Internet Engineering Task Force P. Savola Internet-Draft CSC/FUNET Expires:
October 15,November 17, 2004 R. Lehtonen TeliaSonera D. Meyer Apr 16,May 19, 2004 PIM-SM Multicast Routing Security Issues and Enhancements draft-ietf-mboned-mroutesec-00.txtdraft-ietf-mboned-mroutesec-01.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt.http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 15,November 17, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This memo describes security threats for the larger (intra-domain, or inter-domain) multicast routing infrastructures. Only Protocol Independent Multicast - Sparse Mode (PIM-SM) is analyzed, in its three main operational modes: the traditional Any Source Multicast (ASM) model, Source-Specific Multicast (SSM) model, and the ASM model enhanced by the Embedded-RP group-to-RP mapping mechanism. This memo also describes enhancements to the protocol operations to mitigate the identified threats. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Threats to Multicast Routing . . . . . . . . . . . . . . . . . 4 3.1 Receiver-based Attacks . . . . . . . . . . . . . . . . . . 45 3.1.1 Joins to Different Groups . . . . . . . . . . . . . . 5 3.2 Source-based Attacks . . . . . . . . . . . . . . . . . . . 6 3.2.1 Sending Multicast to Empty Groups . . . . . . . . . . 6 3.2.2 Disturbing Existing Group by Sending to It . . . . . . 78 3.3 Aggravating Factors to the Threats . . . . . . . . . . . . 8 3.3.1 Distant RP/Source Problem . . . . . . . . . . . . . . 9 3.3.2 No Receiver Information in PIM Joins . . . . . . . . . 9 4. Threat Analysis . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Summary of the Threats . . . . . . . . . . . . . . . . . . 9 4.2 Enhancements for Threat Mitigation . . . . . . . . . . . . 10 5. PIM Security Enhancements . . . . . . . . . . . . . . . . . . 11 5.1 Remote Routability Signalling . . . . . . . . . . . . . . 11 5.2 Rate-limiting Possibilities . . . . . . . . . . . . . . . 12 5.3 Specific Rate-limiting Suggestions . . . . . . . . . . . . 13 5.3.1 Group Management Protocol Rate-limiter . . . . . . . . 13 5.3.2 Source Transmission Rate-limiter . . . . . . . . . . . 14 5.3.3 PIM Signalling Rate-limiter . . . . . . . . . . . . . 14 5.3.4 Unicast-decapsulation Rate-limiter . . . . . . . . . . 14 5.3.5 PIM Register Rate-limiter . . . . . . . . . . . . . . 15 5.3.6 MSDP Source-Active Rate-limiter . . . . . . . . . . . 15 6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 1516 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 1516 9.2 Informative References . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17 A. RPF Considers Interface, Not Neighbor . . . . . . . . . . . . 17 B. Return Routability Extensions . . . . . . . . . . . . . . . . 18 B.1 Sending PIM-Prune Messages Down the Tree . . . . . . . . . 18 B.2 Analysing Multicast Group Traffic at DR . . . . . . . . . 19 B.3 Comparison of the Above Approaches . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . 20 1. Introduction This memo describes security threats to the Protocol Independent Multicast - Sparse Mode (PIM-SM) multicast routing infrastructures, and suggests ways to make these architectures more resistant to the described threats. Only attacks which have an effect on the multicast routing (whether intra- or inter-domain) are considered. For example, attacks where the hosts are specifically targeting the Designated Router (DR) or other routers on the link, or where hosts are disrupting other hosts on the same link are out of scope. Similarly, ensuring confidentiality, authentication and integrity of multicast groups and traffic is out of the scope . PIM builds on a model where Reverse Path Forwarding (RPF) check is (among other things) used to ensure loop-free properties of the multicast distribution trees. As a side effect, this limits the effect of using forged source addresses, which is often used as a component in unicast-based attacks. However, a host can still spoof an address within the same subnet, or spoof the source of a unicast-encapsulated PIM Register messages, which a host may send on its own. We consider PIM-SM  operating in the traditional Any Souce Multicast (ASM) model (including the use of Multicast Source Discovery Protocol (MSDP)  for source discovery), in Source-Specific Multicast  (SSM) model, and the Embedded-RP  group-to-RP mapping mechanism in ASM model. If the Bidirectional-PIM enhancements are globally significant, and have implications, they could also be considered, but are out of scope for now. 2. Terminology ASM "ASM"  is used to refer to the traditional Any Source Multicast model with multiple PIM domains and a signalling mechanism (MSDP) to exchange information about active sources between them. SSM "SSM"  is used to refer to Source-Specific Multicast. SSM channel SSM channel (S, G) identifies the multicast delivery tree associated with a source address S and a SSM destination address G. Embedded-RP "Embedded-RP" refers to the ASM model where the Embedded-RP mapping mechanism is used to find the RP for a group, and MSDP is not used. Target Router "Target Router" is used to refer to either the RP processing a packet (ASM or Embedded-RP), or the DR that is receiving (Source, Group) (or (S,G)) joins, (in all models). 