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INFORMATIONAL
Errata Exist
Network Working Group                                          E. Davies
Request for Comments: 4890                                    Consultant
Category: Informational                                       J. Mohacsi
                                                          NIIF/HUNGARNET
                                                                May 2007


       Recommendations for Filtering ICMPv6 Messages in 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 IETF Trust (2007).

Abstract

   In networks supporting IPv6, the Internet Control Message Protocol
   version 6 (ICMPv6) plays a fundamental role with a large number of
   functions, and a correspondingly large number of message types and
   options.  ICMPv6 is essential to the functioning of IPv6, but there
   are a number of security risks associated with uncontrolled
   forwarding of ICMPv6 messages.  Filtering strategies designed for the
   corresponding protocol, ICMP, in IPv4 networks are not directly
   applicable, because these strategies are intended to accommodate a
   useful auxiliary protocol that may not be required for correct
   functioning.

   This document provides some recommendations for ICMPv6 firewall
   filter configuration that will allow propagation of ICMPv6 messages
   that are needed to maintain the functioning of the network but drop
   messages that are potential security risks.















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Classifying ICMPv6 Messages  . . . . . . . . . . . . . . . . .  6
     2.1.  Error and Informational ICMPv6 Messages  . . . . . . . . .  6
     2.2.  Addressing of ICMPv6 . . . . . . . . . . . . . . . . . . .  6
     2.3.  Network Topology and Address Scopes  . . . . . . . . . . .  7
     2.4.  Role in Establishing and Maintaining Communication . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     3.1.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . .  9
     3.2.  Probing . . . . . . . . . . . . . . . . . . . . . . . . . . 9
     3.3.  Redirection Attacks . . . . . . . . . . . . . . . . . . . . 9
     3.4.  Renumbering Attacks  . . . . . . . . . . . . . . . . . . . 10
     3.5.  Problems Resulting from ICMPv6 Transparency  . . . . . . . 10
   4.  Filtering Recommendations  . . . . . . . . . . . . . . . . . . 10
     4.1.  Common Considerations  . . . . . . . . . . . . . . . . . . 11
     4.2.  Interaction of Link-Local Messages with
           Firewall/Routers and Firewall/Bridges  . . . . . . . . . . 12
     4.3.  Recommendations for ICMPv6 Transit Traffic . . . . . . . . 13
       4.3.1.  Traffic That Must Not Be Dropped . . . . . . . . . . . 14
       4.3.2.  Traffic That Normally Should Not Be Dropped  . . . . . 14
       4.3.3.  Traffic That Will Be Dropped Anyway -- No Special
               Attention Needed . . . . . . . . . . . . . . . . . . . 15
       4.3.4.  Traffic for Which a Policy Should Be Defined . . . . . 16
       4.3.5.  Traffic That Should Be Dropped Unless a Good Case
               Can Be Made  . . . . . . . . . . . . . . . . . . . . . 17
     4.4.  Recommendations for ICMPv6 Local Configuration Traffic . . 18
       4.4.1.  Traffic That Must Not Be Dropped . . . . . . . . . . . 18
       4.4.2.  Traffic That Normally Should Not Be Dropped  . . . . . 19
       4.4.3.  Traffic That Will Be Dropped Anyway -- No Special
               Attention Needed . . . . . . . . . . . . . . . . . . . 19
       4.4.4.  Traffic for Which a Policy Should Be Defined . . . . . 20
       4.4.5.  Traffic That Should Be Dropped Unless a Good Case
               Can Be Made  . . . . . . . . . . . . . . . . . . . . . 21
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Notes on Individual ICMPv6 Messages . . . . . . . . . 24
     A.1.  Destination Unreachable Error Message  . . . . . . . . . . 24
     A.2.  Packet Too Big Error Message . . . . . . . . . . . . . . . 24
     A.3.  Time Exceeded Error Message  . . . . . . . . . . . . . . . 25
     A.4.  Parameter Problem Error Message  . . . . . . . . . . . . . 25
     A.5.  ICMPv6 Echo Request and Echo Response  . . . . . . . . . . 26
     A.6.  Neighbor Solicitation and Neighbor Advertisement
           Messages . . . . . . . . . . . . . . . . . . . . . . . . . 26
     A.7.  Router Solicitation and Router Advertisement Messages  . . 27
     A.8.  Redirect Messages  . . . . . . . . . . . . . . . . . . . . 27



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     A.9.  SEND Certificate Path Messages . . . . . . . . . . . . . . 27
     A.10. Multicast Listener Discovery Messages  . . . . . . . . . . 27
     A.11. Multicast Router Discovery Messages  . . . . . . . . . . . 28
     A.12. Router Renumbering Messages  . . . . . . . . . . . . . . . 28
     A.13. Node Information Query and Reply . . . . . . . . . . . . . 28
     A.14. Mobile IPv6 Messages . . . . . . . . . . . . . . . . . . . 28
     A.15. Unused and Experimental Messages . . . . . . . . . . . . . 29
   Appendix B.  Example Script to Configure ICMPv6 Firewall Rules . . 30

1.  Introduction

   When a network supports IPv6 [RFC2460], the Internet Control Message
   Protocol version 6 (ICMPv6) [RFC4443] plays a fundamental role
   including being an essential component in establishing and
   maintaining communications both at the interface level and for
   sessions to remote nodes.  This means that overly aggressive
   filtering of ICMPv6 by firewalls may have a detrimental effect on the
   establishment and maintenance of IPv6 communications.  On the other
   hand, allowing indiscriminate passage of all ICMPv6 messages can be a
   major security risk.  This document recommends a set of rules that
   seek to balance effective IPv6 communication against the needs of
   site security.

   In a few cases, the appropriate rules will depend on whether the
   firewall is protecting

   o  an individual host,

   o  an end site where all ICMPv6 messages originate or terminate
      within the site, or

   o  a transit site such as an Internet Service Provider's site where
      some ICMPv6 messages will be passing through.

   The document suggests alternative rules appropriate to each situation
   where it is relevant.  It also notes some situations where
   alternative rules could be adopted according to the nature of the
   work being carried out on the site and consequent security policies.
   In general, Internet Service Providers should not filter ICMPv6
   messages transiting their sites so that all the necessary
   communication elements are available to their customers to decide and
   filter according to their policy.

   Readers familiar with ICMPv6 can skip to the recommended filtering
   rules in Section 4 and an example configuration script for Linux
   Netfilter in Appendix B.





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   ICMPv6 has a large number of functions defined in a number of sub-
   protocols, and there are a correspondingly large number of messages
   and options within these messages.  The functions currently defined
   fall into a number of categories:

   Returning Error Messages

         *  Returning error messages to the source if a packet could not
            be delivered.  Four different error messages, each with a
            number of sub-types, are specified in [RFC4443].

   Connection Checking

         *  Simple monitoring of connectivity through echo requests and
            responses used by the ping and traceroute utilities.  The
            Echo Request and Echo Response messages are specified in
            [RFC4443].

   Discovery Functions

         *  Finding neighbors (both routers and hosts) connected to the
            same link and determining their IP and link layer addresses.
            These messages are also used to check the uniqueness of any
            addresses that an interface proposes to use (Duplicate
            Address Detection - DAD).  Four messages -- Neighbor
            Solicitation (NS), Neighbor Advertisement (NA), Router
            Solicitation (RS) and Router Advertisement (RA) -- are
            specified in [RFC2461].

         *  Ensuring that neighbors remain reachable using the same IP
            and link layer addresses after initial discovery (Neighbor
            Unreachability Discovery - NUD) and notifying neighbors of
            changes to link layer addresses.  Uses NS and NA [RFC2461].

         *  Finding routers and determining how to obtain IP addresses
            to join the subnets supported by the routers.  Uses RS and
            RA [RFC2461].

         *  If stateless autoconfiguration of hosts is enabled,
            communicating prefixes and other configuration information
            (including the link Maximum Transmission Unit (MTU) and
            suggested hop limit default) from routers to hosts.  Uses RS
            and RA [RFC2462].

