RFC2588 - IP Multicast and Firewalls

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　　Network Working Group R. Finlayson
Request for Comments: 2588 LIVE.COM
Category: Informational May 1999

IP Multicast and Firewalls

Status of this Memo

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

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

1. Abstract

Many organizations use a firewall computer that acts as a security
gateway between the public Internet and their private, internal
'intranet'. In this document, we discuss the issues surrounding the
traversal of IP multicast traffic across a firewall, and describe
possible ways in which a firewall can implement and control this
traversal. We also explain why some firewall mechanisms - such as
SOCKS - that were designed specifically for unicast traffic, are less
appropriate for multicast.

2. Introduction

A firewall is a security gateway that controls access between a
private adminstrative domain (an 'intranet') and the public Internet.
This document discusses how a firewall handles IP multicast [1]
traffic.

We assume that the external side of the firewall (on the Internet)
has access to IP multicast - i.e., is on the public "Multicast
Internet" (aka. "MBone"), or perhaps some other multicast network.

We also assume that the *internal* network (i.e., intranet) supports
IP multicast routing. This is practical, because intranets tend to
be centrally administered. (Also, many corporate intranets already
use multicast internally - for training, meetings, or corporate
announcements.) In contrast, some previously proposed firewall
mechanisms for multicast (e.g., [2]) have worked by sending *unicast*
packets within the intranet. Such mechanisms are usually
inappropriate, because they scale poorly and can cause excessive
network traffic within the intranet. Instead, it is better to rely

upon the existing IP multicast routing/delivery mechanism, rather
than trying to replace it with unicast.

This document addresses scenarios where a multicast session is
carried - via multicast - on both sides of the firewall. For
instance, (i) a particular public MBone session may be relayed onto
the intranet (e.g., for the benefit of employees), or (ii) a special
internal communication (e.g., announcing a new product) may be
relayed onto the public MBone. In contrast, we do not address the
case of a roaming user - outside the firewall - who wishes to access
a private internal multicast session, using a virtual private
network. (Such "road warrior" scenarios are outside the scope of
this document.)

As noted by Freed and Carosso [3], a firewall can act in two
different ways:

1/ As a "protocol end point". In this case, no internal node
(other than the firewall) is directly accessible from the
external Internet, and no external node (other than the
firewall) is directly accessible from within the intranet.
Such firewalls are also known as "application-level gateways".
2/ As a "packet filter". In this case, internal and external
nodes are visible to each other at the IP level, but the
firewall filters out (i.e., blocks passage of) certain packets,
based on their header or contents.

In the remainder of this document, we assume the first type of
firewall, as it is the most restrictive, and generally provides the
most security. For multicast, this means that:

(i) A multicast packet that's sent over the Internet will never
be seen on the intranet (and vice versa), unless such packets
are explicitly relayed by the firewall, and
(ii) The IP source address of a relayed multicast packet will be
that of the firewall, not that of the packet's original
sender. To work correctly, the applications and protocols
being used must take this into account. (Fortunately, most
modern multicast-based protocols - for instance, RTP [4] -
are designed with such relaying in mind.)

3. Why Multicast is Different

When considering the security implications of IP multicast, it is
important to note the fundamental way in which multicast
communication differs from unicast.

Unicast communication consists of a 'conversation' between an
explicit pair of participants. It therefore makes sense for the
security of unicast communication to be based upon these participants
(e.g., by authenticating each participant). Furthermore, 'trust'
within unicast communication can be based upon trust in each
participant, as well as upon trust in the data.

Multicast communication, on the other hand, involves a arbitrary
sized, potentially varying set of participants, whose membership
might never be fully known. (This is a feature, not a bug!) Because
of this, the security of multicast communication is based not upon
its participants, but instead, upon its *data*. In particular,
multicast communication is authenticated by authenticating packet
data - e.g., using digital signatures - and privacy is obtained by
encrypting this data. And 'trust' within multicast communication is
based solely upon trust in the data.

4. Multicast-Related Threats and Countermeasures

The primary threat arising from relaying multicast across a firewall
is therefore "bad data" - in particular:

(i) damaging data flowing from the Internet onto the intranet, or
(ii) sensitive data inadvertently flowing from the intranet onto
the external Internet.

