IP Multicast and Firewalls
RFC 2588

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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

Finlayson                    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