Internet DRAFT - draft-davies-v6ops-icmpv6-filtering-bcp
draft-davies-v6ops-icmpv6-filtering-bcp
IPv6 Operations E. Davies
Internet-Draft Consultant
Expires: January 12, 2006 J. Mohacsi
NIIF/HUNGARNET
July 11, 2005
Best Current Practice for Filtering ICMPv6 Messages in Firewalls
draft-davies-v6ops-icmpv6-filtering-bcp-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
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. A number of security risks are associated with uncontrolled
forwarding of ICMPv6 messages, and it is desirable to configure site
firewalls to intercept inappropriate usages of ICMPv6 which might
allow an attacker outside a site to probe or compromise the site
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network. On the other hand, compared with IPv4 and the corresponding
protocol ICMP, ICMPv6 is essential to the functioning of IPv6 rather
than a useful auxiliary. Hence too aggressive filtering of ICMPv6
messages can be detrimental to the establishment of IPv6
communications. This means that effective filtering of ICMPv6
requires a more complex configuration than was needed for ICMP. 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
which are potential security risks.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Classifying ICMPv6 Messages . . . . . . . . . . . . . . . . 5
2.1 Error and Informational ICMPv6 Messages . . . . . . . . . 5
2.2 Addressing of ICMPv6 . . . . . . . . . . . . . . . . . . . 6
2.3 Network Topology and Address Scopes . . . . . . . . . . . 6
2.4 Role in Establishing Communication . . . . . . . . . . . . 6
3. Security Concerns for ICMPv6 Controllable by Firewall
Configuration . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 Denial of Service Attacks . . . . . . . . . . . . . . . . 7
3.2 Probing . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Redirection Attacks . . . . . . . . . . . . . . . . . . . 8
3.4 Renumbering Attacks . . . . . . . . . . . . . . . . . . . 8
4. Filtering Recommendations . . . . . . . . . . . . . . . . . 8
4.1 Common Considerations . . . . . . . . . . . . . . . . . . 8
4.2 ICMPv6 Echo Request and Echo Response . . . . . . . . . . 9
4.3 Destination Unreachable Error Message . . . . . . . . . . 10
4.4 Packet Too Big Error Message . . . . . . . . . . . . . . . 10
4.5 Time Exceeded Error Message . . . . . . . . . . . . . . . 11
4.6 Parameter Problem Error Message . . . . . . . . . . . . . 12
4.7 Neighbor Solicitation and Neighbor Advertisement
Messages . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.8 Router Solicitation and Router Advertisement Messages . . 14
4.9 Redirect Messages . . . . . . . . . . . . . . . . . . . . 14
4.10 Multicast Listener Discovery Messages . . . . . . . . . 14
4.11 Router Renumbering Messages . . . . . . . . . . . . . . 15
4.12 Node Information Query and Reply . . . . . . . . . . . . 15
4.13 Mobile IPv6 Messages . . . . . . . . . . . . . . . . . . 16
4.14 Unused and Experimental Messages . . . . . . . . . . . . 16
4.15 Problems Resulting from ICMPv6 Transparency . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 17
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1 Normative References . . . . . . . . . . . . . . . . . . . 17
8.2 Informative References . . . . . . . . . . . . . . . . . . 18
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . 20
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1. Introduction
When a network supports IPv6 [RFC2460], the Internet Control Message
Protocol version 6 (ICMPv6) [RFC2463], [I-D.ietf-ipngwg-icmp-v3]
plays a fundamental role including being an essential component in
establishing communications both at the interface level and for
sessions to remote nodes. This means that overly aggressive
filtering of ICMPv6 may have a detrimental effect on the
establishment 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 which seek to balance
effective IPv6 communication against the needs of site security.
[Author's note: The new versions of RFC2461, RFC2462 and RFC2463 have
been taken into account in this draft, but not necessarily referenced
as yet.]