3. Threats to Multicast Routing We make the broad assumption that the multicast routing networks are reasonably trusted. That is, we assume that the multicast routers themselves are well-behaved, in the same sense that unicast routers are expected to behave well, and are not a significant source of abuse. While this assumption is not entirely correct, it simplifies the analysis of threat models. The threats caused by misbehaving multicast routers (including fake multicast routers) are not considered in this memo. Also RP discovery mechanisms like BSR and Auto-RP are out of the scope. In general, the threat model would then be similar to . As the threats described in this memo are mainly Denial of Service (DoS) attacks, it may be useful to note that the attackers will try to find a scarce resource anywhere in the control or data plane, as described in . 3.1 Receiver-based Attacks These attacksThere are often referredmultiple threats relating to the use of host-to-router signalling protocols -- such as control plane attacks andInternet Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD) -- but these are outside the aimscope of this memo. PIM-SM can also be abused in the attackercases where RPF checks are not applicable, in particular, in the stub LAN networks -- as spoofing the on-link traffic is usuallyvery simple. For example, a host could get elected to increasebecome DR for the amountsubnet, but not perform any of multicast state information in routers above a manageable level. One should note that hostsits functions. A host can also originate PIM messages (e.g.easily make PIM Joins) as long as their source address passesrouters on the RPF checks.link stop forwarding multicast by sending PIM Assert messages. This implies that a willful attacker will be able to circumvent many of the potential rate-limiting functions performed at the DR (as one can always send the messages yourself). The PIM-SM specification, however, states that these messages should only be accepted from known PIM neighbors ;neighbors; if this is performed, the hosts would first have to establish a PIM adjacency with the router. Typically, adjacencies are formed with anyone on the link, so a willful attacker would have a high probability of success in forming a protocol adjacency. These are described at some length in , but are also considered out of scope of this memo. 3.1 Receiver-based Attacks These attacks are often referred to as control plane attacks and the aim of the attacker is usually to increase the amount of multicast state information in routers above a manageable level. One should also note that even if a host joins to a group multiple times, the DR only sends one PIM Join message, without waiting for any acknowledgement; the next message is only sent after the timer expires or the state changes at the DR. Also, if the host uses IGMPv3  or MLDv2 , it is able to join multiple sources for the same group and each of these joins for the same group generates new PIM (S,G) Joins to the DR adjacent to the source. 3.1.1 Joins to Different Groups Description of the threat: Join Flooding. Join Flooding occurs when a host tries to join, once or a couple of times, to a group or a SSM channel, and the DR generates a PIM Join to the Target Router. The group/SSM channel or the Targer Router may or may not exist. Example of this is a host trying to join different, non-existent groups at a very rapid pace, trying to overload the routers on the path with an excessive amount of (*/S,G) state (also referred to as "PIM State"), or the Target Router with an excessive number of packets. This kind of joining causes PIM state to be created, but this state is relatively short-lived (260 seconds by default, which is the default time that the state is active at DR in the absence of IGMP/ MLD Reports/Leaves). It should also be noted that the host can join a number of different SSM channels with only one IGMPv3/MLDv2 Report as the protocol allows to include multiple sources in the same message. However, even short-lived state may be harmful when the intent is to cause as much state as possible. The host can continue to send IGMP/ MLD Reports to these groups to make the state attack more long-lived. This results in: o ASM: a (*,G) join is sent to an intra-domain RP, causing state on that path; in turn, that RP joins