         *  When using SEcure Neighbor Discovery (SEND) to authenticate
            a router attached to a link, the Certificate Path
            Solicitation and Advertisement messages specified in
            [RFC3971] are used by hosts to retrieve the certificates



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            documenting the trust chain between a trust anchor and the
            router from the router.

         *  Determining the MTU along a path.  The Packet Too Big error
            message is essential to this function [RFC1981].

         *  Providing a means to discover the IPv6 addresses associated
            with the link layer address of an interface (the inverse of
            Neighbor Discovery, where the link layer address is

            discovered given an IPv6 address).  Two messages, Inverse
            Neighbor Discovery Solicitation and Advertisement messages,
            are specified in [RFC3122].

         *  Communicating which multicast groups have listeners on a
            link to the multicast capable routers connected to the link.
            Uses messages Multicast Listener Query, Multicast Listener
            Report (two versions), and Multicast Listener Done (protocol
            version 1 only) as specified in Multicast Listener Discovery
            MLDv1 [RFC2710] and MLDv2 [RFC3810].

         *  Discovering multicast routers attached to the local link.
            Uses messages Multicast Router Advertisement, Multicast
            Router Solicitation, and Multicast Router Termination as
            specified in Multicast Router Discovery [RFC4286].

   Reconfiguration Functions

         *  Redirecting packets to a more appropriate router on the
            local link for the destination address or pointing out that
            a destination is actually on the local link even if it is
            not obvious from the IP address (where a link supports
            multiple subnets).  The Redirect message is specified in
            [RFC2461].

         *  Supporting renumbering of networks by allowing the prefixes
            advertised by routers to be altered.  Uses NS, NA, RS and RA
            together with the Router Renumbering message specified in
            [RFC2894].

   Mobile IPv6 Support

         *  Providing support for some aspects of Mobile IPv6 especially
            dealing with the IPv6 Mobile Home Agent functionality
            provided in routers and needed to support a Mobile node
            homed on the link.  The Home Agent Address Discovery Request
            and Reply and the Mobile Prefix Solicitation and
            Advertisement messages are specified in [RFC3775].



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   Experimental Extensions

         *  An experimental extension to ICMPv6 specifies the ICMP Node
            Information Query and Response messages that can be used to
            retrieve some basic information about nodes [RFC4620].

         *  The SEAmless IP MOBility (SEAMOBY) working group specified a
            pair of experimental protocols that use an ICMPv6 message
            specified in [RFC4065] to help in locating an access router
            and moving the attachment point of a mobile node from one
            access router to another.

   Many of these messages should only be used in a link-local context
   rather than end-to-end, and filters need to be concerned with the
   type of addresses in ICMPv6 packets as well as the specific source
   and destination addresses.

   Compared with the corresponding IPv4 protocol, ICMP, ICMPv6 cannot be
   treated as an auxiliary function with packets that can be dropped in
   most cases without damaging the functionality of the network.  This
   means that firewall filters for ICMPv6 have to be more carefully
   configured than was the case for ICMP, where typically a small set of
   blanket rules could be applied.

2.  Classifying ICMPv6 Messages

2.1.  Error and Informational ICMPv6 Messages

   ICMPv6 messages contain an eight-bit Type field interpreted as an
   integer between 0 and 255.  Messages with Type values less than or
   equal to 127 are Error messages.  The remainder are Informational
   messages.  In general terms, Error messages with well-known
   (standardized) Type values would normally be expected to be allowed
   to be sent to or pass through firewalls, and may be essential to the
   establishment and maintenance of communications (see Section 2.4)
   whereas Informational messages will generally be the subject of
   policy rules, and those passing through end site firewalls can, in
   many but by no means all cases, be dropped without damaging IPv6
   communications.

2.2.  Addressing of ICMPv6

   ICMPv6 messages are sent using various kinds of source and
   destination address types and scopes.  The source address is usually
   a unicast address, but during address autoconfiguration message
   exchanges, the unspecified address (::) is also used as a source
   address [RFC2462].




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   Multicast Listener Discovery (MLD) Report and Done messages are sent
   with a link-local address as the IPv6 source address, if a valid
   address is available on the interface.  If a valid link-local address
   is not available (e.g., one has not been configured), the message is
   sent with the unspecified address (::) as the IPv6 source address.
   Subsequently, the node will generate new MLD Report messages with
   proper link-local source address once it has been configured
   [RFC3590].

   The destination address can be either a well-known multicast address,
   a generated multicast address such as the solicited-node multicast
   address, an anycast address, or a unicast address.  While many ICMPv6
   messages use multicast addresses most of the time, some also use
   unicast addresses.  For instance, the Router Advertisement messages
   are sent to the all-nodes multicast address when unsolicited, but can
   also be sent to a unicast address in response to a specific Router
   Solicitation, although this is rarely seen in current
   implementations.

2.3.  Network Topology and Address Scopes

   ICMPv6 messages can be classified according to whether they are meant
   for end-to-end communications or local communications within a link.
   There are also messages that we can classify as 'any-to-end', which
   can be sent from any point within a path back to the source;
   typically, these are used to announce an error in processing the
   original packet.  For instance, the address resolution messages are
   solely for local communications [RFC2461], whereas the Destination
   Unreachable messages are any-to-end in nature.  Generally, end-to-end
   and any-to-end messages might be expected to pass through firewalls
   depending on policies but local communications must not.

   Local communications will use link-local addresses in many cases but
   may also use global unicast addresses when configuring global
   addresses, for example.  Also, some ICMPv6 messages used in local
   communications may contravene the usual rules requiring compatible
   scopes for source and destination addresses.

2.4.  Role in Establishing and Maintaining Communication

   Many ICMPv6 messages have a role in establishing or maintaining
   communications to and from the firewall and such messages have to be
   accepted by firewalls for local delivery.  Generally, a firewall will
   also be acting as a router so that all the messages that might be
   used in configuring a router interface need to be accepted and
   generated.  These messages should not transit through a firewall that
   is also acting as a router as they are normally intended for use
   within a link.



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   On the other hand, most ICMPv6 error messages traveling end-to-end or
   any-to-end are essential to the establishment and maintenance of
   communications.  These messages must be passed through firewalls and
   might also be sent to and from firewalls to assist with establishment
   and maintenance of communications.  For example, the Packet Too Big
   error message is needed to determine the MTU along a path both when a
   communication session is established initially and later if the path
   is rerouted during the session.

   The remaining ICMPv6 messages that are not associated with
   communication establishment or maintenance will normally be
   legitimately attempting to pass through a firewall from inside to out
   or vice versa, but in most cases decisions as to whether or not to
   allow them to pass can be made on the basis of local policy without
   interfering with IPv6 communications.

   The filtering rules for the various message roles will generally be
   different.

3.  Security Considerations

   This memo recommends filtering configurations for firewalls designed
   to minimize the security vulnerabilities that can arise in using the
   many different sub-protocols of ICMPv6 in support of IPv6
   communication.

   A major concern is that it is generally not possible to use IPsec or
   other means to authenticate the sender and validate the contents of
   many ICMPv6 messages.  To a large extent, this is because a site can
   legitimately expect to receive certain error and other messages from
   almost any location in the wider Internet, and these messages may
   occur as a result of the first message sent to a destination.
   Establishing security associations with all possible sources of
   ICMPv6 messages is therefore impossible.

   The inability to establish security associations to protect some
   messages that are needed to establish and maintain communications
   means that alternative means have to be used to reduce the
   vulnerability of sites to ICMPv6-based attacks.  The most common way
   of doing this is to establish strict filtering policies in site
   firewalls to limit the unauthenticated ICMPv6 messages that can pass
   between the site and the wider Internet.  This makes control of
   ICMPv6 filtering a delicate balance between protecting the site by
   dropping some of the ICMPv6 traffic passing through the firewall and
   allowing enough of the traffic through to make sure that efficient
   communication can be established.





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   SEND [RFC3971] has been specified as a means to improve the security
   of local ICMPv6 communications.  SEND sidesteps security association
   bootstrapping problems that would result if IPsec was used.  SEND
   affects only link-local messages and does not limit the filtering
   that firewalls can apply, and its role in security is therefore not
   discussed further in this document.

