics .............26
           4.3.2. Consistency Checks .................................27
           4.3.3. Enhance Router Robustness through
                  Operational Adjustments ............................28
           4.3.4. Proper Handling of Router Resource Exhaustion ......28
      4.4. End-System Implementation Issues ..........................29
           4.4.1. State Lookup Complexity ............................29
           4.4.2. Operational Issues .................................30
   5. Conclusions ....................................................30
   6. Security Considerations ........................................31
   7. Acknowledgements ...............................................31
   8. Normative References ...........................................31
   9. Informative References .........................................32
   Appendix A. IAB Members at the Time of This Writing ...............36
   6. Security Considerations ........................................31
   7. Acknowledgements ...............................................31
   8. Normative References ...........................................31
   9. Informative References .........................................32
   Appendix A. IAB Members at the Time of This Writing ...............36
1. Introduction
1. 介绍

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 强调可能的攻击途径,并通过这样做鼓励协议设计者和网络工程师进行更稳健的设计。

o To discuss partial solutions that reduce the effectiveness of attacks.

o 讨论降低攻击有效性的部分解决方案。

o To highlight how some partial solutions can be taken advantage of by attackers to perpetrate alternative attacks.

o 强调攻击者如何利用部分解决方案实施替代攻击。

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.


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
2. 拒绝服务威胁概述

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
2.1. 对终端系统的DoS攻击

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
2.1.1. 利用低劣的软件质量

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.


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
2.1.2. 应用程序资源耗尽

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 可用内存。

o The CPU cycles available.

o 可用的CPU周期。

o The disk space available to the application.

o 应用程序可用的磁盘空间。

o The number of processes or threads or both that the application is permitted to use.

o 允许应用程序使用的进程数或线程数或两者。

o The configured maximum number of simultaneous connections the application is permitted.

o 允许应用程序同时连接的最大配置数。

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.


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
2.1.3. 操作系统资源耗尽

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.

可能操作系统上最著名的DoS攻击是TCP SYN洪水[19],它本质上是一种内存耗尽攻击。攻击者向受害者发送大量TCP SYN数据包,请求连接设置,但随后未完成连接设置。受害者实例化状态以处理传入连接。如果攻击者实例化状态的速度比受害者超时的速度快,那么受害者将耗尽可用于保存TCP状态的内存,因此无法为合法的TCP连接设置尝试提供服务。在某些实现中,由于为半开放连接使用了一个小的专用存储空间,这使得攻击比其他方式更容易,从而加剧了此问题。对于编码错误的操作系统,资源耗尽也可能导致系统崩溃。

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 this

另一种TCP DoS攻击是Ack洪水[23],它本质上是针对受害者的CPU耗尽攻击。攻击者向受害者发送TCP数据包,假装来自从未建立的连接。具有大量未完成连接的繁忙服务器需要检查数据包对应的连接。一些TCP实现实现了这一点

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 随着传入流量的增加,操作系统所做的有用工作应该增加,直到某些资源(如CPU)饱和。

o From this point on, as incoming traffic continues to increase the useful work done should be constant.

o 从这一点开始,随着传入流量的不断增加,所做的有用工作应该是不变的。

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
2.1.4. 触发锁定和配额耗尽

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.


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
2.2. 路由器上的DoS攻击

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
2.2.1. 通过路由协议攻击路由器

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].


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
2.2.2. 基于IP组播的DoS攻击

There are essentially two forms of IP multicast: traditional Any-Source Multicast (ASM), as specified in RFC 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.

IP多播基本上有两种形式:RFC 1112[4]中规定的传统任意源多播(ASM),其中多个源可以发送到同一个多播组;以及源特定多播(SSM),其中接收方必须同时指定IP源地址和组地址。这两种形式的多播提供了完全不同的拒绝服务可能性。

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
2.2.3. 对路由器转发引擎的攻击

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.


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
2.3. 对正在进行的通信的攻击

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 not


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
2.4. 使用受害者自己的资源进行攻击

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.

