IPv6 Operations E. Davies
Internet-Draft Consultant
Expires: April 17, 2006 J. Mohacsi
NIIF/HUNGARNET
October 14, 2005
Best Current Practice for Filtering ICMPv6 Messages in Firewalls
draft-ietf-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. 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.
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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 . . . . . . . . . . . . . . . . . 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 Communication . . . . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
3.1. Denial of Service Attacks . . . . . . . . . . . . . . . . 8
3.2. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Redirection Attacks . . . . . . . . . . . . . . . . . . . 9
3.4. Renumbering Attacks . . . . . . . . . . . . . . . . . . . 9
3.5. Problems Resulting from ICMPv6 Transparency . . . . . . . 9
4. Filtering Recommendations . . . . . . . . . . . . . . . . . . 10
4.1. Common Considerations . . . . . . . . . . . . . . . . . . 10
4.2. Recommendations for ICMPv6 Transit Traffic . . . . . . . . 12
4.2.1. Traffic that Must NOT be Dropped . . . . . . . . . . . 12
4.2.2. Traffic that Normally Should Not be Dropped . . . . . 12
4.2.3. Traffic that May be Dropped but will be Caught
Anyway . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.4. Traffic for which a Dropping Policy Should be
Defined . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.5. Traffic which Should be Dropped Unless a Good Case
can be Made . . . . . . . . . . . . . . . . . . . . . 14
4.3. Recommendationd for ICMPv6 Local Configuration Traffic . . 15
4.3.1. Traffic that Must NOT be Dropped . . . . . . . . . . . 15
4.3.2. Traffic that Normally Should Not be Dropped . . . . . 16
4.3.3. Traffic that May be Dropped but will be Caught
Anyway . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3.4. Traffic for which a Dropping Policy Should be
Defined . . . . . . . . . . . . . . . . . . . . . . . 16
4.3.5. Traffic which Should be Dropped Unless a Good Case
can be Made . . . . . . . . . . . . . . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Normative References . . . . . . . . . . . . . . . . . . . 18
7.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Notes on Individual ICMPv6 Messages . . . . . . . . . 19
A.1. Destination Unreachable Error Message . . . . . . . . . . 19
A.2. Packet Too Big Error Message . . . . . . . . . . . . . . . 20
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A.3. Time Exceeded Error Message . . . . . . . . . . . . . . . 20
A.4. Parameter Problem Error Message . . . . . . . . . . . . . 21
A.5. ICMPv6 Echo Request and Echo Response . . . . . . . . . . 22
A.6. Neighbor Solicitation and Neighbor Advertisement
Messages . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.7. Router Solicitation and Router Advertisement Messages . . 22
A.8. Redirect Messages . . . . . . . . . . . . . . . . . . . . 22
A.9. SEND Certificate Path Messages . . . . . . . . . . . . . . 23
A.10. Multicast Listener Discovery Messages . . . . . . . . . . 23
A.11. Multicast Router Discovery Messages . . . . . . . . . . . 23
A.12. Router Renumbering Messages . . . . . . . . . . . . . . . 23
A.13. Node Information Query and Reply . . . . . . . . . . . . . 24
A.14. Mobile IPv6 Messages . . . . . . . . . . . . . . . . . . . 24
A.15. Unused and Experimental Messages . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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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, each with a number of
sub-types 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)) . 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 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 trust chain between a trust anchor and the router
certificate from the router.
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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
address (where a link supports multiple subnets). 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 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].
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 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 [I-D.ietf-magma-mrdisc].
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 An experimental extension to ICMPv6 specifies the ICMP Node
Information Query and Response messages which can be used to
retrieve some basic information about nodes [I-D.ietf-ipngwg-icmp-
name-lookups].
o The SEAmless IP MOBility (seamoby) working group specified a pair
of experimental protocols which 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.
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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 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].
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.
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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. Generally a firewall will also by
acting as a router so that all the messages that might be used in
configuring a router interface need to be accepted and generated.
