Network Working Group                                           F. Baker
Request for Comments: 3704                                 Cisco Systems
Updates: 2827                                                  P. Savola
BCP: 84                                                        CSC/FUNET
Category: Best Current Practice                               March 2004
Network Working Group                                           F. Baker
Request for Comments: 3704                                 Cisco Systems
Updates: 2827                                                  P. Savola
BCP: 84                                                        CSC/FUNET
Category: Best Current Practice                               March 2004

Ingress Filtering for Multihomed Networks


Status of this Memo


This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited.


Copyright Notice


Copyright (C) The Internet Society (2004). All Rights Reserved.




BCP 38, RFC 2827, is designed to limit the impact of distributed denial of service attacks, by denying traffic with spoofed addresses access to the network, and to help ensure that traffic is traceable to its correct source network. As a side effect of protecting the Internet against such attacks, the network implementing the solution also protects itself from this and other attacks, such as spoofed management access to networking equipment. There are cases when this may create problems, e.g., with multihoming. This document describes the current ingress filtering operational mechanisms, examines generic issues related to ingress filtering, and delves into the effects on multihoming in particular. This memo updates RFC 2827.

BCP 38,RFC 2827,旨在通过拒绝具有伪造地址的流量访问网络,限制分布式拒绝服务攻击的影响,并帮助确保流量可追踪到其正确的源网络。作为保护互联网免受此类攻击的副作用,实施该解决方案的网络还可以保护自己免受此类攻击和其他攻击,例如对网络设备的欺骗管理访问。在某些情况下,这可能会产生问题,例如,多宿主。本文档描述了当前的入口过滤操作机制,检查了与入口过滤相关的一般问题,并特别深入研究了对多宿主的影响。本备忘录更新了RFC 2827。

Table of Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Different Ways to Implement Ingress Filtering  . . . . . . . .  4
       2.1 Ingress Access Lists . . . . . . . . . . . . . . . . . . .  4
       2.2 Strict Reverse Path Forwarding . . . . . . . . . . . . . .  5
       2.3 Feasible Path Reverse Path Forwarding  . . . . . . . . . .  6
       2.4 Loose Reverse Path Forwarding  . . . . . . . . . . . . . .  6
       2.5 Loose Reverse Path Forwarding Ignoring Default Routes  . .  7
   3.  Clarifying the Applicability of Ingress Filtering  . . . . . .  8
       3.1 Ingress Filtering at Multiple Levels . . . . . . . . . . .  8
       3.2 Ingress Filtering to Protect Your Own Infrastructure . . .  8
       3.3 Ingress Filtering on Peering Links . . . . . . . . . . . .  9
   4.  Solutions to Ingress Filtering with Multihoming  . . . . . . .  9
       4.1 Use Loose RPF When Appropriate . . . . . . . . . . . . . . 10
       4.2 Ensure That Each ISP's Ingress Filter Is Complete  . . . . 11
       4.3 Send Traffic Using a Provider Prefix Only to That Provider 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Conclusions and Future Work  . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 14
       8.2.  Informative References . . . . . . . . . . . . . . . . . 14
   9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
   10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16
   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Different Ways to Implement Ingress Filtering  . . . . . . . .  4
       2.1 Ingress Access Lists . . . . . . . . . . . . . . . . . . .  4
       2.2 Strict Reverse Path Forwarding . . . . . . . . . . . . . .  5
       2.3 Feasible Path Reverse Path Forwarding  . . . . . . . . . .  6
       2.4 Loose Reverse Path Forwarding  . . . . . . . . . . . . . .  6
       2.5 Loose Reverse Path Forwarding Ignoring Default Routes  . .  7
   3.  Clarifying the Applicability of Ingress Filtering  . . . . . .  8
       3.1 Ingress Filtering at Multiple Levels . . . . . . . . . . .  8
       3.2 Ingress Filtering to Protect Your Own Infrastructure . . .  8
       3.3 Ingress Filtering on Peering Links . . . . . . . . . . . .  9
   4.  Solutions to Ingress Filtering with Multihoming  . . . . . . .  9
       4.1 Use Loose RPF When Appropriate . . . . . . . . . . . . . . 10
       4.2 Ensure That Each ISP's Ingress Filter Is Complete  . . . . 11
       4.3 Send Traffic Using a Provider Prefix Only to That Provider 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Conclusions and Future Work  . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 14
       8.2.  Informative References . . . . . . . . . . . . . . . . . 14
   9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
   10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16
1. Introduction
1. 介绍