[18]中记录了此类攻击的一个示例,攻击者在发送到受害者的UDP回显端口的数据包上伪造源地址。源地址是运行UDP chargen服务器的另一台计算机的地址(chargen服务器将字符模式发送回原始源)。结果是这两台机器以尽可能快的速度来回反弹数据包,使它们之间的网络或其中一个终端系统本身过载。

2.5. DoS Attacks on Local Hosts or Infrastructure
2.5. 对本地主机或基础设施的DoS攻击

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 of


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.

SSID字段无法有效防御此类攻击。除非启用了加密,否则宣布存在与合法基站具有相同网络名称(SSID)的基站(甚至是特设模式主机)是很简单的。即使在la 802.1X和802.11i中添加身份验证和加密,在这方面也可能没有多大帮助。SSID空间是非托管的,因此每个人都可以在SSID字段中自由放置他们想要的任何内容。大多数主机堆栈不能很好地处理这个问题。此外,SSID通常设置为制造商的默认值,使其具有高度可预测性。

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.

最后,由于802.11中的身份验证在堆栈中的较高级别上进行,因此即使使用有线等效隐私(WEP),也可以简单地取消对受害者的身份验证或解除受害者与基站的关联[30]。Bellardo和Savage[15]描述了一些降低此类攻击有效性的简单补救措施。虽然IEEE 802.11w将保护取消验证或解除关联的帧,但这种攻击仍然可能通过伪造关联帧进行。

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.


2.6. DoS Attacks on Sites through DNS
2.6. 通过DNS对站点进行DoS攻击

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 (see Section 2.14) served by DNS, an attacker can also mount a DoS attack by attacking the black-hole server.


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].

最后,如果允许站点的名称服务器中继来自不受信任的第三方的请求,则可能发生数据损坏攻击[40]。攻击者对他希望破坏的数据发出查询,受害者的名称服务器将请求转发给权威名称服务器。请求包含一个16位ID,用于将响应与请求匹配。如果攻击者在正式响应到达之前从权威名称服务器欺骗足够的响应数据包,每个数据包包含伪造响应和不同的DNS ID,然后,其中一个伪造的响应很有可能具有正确的DNS ID。然后,受害者的名称服务器将相信并缓存错误的数据,从而为将来的查询提供错误的响应。如果攻击者使用不同的DNS ID对同一数据发出许多不同的请求,则攻击的概率会进一步增加,因为许多名称服务器实现将使用不同的DNS ID发出中继请求,因此响应只需匹配这些请求ID中的任何一个[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.

对DNS服务使用anycast使其更容易受到欺骗攻击。如果攻击者能够说服ISP接受到其伪DNS服务器的选播路由,则可以安排接收请求并生成伪响应。Anycast DNS还使DNS上的DoS攻击更容易。我们的想法是禁用其中一个DNS服务器,同时维护到该服务器的BGP路由。这会为路由到(现已失效)服务器的任何客户端创建故障。

2.7. DoS Attacks on Links
2.7. 拒绝服务攻击链接

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.


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
2.8. 对防火墙的DoS攻击

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.


2.9. DoS Attacks on IDS Systems
2.9. 入侵检测系统的DoS攻击

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
2.10. 对NTP或通过NTP的DoS攻击

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.

最明显的此类攻击是DoS NTP服务器本身。许多终端系统的时钟精度相当差,因此,如果没有网络时间,它们的时钟自然会漂移。这可能会导致依赖良好时钟的分布式系统出现问题。例如,如果一个常用的修订控制系统认为修改时间戳在将来,它可能会失败。

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
2.11. 体力劳动

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 as


entry points for non-physical DoS, for instance, by reducing the resources available to deal with overcapacity.


2.12. Social Engineering DoS
2.12. 社会工程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
2.13. 法定义务

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
2.14. 垃圾邮件和黑洞列表

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.


3. Attack Amplifiers
3. 攻击放大器

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
3.1. 攻击放大方法

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.