   Firewalls will normally be used to monitor ICMPv6 to control the
   following security concerns:

3.1.  Denial-of-Service Attacks

   ICMPv6 can be used to cause a denial of service (DoS) in a number of
   ways, including simply sending excessive numbers of ICMPv6 packets to
   destinations in the site and sending error messages that disrupt
   established communications by causing sessions to be dropped.  Also,
   if spurious communication establishment or maintenance messages can
   be infiltrated onto a link, it might be possible to invalidate
   legitimate addresses or disable interfaces.

3.2.  Probing

   A major security consideration is preventing attackers from probing
   the site to determine the topology and identify hosts that might be
   vulnerable to attack.  Carefully crafted but, often, malformed
   messages can be used to provoke ICMPv6 responses from hosts thereby
   informing attackers of potential targets for future attacks.
   However, the very large address space of IPv6 makes probing a less
   effective weapon as compared with IPv4 provided that addresses are
   not allocated in an easily guessable fashion.  This subject is
   explored in more depth in [SCAN-IMP].

3.3.  Redirection Attacks

   A redirection attack could be used by a malicious sender to perform
   man-in-the-middle attacks or divert packets either to a malicious
   monitor or to cause DoS by blackholing the packets.  These attacks
   would normally have to be carried out locally on a link using the
   Redirect message.  Administrators need to decide if the improvement
   in efficiency from using Redirect messages is worth the risk of
   malicious use.  Factors to consider include the physical security of
   the link and the complexity of addressing on the link.  For example,
   on an open wireless link, redirection would be a serious hazard due
   to the lack of physical security.  On the other hand, with a wired
   link in a secure building with complex addressing and redundant
   routers, the efficiency gains might well outweigh the small risk of a
   rogue node being connected.




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3.4.  Renumbering Attacks

   Spurious Renumbering messages can lead to the disruption of a site.
   Although Renumbering messages are required to be authenticated with
   IPsec, so that it is difficult to carry out such attacks in practice,
   they should not be allowed through a site boundary firewall.  On the
   other hand, a site may employ multiple "layers" of firewalls.  In
   this case, Renumbering messages might be expected to be allowed to
   transit interior firewalls but not pass across the outer boundary.

3.5.  Problems Resulting from ICMPv6 Transparency

   Because some ICMPv6 error packets need to be passed through a
   firewall in both directions, malicious users can potentially use
   these messages to communicate between inside and outside, bypassing
   administrative inspection.  For example, it might be possible to
   carry out a covert conversation through the payload of ICMPv6 error
   messages or tunnel inappropriate encapsulated IP packets in ICMPv6
   error messages.  This problem can be alleviated by filtering ICMPv6
   errors using a deep packet inspection mechanism to ensure that the
   packet carried as a payload is associated with legitimate traffic to
   or from the protected network.

4.  Filtering Recommendations

   When designing firewall filtering rules for ICMPv6, the rules can be
   divided into two classes:

   o  Rules for ICMPv6 traffic transiting the firewall, with some minor
      variations for

      *  firewalls protecting end sites or individual hosts, and

      *  firewalls protecting transit sites

   o  Rules for ICMPv6 directed to interfaces on the firewall

   Firewalls integrated with an individual host ("end host firewalls")
   can be treated as end site firewalls, but the special considerations
   discussed in Section 4.2 may be relevant because the firewall is not
   a router.










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   This section suggests some common considerations that should be borne
   in mind when designing filtering rules and then categorizes the rules
   for each class.  The categories are:

   o  Messages that must not be dropped: usually because establishment
      or maintenance of communications will be prevented or severely
      impacted.

   o  Messages that should not be dropped: administrators need to have a
      very good reason for dropping this category.

   o  Messages that may be dropped in firewall/routers, but these
      messages may already be targeted to drop for other reasons (e.g.,
      because they are using link-local addresses) or because the
      protocol specification would cause the messages to be rejected if
      they had passed through a router.  Special considerations apply to
      transit traffic if the firewall is not a router as discussed in
      Section 4.2.

   o  Messages that administrators may or may not want to drop depending
      on local policy.

   o  Messages that administrators should consider dropping (e.g., ICMP
      node information name lookup queries).

   More detailed analysis of each of the message types can be found in
   Appendix A.

4.1.  Common Considerations

   Depending on the classification of the message to be filtered (see
   Section 2), ICMPv6 messages should be filtered based on the ICMPv6
   type of the message and the type (unicast, multicast, etc.) and scope
   (link-local, global unicast, etc.) of source and destination
   addresses.  In some cases, it may be desirable to filter on the Code
   field of ICMPv6 error messages.

   Messages that can be authenticated on delivery, probably because they
   contain an IPsec AH header or ESP header with authentication, may be
   subject to less strict policies than messages that cannot be
   authenticated.  In the remainder of this section, we are generally
   considering what should be configured for unauthenticated messages.
   In many cases, it is not realistic to expect more than a tiny
   fraction of the messages to be authenticated.

   Where messages are not essential to the establishment or maintenance
   of communications, local policy can be used to determine whether a
   message should be allowed or dropped.



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   Depending on the capabilities of the firewall being configured, it
   may be possible for the firewall to maintain state about packets that
   may result in error messages being returned or about ICMPv6 packets
   (e.g., Echo Requests) that are expected to receive a specific
   response.  This state may allow the firewall to perform more precise
   checks based on this state, and to apply limits on the number of
   ICMPv6 packets accepted incoming or outgoing as a result of a packet
   traveling in the opposite direction.  The capabilities of firewalls
   to perform such stateful packet inspection vary from model to model,
   and it is not assumed that firewalls are uniformly capable in this
   respect.

   Firewalls that are able to perform deep packet inspection may be able
   to check the header fields in the start of the errored packet that is
   carried by ICMPv6 error messages.  If the embedded packet has a
   source address that does not match the destination of the error
   message, the packet can be dropped.  This provides a partial defense
   against some possible attacks on TCP that use spoofed ICMPv6 error
   messages, but the checks can also be carried out at the destination.
   For further information on these attacks see [ICMP-ATTACKS].

   In general, the scopes of source and destination addresses of ICMPv6
   messages should be matched, and packets with mismatched addresses
   should be dropped if they attempt to transit a router.  However, some
   of the address configuration messages carried locally on a link may
   legitimately have mismatched addresses.  Node implementations must
   accept these messages delivered locally on a link, and administrators
   should be aware that they can exist.

   ICMPv6 messages transiting firewalls inbound to a site may be treated
   differently depending on whether they are addressed to a node on the
   site or to some other node.  For end sites, packets addressed to
   nodes not on the site should be dropped, but would generally be
   forwarded by firewalls on transit sites.

4.2.  Interaction of Link-Local Messages with Firewall/Routers and
      Firewall/Bridges

   Firewalls can be implemented both as IP routers (firewall/routers)
   and as link layer bridges (e.g., Ethernet bridges) that are
   transparent to the IP layer although they will actually be inspecting
   the IP packets as they pass through (firewall/bridges).

   Many of the messages used for establishment and maintenance of
   communications on the local link will be sent with link-local
   addresses for at least one of their source and destination.  Routers
   conforming to the IPv6 standards will not forward these packets;
   there is no need to configure additional rules to prevent these



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   packets traversing a firewall/router, although administrators may
   wish to configure rules that would drop these packets for insurance
   and as a means of monitoring for attacks.  Also, the specifications
   of ICMPv6 messages intended for use only on the local link specify
   various measures that would allow receivers to detect if the message
   had passed through a router, including:

   o  Requiring that the hop limit in the IPv6 header is set to 255 on
      transmission.  Receivers verify that the hop limit is still 255,
      to ensure that the packet has not passed through a router.

   o  Checking that the source address is a link-local unicast address.