to the DR of the source if the source is active. If the source address was specified by the host in the IGMPv3/MLDv2 Report, a (S,G) Join is sent directly to the DR of the source, as with SSM, below. o SSM: a (S,G) join is sent inter-domain to the DR of the source S, causing state on that path. If the source does not exist, the join goes to the closest router on the path to S as possible. o Embedded-RP: a (*,G) join is sent towards an inter/intra-domain RP embedded in the group G, causing state on that path. If the RP does not exist, the join goes to the closest router to the RP as possible. Similarly, an explicit (S,G) join goes to the DR, as with SSM above. That is, SSM and Embedded-RP always enable "inter-domain" state creation. ASM defaults to intra-domain, but can be used for inter-domain state creation as well. If the source or RP (only in case of Embedded-RP) does not exist, the multicast routing protocol does not have any means to remove the distribution tree if the joining host remains active. Worst case attack could be a host remaining active to many different groups (containing either imaginary source or RP). Please note that the imaginary RP problem is related to only Embedded-RP, where the RP address is extracted from the group address, G. For example, if the host is able to generate 100 IGMPv3 (S,G) joins a second, each carrying 10 sources, the amount of state after 260 seconds would be 260 000 state entries -- and 100 packets per second is still a rather easily achievable number. 3.2 Source-based Attacks These attacks are often referred to as "data plane" attacks; however, with traditional ASM and MSDP, these also include an MSDP control plane threat. 3.2.1 Sending Multicast to Empty Groups Description of the threat: Data Flooding. Data Flooding occurs when a host sends data packets to a multicast group or SSM channel for which there are no real subscribers. Note that since unicast-encapsulation is not subject to RPF checks, the hosts can also craft and send these packets themselves, also spoofing the source address of the register messages unless ingress filtering  has been deployed . Examples of this threat are a virus/worm trying to propagate to multicast addresses, an attacker trying to crash routers with excessive MSDP state, or an attacker wishing to overload the RP with encapsulated packets or different groups. This results in: o ASM: the DR unicast-encapsulates the packets in Register messages to the intra-domain RP, which may join to the source and issue a Register-Stop, but continues to get the data. A notification about the active source is sent (unless the group or source is configured to be local) inter-domain with MSDP and propagated globally. o SSM: the DR receives the data, but the data does not propagate from the DR unless someone joins the (S,G) channel. o Embedded-RP: the DR register-encapsulates the packets to the intra/inter-domain RP, which may join to the source and issue a Register-Stop. Data continues to be encapsulated if different groups are used. This yields many potential attacks, especially if at least parts of the multicast forwarding functions are implemented on a "slow" path or CPU in the routers, at least: o The MSDP control plane traffic generated can cause a significant amount of control and data traffic which may overload the routers receiving it. A thorough analysis of MSDP vulnerabilities can be found in , and is only related to the ASM. However, this is the most serious threat at the moment, because MSDP will flood the multicast group information to all multicast domains in Internet including the multicast packet encapsulated to MSDP source-active message. This creates a lot of data and state to be shared by all multicast enabled routers and if the source remains active, the flooding will be repeated every 60 seconds by default. o As a large amount of data is forwarded on the multicast tree; if multicast forwarding is performed on CPU, it may be a serious performance bottleneck, and a way to perform DoS on the path. Similarly, the DR must always be capable of processing (and discarding, if necessary) the multicast packets received from the source. These are potentially present in every model. o If the encapsulation is performed on software, it may be a performance bottleneck, and a way to perform DoS on the DR. Similarly, if the decapsulation is performed on software, it may be a performance bottleneck, and a way to perform DoS on the RP. Note: the decapsulator may know, based on access configuration, a rate-limit or something else, that it doesn't need to decapsulate the packet, avoiding bottlenecks. These threats are related to ASM and Embedded-RP. 3.2.2 Disturbing Existing Group by Sending to It Description of the threat: Group Integrity Violation. Group Integrity Violation