BCP 38, RFC 2827 [1], is designed to limit the impact of distributed denial of service attacks, by denying traffic with spoofed addresses access to the network, and to help ensure that traffic is traceable to its correct source network. As a side effect of protecting the Internet against such attacks, the network implementing the solution also protects itself from this and other attacks, such as spoofed management access to networking equipment. There are cases when this may create problems, e.g., with multihoming. This document describes the current ingress filtering operational mechanisms, examines generic issues related to ingress filtering and delves into the effects on multihoming in particular.

BCP 38,RFC 2827[1]旨在通过拒绝具有伪造地址的流量访问网络来限制分布式拒绝服务攻击的影响,并帮助确保流量可追踪到其正确的源网络。作为保护互联网免受此类攻击的副作用,实施该解决方案的网络还可以保护自己免受此类攻击和其他攻击,例如对网络设备的欺骗管理访问。在某些情况下,这可能会产生问题,例如,多宿主。本文档描述了当前的入口过滤操作机制,检查了与入口过滤相关的一般问题,并特别深入研究了对多归宿的影响。

RFC 2827 recommends that ISPs police their customers' traffic by dropping traffic entering their networks that is coming from a source address not legitimately in use by the customer network. The filtering includes but is in no way limited to the traffic whose source address is a so-called "Martian Address" - an address that is reserved [3], including any address within,,,,,, or

RFC 2827建议ISP通过丢弃从客户网络未合法使用的源地址进入其网络的流量来监控其客户的流量。过滤包括但不限于源地址为所谓“火星地址”(Martian address)的流量,该地址为保留地址[3],包括0.0.0.0/8、、、、、或240.0.0.0/4范围内的任何地址。

The reasoning behind the ingress filtering procedure is that Distributed Denial of Service Attacks frequently spoof other systems' source addresses, placing a random number in the field. In some attacks, this random number is deterministically within the target network, simultaneously attacking one or more machines and causing those machines to attack others with ICMP messages or other traffic; in this case, the attacked sites can protect themselves by proper filtering, by verifying that their prefixes are not used in the source addresses in packets received from the Internet. In other attacks, the source address is literally a random 32 bit number, resulting in the source of the attack being difficult to trace. If the traffic leaving an edge network and entering an ISP can be limited to traffic it is legitimately sending, attacks can be somewhat mitigated: traffic with random or improper source addresses can be suppressed before it does significant damage, and attacks can be readily traced back to at least their source networks.


This document is aimed at ISP and edge network operators who 1) would like to learn more of ingress filtering methods in general, or 2) are already using ingress filtering to some degree but who would like to expand its use and want to avoid the pitfalls of ingress filtering in the multihomed/asymmetric scenarios.


In section 2, several different ways to implement ingress filtering are described and examined in the generic context. In section 3, some clarifications on the applicability of ingress filtering methods are made. In section 4, ingress filtering is analyzed in detail from the multihoming perspective. In section 5, conclusions and potential future work items are identified.


2. Different Ways to Implement Ingress Filtering
2. 实现入口过滤的不同方法

This section serves as an introduction to different operational techniques used to implement ingress filtering as of writing this memo. The mechanisms are described and analyzed in general terms, and multihoming-specific issues are described in Section 4.