攻击放大器的另一种变体涉及通过重传进行放大。这是典型的TCP放大攻击,称为“bang.c”。攻击者向任意TCP服务器发送带有受害者源地址的伪造TCP SYN。服务器将用发送给受害者的SYN | ACK进行响应,当没有收到完成握手的最终ACK时,SYN | ACK将被重新传输多次。通常,此攻击使用大量任意选择的服务器作为反射器。为了使攻击成功,反射器不得接收来自受害者的响应SYN | ACK的RST。但是,如果攻击流量足以淹没服务器或服务器的访问链路,则数据包丢失将确保许多反射器不会收到响应其SYN | ACK的RST,从而继续重新传输。防火墙在不发送RST的情况下无声地丢弃传入的SYN | ACK,这会加剧攻击。

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".

要发起此攻击,攻击者会发送一条协议消息,并将有效负载内的IP地址设置为指向攻击目标。邮件收件人将生成该IP地址的后续通信量。根据协议的不同,这种攻击可以提供大量的放大特性。在SIP的特定情况下,如果呼叫者呼叫高带宽媒体源(例如视频服务器或流式音频服务器),单个SIP INVITE数据包(通常为几百字节)可以产生几乎连续的媒体数据包流,其速率从每秒几kbit到每秒兆比特不等。这种特殊的攻击被称为“语音锤”。

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 an


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
3.2. 缓解攻击放大的策略

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 尽可能执行入口过滤[7][39],以防止源地址欺骗。

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 避免设计能够返回比请求大小大得多的响应的协议或机制,除非执行握手以验证客户端的源地址。这样的握手需要包含一个不可预测的nonce,该nonce足够安全,以减轻协议的放大效应。

o All retransmission during initial connection setup should be performed by the client.

o 初始连接设置期间的所有重传应由客户端执行。

o Proxies should not arbitrarily relay requests to destinations chosen by a client.

o 代理不应该任意地将请求中继到客户端选择的目的地。

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.

o 避免向第三方连接发送信号。通过信令协议建立的任何不可避免的第三方连接都应该在发送重要数据之前包含轻量级验证。

4. DoS Mitigation Strategies
4. 拒绝服务的缓解策略

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.

1. 避免允许比一般资源消耗更好的攻击。

2. Minimizing the extent to which generic resource consumption attacks crowd out legitimate users.

2. 将一般资源消耗攻击排挤合法用户的程度降至最低。

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 Design
4.1. 协议设计
4.1.1. Don't Hold State for Unverified Hosts
4.1.1. 不保留未验证主机的状态

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.

从终端系统服务器的角度来看,一个简单的目标是避免在没有与客户机完成握手以验证其地址的情况下实例化状态,并尽可能将工作和状态保持推送到客户机。有许多技术可以用来实现这一点,包括SYN cookies[2][14]。所有客户机-服务器协议可能都应设计为允许使用此类技术,但该机制的启用通常应由服务器自行决定,以避免在正常情况下进行不必要的工作。

4.1.2. Make It Hard to Simulate a Legitimate User
4.1.2. 使模拟合法用户变得困难

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].

1. 谜题:迫使攻击者进行一些计算,这些计算对于单个用户来说并不繁重,但总体来说成本太高[14]。

2. Reverse Turing tests: specialized puzzles that are hard for machines to do but easy for humans, thus making automated attacks hard [13].

2. 反向图灵测试:机器很难完成但人类很容易完成的特殊谜题,因此很难进行自动攻击[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.

3. 可达性测试:强制建议的客户端证明它可以在给定的IP地址接收流量。这使得追踪攻击者变得更容易。

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.


4.1.3. Graceful Routing Degradation
4.1.3. 优美路由退化

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
4.1.4. 自动配置和身份验证

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.


4.2. Network Design and Configuration
4.2. 网络设计与配置

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
4.2.1. 冗余与分布式服务

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
4.2.2. 验证路由邻接

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 be


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.

对于BGP,这至少意味着使用TCP MD5[9]或IPsec身份验证,并结合GTSM[8]来防止eBGP与非直接邻居关联。在将来,这可能意味着路由信息本身的身份验证会更好。

4.2.3. Isolate Router-to-Router Traffic
4.2.3. 隔离路由器到路由器的流量

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
4.3. 路由器实现问题

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
4.3.1. 检查协议语法和语义

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?

1. 谁发的信息?

2. Does the content conform to the protocol format?

2. 内容是否符合协议格式?