occurs when a host sends packets to a group or SSM channel, which already exists, to disturb the users of the existing group/SSM channel. The SSM service model prevents injection of packets to (S,G) channels, avoiding this problem. However, if the source address can be spoofed to be a topologically-correct address, it's possible to get the packet into the distribution tree -- typically only those hosts which are on-link with the source are able to perform this, so this is not really relevant in the scope of this memo. With ASM and Embedded-RP sources can inject forged traffic through RPs, which provide the source discovery for the group. The RP(s) send the traffic over the shared tree towards receivers (routers with (*,G) state). DR then forwards the forged traffic to receivers unless the legitimate recipients are able to filter out unwanted sources, e.g., using MSF API . Typically this is not used or supported by the applications using these protocols. Note that with ASM and Embedded-RP, the RP may exert some form of control on who can send to a group, as the first packets are unicast-encapsulated in register packets to the RP -- if the RP drops the packet based on access-list, rate-limiter or something else, it doesn't get injected to an existing group. With ASM this "source control" is distributed across all the PIM domains, which significantly decreases its applicability. Embedded-RP enables easier control because source discovery is done through single RP per group. As a result, for this attack to succeed, the RP must decapsulate the packets, causing the propagation of data and the integrity violation. 3.3 Aggravating Factors to the Threats This section describes a few factors, which aggravate the threats described in Section 3.1 and Section 3.2. These could also be viewed as individual threats on their own. There are multiple threats relating to the use of host-to-router signalling protocols -- such as Internet Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD) -- but these are outside the scope of this memo. PIM-SM can also be abused in the cases where RPF checks are not applicable, in particular, in the stub LAN networks -- as spoofing the on-link traffic is very simple. For example, a host would get elected to become DR for the subnet, but not perform any of its functions. These are described at some length in , but are also considered out of scope of this memo.3.3.1 Distant RP/Source Problem In the shared tree model, if the RP or a source is distant (topologically), then joins will travel to the distant RP or source and keep the state information in the path active, even if the data is being delivered locally. Note that this problem will be exacerbated if the RP/source space is global; if a router is registering to a RP/source that is not in the local domain (say, fielded by the site's direct provider), then the routing domain is flat. Also note that PIM assumes that the addresses used in PIM messages are valid. However, there is no way to ensure this, and using non-existent S or G in (*,G) or (S,G) -messages will cause the signalling to be set up, even though one cannot reach the address. This will be analysed at more length in Section 5.1. 3.3.2 No Receiver Information in PIM Joins Only DRs, which are directly connected to receivers, know the exact receiver information (e.g. IP address). PIM does not forward that information further in the multicast distribution tree. Therefore individual routers (e.g. domain edge routers) are not able to make policy decisions on who can be connected to the distribution tree. 4. Threat Analysis 4.1 Summary of the Threats Trying to summarize the severity of the major classes of threats with respect to each multicast usage model, we have a matrix of resistance to different kinds of threats: +----------------+------------------+-----------------+ | Forged Join | Being a Source | Group Integrity | +-------------+----------------+------------------+-----------------+ | ASM | bad 1) | very bad | bad/mediocre | +-------------+----------------+------------------+-----------------+ | SSM | bad | very good | very good | +-------------+----------------+------------------+-----------------+ | Embedded-RP | bad 1),2) | good/mediocre 3) | good | +-------------+----------------+------------------+-----------------+ Notes: 1) in ASM host can directly join also (S,G) groups with IGMPv3/MLDv2 and thus have same characteristics as SSM (also allows inter-domain state to be created). 2) allows inter-domain shared state to be created. 3) Embedded-RP allows a host to determine the RP for a given group (or set of groups), which in turn allows that host to mount a PIM register attack. In this case, the host can mount the attack without implementing any of the PIM register machinery. 