There are at least five ways one can implement RFC 2827, with varying impacts. These include (the names are in relatively common usage):

实施RFC 2827的方法至少有五种,影响各不相同。这些名称包括(这些名称的用法比较常见):

o Ingress Access Lists

o 入口访问列表

o Strict Reverse Path Forwarding

o 严格反向路径转发

o Feasible Path Reverse Path Forwarding

o 可行路径反向路径转发

o Loose Reverse Path Forwarding

o 松散反向路径转发

o Loose Reverse Path Forwarding ignoring default routes

o 松散反向路径转发忽略默认路由

Other mechanisms are also possible, and indeed, there are a number of techniques that might profit from further study, specification, implementation, and/or deployment; see Section 6. However, these are out of scope.


2.1. Ingress Access Lists
2.1. 入口访问列表

An Ingress Access List is a filter that checks the source address of every message received on a network interface against a list of acceptable prefixes, dropping any packet that does not match the filter. While this is by no means the only way to implement an ingress filter, it is the one proposed by RFC 2827 [1], and in some sense the most deterministic one.

入口访问列表是一个过滤器,它根据可接受的前缀列表检查网络接口上接收的每条消息的源地址,丢弃任何与过滤器不匹配的数据包。虽然这绝不是实现入口过滤器的唯一方法,但它是RFC 2827[1]提出的方法,并且在某种意义上是最确定的方法。

However, Ingress Access Lists are typically maintained manually; for example, forgetting to have the list updated at the ISPs if the set of prefixes changes (e.g., as a result of multihoming) might lead to discarding the packets if they do not pass the ingress filter.


Naturally, this problem is not limited to Ingress Access Lists -- it is inherent to Ingress Filtering when the ingress filter is not complete. However, usually Ingress Access Lists are more difficult to maintain than the other mechanisms, and having an outdated list can prevent legitimate access.


2.2. Strict Reverse Path Forwarding
2.2. 严格反向路径转发

Strict Reverse Path Forwarding (Strict RPF) is a simple way to implement an ingress filter. It is conceptually identical to using access lists for ingress filtering, with the exception that the access list is dynamic. This may also be used to avoid duplicate configuration (e.g., maintaining both static routes or BGP prefix-list filters and interface access-lists). The procedure is that the source address is looked up in the Forwarding Information Base (FIB) - and if the packet is received on the interface which would be used to forward the traffic to the source of the packet, it passes the check.

严格反向路径转发(Strict RPF)是实现入口过滤器的一种简单方法。它在概念上与使用访问列表进行入口过滤相同,只是访问列表是动态的。这也可用于避免重复配置(例如,维护静态路由或BGP前缀列表过滤器和接口访问列表)。程序是在转发信息库(FIB)中查找源地址,如果在用于将流量转发到数据包源的接口上接收到数据包,则通过检查。

Strict Reverse Path Forwarding is a very reasonable approach in front of any kind of edge network; in particular, it is far superior to Ingress Access Lists when the network edge is advertising multiple prefixes using BGP. It makes for a simple, cheap, fast, and dynamic filter.


But Strict Reverse Path Forwarding has some problems of its own. First, the test is only applicable in places where routing is symmetrical - where IP datagrams in one direction and responses from the other deterministically follow the same path. While this is common at edge network interfaces to their ISP, it is in no sense common between ISPs, which normally use asymmetrical "hot potato" routing. Also, if BGP is carrying prefixes and some legitimate prefixes are not being advertised or not being accepted by the ISP under its policy, the effect is the same as ingress filtering using an incomplete access list: some legitimate traffic is filtered for lack of a route in the filtering router's Forwarding Information Base.


There are operational techniques, especially with BGP but somewhat applicable to other routing protocols as well, to make strict RPF work better in the case of asymmetric or multihomed traffic. The ISP assigns a better metric which is not propagated outside of the router, either a vendor-specific "weight" or a protocol distance to prefer the directly received routes. With BGP and sufficient machinery in place, setting the preferences could even be automated, using BGP Communities [2]. That way, the route will always be the best one in the FIB, even in the scenarios where only the primary connectivity would be used and typically no packets would pass


through the interface. This method assumes that there is no strict RPF filtering between the primary and secondary edge routers; in particular, when applied to multihoming to different ISPs, this assumption may fail.


