Internet DRAFT - draft-chown-mboned-multicast-filtering
draft-chown-mboned-multicast-filtering
Mboned T. Chown
Internet-Draft University of Southampton
Intended status: Informational L. Giuliano
Expires: September 13, 2012 Juniper Networks
March 12, 2012
Multicast Filtering Practices
draft-chown-mboned-multicast-filtering-02
Abstract
Operators of multicast networks may apply various filters to
multicast traffic at boundary routers or on MSDP peerings. The aim
of this text is to discuss appropriate filtering policies, as well as
documenting existing filtering practices, with a view to generating
some discussion towards producing guidance on best filtering
practice.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. General Discussion of Policies . . . . . . . . . . . . . . . . 4
2.1. Multicast Addressing and Domain Borders . . . . . . . . . 4
2.2. MSDP SA Policy and Policers . . . . . . . . . . . . . . . 4
2.3. Multicast Scoping . . . . . . . . . . . . . . . . . . . . 5
2.4. PIM Policy . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. IGMP Policy . . . . . . . . . . . . . . . . . . . . . . . 6
2.6. SAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Summary of Reported Filtering Practices . . . . . . . . . . . 6
3.1. Border and MSDP Filtering . . . . . . . . . . . . . . . . 7
3.2. Organisational filtering . . . . . . . . . . . . . . . . . 9
3.3. Subnet filtering . . . . . . . . . . . . . . . . . . . . . 9
4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. Informative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Multicast filtering can be applied at a multicast boundary or on an
MSDP peering as a means to prevent unintended leakage of multicast
traffic beyond its desired scope. An informal discussion of
filtering practices suggested that those practices vary from
organisation to organisation. The aim of this text is to gather and
document commonly used existing filtering practices. Whether it is
then possible to draw up a definitive best practice is to be
determined; it is quite possible that due to the shifting nature of
the target that a point-in-time recommendation would quickly be
overtaken by events. For example, the recent addition of unicast
prefix-based IPv4 multicast addresses [RFC6034] meant that filtering
of all of 234.0.0.0/8 became undesirable. However, general
principles may remain valid over time.
The text begins with a discussion of appropriate policies in Section
2, followed by Section 3 in which we document a summary of reported
practices at border, organisational and subnet scopes. We then draw
some conclusions in Section 3.
For sites on academic research networks, some examples of filtering
recommendations already exists, e.g. in documentation [I2multicast]
from the Internet2 Multicast WG, and in the JANET IPv4 Multicast
Guide [JANETmulticast]. There is also a more specific proposal for
the Rutgers network [RutgersProposal], which includes a good
discussion of organisational-local scope address usage within its
network as a whole.
When determining filtering policies, one needs to consider how strict
to be; some ranges are not supposed to be used, but there may be no
harm per se in accepting them. There are certainly some ranges that
should not be filtered, such as the newly assigned 234.0.0.0/8 range
mentioned above, and the GLOP range under 233.0.0.0/8.
An additional resource is the registry of IPv4 multicast address
space held by IANA [IANAmulticast]. This registry should be a
definitive guide to the formal use of ranges of addresses within the
overall IPv4 multicast address space. A similar registry is
maintained for IPv6 multicast address space [IANA6multicast].
This text is still quite an early draft, aimed at soliciting
feedback, both on content and whether the goal of the draft is
actually worthwhile. Different sites may have different
requirements. There may also be issues with handling scope
boundaries that need to be considered. So there may be general
principles that could be captured in a document such as this, even if
specific filtering rules are not included.
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2. General Discussion of Policies
In this section we discuss how we might believe filters to be applied
with respect to various multicast protocol functions. This may
include rate limiting and policing in addition to straight filtering.
In the following section we summarise actual filtering practices that
have been reported.
2.1. Multicast Addressing and Domain Borders
To date, IPv4 multicast addresses have been assigned from the
following ranges for global usage: 224.0/16, 224.1/16, 224.2/16,
224.3/16, 224.4/16, 232/8, 233/8 and 234/8. All other IPv4 multicast
addresses are therefore reserved, unassigned or scoped, and as such,
have no legitimate reason for use on the Internet. Therefore we
recommend operational filters that permit these address ranges and
block all others at domain borders.
It should be noted that some networks do use multicast addresses
outside of these ranges for internal purposes. For example, 239/8,
which is administratively scoped for Organization-Local usage, is
functionally analogous to RFC 1918 unicast IPv4 addresses, and is
often used by networks to support internal customers or
infrastructure services. As such, these types of addresses may be
used within a domain, but should never be allowed to cross domain
borders.