To avert this threat, the intranet's security administrator must
establish, in advance, a security policy that decides:

(i) Which multicast groups (and corresponding UDP ports) contain
data that can safely be relayed from the Internet onto the
intranet. For example, the security administrator might
choose to permit the relaying of an MBone lecture, knowing
that the data consists only of audio/video (& to safe ports).
(ii) Which multicast groups (and corresponding UDP ports) will not
contain sensitive internal information (that should therefore
not be relayed from the intranet onto the Internet). This,
of course, requires placing trust in the applications that
internal users will use to participate in these groups. For
example, if users use an audio/video 'viewer' program to
participate in an MBone session, then this program must be
trusted not to be a "Trojan Horse". (This requirement for
"trusted applications" is by no means specific to multicast,
of course.)

Once such a security policy has been established, it is then the job
of the firewall to implement this policy.

5. What Firewalls Need to Do

In short, a firewall must do three things in order to handle
multicast:

1/ Support the chosen multicast security policy (which establishes
particular multicast groups as being candidates to be relayed),
2/ Determine (dynamically) when each candidate group should be
relayed, and
3/ Relay each candidate group's data across the firewall (and then
re-multicast it at the far end).

These three tasks are described in more detail in the next three
sections.

Note that because a firewall is often a convenient place to
centralize the administration of the intranet, some firewalls might
also perform additional administrative functions - for example,
auditing, accounting, and resource monitoring. These additional
functions, however, are outside the scope of this document, because
they are not specifically *firewall*-related. They are equally
applicable to an administrative domain that is not firewalled.

6. Supporting a Multicast Security Policy

As noted above, a multicast security policy consists of specifying
the set of allowed multicast groups (& corresponding UDP ports) that
are candidates to be relayed across the firewall. There are three
basic ways in which a firewall can support such a policy:

1/ Static configuration. The firewall could be configured, in
advance, with the set of candidate groups/ports - for example,
in a configuration file.
2/ Explicit dynamic configuration. The set of candidate
groups/ports could be set (and updated) dynamically, based upon
an explicit request from one or more trusted clients
(presumably internal). For example, the firewall could contain
a 'remote control' mechanism that allows these trusted clients
- upon authentication - to update the set of candidate
groups/ports.
3/ Implicit dynamic configuration. The set of candidate
groups/ports could be determined implicitly, based upon the
contents of some pre-authorized multicast group/port, such as a
"session directory". Suppose, for example, that the security
policy decides that the default MBone SAP/SDP session directory
[5] may be relayed, as well as any sessions that are announced
in this directory. A 'watcher' process, associated with the
firewall, would watch this directory, and use its contents to

dynamically update the set of candidates.

Notes:

(i) Certain ranges of multicast addresses are defined to be
"administratively scoped" [6]. Even though the firewall
does not act as a true multicast router, the multicast
security policy should set up and respect administrative
scope boundaries.
(ii) As noted in [2], certain privileged UDP ports may be
considered dangerous, even with multicast. The multicast
security policy should check that such ports do not become
candidates for relaying.
(iii) Even if sessions announced in a session directory are
considered automatic candidates for relaying (i.e., case 3/
above), the firewall's 'watcher' process should still
perform some checks on incoming announcements. In
particular, it should ensure that each session's 'group'
address really is a multicast address, and (as noted above)
it should also check that the port number is within a safe
range. Depending on the security policy, it may also wish
to prevent any *locally* created session announcements from
becoming candidates (or being relayed).

7. Determining When to Relay Candidate Groups

If a multicast group becomes a candidate to be relayed across the
firewall, the actual relaying should *not* be done continually, but
instead should be done only when there is actual interest in having
this group relayed. The reason for this is two-fold. First,
relaying a multicast group requires that one or both sides of the
firewall join the group; this establishes multicast routing state
within the network. This is inefficient if there is no current
interest in having the group relayed (especially for
Internet->intranet relaying). Second, the act of relaying an
unwanted multicast group consumes unnecessary resources in the
firewall itself.

The best way for the firewall to determine when a candidate group
should be relayed is for it to use actual multicast routing
information, thereby acting much as if it were a real 'inter-domain'
multicast router. If the intranet consists of a single subnet only,
then the firewall could listen to IGMP requests to learn when a
candidate group has been joined by a node on this subnet. If,
however, the intranet consists of more than one subnet, then the
firewall can learn about candidate group memberships by listening to
"Domain Wide Multicast Group Membership Reports" [7]. Unfortunately,
this mechanism has only recently been defined, and is not yet used by

most routers.

Another, albeit less desirable, way for the firewall to learn when
candidate multicast groups have been joined is for the firewall to
periodically 'probe' each of these groups. Such a probe can be
performed by sending an ICMP ECHO request packet to the group, and
listening for a response (with some timeout interval). This probing
scheme is practical provided that the set of candidate groups is
reasonably small, but it should be used only on the intranet, not on
the external Internet. One significant drawback of this approach is
that some operating systems - most notably Windows 95 - do not
respond to multicast ICMP ECHOs. However, this approach has been
shown to work on a large, all-Unix network.