   Accordingly, it is not essential to configure firewall/router rules
   to drop out-of-specification packets of these types.  If they have
   non-link-local source and destination addresses, allowing them to
   traverse the firewall/router, they would be rejected because of the
   checks performed at the destination.  Again, firewall administrators
   may still wish to configure rules to log or drop such out-of-
   specification packets.

   For firewall/bridges, slightly different considerations apply.  The
   physical links on either side of the firewall/bridge are treated as a
   single logical link for the purposes of IP.  Hence, the link local
   messages used for discovery functions on the link must be allowed to
   transit the transparent bridge.  Administrators should ensures that
   routers and hosts attached to the link containing the firewall/bridge
   are built to the correct specifications so that out-of-specification
   packets are actually dropped as described in the earlier part of this
   section.

   An end host firewall can generally be thought of as a special case of
   a firewall/bridge, but the only link-local messages that need to be
   allowed through are those directed to the host's interface.

4.3.  Recommendations for ICMPv6 Transit Traffic

   This section recommends rules that should be applied to ICMPv6
   traffic attempting to transit a firewall.












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4.3.1.  Traffic That Must Not Be Dropped

   Error messages that are essential to the establishment and
   maintenance of communications:

   o  Destination Unreachable (Type 1) - All codes
   o  Packet Too Big (Type 2)
   o  Time Exceeded (Type 3) - Code 0 only
   o  Parameter Problem (Type 4) - Codes 1 and 2 only

   Appendix A.4 suggests some more specific checks that could be
   performed on Parameter Problem messages if a firewall has the
   necessary packet inspection capabilities.

   Connectivity checking messages:

   o  Echo Request (Type 128)
   o  Echo Response (Type 129)

   For Teredo tunneling [RFC4380] to IPv6 nodes on the site to be
   possible, it is essential that the connectivity checking messages are
   allowed through the firewall.  It has been common practice in IPv4
   networks to drop Echo Request messages in firewalls to minimize the
   risk of scanning attacks on the protected network.  As discussed in
   Section 3.2, the risks from port scanning in an IPv6 network are much
   less severe, and it is not necessary to filter IPv6 Echo Request
   messages.

4.3.2.  Traffic That Normally Should Not Be Dropped

   Error messages other than those listed in Section 4.3.1:

   o  Time Exceeded (Type 3) - Code 1
   o  Parameter Problem (Type 4) - Code 0

   Mobile IPv6 messages that are needed to assist mobility:

   o  Home Agent Address Discovery Request (Type 144)
   o  Home Agent Address Discovery Reply (Type 145)
   o  Mobile Prefix Solicitation (Type 146)
   o  Mobile Prefix Advertisement (Type 147)

   Administrators may wish to apply more selective rules as described in
   Appendix A.14 depending on whether the site is catering for mobile
   nodes that would normally be at home on the site and/or foreign
   mobile nodes roaming onto the site.





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4.3.3.  Traffic That Will Be Dropped Anyway -- No Special Attention
        Needed

   The messages listed in this section are all involved with local
   management of nodes connected to the logical link on which they were
   initially transmitted.  All these messages should never be propagated
   beyond the link on which they were initially transmitted.  If the
   firewall is a firewall/bridge rather than a firewall/router, these
   messages should be allowed to transit the firewall as they would be
   intended for establishing communications between the two physical
   parts of the link that are bridged into a single logical link.

   During normal operations, these messages will have destination
   addresses, mostly link local but in some cases global unicast
   addresses, of interfaces on the local link.  No special action is
   needed to filter messages with link-local addresses in a firewall/
   router.  As discussed in Section 4.1, these messages are specified so
   that either the receiver is able to check that the message has not
   passed through a router or it will be dropped at the first router it
   encounters.

   Administrators may also wish to consider providing rules in firewall/
   routers to catch illegal packets sent with hop limit = 1 to avoid
   ICMPv6 Time Exceeded messages being generated for these packets.

   Address Configuration and Router Selection messages (must be received
   with hop limit = 255):

   o  Router Solicitation (Type 133)
   o  Router Advertisement (Type 134)
   o  Neighbor Solicitation (Type 135)
   o  Neighbor Advertisement (Type 136)
   o  Redirect (Type 137)
   o  Inverse Neighbor Discovery Solicitation (Type 141)
   o  Inverse Neighbor Discovery Advertisement (Type 142)

   Link-local multicast receiver notification messages (must have link-
   local source address):

   o  Listener Query (Type 130)
   o  Listener Report (Type 131)
   o  Listener Done (Type 132)
   o  Listener Report v2 (Type 143)








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   SEND Certificate Path notification messages (must be received with
   hop limit = 255):

   o  Certificate Path Solicitation (Type 148)
   o  Certificate Path Advertisement (Type 149)

   Multicast Router Discovery messages (must have link-local source
   address and hop limit = 1):

   o  Multicast Router Advertisement (Type 151)
   o  Multicast Router Solicitation (Type 152)
   o  Multicast Router Termination (Type 153)

4.3.4.  Traffic for Which a Policy Should Be Defined

   The message type that the experimental Seamoby protocols are using
   will be expected to have to cross site boundaries in normal
   operation.  Transit sites must allow these messages to transit the
   site.  End site administrators should determine if they need to
   support these experiments and otherwise messages of this type should
   be dropped:

   o  Seamoby Experimental (Type 150)

   Error messages not currently defined by IANA:
   o  Unallocated Error messages (Types 5-99 inclusive and 102-126
      inclusive)

   The base ICMPv6 specification suggests that error messages that are
   not explicitly known to a node should be forwarded and passed to any
   higher-level protocol that might be able to interpret them.  There is
   a small risk that such messages could be used to provide a covert
   channel or form part of a DoS attack.  Administrators of end sites
   should be aware of this and determine whether they wish to allow
   these messages through the firewall.  Firewalls protecting transit
   sites must allow all types of error messages to transit the site but
   may adopt different policies for error messages addressed to nodes
   within the site.

   All informational messages with types not explicitly assigned by
   IANA, currently:

   o  Unallocated Informational messages (Types 154-199 inclusive and
      202-254 inclusive).

   Note that the base ICMPv6 specification requires that received
   informational messages with unknown types must be silently discarded.
   Transit sites must allow these messages to transit the site.  End



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   site administrators can either adopt a policy of allowing all these
   messages through the firewall, relying on end hosts to drop
   unrecognized messages, or drop all such messages at the firewall.
   Different policies could be adopted for inbound and outbound
   messages.

   If administrators choose to implement policies that drop currently
   unallocated error or informational messages, it is important to
   review the set of messages affected in case new message types are
   assigned by IANA.

4.3.5.  Traffic That Should Be Dropped Unless a Good Case Can Be Made

   Node Information enquiry messages should generally not be forwarded
   across site boundaries.  Some of these messages will be using non-
   link-local unicast addresses so that they will not necessarily be
   dropped by address scope limiting rules:

   o  Node Information Query (Type 139)
   o  Node Information Response (Type 140)

   Router Renumbering messages should not be forwarded across site
   boundaries.  As originally specified, these messages may use a site
   scope multicast address or a site local unicast address.  They should
   be caught by standard rules that are intended to stop any packet with
   a multicast site scope or site local destination being forwarded
   across a site boundary provided these are correctly configured.
   Since site local addresses have now been deprecated, it seems likely
   that changes may be made to allow the use of unique local addresses
   or global unicast addresses.  Should this happen, it will be
   essential to explicitly filter these messages at site boundaries.  If
   a site has internal as well as boundary firewalls, individual
   policies should be established for the internal firewalls depending
   on whether or not the site wishes to use Router Renumbering:

   o  Router Renumbering (Type 138)

   Messages with types in the experimental allocations:

   o  Types 100, 101, 200, and 201.

   Messages using the extension type numbers until such time as ICMPv6
   needs to use such extensions:

   o  Types 127 and 255.






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4.4.  Recommendations for ICMPv6 Local Configuration Traffic

   This section recommends filtering rules for ICMPv6 traffic addressed
   to an interface on a firewall.  For a small number of messages, the
   desired behavior may differ between interfaces on the site or private
   side of the firewall and the those on the public Internet side of the
   firewall.