This type of communication establishment messages should not be
passed through a firewall as they are normally intended for use
within a link.
On the other hand, most ICMPv6 error messages travelling end-to-end
or any-to-end are essential to the establishment of communications.
These messages must be passed through firewalls and might also be
sent to and from firewalls to assist with establishment of
communications. For example the Packet Too Big error message is
needed to establish the MTU along a path.
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 in
most cases 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.
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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 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 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.
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
which firewalls can apply and its role in security is therefore not
discussed further in this document.
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 numbers of ICMPv6 packets to
destinations in the site and sending error messages which disrupt
established communications by causing sessions to be dropped. Also
if spurious communication establishment messages can be passed on to
link it might be possible to invalidate legitimate addresses or
disable interfaces.
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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. 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 [I-D.chown-v6ops-port-scanning-implications].
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 a 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.
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. Renumbering
messages are required to be authenticated with IPsec so that it is
difficult to carry out such attacks in practice.
3.5. 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
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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
o Rules for ICMPv6 directed to interfaces on the firewall
This section suggests some common considerations which 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
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 but it is not essential because they
would normally be dropped for other reasons (e.g., because they
would be using link-local addresses) or the protocol specification
would cause them to be rejected if they had passed through a
router.
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 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.
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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.
Many of the messages used for establishment of communications on the
local link will be sent with link-local addresses for at least one of
their source and destination. Routers (and firewalls) conforming to
the IPv6 standards will not forward these packets; there is no need
to configure additional rules to prevent these packets traversing the
firewall/router. Also the specifications of ICMPv6 messages intended
for use only on the local link specify various measures which would
allow receivers to detect if the message had passed through a
firewall/router, including:
o Requiring that the hop count in the IPv6 header is set to 255 on
transmission. On reception the hop count is required to be still
255 which would not be the case if the packet had passed through a
firewall/router.
o Checking that the source address is a link-local unicast address.
Accordingly it is not essential to configure firewall rules to drop
illegal packets of these types. If they have non-link-local source
and destination addresses, allowing them to traverse the firewall,
they would be rejected because of the checks performed at the
destination. However, firewall administrators may still wish to log
or drop such illegal packets.
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.
Firewalls which are able to perform deep packet inspection may be
able to check the header fields in the start of the errored packet
which is carried by ICMPv6 error messages. If the embedded packet
has a source address which does not match the destination of the
error message the packet can be dropped. This provides a partial
defence against some possible attacks on TCP that use spoofed ICMPv6
error messages, but the checks can also be carried out at the
destination.
In general, the scopes of source and destination addresses of ICMPv6
messages should be matched, and packets with mismatched addresses
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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 need to
avoid over-zealous filtering of these messages delivered locally on a
link.
4.2. Recommendations for ICMPv6 Transit Traffic
This section recommends rules that should be applied to ICMPv6
traffic attempting to transit a firewall.
4.2.1. Traffic that Must NOT be Dropped
Error messages that are essential to the establishment 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 [I-D.huitema-v6ops-teredo] 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.
4.2.2. Traffic that Normally Should Not be Dropped
Error messages other than those listed in Section 4.2.1
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
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nodes which would normally be at home on the site and/or foreign
mobile nodes roaming onto the site.
4.2.3. Traffic that May be Dropped but will be Caught Anyway
The messages listed in this section are all involved with local
management of nodes connected to the link on which they were
initially transmitted. All these messages should never be propagated
beyond the link on which they were initially transmitted. 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. 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 firewall/router or it
will be dropped at the first router it encounters. Administrators
may wish to consider providing rules to catch illegal packets sent
with Hop Count = 1 to avoid ICMPv6 Time Exceeded messages being
generated for these packets.