3. Was the message sent with correct timing?

3. 消息发送的时间是否正确?

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.

协议语法验证的第一步是确保传入消息由合法方发送。执行此检查有多种方法。可以验证消息的源IP地址,甚至MAC地址。利用eBGP对等点通常直接连接的事实,还可以检查数据包中的TTL值,并丢弃TTL小于某个最大值(通常为最大TTL-1)的任何BGP更新数据包[8]。只要可用,也应使用加密身份验证来验证传入消息是否确实来自预期的发送方。对于BGP,这至少意味着使用TCP MD5[9]或IPsec身份验证。

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
4.3.2. 一致性检查

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 the


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
4.3.3. 通过操作调整增强路由器的健壮性

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.

另一个有用的配置调整示例是调整BGP的KeepAlive和Hold定时器值,以最小化BGP对等会话重置。以前的测量表明,重流量负载(如蠕虫引起的负载)会导致BGP KeepAlive消息延迟或丢弃,进而导致BGP对等会话中断。这种由负载引起的会话中断和重新建立可能会导致在最需要稳定路由的期间BGP更新过多。

4.3.4. Proper Handling of Router Resource Exhaustion
4.3.4. 正确处理路由器资源耗尽

In addition to the resource exhaustion problems that are generally apply to all end-systems, as described in Section 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.


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 Issues
4.4. 终端系统实施问题
4.4.1. State Lookup Complexity
4.4.1. 状态查找复杂性

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.

任何实例化每个连接状态的系统都应该非常小心地以攻击者无法控制性能的方式实现状态查找机制。实现这一点的一种方法是使用哈希表,其中哈希机制的键控方式使得攻击者无法在同一个哈希桶中实例化大量流。 Avoid Livelock 避免活锁

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.

大多数操作系统使用网络中断来接收来自网络的数据,如果主机只花费少量时间处理网络流量,这是一个很好的解决方案。然而,这使得主机对livelock开放[33],在高负载情况下,操作系统将所有时间都花在处理中断上,而没有时间做处理应用程序级流量所需的工作。服务器操作系统应该考虑在重载时使用网络轮询,而不是中断驱动,并且应该仔细地架构,以便尽可能合理地处理OS接收的流量完成或非常便宜地丢弃。 Use Unpredictable Values for Session IDs 对会话ID使用不可预测的值

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.


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 Issues
4.4.2. 业务问题 Eliminate Bad Traffic Early 尽早消除不良交通

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].

许多DoS攻击是一般带宽消耗攻击,通过阻塞将受害者服务器连接到Internet的链接进行操作。在服务器上过滤这些攻击没有任何好处,因为流量已经通过了稀缺资源的链接。这些流需要在更靠近攻击者的某个点进行过滤。在可能的情况下,运营商应过滤掉明显的不良流量。特别是,它们应该执行入口过滤[7]。 Establish a Monitoring Framework 建立监测框架

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
5. 结论

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.


6. Security Considerations
6. 安全考虑

This entire document is about security.


7. Acknowledgements
7. 致谢

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.

我们非常感谢Vern Paxson、Paul Vixie、Rob Thomas、Dag Song、George Jones、Jari Arkko、Geoff Huston和Barry Greene对本文件早期版本的建设性意见。

8. Normative References
8. 规范性引用文件

[1] J. Abley, "Hierarchical Anycast for Global Service Distribution",

[1] J.Abley,“全球服务分发的分层选播”,。

[2] D.J. Bernstein, "SYN Cookies",

[2] D.J.Bernstein,“SYN Cookies”,

[3] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918, September 2000.

[3] 陈,E,“BGP-4的路由刷新能力”,RFC 2918,2000年9月。

[4] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989.

[4] Deering,S.,“IP多播的主机扩展”,STD 5,RFC 1112,1989年8月。

[5] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.

[5] Dierks,T.和E.Rescorla,“传输层安全(TLS)协议版本1.1”,RFC 4346,2006年4月。

[6] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, August 2006.