4.2 Enhancements for Threat Mitigation There are several desirable actions ("requirements") which could be considered to mitigate these threats; these are listed below. A few more concrete suggestions are presented later in the section. o Inter-domain MSDP (ASM) should be retired to avoid attacks; or, if this is not reasonable, the DRs should rate-limit the unicast-encapsulation (note that the hosts can circumvent this) and (more importantly) the RPs should rate-limit the unicast-decapsulation especially from different sources, or MSDP must rate-limit the MSDP data generation for new sources. o DRs should rate-limit PIM Joins and Prunes somehow; there are multiple possibilities how exactly this should be considered (i.e., which variables to take into the consideration). o DRs could rate-limit unicast-encapsulation somehow; there are multiple ways to perform this. Note that the hosts can avoid this by performing the unicast-encapsulation themselves if so inclined. o RPs could rate-limit unicast-decapsulation somehow; there are multiple ways to perform this. Note that if the source of the unicast packets is spoofed by the host, this may have an effect on how e.g. rate-limiters behave. o RPs should rate limit the MSDP SA messages coming from MSDP peers. o RPs could limit or even disable the SA cache size. However, this could have negative effects on normal operation. o RPs should provide good interfaces to reject packets which are not interesting; for example, if an Embedded-RP group is not configured to be allowed in the RP, the unicast-encapsulated packets would not even be decapsulated. o DRs could rate-limit the multicast traffic somehow to reduce the disturbing possibilities; there are multiple possibilities how exactly this should be considered. o DRs should rate-limit the number of groups/SSM channels that can be created by a given source, S. 5. PIM Security Enhancements This section includes more in-depth description of the above-mentioned rate-limiting etc. functions as well as description of the remote routability signalling issue. 5.1 Remote Routability Signalling As described in Section 3.3.1, non-existent DRs or RPs may cause some problems when setting up multicast state. There seem to be a couple of different approaches to mitigate this, especially if rate-limiting is not extensively deployed. With ASM and Embedded-RP, Register message delivery could be ensured somehow. For example: 1) At the very least, receiving an ICMP unreachable message (of any flavor) should cause the DR to stop the Register packets -- as the RP will not be receiving them anyway. (However, one should note that easy spoofing of such ICMP messages could cause a DoS on legitimate traffic.) 2) An additional method could be implementing a timer on the RPs so that unless nothing is heard back from the RP within a defined time period, the flow of Register messages would stop. (Currently, the RPs are not required to answer back, unless they want to join to the source.) 3) An extreme case would be performing some form of return routability check prior to starting the register messages: first a packet would be sent to the RP, testing its existence and willingness to serve, and also proving to the RP that the sender of the "bubble" and the sender of the registers are the same and the source address is not forged (i.e., the RP would insert a cookie in the bubble, and it would have to be present in the register message.) It would be desirable to have some kind of state management for PIM Joins (and other messages) as well, for example, a "Join Ack" which could be used to ensure that the path to the source/RP actually exists. However, this is very difficult if not impossible with the current architecture: PIM messages are sent hop-by-hop, and there is not enough information to trace back the replies to e.g., notify the routers in the middle to release the corresponding state, and to nofify the DR that the path did not exist. Appendix B discusses this receiver-based remote routability signalling in more detail. 5.2 Rate-limiting Possibilities There seem to be many ways to implement rate-limiting (for signalling, data encapsulation and multicast traffic) at the DRs or RPs -- the best approach likely depends on the threat model; factors in the evaluation might be e.g.: o Whether the host is willfully maliscious, uncontrolled (e.g., virus/worm), or a regular user just doing something wrong. o Whether the threat is aimed towards a single group, a single RP handling the group, or the (multicast) routing infrastructure in general. o Whether the host on a subnet is spoofing its address (but still as one which fulfills the RPF checks of the DR) or not. o Whether the host may generate the PIM join (and similar) messages itself to avoid rate-limiters at the DR if possible. o Whether unicast