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
are:
o Returning error messages to the source if a packet could not be
delivered. Four different error messages are specified in
[RFC2463].
o 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 [RFC2463].
o 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)) - DAD can be turned off if the network administrator
believes that the configuration method used is bound to generate
unique addresses. Four messages - Neighbor Solicitation (NS),
Neighbor Advertisement (NA), Router Solicitation (RS) and Router
Advertisement (RA) - are specified in [RFC2461].
o 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].
o Finding routers and determining how to obtain IP addresses to join
the subnets supported by the routers. Uses RS and RA [RFC2461].
o If stateless auto-configuration of hosts is enabled, communicating
prefixes and other configuration information (including the link
MTU and suggested hop count default) from routers to hosts. Uses
RS and RA [RFC2462].
o 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
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address (where a link supports multiple subnets). This facility
could be used by a malicious sender to divert packets and nodes
should provide configuration options to prevent the messages being
sent by routers and acted on by hosts. The redirect message is
specified in [RFC2461].
o 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].
o Determining the Maximum Transmission Unit (MTU) along a path. The
Packet Too Big error message is essential to this function
[RFC1981].
o 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 (version 1 only) as
specified in Multicast Listener Discovery MLDv1 [RFC2710] and
MLDv2[RFC3810].
o 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 Mobile
Prefix Solicitation and Advertisement messages are specified in
[RFC3775]
o ICMPv6 can provide some basic information about nodes to
interested parties [I-D.ietf-ipngwg-icmp-name-lookups].
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
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to be sent to or pass through firewalls, and may be essential to the
establishment of communications (see Section 2.4 whereas
Informational Messages will generally be the subject of policy rules,
and those passing through 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. 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].
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 sometimes. 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.
2.3 Network Topology and Address Scopes
ICMPv6 messages can be classified according to whether they are meant
for end-to-end communications or 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 for example when configuring
global addresses. Also some ICMPv6 messages in local communications
may contravene the usual rules requiring compatible scopes for source
and destination addresses.
2.4 Role in Establishing Communication
Many ICMPv6 messages have a role in establishing communications to
and from the firewall and such messages have to be accepted by
firewalls for local delivery. Which messages need to be accepted
depend on whether the firewall is also acting as a router. This type
of communication establishment messages should not be passed through
a firewall as they are normally intended for use within a link.
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On the other hand, some ICMPv6 error messages, which are sent either
end-to-end or any-to-end. are essential to the establishment of
communications. For example the Packet Too Big error message is
needed to establish the MTU along a path. These messages must be
passed through firewalls and might also be sent to and from firewalls
to assist with establishment of communications.
The remaining ICMPv6 messages which are not associated with
communication establishment will normally be legitimately attempting
to pass through a firewall from inside to out or vice versa, but
decisions as to whether to allow them to pass or not can be made on
the basis of local policy without interfering with the establishment
of IPv6 communications.
The filtering rules for the various message roles will generally be
different.
3. Security Concerns for ICMPv6 Controllable by Firewall Configuration
A major concern with most ICMPv6 messages is that it is generally not
possible to use IPsec or other means to authenticate the sender and
validate the contents of some 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 communications means that
alternative means have to 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 most of the
ICMPv6 traffic passing through the firewall and allowing enough of
the traffic through to make sure that efficient communication can be
established.
Firewalls will normally be concerned 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 number sof ICMPv6 packets to
destinations in the site and sending error messages which disrupt
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established commnuications by causing sessions to be dropped. Also
if spurious communication establishment messages can be passed on to
link it might be possible to disrupt established communications.
3.2 Probing
A major security consideration is preventing attackers 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.
3.3 Redirection Attacks
These attacks would normally have to be carried out locally on a
link, but it is important to ensure that Redirect messages are not
allowed through the firewall. Redirection could be used simply for
DoS as well as endeavouring to redirect traffic to a compromised
node.
3.4 Renumbering Attacks
Spurious Renumbering messages could lead to the disruption of a site
and should not be allowed through a firewall in general.
4. Filtering Recommendations
4.1 Common Considerations
Depending on the classification of the message to be fitered (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, where deeper packet inspection is
possible, it may be desirable to filter on, for example, the Code
field of ICMPv6 error messages.
Messages that are authenticated by means of an IPsec AH or ESP header
may be subject to less strict policies than unauthenticated messages.