[6] Fenner,B.,Handley,M.,Holbrook,H.,和I.Kouvelas,“协议独立多播-稀疏模式(PIM-SM):协议规范(修订版)”,RFC 4601,2006年8月。

[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.

[7] Ferguson,P.和D.Senie,“网络入口过滤:击败利用IP源地址欺骗的拒绝服务攻击”,BCP 38,RFC 2827,2000年5月。

[8] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL Security Mechanism (GTSM)", RFC 3682, February 2004.

[8] Gill,V.,Heasley,J.,和D.Meyer,“广义TTL安全机制(GTSM)”,RFC 3682,2004年2月。

[9] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.

[9] Heffernan,A.,“通过TCP MD5签名选项保护BGP会话”,RFC 2385,1998年8月。

[10] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

[10] Rekhter,Y.,Li,T.,和S.Hares,“边境网关协议4(BGP-4)”,RFC 42712006年1月。

[11] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route Flap Damping", RFC 2439, November 1998.

[11] Villamizar,C.,Chandra,R.,和R.Govindan,“BGP路线襟翼阻尼”,RFC 2439,1998年11月。

[12] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector Multicast Routing Protocol", RFC 1075, November 1988.

[12] Waitzman,D.,Partridge,C.和S.Deering,“距离向量多播路由协议”,RFC 1075,1988年11月。

[13] L. von Ahn, M. Blum, N. Hopper, and J. Langford. CAPTCHA: Using hard AI problems for security. In Proceedings of Eurocrypt, 2003.

[13] L.冯·安、M.布鲁姆、N.霍珀和J.朗福德。验证码:使用人工智能硬问题进行安全性验证。《欧洲密码会议录》,2003年。

9. Informative References
9. 资料性引用

[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.

[14] T.Aura,P.Nikander,J.Leiwo,“具有客户端谜题的拒绝服务认证”,载于B.Christianson,B.Crispo和M.Roe,编辑,第八届安全协议国际研讨会论文集,计算机科学讲稿,英国剑桥,2000年4月。

[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.

[15] J.Bellardo,S.Savage,“802.11拒绝服务攻击:真正的漏洞和实际解决方案”,USENIX安全研讨会论文集,华盛顿特区,2003年8月。

[16] S.M. Bellovin, "Security Problems in the TCP/IP Protocol Suite", Computer Communication Review, Vol. 19, No. 2, pp. 32-48, April 1989.

[16] S.M.Bellovin,“TCP/IP协议套件中的安全问题”,《计算机通信评论》,第19卷,第2期,第32-48页,1989年4月。

[17] CCAIS/RNP Alertas do Cais ALR-19112002a, "Vulnerability in the sending requests control of Bind versions 4 and 8 allows DNS spoofing",

[17] CCAIS/RNP Alertas do Cais ALR-19112002a,“绑定版本4和8的发送请求控制中存在允许DNS欺骗的漏洞”,

[18] CERT Advisory CA-1996-01, "UDP Port Denial-of-Service Attack", Feb 1996.

[18] CERT咨询CA-1996-01,“UDP端口拒绝服务攻击”,1996年2月。

[19] CERT Advisory CA-1996-21, "TCP SYN Flooding and IP Spoofing Attacks", Sept 1996.

[19] CERT咨询CA-1996-21,“TCP SYN洪泛和IP欺骗攻击”,1996年9月。

[20] CERT Advisory CA-2001-09, "Statistical Weaknesses in TCP/IP Initial Sequence Numbers", May 2001.

[20] CERT咨询CA-2001-09,“TCP/IP初始序列号的统计缺陷”,2001年5月。

[21] CERT Advisory CA-1996-26, "Denial-of-Service Attack via ping", Dec 1996.

[21] CERT咨询CA-1996-26,“通过ping进行的拒绝服务攻击”,1996年12月。

[22] CERT Advisory CA-1998-01, "Smurf IP Denial-of-Service Attacks",, Jan 1998.

[22] 证书咨询CA-1998-01,“Smurf IP拒绝服务攻击”,,1998年1月。

[23] CERT Incident Note IN-2000-05, "'mstream' Distributed Denial of Service Tool", May 2000.