RPF checks are applied on the link (i.e., whether the host can send unicast-encapsulated register-messages on its own). o Whether blocking the misbehaving host on a subnet is allowed to also block other, legitimate hosts on the same subnet. o Whether these mechanisms would cause false positives on links with only properly working hosts if many of them are receivers or senders. As should be obvious, there are many different scenarios here which seem to call for different kinds of solutions. For example, the rate-limiting could be performed based on: 1. multicast address, or the RP where the multicast address maps to 2. source address 3. the (source address, multicast address) -pair (or the RP which maps to the multicast address) 4. data rate in case of rate limiting the source 5. everything (multicast groups and sources would not be distinguished at all) In the above, we make an assumption that rate-limiting would be performed per-interface (on DRs) if a more fine-grained filter is not being used. It should be noted that some of the rate limiting functions can be used as a tool for DoS against legimate multicast users. Therefore several parameters for rate limiting should be used to prevent such operation. The next revisions of this document (or separated in other documents, if appropriate) will include more explicit discussion of the best ways to perform rate-limiting, especially considering the effects on the legimate traffic. 5.3 Specific Rate-limiting Suggestions These suggestions take two forms: limiters designed to be run on all the edge networks, preventing or limiting an attack in the first place, and the limiters designed to be run at the border of PIM domains or at the RPs, which should provide protection in case edge-based limiting fails or was not implemented, or when additional control is required. Almost none of the suggested rate-limiters take legitimate users into account. That is, for example, being able to allow some hosts on a link to transmit/receive, while disallowing others, is very challenging to do right, because the attackers can easily circumvent such systems. Therefore the intent is to limit the damage to only one link, one DR, or one RP -- and avoid the more global effects on the Internet multicast architecture. It could also be possible to perform white-listing of groups, sources, or (S,G) -pairs from the rate-limiters -- so that packets related to these would not be counted towards the limits (e.g., the case of an aggressive but legitimate source, while not not desiring to modify the limiting parameters for all the traffic. 5.3.1 Group Management Protocol Rate-limiter A token-bucket -based rate-limiter to all Group Management Protocols (IGMP, MLD), which would limit the average rate of accepted groups or sources, on the specific interface, to G_MAX per second, with a bucket of G_LONG. Example values could be G_MAX=1 and G_LONG=20. Note that e.g., an IGMPv3 join with two included sources for one group would count as two groups/sources. This would be the first-order defense against state-creation attacks from the hosts. However, as it cannot be guaranteed that all the routers would implement something like this, other kinds of protections would be useful as well. This harms legitimate receivers on the same link as an attacker as well. 5.3.2 Source Transmission Rate-limiter A token-bucket -based rate-limiter which would limit the multicast data transmission (excluding link-local groups) on a specific interface to GSEND_MAX groups per second, with a bucket of GSEND_LONG. Example values could be GSEND_MAX=10 and GSEND_LONG=20. This would be the first-order defense against data flooding attacks. However, as it cannot be guaranteed that all routers would implement something like this, and as the RP (if SSM is not used) could be loaded from multiple senders, additional protections are needed as well. This harms legitimate senders on the same link as an attacker as well. This does not protect from a host sending a lot of traffic to the same group; this only harms the DR and the RP of the group, and is similar to unicast DDoS attacks against one source, and is not considered critical for the overall security. 