2.2. MSDP SA Policy and Policers
MSDP policies should include filters that allow SAs only from the
assigned ASM IPv4 multicast address ranges: 224.0/16, 224.1/16,
224.2/16, 224.3/16, 224.4/16, 233/8 and 234/8. The 232/8 address
range is reserved for SSM only, and thus, should never appear in
MSDP.
The most common multicast attack vector that has appeared on the
Internet has been MSDP SA storms. In nearly all cases, these were
triggered by Internet worms that probed large blocks of addresses in
an attempt to scan vulnerable ports. Typically, these worms were
intended to scan only unicast addresses at random, however, the worm
coders accidentally included multicast addresses in the random pools
of destinations to scan. When port scans with destination addresses
of multicast addresses occur on multicast-enabled network, these
packets generate PIM register messages and, consequently, MSDP SA
messages according to standard PIM and MSDP procedures, resulting in
a flood of SA messages across the Internet that can put great strain
on MSDP-speaking routers.
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MSDP filters that allow SAs from only the assigned ranges do help to
reduce the potential pool for these accidental attacks. However, the
addition of MSDP policers provides much stronger protection from SA
storms. MSDP policers can be applied on a per-peer and per-source
basis. Per-peer policers are used to detect when the number of SAs
received from an MSDP peer exceeds a certain configured threshold.
These policers are functionally analogous to BGP max prefix limits
applied on BGP peers, which alert an operator when the number of
routes received from a BGP peer exceeds a certain configured
threshold. Typically, this type of per-peer threshold is determined
by observing the number of SAs that are normally advertised by a peer
and then selecting some multiple of that to be a limit. For example,
if a peer normally advertises 2k SAs, an operator may set the per-
peer threshold to 5k.
The challenge with per-peer limits is that they are not granular
enough to determine good SAs from bad ones. As such, these can
actually lower the bar needed to launch a denial of service attack.
For example, with a 5k per-peer threshold, an attacker need only
generate 5k SAs to trigger the limit and potentially block all the
legitimate SAs from propagating. As such, operators may want to use
per-peer limits to trigger an alarm, rather than tear down the MSDP
session. Additionally, per-source MSDP policers can be used to
provide further granularity between good and bad SAs. Per-source SA
policers define the maximum number of SAs that can be permitted from
the same source (or source range). For example, an operator can use
a per-source limit of 1k, which would prevent any source from
generating more than 1k MSDP SAs. With per-source limits in place, a
heavily distributed attack would be necessary to generate enough MSDP
state to impact routers as no single source can generate enough
state.
2.3. Multicast Scoping
Multicast scoping is used to block multicast packets based on group
address. Scoping filters should be used on all domain borders to
allow only the assigned IPv4 multicast addresses (224.0/16, 224.1/16,
224.2/16, 224.3/16, 224.4/16, 232/8, 233/8 and 234/8) and block all
other multicast groups from ingress/egress these borders.
2.4. PIM Policy
While multicast scoping blocks traffic in the data plane, it does not
affect the control plane. Thus, an illegitimate group could be
joined, with traffic carried across a providers network only to be
dropped at the border by a scoping filter. However, with PIM join
filters, the join could be dropped at the border so that no state is
created in the first place. Therefore, PIM join filters can be used
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for additional protection on all domain borders to allow joins for
only the assigned IPv4 multicast addresses (224.0/16, 224.1/16,
224.2/16, 224.3/16, 224.4/16, 232/8, 233/8 and 234/8) and block joins
for all other multicast groups. Additionally, PIM join filters can
be used to block joins based on the source address of the multicast
source (ie, the S in the (S,G) join). So PIM join filters can also
be configured to prevent joins for illegitimate interdomain sources,
such as RFC 1918 addresses.
PIM register filters can also be used to protect an RP from creating
register state for illegitimate groups by allowing PIM registers on
an RP for only the assigned IPv4 ASM multicast addresses (224.0/16,
224.1/16, 224.2/16, 224.3/16, 224.4/16, 233/8 and 234/8) and blocking
registers for all other multicast groups. PIM register filters can
also be configured to prevent registers for illegitimate interdomain
sources, such as RFC 1918 addresses.
2.5. IGMP Policy
As with PIM join filters, IGMP filters can be used for additional
protection on all receiver LANs to allow IGMP reports for only the
assigned IPv4 multicast addresses (224.0/16, 224.1/16, 224.2/16,
224.3/16, 224.4/16, 232/8, 233/8 and 234/8) and block reports for all
other multicast groups. IGMP filters can also be configured to
prevent IGMPv3 source-included reports for illegitimate interdomain
sources, such as RFC 1918 addresses.