[23] CERT事件说明IN-2000-05,“mstream”分布式拒绝服务工具,2000年5月。

[24] CERT/CC - "Managing the Threat of Denial of Service Attacks",

[24] CERT/CC - "Managing the Threat of Denial of Service Attacks", error, please retry

[25] CERT/CC - "Trends in Denial of Service Attack Technology",

[25] CERT/CC—“拒绝服务攻击技术的发展趋势”,

[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.

[26] D.F.Chang,R.Govindan,J.Heidemann,“路由器对大路由表负载响应的实证研究”,第二届互联网测量研讨会论文集(IMW 2002),2002年。

[27] Cisco Systems, "Configuring the BGP Maximum-Prefix Feature", Cisco Document ID: 25160,

[27] Cisco Systems,“配置BGP最大前缀功能”,Cisco文档ID:25160,

[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.

[28] Scott A Crosby和Dan S Wallach,“通过算法复杂性攻击的拒绝服务”,USENIX安全研讨会论文集,华盛顿特区,2003年8月。

[29] Laurent Joncheray, "Simple Active Attack Against TCP", 5th USENIX Security Symposium, 1995.

[29] Laurent Joncheray,“针对TCP的简单主动攻击”,第五届USENIX安全研讨会,1995年。

[30] M. Lough, "A Taxonomy of Computer Attacks with Applications to Wireless", PhD thesis, Virginia Polytechnic Institute, April 2001.

[30] M.Lough,“应用于无线的计算机攻击分类”,弗吉尼亚理工学院博士论文,2001年4月。

[31] Z. Mao, R. Govindan, G. Varghese, R. Katz, "Route Flap Dampening Exacerbates Internet Routing Convergence", Proceedings of ACM SIGCOMM, 2002.

[31] 毛志强,R.戈文丹,G.瓦盖斯,R.卡茨,“路由抖动抑制加剧了互联网路由收敛”,ACM SIGCOMM会议录,2002年。

[32] Fenner, B., Ed., and D. Meyer, Ed., "Multicast Source Discovery Protocol (MSDP)", RFC 3618, October 2003.

[32] Fenner,B.,Ed.,和D.Meyer,Ed.,“多播源发现协议(MSDP)”,RFC3618,2003年10月。

[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.

[33] 莫格尔,KK。罗摩克里希南,“在中断驱动内核中消除接收活锁”,《计算机系统上的ACM事务》,第15卷,第3期,第217-252页,1997年。

[34] Watson, P., "Slipping in the Window: TCP Reset attacks", Presentation at 2004 CanSecWest,

[34] Watson,P.,“在窗口中滑动:TCP重置攻击”,在2004年CanSecWest上的演讲,

[35] V. Paxson, "An Analysis of Using Reflectors for Distributed Denial-of-Service Attacks", Computer Communication Review 31(3), July 2001.

[35] V.Paxson,“使用反射器进行分布式拒绝服务攻击的分析”,《计算机通信评论》31(3),2001年7月。

[36] Joe Stewart, "DNS Cache Poisoning - The Next Generation", Jan 27 2003,

[36] Joe Stewart,“DNS缓存中毒-下一代”,2003年1月27日,

[37] Stewart, R., Ed., and M. Dalal, Ed., "Improving TCP's Robustness to Blind In-Window Attacks", Work in Progress, June 2006.

[37] Stewart,R.,Ed.,和M.Dalal,Ed.,“提高TCP对窗口盲攻击的鲁棒性”,进展中的工作,2006年6月。

[38] P. Vixie, G. Sneeringer, M. Schleifer, "Events of 21-Oct-2002",

[38] P.Vixie,G.Sneeringer,M.Schleifer,“2002年10月21日事件”,

[39] P. Vixie, "Securing the Edge",

[39] P.Vixie,“保护边缘”,

[40] D. Wessels, "Running An Authoritative-Only BIND Nameserver",

[40] D.Wessels,“运行仅限权威的绑定名称服务器”,。

[41] M. Zalewski, "Strange Attractors and TCP/IP Sequence Number Analysis",

[41] M.Zalewski,“奇怪吸引子和TCP/IP序列号分析”,

[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.