Address Configuration and Router Selection messages (must be received
with Hop Count = 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)
SEND Certificate Path notification messages (must be received with
Hop Count = 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 Count = 1):
o Multicast Router Advertisement (Type 151)
o Multicast Router Solicitation (Type 152)
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o Multicast Router Termination (Type 153)
4.2.4. Traffic for which a Dropping Policy Should be Defined
The message which the experimental Seamoby protocols are using will
be expected to have to cross site boundaries. 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 and 102-126, inclusive)
The base ICMPv6 specification suggests that error messages which 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
through the firewall.
4.2.5. Traffic which 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:
o Router Renumbering (Type 139)
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:
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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.
4.3. Recommendationd 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.
4.3.1. Traffic that Must NOT be Dropped
Error messages that are essential to the establishment 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 4.2.1, 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 which 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 4.3.4).
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)
Link-local multicast receiver notification messages:
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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)
4.3.2. 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
4.3.3. Traffic that May be Dropped but will be Caught Anyway
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:
o Router Renumbering (Type 139)
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.
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)
4.3.4. Traffic for which a Dropping Policy Should be Defined
Redirect messages provide a significant security risk and
administrators should take a case-by-case view of whether firewalls,
routers in general and other nodes should accept these messages:
o Redirect (Type 137)
Conformant nodes must provide configuration controls which allow
nodes to control their behavior with respect to redirection messages
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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 ignored and no filtering is
necessary.
Error messages not currently defined by IANA:
o Unallocated Error messages (Types 5-99 and 102-126, inclusive)
The base ICMPv6 specification suggests that error messages which 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.
4.3.5. Traffic which 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.
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
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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 draft relating to ICMPv6 and IKE.
7. References
7.1. Normative References
[I-D.ietf-ipngwg-icmp-name-lookups]
Crawford, M. and B. Haberman, "IPv6 Node Information
Queries", draft-ietf-ipngwg-icmp-name-lookups-12 (work in
progress), July 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-v3-07 (work in progress),
July 2005.
[I-D.ietf-magma-mrdisc]
Haberman, B. and J. Martin, "Multicast Router Discovery",
draft-ietf-magma-mrdisc-07 (work in progress), May 2005.
[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.
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[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.
7.2. Informative References
[I-D.chown-v6ops-port-scanning-implications]
Chown, T., "IPv6 Implications for TCP/UDP Port Scanning",
draft-chown-v6ops-port-scanning-implications-01 (work in
progress), July 2004.
[I-D.huitema-v6ops-teredo]
Huitema, C., "Teredo: Tunneling IPv6 over UDP through
NATs", draft-huitema-v6ops-teredo-05 (work in progress),
April 2005.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
Appendix A. Notes on Individual ICMPv6 Messages
A.1. 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 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
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part of the process of establishing 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, reject
route to destination.
A.2. 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,
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.
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.
A.3. 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 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
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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.
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 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
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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 [RFC2463]. 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[I-D.huitema-v6ops-teredo]: Teredo
tuneling 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 of communications on
the local link. Firewalls need to generate and accept these messages
to allow them to establish interfaces onto their connected links.
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).
A.7. Router Solicitation and Router Advertisement Messages
ICMPv6 Router Solicitation and Router Advertisement(Type 133 and 134)
messages are essential to the 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.
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
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communications and may be a security vulnerability, particularly if a
link is not physically secured. Conformant nodes are required to
provide configuration controls which suppress the generation of
redirection messages and allow them to be ignored on reception.
Using Redirect messages on a wireless link is particularly hazardous
because of the lack of physical security.
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 which
will allow the node to authenticate the router's claim to provide
routing services for certain prefixes. If a link conected 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 which wish to
join or leave specific multicast groups. Firewalls need to be able
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 [I-D.ietf-magma-mrdisc] (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 which 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 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.
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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.
A.13. Node Information Query and Reply
ICMPv6 Node Information Query and Reply (Type 139 and 140) messages
are sent end-to-end between unicast addresses, and 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 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.
o Administrators may find it desirable to prevent static nodes which
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 which attempt to use these
Type values in a way of which the network administrator (and hence
the firewall) is not aware.
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[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.
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 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
higher layer protocol 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.
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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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