2.6. SAP
In the early years of multicast, routers were often configured to
listen to the SAP group (224.2.127.254) and cache the SDP messages.
This was used as a quick and dirty way of determining if multicast
was working properly. Just by looking at the SAP/SDP cache on a
router and guessing if the number of session entries looked about
right, an operator could quickly determine if multicast flows were
traversing the router. However, this imprecise method of management
and troubleshooting eventually proved dangerous, as improperly
formatted SDP messages occasionally crashed routers as well as
improperly configured SAP sources accidently transmitted the
multicast stream they intended to announce on the SAP group, causing
harm to routers that cached this data. As such, routers should not
be configured to listen to the SAP group and cache SDP messages.
3. Summary of Reported Filtering Practices
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3.1. Border and MSDP Filtering
In this section we summarise IPv4 multicast addresses that are
commonly filtered at site borders or on MSDP peerings. Based on
responses we received from a couple of multicast community lists, it
wasn't clear which filters are applied on border routers and which on
MSDP SA messages. Some sites apply minimal traffic filters, but
heavier MSDP filtering.
A site may choose to filter on addresses or on observed TTLs; it is
now general practice to filter on addresses rather than the TTL
filtering that was common a long time ago.
Some sites choose to route multicast around their unicast firewalls,
for performance or other operational reasons, but this shouldn't
alter the requirement to filter groups appropriately where necessary.
In this section we draw on the small number contributions so far; we
hope to get more inputs in time. In general, many 224.0.0.*
addresses that are used by infrastructure are typically blocked, as
well as some addresses that are global scope but should not be, like
Ghostcast.
The following list includes multicast IPv4 addresses that are being
filtered based on the union of responses received so far (hence the
apparent duplication of certain prefixes). The list of filters
applied by all respondents would be somewhat shorter.
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224.0.1.1 NTP
224.0.1.2 SGI-Dogfight
224.0.1.3 Rwhod
224.0.1.8 SUN NIS+
224.0.1.20 any private experiment
224.0.1.22 SVRLOC
224.0.1.24 microsoft-ds
224.0.1.25 nbc-pro
224.0.1.35 SVRLOC-DA
224.0.1.38 Retrospect
224.0.1.39 cisco-rp-announce
224.0.1.40 cisco-rp-discovery
224.0.1.41 gatekeeper
224.0.1.60 hp-device-disc
224.0.1.65 iapp
224.0.1.76 IAPP lucaent-avaya-ap
224.0.2.1 rwho
224.0.2.2 SUN RPC
224.0.2.3 EPSON-disc-set
224.0.23.1 Ricoh-device-ctrl
224.0.23.2 Ricoh-device-ctrl
224.1.0.1 Cisco Aironet
224.1.0.38 Retrospect
224.2.0.2 Altiris Rapideploy
224.2.0.3 Altiris Rapideploy
224.77.0.0/16 Norton Ghost
224.101.101.101 Sun Sunray
225.1.2.3 Altiris Development Server and Deployment Agent
226.77.0.0/16 Norton Ghost
229.55.150.208 Norton Ghost
231.0.0.0/8 ?
234.21.81.1 Limewire
234.42.42.0/30 ImageCast
234.42.42.32/31 ImageCast
234.42.42.40/30 ImageCast
234.142.142.42/31 ImageCast
234.142.142.44/30 ImageCast
234.142.142.48/28 ImageCast
234.142.142.64/26 ImageCast
234.142.142.128/29 ImageCast
234.142.142.136/30 ImageCast
234.142.142.140/31 ImageCast
234.142.142.142 ImageCast
239.0.0.0/8 Scoped groups
239.252.0.0/14 Scoped groups
239.234.5.6 ECopy ShareScan
One site gave figures for matches/hits on its filters; it may be
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interesting to gather such statistics at other sites.
Different networks make different use of the scoped address space
under 239.0.0.0/8, which may lead to different organisational filters
in different scenarios. Organisation-local scope IPv4 multicast
addressing is described in [RFC2365].
The SSM range 232.0.0.0/8 should not be carried in MSDP peerings;
this is an example of different policy applied at the site border to
an MSDP peering. Usually the filters are probably the same though.
As a general principle, multicast sourced from private address ranges
[RFC1918] or from 169.254.0.0/16, 192.0.2.0/24 or 127.0.0.0/8 should
be dropped, regardless of the multicast destination.