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 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
travelling 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.
Unless otherwise specified, the scopes of source and destination
addresses of ICMPv6 messages should be matched, and packets with
mismatched addresses should be dropped.
4.2 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, even multicast and anycast
addresses [RFC2463]. Echo Requests travel end-to-end but never have
a role in establishing communications. 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 used in combination for monitoring and debugging connectivity.
o It is desirable that Echo Requests are allowed to pass outwards
through firewalls.
o Echo Requests may be allowed to pass inwards only towards selected
hosts which are providing well-known services to the rest of the
Internet.
o If possible, it is desirable to maintain state about Echo Requests
passing through the firewall which can be used to limit the
possible corresponding Echo Responses that may be returned.
o If policy requires, the possible source addresses for Echo
Requests can be limited to certain machines, but in any case the
source address should be an address owned by the site.
o If state is maintained, only one response should be allowed
corresponding to each request. Echo Responses should only be
passed outwards from machines to which Echo Requests can be sent,
and should only be passed inwards towards hosts which are allowed
to originate Echo Requests.
o Both Echo Requets and Echo Responses should be rate limited in
line with [RFC2463] and, if possible, the overall incoming rate
from all sources destined for any given host should be limited to
a value that the host is known to be able to handle.
o Messages with link local addresses in either source or destination
should be dropped, except for messages coming from the inside
directed to the firewall itself.
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o Echo Response messages must have a unicast address as destination
and may have a unicast, or anycast address as source.
4.3 Destination Unreachable Error Message
Destination Unreachable (Type 1) error messages [RFC2463] are sent
any-to-end between unicast addresses. The message can be generated
from any node which a packet traverses on the path when the node is
unable to forward the packet for any reason except congestion.
o Incoming ICMPv6 Destination Unreachable messages may be passed
through the firewall for debugging purposes where they relate to
outgoing IPv6 packets that have previously been sent out through
the firewall. For preference, this should be implemented by means
of a stateful packet inspection mechanism. These messages should
not be sent if the outgoing message was sent to a multicast
address: if possible any error message that is returned from a
multicast address should be discarded.
o Outgoing ICMPv6 Destination Unreachable messages may be generated
and passed out for all packets that have been allowed through the
firewall.
o At most one Destination Unreachable message should be passed
through the firewall in response to each packet that might result
in an error if the firewall is able to manage this.
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 communications. It is a common
practice in IPv4, to refrain from generating ICMP Destination
Unreachable messages to attempt to hide the networking topology
and/or service structure. The same rule can be applied to IPv6 but
this can slow down connection if a host has multiple addresses some
of which are deprecated as when using privacy addresses [RFC3041].
If policy allows the generation of ICMPv6 Destination Unreachable
messages, it is important to 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, reject route to destination.
4.4 Packet Too Big Error Message
Packet Too Big (Type 2) error messages [RFC2463] are sent any-to-end
between unicast addresses. The message can be generated from any
node which 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 of
communications. Since routers are not allowed to fragment packets,
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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 which are too large
or may assume that the route is congested. Effectively parts of the
Internet will become inaccessible.
o It is essential to allow incoming ICMPv6 Packet Too Big messages
as responses to outgoing IPv6 packets so that Path MTU Discovery
will operate properly.
o If possible, stateful packet inspection should be used to limit
incoming Packet Too Big messages to legitimate responses to
outgoing packets and to limit error messages to at most one per
outgoing packet. Note that this message can be geenrated as a
result of messages sent to multicast addresses.
o It is essential to allow outgoing ICMPv6 Packet Too Big messages
to be generated and passed through the firewall if the MTU is
smaller on any link anywhere within the network protected by the
firewall than the MTU on the link between the firewall and the ISP
which connects the site to the rest of the Internet. In transit
networks, it will be necessary to pass Packet Too Big messages
through the network.
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 can be dropped if they
arrive.
4.5 Time Exceeded Error Message
Time Exceeded (Type 3) error messages [RFC2463] 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 any point on the path.
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. It is therefore essential to
generate and forward Code 0 Time Exceeded messages for effective
operation of IPv6 networks.