Another possibility - less desirable still - is for each node to
explicitly notify the firewall whenever it joins, or leaves, a
multicast group. This requires changes to the node's operating
system or libraries, or cooperation from the application. Therefore
this technique, like the previous one, is applicable only within the
intranet, not the external Internet. Note that if multicast
applications are always launched from a special "session directory"
or "channel guide" application, then this application may be the only
one that need be aware of having to contact the firewall.

What makes the latter two approaches ("probing" and "explicit
notification") undesirable is that they duplicate some of the
existing functionality of multicast routing, and in a way that scales
poorly for large networks. Therefore, if possible, firewalls should
attempt to make use of existing multicast routing information: either
IGMP (for a single-subnet intranet), or "Domain Wide Multicast Group
Membership Reports".

In some circumstances, however, the client cannot avoid contacting
the firewall prior to joining a multicast session. In this case, it
may make sense for this contact to also act as a 'notification'
operation. Consider, for example, an RTSP [8] proxy associated with
the firewall. When the proxy receives a request - from an internal
user - to open a remote RTSP session, the proxy might examine the
response from the remote site, to check whether a multicast session
is being launched, and if so, check whether the multicast group(s)
are candidates to be relayed.

8. Relaying Candidate Groups

The actual mechanism that's used to relay multicast packets will
depend upon the nature of the firewall. One common firewall
configuration is to use two nodes: one part of the intranet; the
other part of the external Internet. In this case, multicast packets
would be relayed between these two nodes (and then re-multicast on
the other side) using a tunneling protocol.

A tunneling protocol for multicast should *not* run on top of TCP,
because the reliability and ordering guarantees that TCP provides are
unnecessary for multicast communication (where any reliability is
provided at a higher level), yet would add latency. Instead, a UDP-
based tunneling protocol is a better fit for relaying multicast
packets. (If congestion avoidance is a concern, then the tunnel
traffic could be rate-limited, perhaps on a per-group basis.)

One possible tunneling protocol is the "UDP Multicast Tunneling
Protocol" (UMTP) [9]. Although this protocol was originally designed
as a mechanism for connecting individual client machines to the
MBone, it is also a natural fit for for use across firewalls. UMTP
uses only a single UDP port, in each direction, for its tunneleling,
so an existing firewall can easily be configured to support multicast
relaying, by adding a UMTP implementation at each end, and enabling
the UDP port for tunneling.

Notes:

(i) When multicast packets are relayed from the intranet onto the
external Internet, they should be given the same TTL that
they had when they arrived on the firewall's internal
interface (except decremented by 1). Therefore, the internal
end of the multicast relay mechanism needs to be able to read
the TTL of incoming packets. (This may require special
privileges.) In contrast, the TTL of packets being relayed
in the other direction - from the external Internet onto the
intranet - is usually less important; some default value
(sufficient to reach the whole intranet) will usually
suffice. Thus, the Internet end of the multicast relay
mechanism - which might be less trusted than the intranet end
- need not run with special privileges.
(ii) One end of the multicast tunnel - usually the intranet end -
will typically act as the controller (i.e., "master") of the
tunnel, with the other end - usually the Internet end -
acting as a "slave". For security, the "master" end of the
tunnel should be configured not to accept any commands from
the "slave" (which will often be less trusted).

9. Networks With More Than One Firewall

So far we have assumed that there is only one firewall between the
intranet and the external Internet. If, however, the intranet has
more than one firewall, then it's important that no single multicast
group be relayed by more than one firewall. Otherwise (because
firewalls are assumed to be application-level gateways - not proper
multicast routers), packets sent to any such group would become
replicated on the other side of the firewalls. The set of candidate
groups must therefore be partitioned among the firewalls (so that
exactly one firewall has responsibility for relaying each candidate
group). Clearly, this will require coordination between the
administrators of the respective firewalls.

As a general rule, candidate groups should be assigned - if possible
- to the firewall that is topologically closest to most of the group
members (on both the intranet and the external Internet). For
example, if a company's intranet spans the Atlantic, with firewalls
in New York and London, then groups with mostly North American
members should be assigned to the New York firewall, and groups with
mostly European members should be assigned to the London firewall.
(Unfortunately, even if a group has many internal and external
members on both sides of the Atlantic, only one firewall will be
allowed to relay it. Some inefficiencies in the data delivery tree
are unavoidable in this case.)

