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Network Working Group                                      R. FinlaysonRequest for Comments: 2588                                     LIVE.COMCategory: Informational                                        May 1999IP Multicast and FirewallsStatus 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 relyFinlayson                    Informational                      [Page 1]

RFC 2588               IP Multicast and Firewalls               May 1999   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.Finlayson                    Informational                      [Page 2]

RFC 2588               IP Multicast and Firewalls               May 1999   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.Finlayson                    Informational                      [Page 3]

RFC 2588               IP Multicast and Firewalls               May 19995. 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 toFinlayson                    Informational                      [Page 4]

RFC 2588               IP Multicast and Firewalls               May 1999         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 byFinlayson                    Informational                      [Page 5]

RFC 2588               IP Multicast and Firewalls               May 1999   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.Finlayson                    Informational                      [Page 6]

RFC 2588               IP Multicast and Firewalls               May 19998. 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).Finlayson                    Informational                      [Page 7]

RFC 2588               IP Multicast and Firewalls               May 19999. 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.Finlayson                    Informational                      [Page 8]

RFC 2588               IP Multicast and Firewalls               May 1999   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 insection 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 insection 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 areFinlayson                    Informational                      [Page 9]

RFC 2588               IP Multicast and Firewalls               May 1999   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,RFC1112, 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",RFC 1889,       January 1996.   [5] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",RFC 2327, April 1998.   [6] Meyer, D., "Administratively Scoped IP Multicast",BCP 23,RFC2365 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)",RFC 2326, April 1998.Finlayson                    Informational                     [Page 10]

RFC 2588               IP Multicast and Firewalls               May 1999   [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",RFC 1928, 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/Finlayson                    Informational                     [Page 11]

RFC 2588               IP Multicast and Firewalls               May 199915.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Finlayson                    Informational                     [Page 12]