4.4.1.  Traffic That Must Not Be Dropped

   Error messages that are essential to the establishment and
   maintenance of communications:

   o  Destination Unreachable (Type 1) - All codes
   o  Packet Too Big (Type 2)
   o  Time Exceeded (Type 3) - Code 0 only
   o  Parameter Problem (Type 4) - Codes 1 and 2 only

   Connectivity checking messages:

   o  Echo Request (Type 128)
   o  Echo Response (Type 129)

   As discussed in Section 4.3.1, dropping connectivity checking
   messages will prevent the firewall being the destination of a Teredo
   tunnel and it is not considered necessary to disable connectivity
   checking in IPv6 networks because port scanning is less of a security
   risk.

   There are a number of other sets of messages that play a role in
   configuring the node and maintaining unicast and multicast
   communications through the interfaces of a node.  These messages must
   not be dropped if the node is to successfully participate in an IPv6
   network.  The exception to this is the Redirect message for which an
   explicit policy decision should be taken (see Section 4.4.4).

   Address Configuration and Router Selection messages:

   o  Router Solicitation (Type 133)
   o  Router Advertisement (Type 134)
   o  Neighbor Solicitation (Type 135)
   o  Neighbor Advertisement (Type 136)
   o  Inverse Neighbor Discovery Solicitation (Type 141)
   o  Inverse Neighbor Discovery Advertisement (Type 142)







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   Link-Local Multicast Receiver Notification messages:

   o  Listener Query (Type 130)
   o  Listener Report (Type 131)
   o  Listener Done (Type 132)
   o  Listener Report v2 (Type 143)

   SEND Certificate Path Notification messages:

   o  Certificate Path Solicitation (Type 148)
   o  Certificate Path Advertisement (Type 149)

   Multicast Router Discovery messages:

   o  Multicast Router Advertisement (Type 151)
   o  Multicast Router Solicitation (Type 152)
   o  Multicast Router Termination (Type 153)

4.4.2.  Traffic That Normally Should Not Be Dropped

   Error messages other than those listed in Section 4.4.1:

   o  Time Exceeded (Type 3) - Code 1
   o  Parameter Problem (Type 4) - Code 0

4.4.3.  Traffic That Will Be Dropped Anyway -- No Special Attention
        Needed

   Router Renumbering messages must be authenticated using IPsec, so it
   is not essential to filter these messages even if they are not
   allowed at the firewall/router:

   o  Router Renumbering (Type 138)

   Mobile IPv6 messages that are needed to assist mobility:

   o  Home Agent Address Discovery Request (Type 144)
   o  Home Agent Address Discovery Reply (Type 145)
   o  Mobile Prefix Solicitation (Type 146)
   o  Mobile Prefix Advertisement (Type 147)

   It may be desirable to drop these messages, especially on public
   interfaces, if the firewall is not also providing mobile home agent
   services, but they will be ignored otherwise.







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   The message used by the experimental Seamoby protocols may be dropped
   but will be ignored if the service is not implemented:

   o  Seamoby Experimental (Type 150)

4.4.4.  Traffic for Which a Policy Should Be Defined

   Redirect messages provide a significant security risk, and
   administrators should take a case-by-case approach to whether
   firewalls, routers in general, and other nodes should accept these
   messages:

   o  Redirect (Type 137)

   Conformant nodes must provide configuration controls that allow nodes
   to control their behavior with respect to Redirect messages so that
   it should only be necessary to install specific filtering rules under
   special circumstances, such as if Redirect messages are accepted on
   private interfaces but not public ones.

   If a node implements the experimental Node Information service, the
   administrator needs to make an explicit decision as to whether the
   node should respond to or accept Node Information messages on each
   interface:

   o  Node Information Query (Type 139)
   o  Node Information Response (Type 140)

   It may be possible to disable the service on the node if it is not
   wanted, in which case these messages will be ignored and no filtering
   is necessary.

   Error messages not currently defined by IANA:

   o  Unallocated Error messages (Types 5-99 inclusive and 102-126
      inclusive)

   The base ICMPv6 specification suggests that error messages that are
   not explicitly known to a node should be forwarded and passed to any
   higher-level protocol that might be able to interpret them.  There is
   a small risk that such messages could be used to provide a covert
   channel or form part of a DoS attack.  Administrators should be aware
   of this and determine whether they wish to allow these messages to be
   sent to the firewall.







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4.4.5.  Traffic That Should Be Dropped Unless a Good Case Can Be Made

   Messages with types in the experimental allocations:

   o  Types 100, 101, 200, and 201.

   Messages using the extension type numbers until such time as ICMPv6
   needs to use such extensions:

   o  Types 127 and 255.

   All informational messages with types not explicitly assigned by
   IANA, currently:

   o  Types 154-199 inclusive and 202-254 inclusive.

   Note that the base ICMPv6 specification requires that received
   informational messages with unknown types must be silently discarded.

5.  Acknowledgements

   Pekka Savola created the original IPv6 Security Overview document,
   which contained suggestions for ICMPv6 filter setups.  This
   information has been incorporated into this document.  He has also
   provided important comments.  Some analysis of the classification of
   ICMPv6 messages and the term 'any-to-end' were used by Jari Arkko in
   a document relating to ICMPv6 and IKE.

   The Netfilter configuration script in Appendix B was contributed by
   Suresh Krishnan.

6.  References

6.1.  Normative References

   [RFC1981]       McCann, J., Deering, S., and J. Mogul, "Path MTU
                   Discovery for IP version 6", RFC 1981, August 1996.

   [RFC2460]       Deering, S. and R. Hinden, "Internet Protocol,
                   Version 6 (IPv6) Specification", RFC 2460,
                   December 1998.

   [RFC2461]       Narten, T., Nordmark, E., and W. Simpson, "Neighbor
                   Discovery for IP Version 6 (IPv6)", RFC 2461,
                   December 1998.

   [RFC2462]       Thomson, S. and T. Narten, "IPv6 Stateless Address
                   Autoconfiguration", RFC 2462, December 1998.



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   [RFC2710]       Deering, S., Fenner, W., and B. Haberman, "Multicast
                   Listener Discovery (MLD) for IPv6", RFC 2710,
                   October 1999.

   [RFC2894]       Crawford, M., "Router Renumbering for IPv6",
                   RFC 2894, August 2000.

   [RFC3122]       Conta, A., "Extensions to IPv6 Neighbor Discovery for
                   Inverse Discovery Specification", RFC 3122,
                   June 2001.

   [RFC3590]       Haberman, B., "Source Address Selection for the
                   Multicast Listener Discovery (MLD) Protocol",
                   RFC 3590, September 2003.

   [RFC3775]       Johnson, D., Perkins, C., and J. Arkko, "Mobility
                   Support in IPv6", RFC 3775, June 2004.

   [RFC3810]       Vida, R. and L. Costa, "Multicast Listener Discovery
                   Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC3971]       Arkko, J., Kempf, J., Zill, B., and P. Nikander,
                   "SEcure Neighbor Discovery (SEND)", RFC 3971,
                   March 2005.

   [RFC4065]       Kempf, J., "Instructions for Seamoby and Experimental
                   Mobility Protocol IANA Allocations", RFC 4065,
                   July 2005.

   [RFC4286]       Haberman, B. and J. Martin, "Multicast Router
                   Discovery", RFC 4286, December 2005.

   [RFC4443]       Conta, A., Deering, S., and M. Gupta, "Internet
                   Control Message Protocol (ICMPv6) for the Internet
                   Protocol Version 6 (IPv6) Specification", RFC 4443,
                   March 2006.

   [RFC4620]       Crawford, M. and B. Haberman, "IPv6 Node Information
                   Queries", RFC 4620, August 2006.

6.2.  Informative References

   [ICMP-ATTACKS]  Gont, F., "ICMP attacks against TCP", Work
                   in Progress, October 2006.

   [RFC3041]       Narten, T. and R. Draves, "Privacy Extensions for
                   Stateless Address Autoconfiguration in IPv6",
                   RFC 3041, January 2001.