5.3.3 PIM Signalling Rate-limiter A token-bucket -based rate-limiter which would limit the all multicast PIM messaging, either through a specific interface or globally on the router, to PIM_MAX new entries per second, with a bucket of PIM_LONG. Example values could be 1000 and 10000. This would second-order defense againt PIM state attacks when GMP rate-limiters haven't been implemented or haven't been effective. This limiter might not need to be active by default, as long as the values are configurable. The main applicability for this filter would be applying it at a border of PIM domain in case PIM state attacks are detected. This harms legitimate receivers as well. 5.3.4 Unicast-decapsulation Rate-limiter A simple decapsulation rate-limiter protecting the CPU usage in the router, limiting X pps (the need and limit depends on the router architecture), disregarding the source of the registers. This could also be an additional check to be used before decapsulation and checking the group to throttle the worst of the decapsulation CPU consumption. This limit should have to be quite high, and would hamper the existing legimate sessions as well. 5.3.5 PIM Register Rate-limiter A token-bucket -based rate-limiter for unicast-decapsulation,unicast-decapsulation of PIM Register messages, limiting the decapsulationto DECAP_MAXREG_MAX new groups per second, with a bucket of DECAP_LONG.REG_LONG. If the router has restarted recently, a larger initial bucket should be used. Example values could be DECAP_MAX=1REG_MAX=1 and DECAP_LONG=10REG_LONG=10 (or DECAP_LONG=500REG_LONG=500 after restart). This would be second-order defense against data flooding: if the DRs would not implement appropriate limiters, or if the total number of flooded groups rises too high, the RP should be able to limit the rate with which new groups are created. This does not harm legitimate senders, as long as their group has already been created. 188.8.131.52.6 MSDP Source-Active Rate-limiter A token-bucket -based, source-based rate-limiter, limiting new groups per source to SAG_MAX per second, with a bucket of SAG_LONG. Example values could be SAG_MAX=1 and SAG_LONG=10. This would be a second-order defense, both at the MSDP SA sending and receiving sites, against data flooding and MSDP vulnerabilities in particular. The specific threat being addressed here is a source (or multiple different sources) trying to "probe" (e.g., virus or worm) different multicast addresses.  discusses different MSDP attack prevention mechanisms at length. 6. Security Considerations This memo analyzes the security of PIM routing infrastructures in some detail, and proposes enhancements to mitigate the observed threats. 7. IANA Considerations This memo is for informational purposes and does not introduce new namespaces for the IANA to manage. 8. Acknowledgements Kamil Sarac discussed "return routability" issues at length. Stig Venaas provided feedback to improve the document quality. 9. References 9.1 Normative References  Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode PIM-SM): Protocol Specification (Revised)", draft-ietf-pim-sm-v2-new-09 (work in progress), February 2004.  Fenner, B. and D. Meyer, "Multicast Source Discovery Protocol (MSDP)", RFC 3618, October 2003.  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", draft-ietf-ssm-arch-04 (work in progress), October 2003.  Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address", draft-ietf-mboned-embeddedrp-02draft-ietf-mboned-embeddedrp-04 (work in progress), MarchApril 2004.  Barbir, A., Murphy, S. and Y. Yang, "Generic Threats to Routing Protocols", draft-ietf-rpsec-routing-threats-06 (work in progress), April 2004. 9.2 Informative References  Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989.  Bhattacharyya, S., "An Overview of Source-Specific Multicast (SSM)", RFC 3569, July 2003.  Thaler, D., Fenner, B. and B. Quinn, "Socket Interface Extensions for Multicast Source Filters", RFC 3678, January 2004.  Hardjono, T. and B. Weis, "The Multicast Security Architecture", draft-ietf-msec-arch-05 (work in progress), January 2004.  Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002.  Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", draft-vida-mld-v2-08 (work in progress), December 2003.  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.  Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", draft-savola-bcp38-multihoming-update-03 (work in progress), December 2003.  Rajvaidya, P., Ramachandran, K. and K. Almeroth, "Detection and Deflection of DoS Attacks Against the Multicast Source Discovery Protocol", UCSB Technical Report, May 2003. Authors' Addresses Pekka Savola CSC/FUNET Espoo Finland EMail: email@example.com Rami Lehtonen TeliaSonera Hataanpaan valtatie 20 Tampere 33100 Finland EMail: firstname.lastname@example.org David Meyer EMail: email@example.com Appendix A. RPF Considers Interface, Not Neighbor In most current implementations, the RPF check considers only the incoming interface, and not the upstream neighbor (RPF neighbor). This can result in accepting packets from the "wrong" RPF neighbor (the neighbor is "wrong" since, while the RPF check succeeds and the packet is forwarded, the unicast policy would not have forwarded the packet). This is a problem in the media where more than two routers can connect to, in particular, Ethernet-based Internet Exchanges. Therefore any neighbor on such