2.3. Feasible Path Reverse Path Forwarding
2.3. 可行路径反向路径转发

Feasible Path Reverse Path Forwarding (Feasible RPF) is an extension of Strict RPF. The source address is still looked up in the FIB (or an equivalent, RPF-specific table) but instead of just inserting one best route there, the alternative paths (if any) have been added as well, and are valid for consideration. The list is populated using routing-protocol specific methods, for example by including all or N (where N is less than all) feasible BGP paths in the Routing Information Base (RIB). Sometimes this method has been implemented as part of a Strict RPF implementation.


In the case of asymmetric routing and/or multihoming at the edge of the network, this approach provides a way to relatively easily address the biggest problems of Strict RPF.


It is critical to understand the context in which Feasible RPF operates. The mechanism relies on consistent route advertisements (i.e., the same prefix(es), through all the paths) propagating to all the routers performing Feasible RPF checking. For example, this may not hold e.g., in the case where a secondary ISP does not propagate the BGP advertisement to the primary ISP e.g., due to route-maps or other routing policies not being up-to-date. The failure modes are typically similar to "operationally enhanced Strict RPF", as described above.


As a general guideline, if an advertisement is filtered, the packets will be filtered as well.


In consequence, properly defined, Feasible RPF is a very powerful tool in certain kinds of asymmetric routing scenarios, but it is important to understand its operational role and applicability better.


2.4. Loose Reverse Path Forwarding
2.4. 松散反向路径转发

Loose Reverse Path Forwarding (Loose RPF) is algorithmically similar to strict RPF, but differs in that it checks only for the existence of a route (even a default route, if applicable), not where the route points to. Practically, this could be considered as a "route presence check" ("loose RPF is a misnomer in a sense because there is no "reverse path" check in the first place).


The questionable benefit of Loose RPF is found in asymmetric routing situations: a packet is dropped if there is no route at all, such as to "Martian addresses" or addresses that are not currently routed, but is not dropped if a route exists.


Loose Reverse Path Forwarding has problems, however. Since it sacrifices directionality, it loses the ability to limit an edge network's traffic to traffic legitimately sourced from that network, in most cases, rendering the mechanism useless as an ingress filtering mechanism.


Also, many ISPs use default routes for various purposes such as collecting illegitimate traffic at so-called "Honey Pot" systems or discarding any traffic they do not have a "real" route to, and smaller ISPs may well purchase transit capabilities and use a default route from a larger provider. At least some implementations of Loose RPF check where the default route points to. If the route points to the interface where Loose RPF is enabled, any packet is allowed from that interface; if it points nowhere or to some other interface, the packets with bogus source addresses will be discarded at the Loose RPF interface even in the presence of a default route. If such fine-grained checking is not implemented, presence of a default route nullifies the effect of Loose RPF completely.


One case where Loose RPF might fit well could be an ISP filtering packets from its upstream providers, to get rid of packets with "Martian" or other non-routed addresses.


If other approaches are unsuitable, loose RPF could be used as a form of contract verification: the other network is presumably certifying that it has provided appropriate ingress filtering rules, so the network doing the filtering need only verify the fact and react if any packets which would show a breach in the contract are detected. Of course, this mechanism would only show if the source addresses used are "martian" or other unrouted addresses -- not if they are from someone else's address space.


2.5. Loose Reverse Path Forwarding Ignoring Default Routes
2.5. 松散反向路径转发忽略默认路由

The fifth implementation technique may be characterized as Loose RPF ignoring default routes, i.e., an "explicit route presence check". In this approach, the router looks up the source address in the route table, and preserves the packet if a route is found. However, in the lookup, default routes are excluded. Therefore, the technique is mostly usable in scenarios where default routes are used only to catch traffic with bogus source addresses, with an extensive (or even full) list of explicit routes to cover legitimate traffic.