10. Why SOCKS is Less Appropriate for Multicast

SOCKS [10] is a mechanism for transparently performing unicast
communication across a firewall. A special client library -
simulating the regular 'sockets' library - sits between applications
and the transport level. A conversation between a pair of nodes is
implemented (transparently) as a pair of conversations: one between
the first node and a firewall; the other between the firewall and the
second node.

In contrast, because multicast communication does not involve a
conversation between a pair of nodes, the SOCKS model is less
appropriate. Although multicast communication across a firewall is
implemented as two separate multicast communications (one inside the
firewall; the other outside), the *same* multicast address(es) and
port(s) are used on both sides of the firewall. Thus, multicast
applications running inside the firewall see the same environment as
those running outside, so there is no need for them to use a special
library.

Nonetheless, there has been a proposal [11] to extend SOCKS V5 to
support multicast. This proposal includes two possible modes of
communication:

(i) "MU-mode", uses only *unicast* communication within the
intranet (between the firewall and each internal group
member), and
(ii) "MM-mode", which uses unicast for client-to-firewall relay
control, but uses *multicast* for other communication within
the intranet.

As noted in section 2 above, "MU-mode" would be a poor choice
(unless, for some reason, the intranet does not support multicast
routing at all). If multicast routing is available, there should
rarely be a compelling reason to replace multicast with 'multiple-
unicast'. Not only does this scale badly, but it also requires
(otherwise unnecessary) changes to each application node, because the
multicast service model is different from that of unicast.

On the other hand, "MM-mode" (or some variant thereof) *might* be
useful in environments where a firewall can learn about group
membership only via "explicit notification". In this case each node
might use SOCKS to notify the firewall whenever it joins and leaves a
group. However, as we explained above, this should only be
considered as a last resort - a far better solution is to leverage
off the existing multicast routing mechanism.

It has been suggested [11] that a benefit of using multicast SOCKS
(or an "explicit notification" scheme in general) is that it allows
the firewall to authenticate a client's multicast "join" and "leave"
operations. This, however, does not provide any security, because it
does not prevent other clients within the intranet from joining the
multicast session (and receiving packets), nor from sending packets
to the multicast session. As we noted in section 3 above,
authentication and privacy in multicast sessions is usually obtained
by signing and encrypting the multicast data, not by attempting to
impose low-level restrictions on group membership. We note also that
even if group membership inside the intranet could be restricted, it
would not be possible, in general, to impose any such membership
restrictions on the external Internet.

11. Security Considerations

Once a security policy has been established, the techniques described
in this document can be used to implement this policy. No security
mechanism, however, can overcome a badly designed security policy.
Specifically, network administrators must be confident that the
multicast groups/ports that they designate as being 'safe' really are

free from harmful data. In particular, administrators must be
familiar with the applications that will receive and process
multicast data, and (as with unicast applications) be confident that
they cannot cause harm (e.g., by executing unsafe code received over
the network).

Because it is possible for an adversary to initiate a "denial of
service" attack by flooding an otherwise-legitimate multicast group
with garbage, administrators may also wish to guard against this by
placing bandwidth limits on cross-firewall relaying.

12. Summary

Bringing IP multicast across a firewall requires that the intranet
first establish a multicast security policy that defines which
multicast groups (& corresponding UDP ports) are candidates to be
relayed across the firewall. The firewall implements this policy by
dynamically determining when each candidate group/port needs to be
relayed, and then by doing the actual relaying. This document has
outlined how a firewall can perform these tasks.

13. References

[1] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.

[2] Djahandari, K., Sterne, D. F., "An MBone Proxy for an Application
Gateway Firewall" IEEE Symposium on Security and Privacy, 1997.

[3] Freed, N. and K. Carosso, "An Internet Firewall Transparency
Requirement", Work in Progress.

[4] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC1889,
January 1996.

[5] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
RFC2327, April 1998.

[6] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365 July 1998.

[7] Fenner, B., "Domain Wide Multicast Group Membership Reports",
Work in Progress.

[8] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC2326, April 1998.

[9] Finlayson, R., "The UDP Multicast Tunneling Protocol", Work in
Progress.

[10] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L.
Joned, SOCKS Protocol Version 5", RFC1928, April 1996.

[11] Chouinard, D., "SOCKS V5 UDP and Multicast Extensions", Work in
Progress.

14. Author's Address

Ross Finlayson,
Live Networks, Inc. (LIVE.COM)

EMail: finlayson@live.com
WWW: http://www.live.com/

15. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

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Acknowledgement

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