In certain cases, rate limiting may be desirable, where complete
filtering might not, e.g. in mitigating against SAP [RFC2974] storms,
or against unintended MSDP SA bursts.
Where BSR is deployed, a site should consider dropping BSR packets at
its border, both BSR messages and C-RP messages. Except for
Embedded-RP it probably makes sense to drop PIM register messages at
the site border, unless a site's RP is external.
3.2. Organisational filtering
As described in [RutgersProposal], a site may use multiple
organisational scopes within its site, which may use different blocks
from 239.0.0.0/8, and thus require appropriate filtering at
boundaries, e.g. between metropolitan campuses.
3.3. Subnet filtering
Two respondents are currently filtering uPNP between subnets, and one
is filtering mDNS. One reason for the uPNP filtering was due to
issues with errant Ricoh printers which flood announcements with too-
large TTLs.
Subnet filtering may help protect against other forms of
misconfigured client subnets. One site has networks that consist of
multiple edge routers, where the outside 'LAN side' is strictly meant
to be for local subnets only, and all intranet comms are to go
through the 'WAN interface'. They have had cases where multihomed
client networks were misconfigured, cross-connecting IP subnets with
layer 2 boxes. To prevent multiplication of multicasts, they
configure all edge routers in the intranet to accept multicast
packets from the 'LAN side' only if the source IP of the multicast
packets belongs to the IP subnet of that LAN. So it's a simple
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filter, with no scaling issues.
At least one site is filtering multicast traffic from its wireless
links; this is presumably streamed video or audio content. Multicast
support is required on wireless links for IPv6 operation. At layer
2, multicast IPv6 Router Advertisements may be filtered on ports that
do not have known routers attached.
One site is running per-subnet boundary filters on its wired
multicast-enabled subnets. The list below reflects these. One could
add local scope relative addresses, though in practice the latter
would all fall under 239.255.0.0/16 if it is the smallest scope group
applied to a subnet.
224.0.1.1 NTP
224.0.1.2 SGI-Dogfight
224.0.1.3 Rwhod
224.0.1.8 SUN NIS+
224.0.1.24 microsoft-ds
224.0.1.25 nbc-pro
224.0.1.60 hp-device-disc
224.0.1.76 IAPP
224.0.2.1 rwho
224.0.2.2 SUN RPC
234.21.81.1 Limewire
239.255.0.0/16 subnet scope
The importance of such subnet filtering may depend on TTLs used.
4. Conclusions
This document discusses filters and related mechanisms that should be
applied in multicast deployments, and summarises the reported use of
such multicast filtering practices in the wild. It includes
discussion of various multicast protocols and of reported practices
of applying filters at various scope boundaries. The next iteation
of this draft will include more detailed conclusions.
Further feedback on the text, and the practices reported to date is
welcomed.
5. Security Considerations
There are no extra security consideration for this document.
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6. IANA Considerations
There are no extra IANA consideration for this document.
7. Acknowledgments
The author would like to thank the following people for their
contributions to this text: Scott Bertilson, Alan Buxey, Bruce
Curtis, Andy Gatward, Bob Gerdes, Jeffry J. Handal, Lonnie Leger,
Bert Manfredi, Garry Peirce, William F. Maton Sotomayor, and Stig
Venaas.
8. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC6034] Thaler, D., "Unicast-Prefix-Based IPv4 Multicast
Addresses", RFC 6034, October 2010.
[JANETmulticast]
Price, D., "IPv4 Multicast on JANET", 2006, <http://
www.ja.net/documents/publications/technical-guides/
ipv4-multicast-web.pdf>.
[IANA6multicast]
"IPv6 Multicast Address Space Registry", <http://
www.iana.org/assignments/ipv6-multicast-addresses/
ipv6-multicast-addresses.xml>.
[IANAmulticast]
"IPv4 Multicast Address Space Registry", <http://
www.iana.org/assignments/multicast-addresses/
multicast-addresses.xml>.
[RutgersProposal]
"iDRAFT Proposal for RUNet Administratively Scoped
Multicast", <http://www.td.rutgers.edu/documentation/
Papers/Administratively_Scoped_Multicast_Proposal.pdf>.
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[I2multicast]
"Enabling IP Multicast with Internet2", <http://
noc.net.internet2.edu/i2network/multicast-cookbook.html>.
Authors' Addresses
Tim Chown
University of Southampton
Highfield
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
Leonard Giuliano
Juniper Networks
Email: lenny@juniper.net
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