Like Loose RPF, this is useful in places where asymmetric routing is found, such as on inter-ISP links. However, like Loose RPF, since it sacrifices directionality, it loses the ability to limit an edge network's traffic to traffic legitimately sourced from that network.


3. Clarifying the Applicability of Ingress Filtering
3. 澄清入口过滤的适用性

What may not be readily apparent is that ingress filtering is not applied only at the "last-mile" interface between the ISP and the end user. It's perfectly fine, and recommended, to also perform ingress filtering at the edges of ISPs where appropriate, at the routers connecting LANs to an enterprise network, etc. -- this increases the defense in depth.


3.1. Ingress Filtering at Multiple Levels
3.1. 多级入口过滤

Because of wider deployment of ingress filtering, the issue is recursive. Ingress filtering has to work everywhere where it's used, not just between the first two parties. That is, if a user negotiates a special ingress filtering arrangement with his ISP, he should also ensure (or make sure the ISP ensures) that the same arrangements also apply to the ISP's upstream and peering links, if ingress filtering is being used there -- or will get used, at some point in the future; similarly with the upstream ISPs and peers.


In consequence, manual models which do not automatically propagate the information to every party where the packets would go and where ingress filtering might be applied have only limited generic usefulness.


3.2. Ingress Filtering to Protect Your Own Infrastructure
3.2. 入口过滤以保护您自己的基础架构

Another feature stemming from wider deployment of ingress filtering may not be readily apparent. The routers and other ISP infrastructure are vulnerable to several kinds of attacks. The threat is typically mitigated by restricting who can access these systems.


However, unless ingress filtering (or at least, a limited subset of it) has been deployed at every border (towards the customers, peers and upstreams) -- blocking the use of your own addresses as source addresses -- the attackers may be able to circumvent the protections of the infrastructure gear.


Therefore, by deploying ingress filtering, one does not just help the Internet as a whole, but protects against several classes of threats to your own infrastructure as well.


3.3. Ingress Filtering on Peering Links
3.3. 对等链路上的入口过滤

Ingress filtering on peering links, whether by ISPs or by end-sites, is not really that much different from the more typical "downstream" or "upstream" ingress filtering.


However, it's important to note that with mixed upstream/downstream and peering links, the different links may have different properties (e.g., relating to contracts, trust, viability of the ingress filtering mechanisms, etc.). In the most typical case, just using an ingress filtering mechanism towards a peer (e.g., Strict RPF) works just fine as long as the routing between the peers is kept reasonably symmetric. It might even be considered useful to be able to filter out source addresses coming from an upstream link which should have come over a peering link (implying something like Strict RPF is used towards the upstream) -- but this is a more complex topic and considered out of scope; see Section 6.


4. Solutions to Ingress Filtering with Multihoming
4. 采用多归宿的入口过滤解决方案

First, one must ask why a site multihomes; for example, the edge network might:


o use two ISPs for backing up the Internet connectivity to ensure robustness,

o 使用两个ISP备份Internet连接以确保健壮性,

o use whichever ISP is offering the fastest TCP service at the moment,

o 使用目前提供最快TCP服务的ISP,

o need several points of access to the Internet in places where no one ISP offers service, or

o 在没有ISP提供服务的地方需要多个互联网接入点,或者

o be changing ISPs (and therefore multihoming only temporarily).

o 正在更改ISP(因此多主机只能暂时更改)。

One can imagine a number of approaches to working around the limitations of ingress filters for multihomed networks. Options include:


1. Do not multihome.

1. 不要住在家里。

2. Do not use ingress filters.

2. 不要使用入口过滤器。

3. Accept that service will be incomplete.

3. 接受服务将是不完整的。

4. On some interfaces, weaken ingress filtering by using an appropriate form of loose RPF check, as described in Section 4.1.

4. 在某些接口上,如第4.1节所述,通过使用适当形式的松散RPF检查来削弱入口过滤。

5. Ensure, by BGP or by contract, that each ISP's ingress filter is complete, as described in Section 4.2.

5. 按照第4.2节所述,通过BGP或合同确保每个ISP的入口过滤器完整。

6. Ensure that edge networks only deliver traffic to their ISPs that will in fact pass the ingress filter, as described in Section 4.3.