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   [RFC4380]       Huitema, C., "Teredo: Tunneling IPv6 over UDP through
                   Network Address Translations (NATs)", RFC 4380,
                   February 2006.

   [SCAN-IMP]      Chown, T., "IPv6 Implications for Network Scanning",
                   Work in Progress, March 2007.

   [netfilter]     netfilter.org, "The netfilter.org project",
                   Firewalling, NAT and Packet Mangling for Linux ,
                   2006, <http://www.netfilter.org/>.









































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Appendix A.  Notes on Individual ICMPv6 Messages

A.1.  Destination Unreachable Error Message

   Destination Unreachable (Type 1) error messages [RFC4443] are sent
   any-to-end between unicast addresses.  The message can be generated
   from any node that a packet traverses when the node is unable to
   forward the packet for any reason except congestion.

   Destination Unreachable messages are useful for debugging, but are
   also important to speed up cycling through possible addresses, as
   they can avoid the need to wait through timeouts and hence can be
   part of the process of establishing or maintaining communications.
   It is a common practice in IPv4 to refrain from generating ICMP
   Destination Unreachable messages in an attempt to hide the networking
   topology and/or service structure.  The same idea could be applied to
   IPv6, but this can slow down connection if a host has multiple
   addresses, some of which are deprecated, as they may be when using
   privacy addresses [RFC3041].  If policy allows the generation of
   ICMPv6 Destination Unreachable messages, it is important that nodes
   provide the correct reason code, one of: no route to destination,
   administratively prohibited, beyond scope of source address, address
   unreachable, port unreachable, source address failed ingress/egress
   policy, or reject route to destination.

A.2.  Packet Too Big Error Message

   Packet Too Big (Type 2) error messages [RFC4443] are sent any-to-end
   between unicast addresses.  The message can be generated from any
   node that a packet traverses on the path when the node is unable to
   forward the packet because the packet is too large for the MTU of the
   next link.  This message is vital to the correct functioning of Path
   MTU Discovery and hence is part of the establishment and maintenance
   of communications.  Since routers are not allowed to fragment
   packets, informing sources of the need to fragment large packets is
   more important than for IPv4.  If these messages are not generated
   when appropriate, hosts will continue to send packets that are too
   large or may assume that the route is congested.  Effectively, parts
   of the Internet will become inaccessible.

   If a network chooses to generate packets that are no larger than the
   Guaranteed Minimum MTU (1280 octets) and the site's links to the
   wider Internet have corresponding MTUs, Packet Too Big messages
   should not be expected at the firewall and could be dropped if they
   arrive.






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A.3.  Time Exceeded Error Message

   Time Exceeded (Type 3) error messages [RFC4443] can occur in two
   contexts:

   o  Code 0 are generated at any node on the path being taken by the
      packet and sent, any-to-end between unicast addresses, if the Hop
      Limit value is decremented to zero at that node.

   o  Code 1 messages are generated at the destination node and sent
      end-to-end between unicast addresses if all the segments of a
      fragmented message are not received within the reassembly time
      limit.

   Code 0 messages can be needed as part of the establishment of
   communications if the path to a particular destination requires an
   unusually large number of hops.

   Code 1 messages will generally only result from congestion in the
   network, and it is less essential to propagate these messages.

A.4.  Parameter Problem Error Message

   The great majority of Parameter Problem (Type 4) error messages will
   be generated by the destination node when processing destination
   options and other extension headers, and hence are sent end-to-end
   between unicast addresses.  Exceptionally, these messages might be
   generated by any node on the path if a faulty or unrecognized hop-by-
   hop option is included or from any routing waypoint if there are
   faulty or unrecognized destination options associated with a Type 0
   routing header.  In these cases, the message will be sent any-to-end
   using unicast source and destination addresses.

   Parameter Problem Code 1 (Unrecognized Next Header) and Code 2
   (Unrecognized IPv6 Option) messages may result if a node on the path
   (usually the destination) is unable to process a correctly formed
   extension header or option.  If these messages are not returned to
   the source, communication cannot be established, as the source would
   need to adapt its choice of options probably because the destination
   does not implement these capabilities.  Hence, these messages need to
   be generated and allowed for effective IPv6 communications.

   Code 0 (Erroneous Header) messages indicate a malformed extension
   header generally as a result of incorrectly generated packets.
   Hence, these messages are useful for debugging purposes, but it is
   unlikely that a node generating such packets could establish
   communications without human intervention to correct the problem.




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   Code 2 messages, only, can be generated for packets with multicast
   destination addresses.

   It is possible that attackers may seek to probe or scan a network by
   deliberately generating packets with unknown extension headers or
   options or with faulty headers.  If nodes generate Parameter Problem
   error messages in all cases and these outgoing messages are allowed
   through firewalls, the attacker may be able to identify active
   addresses that can be probed further or learn about the network
   topology.  The vulnerability could be mitigated whilst helping to
   establish communications if the firewall was able to examine such
   error messages in depth and was configured to only allow Parameter
   Problem messages for headers that had been standardized but were not
   supported in the protected network.  If the network administrator
   believes that all nodes in the network support all legitimate
   extension headers, then it would be reasonable to drop all outgoing
   Parameter Problem messages.  Note that this is not a major
   vulnerability in a well-designed IPv6 network because of the
   difficulties of performing scanning attacks (see Section 3.2).

A.5.  ICMPv6 Echo Request and Echo Response

   Echo Request (Type 128) uses unicast addresses as source addresses,
   but may be sent to any legal IPv6 address, including multicast and
   anycast addresses [RFC4443].  Echo Requests travel end-to-end.
   Similarly, Echo Responses (Type 129) travel end-to-end and would have
   a unicast address as destination and either a unicast or anycast
   address as source.  They are mainly used in combination for
   monitoring and debugging connectivity.  Their only role in
   establishing communication is that they are required when verifying
   connectivity through Teredo tunnels [RFC4380]: Teredo tunneling to
   IPv6 nodes on the site will not be possible if these messages are
   blocked.  It is not thought that there is a significant risk from
   scanning attacks on a well-designed IPv6 network (see Section 3.2),
   and so connectivity checks should be allowed by default.

A.6.  Neighbor Solicitation and Neighbor Advertisement Messages

   ICMPv6 Neighbor Solicitation and Neighbor Advertisement (Type 135 and
   136) messages are essential to the establishment and maintenance of
   communications on the local link.  Firewalls need to generate and
   accept these messages to allow them to establish and maintain
   interfaces onto their connected links.








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   Note that the address scopes of the source and destination addresses
   on Neighbor Solicitations and Neighbor Advertisements may not match.
   The exact functions that these messages will be carrying out depends
   on the mechanism being used to configure IPv6 addresses on the link
   (Stateless, Stateful, or Static configuration).

A.7.  Router Solicitation and Router Advertisement Messages

   ICMPv6 Router Solicitation and Router Advertisement (Type 133 and
   134) messages are essential to the establishment and maintenance of
   communications on the local link.  Firewalls need to generate (since
   the firewall will generally be behaving as a router) and accept these
   messages to allow them to establish and maintain interfaces onto
   their connected links.

A.8.  Redirect Messages

   ICMPv6 Redirect Messages (Type 137) are used on the local link to
   indicate that nodes are actually link-local and communications need
   not go via a router, or to indicate a more appropriate first-hop
   router.  Although they can be used to make communications more
   efficient, they are not essential to the establishment of
   communications and may be a security vulnerability, particularly if a
   link is not physically secured.  Conformant nodes are required to
   provide configuration controls that suppress the generation of
   Redirect messages and allow them to be ignored on reception.  Using
   Redirect messages on, for example, a wireless link without link level
   encryption/authentication is particularly hazardous because the link
   is open to eavesdropping and packet injection.

A.9.  SEND Certificate Path Messages

   SEND [RFC3971] uses two messages (Certificate Path Solicitation and
   Advertisement - Types 148 and 149) sent from nodes to supposed
   routers on the same local link to obtain a certificate path that will
   allow the node to authenticate the router's claim to provide routing
   services for certain prefixes.  If a link connected to a firewall/
   router is using SEND, the firewall must be able to exchange these
   messages with nodes on the link that will use its routing services.