Code 1 messages will generally only result from congestion in the
network and it is less essential to propagate these messages.
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o It is essential to allow incoming ICMPv6 Time Exceeded Code 0
messages to be able discover destination systems not reachable due
to a low Hop Limit value in the outgoing packets, so that the Hop
Limit can be selectively increased for these packets.
o Incoming ICMPv6 Time Exceeded Code 1 messages may be enabled by
policy if desired.
o If possible, stateful packet inspection should be used to limit
incoming Time Exceeded messages to legitimate responses to
outgoing packets and to limit error messages to at most one per
outgoing packet.
o It is essential to generate and allow outgoing ICMPv6 Time
Exceeded Code 0 messages to ensure that communications can be
established to sites which generate packets with small hop limits
or are traversing very long paths.
o Generation of outgoing ICMPv6 Time Exceeded Code 1 messages and
allowing them through the firewall may be enabled by policy if
desired.
4.6 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.
Code 2 messages, only, can be generated for packets with multicast
destinatio addresses.
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o It is essential that incoming ICMPv6 Parameter Problem messages
with Code 1 or Code 2 are allowed as responses to outgoing IPv6
packets to allow establishment of communications to sites which do
not support unusual options.
o Incoming ICMPv6 Parameter Problem messages with Code 1 may be
allowed by policy if desired.
o If possible, stateful packet inspection should be used to limit
incoming Parameter Problem messages to legitimate responses to
outgoing packets and to limit error messages to at most one per
outgoing packet.
o It is essential to generate and allow outgoing ICMPv6 Parameter
Problem Code 1 and Code 2 messages to ensure that communications
can be established if it is known that nodes are not able to
support certain options or extension headers. Some risks
associated with blanket generation and forwarding of responses of
this type are discussed at the end of this section.
o Generation of outgoing ICMPv6 Parameter Problem Code 0 messages
and allowing them through the firewall may be enabled by policy if
desired.
It is possible that attackers may seek to probe or scan a network by
deliberately generating packets with unknown extension headers or
options, or 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 which 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.
4.7 Neighbor Solicitation and Neighbor Advertisement Messages
ICMPv6 Neighbor Solicitation and Neighbor Advertisement (Type 135 and
136) messages are essential to establishment of communications on the
local link. Firewalls need to generate and accept these messages to
allow them to establish interfaces onto their connected links.
o Firewalls must accept and generate ICMPv6 neighbor Solicitation
and Neighbor Advertisement messages for local delivery on all
interfaces as described in [RFC2461] and [RFC2462].
o Firewalls must not allow any Neighbor Solicitation or Neighbor
Advertisement messages to pass through the firewall, either
incoming or outgoing.
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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 which these messages will be carrying out depends
on the mechanism being used to configure IPv6 addresses on the link
(Stateless, Stateful or Static configuration).
4.8 Router Solicitation and Router Advertisement Messages
ICMPv6 Router Solicitation and Router Advertisement(Type 133 and 134)
messages are essential to establishment 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 interfaces onto their connected links.
o Firewalls must accept and generate ICMPv6 Router Solicitation and
Router Advertisement messages for local delivery on all interfaces
as described in [RFC2461] and [RFC2462].
o Firewalls must not allow any Router Solicitation or Router
Advertisement messages to pass through the firewall, either
incoming or outgoing.
4.9 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.
o Firewalls should generally implement a policy of not generating or
accepting ICMPv6 Redirect messages for local delivery on any
interface.
o Firewalls must not allow ICMPv6 Redirect messages to pass through
the firewall, either incoming or outgoing.
4.10 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 which wish to
join or leave specific multicast groups.
o Firewalls must be able to generate ICMPv6 Listener Report messages
on their interfaces for Solicited Node Multicast Groups as part of
the address configuration process. They may also have to generate
ICMPv6 Listener Report version 2 messages and Listener Done
messages for the same reason [Author's Note: The current versions
of the standards are not specific about this.].