a link could inject any PIM signalling as long as a route matching the address used in the signalling is going through the interface. However, one should note that for PIM signalling to be accepted, a PIM adjancency must have been established. However, typically, this does not help much against willful attackers, as typically PIM adjacencies are formed with anyone on the link. Still, the requirement is that the neighbor who has enabled PIM in the concerned interface. I.e., in most cases, the threat is limited to attackers within the operators in the exchange, not third parties. On the other hand, data plane forwarding has no such checks -- and having such checks would require one to look at the link-layer addresses used; that is, this checking is not as feasible as one might hope. Appendix B. Return Routability Extensions The multicast state information is built from the receiver side and it can be currently pruned only by the receiver side DR. If the RP or the source for the group is non-existent, the state can't be pruned by the DR without return routability extensions to provide such information. There might be also need to remove the state in some cases when there is no multicast traffic sent to that group. This section discusses about the alternative ways to remove the unused state information in the routers, so that it can't be used in state based DoS attacks. Note that rate limiting PIM Joins gives some protection against the state attacks. B.1 Sending PIM-Prune Messages Down the Tree When a router discovers the non-existancenon-existence of the RP or the source (XXX: does it actually know if there is RP/Source or not),source, it can create a PIM-Prune message and send it back to the join originator. However, since it does not know the unicast IP address of join originator DR, it cannot directly unicast it to that router. A possible alternative is to use a link-local multicast group address (e.g., all-pim routers local multicast address) to pass this information back toward the joining DR. Since the routers from this current router all the way back to the joining DR has forwarding state entry for the group, they can use this state information to see how to forward the PIM-Prune message back. Each on-tree router, in addition to forwarding the PIM-Prune message, can also prune the state from their state tables. This way, the PIM-Prune message will go back to the DR by following the so far created multicast forwarding state information. In addition, if we use some sort of RPF checks during this process, we can also make it more difficult to inject such PIM-Prune messages maliciously. A potential abuse scenario may involve an attacker having access to a router on the direct path to send such PIM-Prune messages down the tree branch so as to prune the branch from the tree. But such an attacker can currently achieve the same effect by sending PIM-Prune message toward the source from the same point on the tree. So, the proposed mechanism does not really aggravate the situation. One visible overhead in this new scenario might be that someone can send bogus join messages to create redundant PIM-Join and PIM-Prune messages in the network. B.2 Analysing Multicast Group Traffic at DR Another possible way to remove the unused state information would be to analyse individual group traffic at the DR and if there is no multicast traffic for a certain group within a certain time limit, the state should be removed. In here, if the receiver is malicious and wants to create states in the network, then it can send joins to different groups and create states on routers for each of these different groups until the DR decides that the groups are inactive and initiates the prune process. In addition, during the prune process, the routers will again process all these prune messages and therefore will be spending time. B.3 Comparison of the Above Approaches Both of these solutions have the same problem of renewing the multicast state information. DR shouldn't permanently block the state building for that group, but to restrict the PIM Joins if it notices that the receiver is abusing the system. One additional option is to block the PIM Joins to the non-existent source/RP for a certain time. In the first approach (sending PIM-Prunes down the tree), part of the goal was to prune the states in the routers much sooner than in the second approach (e.g. goal is to make sure that the routers will not be keeping unnecessary states for long time). The second approach works also for DoS attacks related to the existing source/RP addresses and could be more quickly implemented and deployed in the network and does not have any relationship related to the other deployments (no need to change all PIM routers). 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