A.10.  Multicast Listener Discovery Messages

   Multicast Listener Discovery (MLD) version 1 [RFC2710] (Listener
   Query, Listener Report, and Listener Done - Types 130, 131, and 132)
   and version 2 [RFC3810] (Listener Query and Listener Report version 2
   - Types 130 and 143) messages are sent on the local link to
   communicate between multicast-capable routers and nodes that wish to
   join or leave specific multicast groups.  Firewalls need to be able



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   to generate Listener messages in order to establish communications
   and may generate all the messages if they also provide multicast
   routing services.

A.11.  Multicast Router Discovery Messages

   Multicast Router Discovery [RFC4286] (Router Advertisement, Router
   Solicitation, and Router Termination - Types 151, 152, and 153)
   messages are sent by nodes on the local link to discover multicast-
   capable routers on the link, and by multicast-capable routers to
   notify other nodes of their existence or change of state.  Firewalls
   that also act as multicast routers need to process these messages on
   their interfaces.

A.12.  Router Renumbering Messages

   ICMPv6 Router Renumbering (Type 138) command messages can be received
   and results messages sent by routers to change the prefixes that they
   advertise as part of Stateless Address Configuration [RFC2461],
   [RFC2462].  These messages are sent end-to-end to either the all-
   routers multicast address (site or local scope) or specific unicast
   addresses from a unicast address.

   Router Renumbering messages are required to be protected by IPsec
   authentication since they could be readily misused by attackers to
   disrupt or divert site communications.  Renumbering messages should
   generally be confined to sites for this reason.

A.13.  Node Information Query and Reply

   ICMPv6 Node Information Query and Reply (Type 139 and 140) messages
   defined in [RFC4620] are sent end-to-end between unicast addresses,
   and they can also be sent to link-local multicast addresses.  They
   can, in theory, be sent from any node to any other, but it would
   generally not be desirable for nodes outside the local site to be
   able to send queries to nodes within the site.  Also, these messages
   are not required to be authenticated.

A.14.  Mobile IPv6 Messages

   Mobile IPv6 [RFC3775] defines four ICMPv6 messages that are used to
   support mobile operations: Home Agent Address Discovery Request, Home
   Agent Address Discovery Reply, Mobile Prefix Solicitation, and ICMP
   Mobile Prefix Advertisement (Type 144, 145, 146, and 147) messages.
   These messages are sent end-to-end between unicast addresses of a
   mobile node and its home agent.  They must be expected to be sent
   from outside a site and must traverse site-boundary firewalls to
   reach the home agent in order for Mobile IPv6 to function.  The two



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   Mobile prefix messages should be protected by the use of IPsec
   authentication.

   o  If the site provides home agents for mobile nodes, the firewall
      must allow incoming Home Agent Address Discovery Request and
      Mobile Prefix Solicitation messages, and outgoing Home Agent
      Address Discovery Reply and ICMP Mobile Prefix Advertisement
      messages.  It may be desirable to limit the destination addresses
      for the incoming messages to links that are known to support home
      agents.

   o  If the site is prepared to host roaming mobile nodes, the firewall
      must allow outgoing Home Agent Address Discovery Request and
      Mobile Prefix Solicitation messages, and incoming Home Agent
      Address Discovery Reply and ICMP Mobile Prefix Advertisement
      messages.

   o  Administrators may find it desirable to prevent static nodes that
      are normally resident on the site from behaving as mobile nodes by
      dropping Mobile IPv6 messages from these nodes.

A.15.  Unused and Experimental Messages

   A large number of ICMPv6 Type values are currently unused.  These
   values have not had a specific function registered with IANA.  This
   section describes how to treat messages that attempt to use these
   Type values in a way of which the network administrator (and hence
   the firewall) is not aware.

   [RFC4443] defines a number of experimental Type values for ICMPv6
   Error and Informational messages, which could be used in site-
   specific ways.  These messages should be dropped by transit networks
   and at site edges.  They should also not be propagated within sites
   unless the network administrator is explicitly made aware of usage.

   The codes reserved for future extension of the ICMPv6 Type space
   should currently be dropped as this functionality is as yet
   undefined.

   Any ICMPv6 Informational messages of which the firewall is not aware
   should be allowed to transit through the firewall but should not be
   accepted for local delivery on any of its interfaces.

   Unknown ICMPv6 Error messages should be allowed to pass through
   transit networks.  At end site boundaries any incoming ICMPv6 Error
   messages of which the firewall is not aware may be allowed through
   the firewall in line with the specification in [RFC4443], which
   requests delivery of unknown error messages to higher-layer protocol



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   processes.  However, administrators may wish to disallow forwarding
   of these incoming messages as a potential security risk.  Unknown
   outgoing Error messages should be dropped as the administrator should
   be aware of all messages that could be generated on the site.

   Also, the SEAMOBY working group has had an ICMPv6 message (Type 150)
   allocated for experimental use in two protocols.  This message is
   sent end-to-end and may need to pass through firewalls on sites that
   are supporting the experimental protocols.

Appendix B.  Example Script to Configure ICMPv6 Firewall Rules

   This appendix contains an example script to implement most of the
   rules suggested in this document when using the Netfilter packet
   filtering system for Linux [netfilter].  When used with IPv6, the
   'ip6tables' command is used to configure packet filtering rules for
   the Netfilter system.  The script is targeted at a simple enterprise
   site that may or may not support Mobile IPv6.

   #!/bin/bash
   # Set of prefixes on the trusted ("inner") side of the firewall
   export INNER_PREFIXES="2001:DB8:85::/60"
   # Set of hosts providing services so that they can be made pingable
   export PINGABLE_HOSTS="2001:DB8:85::/64"
   # Configuration option: Change this to 1 if errors allowed only for
   # existing sessions
   export STATE_ENABLED=0
   # Configuration option: Change this to 1 if messages to/from link
   # local addresses should be filtered.
   # Do not use this if the firewall is a bridge.
   # Optional for firewalls that are routers.
   export FILTER_LINK_LOCAL_ADDRS=0
   # Configuration option: Change this to 0 if the site does not support
   # Mobile IPv6 Home Agents - see Appendix A.14
   export HOME_AGENTS_PRESENT=1
   # Configuration option: Change this to 0 if the site does not support
   # Mobile IPv6 mobile nodes being present on the site -
   # see Appendix A.14
   export MOBILE_NODES_PRESENT=1

   ip6tables -N icmpv6-filter
   ip6tables -A FORWARD -p icmpv6 -j icmpv6-filter

   # Match scope of src and dest else deny
   # This capability is not provided for in base ip6tables functionality
   # An extension (agr) exists which may support it.
   #@TODO@




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   # ECHO REQUESTS AND RESPONSES
   # ===========================

   # Allow outbound echo requests from prefixes which belong to the site
   for inner_prefix in $INNER_PREFIXES
   do
     ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
           --icmpv6-type echo-request -j ACCEPT
   done

   # Allow inbound echo requests towards only predetermined hosts
   for pingable_host in $PINGABLE_HOSTS
   do
     ip6tables -A icmpv6-filter -p icmpv6 -d $pingable_host \
           --icmpv6-type echo-request -j ACCEPT
   done

   if [ "$STATE_ENABLED" -eq "1" ]
   then
     # Allow incoming and outgoing echo reply messages
     # only for existing sessions
     ip6tables -A icmpv6-filter -m state -p icmpv6 \
           --state ESTABLISHED,RELATED --icmpv6-type \
         echo-reply -j ACCEPT
   else
     # Allow both incoming and outgoing echo replies
     for pingable_host in $PINGABLE_HOSTS
     do
       # Outgoing echo replies from pingable hosts
       ip6tables -A icmpv6-filter -p icmpv6 -s $pingable_host \
           --icmpv6-type echo-reply -j ACCEPT
     done
     # Incoming echo replies to prefixes which belong to the site
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
           --icmpv6-type echo-reply -j ACCEPT
     done
   fi