6. 如第4.3节所述,确保边缘网络仅向其ISP提供实际上将通过入口过滤器的流量。

The first three of these are obviously mentioned for completeness; they are not and cannot be viable positions; the final three are considered below.


The fourth and the fifth must be ensured in the upstream ISPs as well, as described in Section 3.1.


Next, we now look at the viable ways for dealing with the side-effects of ingress filters.


4.1. Use Loose RPF When Appropriate
4.1. 适当时使用松散的RPF

Where asymmetric routing is preferred or is unavoidable, ingress filtering may be difficult to deploy using a mechanism such as strict RPF which requires the paths to be symmetrical. In many cases, using operational methods or feasible RPF may ensure the ingress filter is complete, like described below. Failing that, the only real options are to not perform ingress filtering, use a manual access-list (possibly in addition to some other mechanisms), or to using some form of Loose RPF check.


Failing to provide any ingress filter at all essentially trusts the downstream network to behave itself, which is not the wisest course of action. However, especially in the case of very large networks of even hundreds or thousands of prefixes, maintaining manual access-lists may be too much to ask.


The use of Loose RPF does not seem like a good choice between the edge network and the ISP, since it loses the directionality of the test. This argues in favor of either using a complete filter in the upstream network or ensuring in the downstream network that packets the upstream network will reject will never reach it.


Therefore, the use of Loose RPF cannot be recommended, except as a way to measure whether "martian" or other unrouted addresses are being used.


4.2. Ensure That Each ISP's Ingress Filter Is Complete
4.2. 确保每个ISP的入口过滤器都是完整的

For the edge network, if multihoming is being used for robustness or to change routing from time to time depending on measured ISP behavior, the simplest approach will be to ensure that its ISPs in fact carry its addresses in routing. This will often require the edge network to use provider-independent prefixes and exchange routes with its ISPs with BGP, to ensure that its prefix is carried upstream to the major transit ISPs. Of necessity, this implies that the edge network will be of a size and technical competence to qualify for a separate address assignment and an autonomous system number from its RIR.


There are a number of techniques which make it easier to ensure the ISP's ingress filter is complete. Feasible RPF and Strict RPF with operational techniques both work quite well for multihomed or asymmetric scenarios between the ISP and an edge network.


When a routing protocol is not being used, but rather the customer information is generated from databases such as Radius, TACACS, or Diameter, the ingress filtering can be the most easily ensured and kept up-to-date with Strict RPF or Ingress Access Lists generated automatically from such databases.


4.3. Send Traffic Using a Provider Prefix Only to That Provider
4.3. 仅使用提供程序前缀向该提供程序发送流量

For smaller edge networks that use provider-based addressing and whose ISPs implement ingress filters (which they should do), the third option is to route traffic being sourced from a given provider's address space to that provider.


This is not a complicated procedure, but requires careful planning and configuration. For robustness, the edge network may choose to connect to each of its ISPs through two or more different Points of Presence (POPs), so that if one POP or line experiences an outage, another link to the same ISP can be used. Alternatively, a set of tunnels could be configured instead of multiple connections to the same ISP [4][5]. This way the edge routers are configured to first inspect the source address of a packet destined to an ISP and shunt it into the appropriate tunnel or interface toward the ISP.


If such a scenario is applied exhaustively, so that an exit router is chosen in the edge network for every prefix the network uses, traffic originating from any other prefix can be summarily discarded instead of sending it to an ISP.


5. Security Considerations
5. 安全考虑

Ingress filtering is typically performed to ensure that traffic arriving on one network interface legitimately comes from a computer residing on a network reachable through that interface.


The closer to the actual source ingress filtering is performed, the more effective it is. One could wish that the first hop router would ensure that traffic being sourced from its neighboring end system was correctly addressed; a router further away can only ensure that it is possible that there is such a system within the indicated prefix. Therefore, ingress filtering should be done at multiple levels, with different level of granularity.