[42] 贝聿铭、赵晓阳、王磊、梅西、曼金、吴福生和张磊。通过断言方法提高BGP的收敛性。在过程中。IEEE信息网,2002年6月。

[43] Chavali, S., Radoaca, V., Miri, M., Fang, L., and S. Hares, "Peer Prefix Limits Exchange in BGP", Work in Progress, April 2004.

[43] Chavali,S.,Radoaca,V.,Miri,M.,Fang,L.,和S.Hares,“对等前缀限制BGP中的交换”,正在进行的工作,2004年4月。

[44] X. Zhao, D. Massey, A. Mankin, S.F. Wu, D. Pei, L. Wang, L. Zhang, "BGP Multiple Origin AS (MOAS) Conflicts",, 2001.

[44] X.Zhao,D.Massey,A.Mankin,S.F.Wu,D.Pei,L.Wang,L.Zhang,“BGP多源AS(MOAS)冲突”,, 2001.

[45] Cisco Systems, "Building Security Into the Hardware", Paris-Sept-04/SE14-BUILDING-SECURITY-INTO-THE-HARDWARE-c1_8_30_04.pdf, 2004.

[45] Cisco Systems,“在硬件中构建安全性”, 巴黎9月4日/SE14-BUILDING-SECURITY-INTO-THE-HARDWARE-c1_8_30_04.pdf,2004年。

[46] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006.

[46] Ilonen,T.和C.Lonvick,“安全外壳(SSH)协议架构”,RFC 42512006年1月。

[47] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)", RFC 3768, April 2004.

[47] Hinden,R.,“虚拟路由器冗余协议(VRRP)”,RFC 3768,2004年4月。

[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.

[48] Harrington,D.,Presohn,R.,和B.Wijnen,“描述简单网络管理协议(SNMP)管理框架的体系结构”,STD 62,RFC 3411,2002年12月。

[49] Malkin, G. and A. Harkin, "TFTP Timeout Interval and Transfer Size Options", RFC 2349, May 1998.

[49] Malkin,G.和A.Harkin,“TFTP超时间隔和传输大小选项”,RFC 2349,1998年5月。

[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.

[50] Rosenberg,J.,Schulzrinne,H.,Camarillo,G.,Johnston,A.,Peterson,J.,Sparks,R.,Handley,M.,和E.Schooler,“SIP:会话启动协议”,RFC 3261,2002年6月。

[51] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.

[51] Handley,M.,Jacobson,V.,和C.Perkins,“SDP:会话描述协议”,RFC4566,2006年7月。

[52] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.

[52] Schulzrinne,H.,Casner,S.,Frederick,R.,和V.Jacobson,“RTP:实时应用的传输协议”,STD 64,RFC 35502003年7月。

[53] Hedrick, C., "Routing Information Protocol", RFC 1058, June 1988.

[53] Hedrick,C.,“路由信息协议”,RFC 1058,1988年6月。

Appendix A. IAB Members at the Time of This Writing

o Bernard Aboba

o 伯纳德·阿博巴

o Loa Andersson

o 安徒生酒店

o Brian Carpenter

o 布莱恩·卡彭特

o Leslie Daigle

o 莱斯利·戴格尔

o Elwyn Davies

o 埃尔温·戴维斯

o Kevin Fall

o 凯文·法尔

o Olaf Kolkman

o 奥拉夫·科尔克曼

o Kurtis Lindvist

o 库蒂斯·林德维斯特

o David Meyer

o 大卫·梅耶尔

o David Oran

o 大卫·奥兰

o Eric Rescorla

o 埃里克·雷斯科拉

o Dave Thaler

o 戴夫·泰勒

o Lixia Zhang

o 张丽霞

Authors' Addresses


Mark J. Handley, Ed. UCL Gower Street London WC1E 6BT UK

马克·J·汉德利,英国伦敦大学学院高尔街WC1E 6BT


Eric Rescorla, Ed. Network Resonance 2483 E. Bayshore #212 Palo Alto 94303 USA

Eric Rescorla,Ed.网络共振2483 E.Bayshore#212 Palo Alto 94303美国


Internet Architecture Board IAB



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