   # Deny icmps to/from link local addresses
   # If the firewall is a router:
   #    These rules should be redundant as routers should not forward
   #    link local addresses but to be sure...
   # DO NOT ENABLE these rules if the firewall is a bridge
   if [ "$FILTER_LINK_LOCAL_ADDRS" -eq "1" ]
   then
     ip6tables -A icmpv6-filter -p icmpv6 -d fe80::/10 -j DROP



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     ip6tables -A icmpv6-filter -p icmpv6 -s fe80::/10 -j DROP
   fi

   # Drop echo replies which have a multicast address as a
   # destination
   ip6tables -A icmpv6-filter -p icmpv6 -d ff00::/8 \
           --icmpv6-type echo-reply -j DROP

   # DESTINATION UNREACHABLE ERROR MESSAGES
   # ======================================

   if [ "$STATE_ENABLED" -eq "1" ]
   then
     # Allow incoming destination unreachable messages
     # only for existing sessions
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -m state -p icmpv6 \
            -d $inner_prefix \
            --state ESTABLISHED,RELATED --icmpv6-type \
            destination-unreachable -j ACCEPT
     done
   else
     # Allow incoming destination unreachable messages
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type destination-unreachable -j ACCEPT
     done
   fi

   # Allow outgoing destination unreachable messages
   for inner_prefix in $INNER_PREFIXES
   do
     ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type destination-unreachable -j ACCEPT
   done

   # PACKET TOO BIG ERROR MESSAGES
   # =============================

   if [ "$STATE_ENABLED" -eq "1" ]
   then
     # Allow incoming Packet Too Big messages
     # only for existing sessions
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -m state -p icmpv6 \



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            -d $inner_prefix \
            --state ESTABLISHED,RELATED \
            --icmpv6-type packet-too-big \
            -j ACCEPT
     done
   else
     # Allow incoming Packet Too Big messages
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type packet-too-big -j ACCEPT
     done
   fi

   # Allow outgoing Packet Too Big messages
   for inner_prefix in $INNER_PREFIXES
   do
     ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type packet-too-big -j ACCEPT
   done

   # TIME EXCEEDED ERROR MESSAGES
   # ============================

   if [ "$STATE_ENABLED" -eq "1" ]
   then
     # Allow incoming time exceeded code 0 messages
     # only for existing sessions
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -m state -p icmpv6 \
            -d $inner_prefix \
            --state ESTABLISHED,RELATED --icmpv6-type packet-too-big \
            -j ACCEPT
     done
   else
     # Allow incoming time exceeded code 0 messages
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type ttl-zero-during-transit -j ACCEPT
     done
   fi

   #@POLICY@
   # Allow incoming time exceeded code 1 messages
   for inner_prefix in $INNER_PREFIXES
   do



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   ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type ttl-zero-during-reassembly -j ACCEPT
   done

   # Allow outgoing time exceeded code 0 messages
   for inner_prefix in $INNER_PREFIXES
   do
   ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type ttl-zero-during-transit -j ACCEPT
   done

   #@POLICY@
   # Allow outgoing time exceeded code 1 messages
   for inner_prefix in $INNER_PREFIXES
   do
   ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type ttl-zero-during-reassembly -j ACCEPT
   done


   # PARAMETER PROBLEM ERROR MESSAGES
   # ================================

   if [ "$STATE_ENABLED" -eq "1" ]
   then
     # Allow incoming parameter problem code 1 and 2 messages
     # for an existing session
     for inner_prefix in $INNER_PREFIXES
     do
       ip6tables -A icmpv6-filter -m state -p icmpv6 \
            -d $inner_prefix \
            --state ESTABLISHED,RELATED --icmpv6-type \
            unknown-header-type \
            -j ACCEPT
       ip6tables -A icmpv6-filter -m state -p icmpv6 \
            -d $inner_prefix \
            --state ESTABLISHED,RELATED \
            --icmpv6-type unknown-option \
            -j ACCEPT
     done
   fi

   # Allow outgoing parameter problem code 1 and code 2 messages
   for inner_prefix in $INNER_PREFIXES
   do
     ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type unknown-header-type -j ACCEPT
     ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \



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            --icmpv6-type unknown-option -j ACCEPT
   done

   #@POLICY@
   # Allow incoming and outgoing parameter
   # problem code 0 messages
   for inner_prefix in $INNER_PREFIXES
   do
     ip6tables -A icmpv6-filter -p icmpv6 \
            --icmpv6-type bad-header \
            -j ACCEPT
   done

   # NEIGHBOR DISCOVERY MESSAGES
   # ===========================

   # Drop NS/NA messages both incoming and outgoing
   ip6tables -A icmpv6-filter -p icmpv6 \
            --icmpv6-type neighbor-solicitation -j DROP
   ip6tables -A icmpv6-filter -p icmpv6 \
            --icmpv6-type neighbor-advertisement -j DROP

   # Drop RS/RA messages both incoming and outgoing
   ip6tables -A icmpv6-filter -p icmpv6 \
            --icmpv6-type router-solicitation -j DROP
   ip6tables -A icmpv6-filter -p icmpv6 \
            --icmpv6-type router-advertisement -j DROP

   # Drop Redirect messages both incoming and outgoing
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type redirect -j DROP

   # MLD MESSAGES
   # ============

   # Drop incoming and outgoing
   # Multicast Listener queries (MLDv1 and MLDv2)
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 130 -j DROP

   # Drop incoming and outgoing Multicast Listener reports (MLDv1)
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 131 -j DROP

   # Drop incoming and outgoing Multicast Listener Done messages (MLDv1)
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 132 -j DROP

   # Drop incoming and outgoing Multicast Listener reports (MLDv2)
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 143 -j DROP

   # ROUTER RENUMBERING MESSAGES



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   # ===========================

   # Drop router renumbering messages
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 138 -j DROP

   # NODE INFORMATION QUERIES
   # ========================

   # Drop node information queries (139) and replies (140)
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 139 -j DROP
   ip6tables -A icmpv6-filter -p icmpv6 --icmpv6-type 140 -j DROP


   # MOBILE IPv6 MESSAGES
   # ====================

   # If there are mobile ipv6 home agents present on the
   # trusted side allow
   if [ "$HOME_AGENTS_PRESENT" -eq "1" ]
   then
     for inner_prefix in $INNER_PREFIXES
     do
       #incoming Home Agent address discovery request
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type 144 -j ACCEPT
       #outgoing Home Agent address discovery reply
       ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type 145 -j ACCEPT
       #incoming Mobile prefix solicitation
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type 146 -j ACCEPT
       #outgoing Mobile prefix advertisement
       ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type 147 -j ACCEPT
     done
   fi

   # If there are roaming mobile nodes present on the
   # trusted side allow
   if [ "$MOBILE_NODES_PRESENT" -eq "1" ]
   then
     for inner_prefix in $INNER_PREFIXES
     do
       #outgoing Home Agent address discovery request
       ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type 144 -j ACCEPT
       #incoming Home Agent address discovery reply
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \



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            --icmpv6-type 145 -j ACCEPT
       #outgoing Mobile prefix solicitation
       ip6tables -A icmpv6-filter -p icmpv6 -s $inner_prefix \
            --icmpv6-type 146 -j ACCEPT
       #incoming Mobile prefix advertisement
       ip6tables -A icmpv6-filter -p icmpv6 -d $inner_prefix \
            --icmpv6-type 147 -j ACCEPT
     done
   fi

   # DROP EVERYTHING ELSE
   # ====================

   ip6tables -A icmpv6-filter -p icmpv6 -j DROP

        Example Netfilter Configuration Script for ICMPv6 Filtering

Authors' Addresses

   Elwyn B. Davies
   Consultant
   Soham, Cambs
   UK

   Phone: +44 7889 488 335
   EMail: elwynd@dial.pipex.com


   Janos Mohacsi
   NIIF/HUNGARNET
   Victor Hugo u. 18-22
   Budapest,   H-1132
   Hungary

   Phone: +36 1 4503070
   EMail: mohacsi@niif.hu















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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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