It bears to keep in mind that while one goal of ingress filtering is to make attacks traceable, it is impossible to know whether the particular attacker "somewhere in the Internet" is being ingress filtered or not. Therefore, one can only guess whether the source addresses have been spoofed or not: in any case, getting a possible lead -- e.g., to contact a potential source to ask whether they're observing an attack or not -- is still valuable, and more so when the ingress filtering gets more and more widely deployed.


In consequence, every administrative domain should try to ensure a sufficient level of ingress filtering on its borders.


Security properties and applicability of different ingress filtering types differ a lot.


o Ingress Access Lists require typically manual maintenance, but are the most bulletproof when done properly; typically, ingress access lists are best fit between the edge and the ISP when the configuration is not too dynamic if strict RPF is not an option, between ISPs if the number of used prefixes is low, or as an additional layer of protection.

o 入口访问列表通常需要手动维护,但如果操作得当,则最安全;通常,如果没有严格的RPF选项,则当配置不是太动态时,入口访问列表最适合在边缘和ISP之间使用;如果使用的前缀数量较少,则在ISP之间使用;或者作为额外的保护层使用。

o Strict RPF check is a very easy and sure way to implement ingress filtering. It is typically fit between the edge network and the ISP. In many cases, a simple strict RPF can be augmented by operational procedures in the case of asymmetric traffic patterns, or the feasible RPF technique to also account for other alternative paths.

o 严格的RPF检查是实现入口过滤的一种非常简单和可靠的方法。它通常适用于边缘网络和ISP之间。在许多情况下,简单的严格RPF可以通过不对称流量模式下的操作程序或可行的RPF技术来增强,以同时考虑其他备选路径。

o Feasible Path RPF check is an extension of Strict RPF. It is suitable in all the scenarios where Strict RPF is, but multihomed or asymmetric scenarios in particular. However, one must remember that Feasible RPF assumes the consistent origination and

o 可行路径RPF检查是严格RPF的一个扩展。它适用于所有需要严格RPF的场景,尤其是多宿主或不对称场景。然而,必须记住,可行的RPF假设始发和终止的一致性

propagation of routing information to work; the implications of this must be understood especially if a prefix advertisement passes through third parties.


o Loose RPF primarily filters out unrouted prefixes such as Martian addresses. It can be applied in the upstream interfaces to reduce the size of DoS attacks with unrouted source addresses. In the downstream interfaces it can only be used as a contract verification, that the other network has performed at least some ingress filtering.

o 松散的RPF主要过滤掉未路由的前缀,如火星地址。它可以应用于上游接口,以减少源地址未路由的DoS攻击的规模。在下游接口中,它只能用作合同验证,以确认其他网络至少执行了一些入口过滤。

When weighing the tradeoffs of different ingress filtering mechanisms, the security properties of a more relaxed approach should be carefully considered before applying it. Especially when applied by an ISP towards an edge network, there don't seem to be many reasons why a stricter form of ingress filtering would not be appropriate.


6. Conclusions and Future Work
6. 结论和今后的工作

This memo describes ingress filtering techniques in general and the options for multihomed networks in particular.


It is important for ISPs to implement ingress filtering to prevent spoofed addresses being used, both to curtail DoS attacks and to make them more traceable, and to protect their own infrastructure. This memo describes mechanisms that could be used to achieve that effect, and the tradeoffs of those mechanisms.