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o Firewalls which are also acting as multicast routers need to be
able to receive both types of ICMPv6 Listener Report and Listener
Done messages, and generate ICMPv6 Listener Query messages on
relevant interfaces.
o Firewalls must not allow any of the MLD messages to pass throigh
the firewall, wither incoming or outgoing.
4.11 Router Renumbering Messages
ICMPv6 Router Renumbering (Type 138) command messages can be received
and results messages sent by routers to change the prefixes which
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
be confined to sites for this reason.
o Firewalls which are acting as routers on sites which implement
router renumbering functionality must accept for local delivery
and generate appropriate ICMPv6 Router Renumbering messages on
inward facing interfaces. Such messages must be protected by
IPsec authentication as described in [RFC2894].
o Firewalls must not accept ICMPv6 Router Renumbering messages on
outward facing interfaces.
o Firewalls must not allow any Router Renumbering messages to pass
through the firewall, either incoming or outgoing.
4.12 Node Information Query and Reply
ICMPv6 Node Information Query and Reply (Type 139 and 140) messages
are sent end-to-end between unicast 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.
o Firewall policy may allow ICMPv6 Node Information Query messages
to be accepted for local delivery on inward facing interfaces and
allow generation of corresponding Node Information reply messages
on these interfaces.
o Firewalls must not accept or generate ICMPv6 Node Information
messages on outward facing interfaces.
o Firewalls must not allow any ICMPv6 Node Information messages to
pass through the firewall, either incoming or outgoing.
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4.13 Mobile IPv6 Messages
Mobile IPv6 [RFC3775] defines four ICMPv6 messages which 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. The two 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. Administrators may find it desirable to prevent fixed
nodes which are normally resident on the site from behaving as
mobile nodes by dropping outgoing messages from these nodes.
o If possible, stateful packet inspection should be used to match
incoming and outgoing messages.
4.14 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 which attempt to use these
Type values in a way of which the network administrator (and hence
the firewall) is not aware.
[I-D.ietf-ipngwg-icmp-v3] defines a number of experimental Type
values for ICMPv6 Error and Informational messages, which could be
used in site specific ways. These values should be treated in the
same way as values which are not registered by IANA unless the
network administrator is explicitly made aware of usage.
Any ICMPv6 Informational messages of which the firewall is not aware
should not be allowed to pass through the firewall or be accepted for
local delivery on any of its interfaces.
Any incoming ICMPv6 Error messages of which the firewall is not aware
may be allowed through the firewall in line with the specification in
[RFC2463], which requests delivery of unknown error messages to
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higher layer protocol processes. However, administrators may wish to
disallow forwarding of these incoming messages as a potential
security risk. Unknown outgoing Error messages must be dropped as
the administrator should be aware of all messages that could be
generated on the site.
4.15 Problems Resulting from ICMPv6 Transparency
Because some ICMPv6 error packets need to be passed through a
firewall in both directions. This means that the ICMPv6 error
packets can be exchanged between inside and outside without any
filtering.
Using this feature, malicious users can communicate between the
inside and outside of a firewall bypassing the administrator's
inspection (proxy, firewall etc.). For example in 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 stateful packet inspection mechanism to ensure
that the packet carried as a payload is associated with legitimate
traffic to or from the protected network.
5. IANA Considerations
There are no IANA considerations defined in this document.
6. 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. Some analysis
of the classification of ICMPv6 messages and the term 'any-to-end'
were used by Jari Arkko in a draft relating to ICMPv6 and IKE.
7. 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.
8. References
8.1 Normative References
[I-D.ietf-ipngwg-icmp-name-lookups]
Crawford, M. and B. Haberman, "IPv6 Node Information
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Queries", draft-ietf-ipngwg-icmp-name-lookups-11 (work in
progress), May 2005.
[I-D.ietf-ipngwg-icmp-v3]
Conta, A., "Internet Control Message Protocol (ICMPv6)for
the Internet Protocol Version 6 (IPv6) Specification",
draft-ietf-ipngwg-icmp-v3-06 (work in progress),
November 2004.
[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.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[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.
[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.
8.2 Informative References
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
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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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Davies & Mohacsi Expires January 12, 2006 [Page 20]