To summarize:


o Ingress filtering should always be done between the ISP and a single-homed edge network.

o 应始终在ISP和单个托管边缘网络之间进行入口过滤。

o Ingress filtering with Feasible RPF or similar Strict RPF techniques could almost always be applied between the ISP and multi-homed edge networks as well.

o 使用可行的RPF或类似的严格RPF技术的入口过滤几乎总是可以应用于ISP和多宿边缘网络之间。

o Both the ISPs and edge networks should verify that their own addresses are not being used in source addresses in the packets coming from outside their network.

o ISP和边缘网络都应验证其自身的地址未在来自其网络外部的数据包的源地址中使用。

o Some form of ingress filtering is also reasonable between ISPs, especially if the number of prefixes is low.

o ISP之间的某种形式的入口过滤也是合理的,特别是在前缀数量较少的情况下。

This memo will lower the bar for the adoption of ingress filtering especially in the scenarios like asymmetric/multihomed networks where the general belief has been that ingress filtering is difficult to implement.


One can identify multiple areas where additional work would be useful:


o Specify the mechanisms in more detail: there is some variance between implementations e.g., on whether traffic to multicast destination addresses will always pass the Strict RPF filter or not. By formally specifying the mechanisms the implementations might get harmonized.

o 更详细地指定机制:实现之间存在一些差异,例如,到多播目标地址的流量是否总是通过严格的RPF过滤器。通过正式指定机制,实现可能会得到协调。

o Study and specify Routing Information Base (RIB) -based RPF mechanisms, e.g., Feasible Path RPF, in more detail. In particular, consider under which assumptions these mechanisms work as intended and where they don't.

o 更详细地研究和指定基于路由信息库(RIB)的RPF机制,例如可行路径RPF。特别是,在这些假设下考虑这些机制的工作意图和他们没有。

o Write a more generic note on the ingress filtering mechanisms than this memo, after the taxonomy and the details or the mechanisms (points above) have been fleshed out.

o 在分类法和细节或机制(上面的要点)充实之后,就入口过滤机制编写一份比本备忘录更一般的说明。

o Consider the more complex case where a network has connectivity with different properties (e.g., peers and upstreams), and wants to ensure that traffic sourced with a peer's address should not be accepted from the upstream.

o 考虑网络具有不同属性(例如,对等体和上游流)的更复杂的情况,并且希望不应该从上游接收来自对等体地址的流量。

7. Acknowledgements
7. 致谢

Rob Austein, Barry Greene, Christoph Reichert, Daniel Senie, Pedro Roque, and Iljitsch van Beijnum reviewed this document and helped in improving it. Thomas Narten, Ted Hardie, and Russ Housley provided good feedback which boosted the document in its final stages.

Rob Austein、Barry Greene、Christoph Reichert、Daniel Senie、Pedro Roque和Iljitsch van Beijnum审查了该文件,并帮助改进了该文件。Thomas Narten、Ted Hardie和Russ Housley提供了良好的反馈,推动了文件的最后阶段。

8. References
8. 工具书类
8.1. Normative References
8.1. 规范性引用文件

[1] 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.

[1] 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。

8.2. Informative References
8.2. 资料性引用

[2] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities Attribute", RFC 1997, August 1996.

[2] Chandrasekeran,R.,Traina,P.和T.Li,“BGP社区属性”,RFC 1997,1996年8月。

[3] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.

[3] IANA,“特殊用途IPv4地址”,RFC 3330,2002年9月。

[4] Bates, T. and Y. Rekhter, "Scalable Support for Multi-homed Multi-provider Connectivity", RFC 2260, January 1998.

[4] Bates,T.和Y.Rekhter,“多宿多提供商连接的可扩展支持”,RFC 2260,1998年1月。

[5] Hagino, J. and H. Snyder, "IPv6 Multihoming Support at Site Exit Routers", RFC 3178, October 2001.

[5] Hagino,J.和H.Snyder,“站点出口路由器的IPv6多主支持”,RFC 3178,2001年10月。

9. Authors' Addresses
9. 作者地址

Fred Baker Cisco Systems Santa Barbara, CA 93117 US



Pekka Savola CSC/FUNET Espoo Finland


10. Full Copyright Statement
10. 完整版权声明

Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78 and except as set forth therein, the authors retain all their rights.

版权所有(C)互联网协会(2004年)。本文件受BCP 78中包含的权利、许可和限制的约束,除其中规定外,作者保留其所有权利。



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