Dear Elwyn and Janos,
I am the assigned Gen-ART reviewer for this draft. For
background on Gen-ART, please see the FAQ
at<http://www.alvestrand.no/ietf/gen/art/gen-art-FAQ.html>. (or ask Elwyn,
who is also a Gen-ART reviewer).
Summary - This document is almost ready for publication as an Informational
RFC.
I did find a small number of questions, tagged as
"Spencer/More-than-Editorial", which appear below, that I'd like you to
consider along with any other Last Call comments you may receive.
There are also editorial notes tagged as "Spencer/Editorial".
Thanks,
Spencer Dawkins
Throughout the document - weasel-wording about "site local" appears in a
number of places. Do we have a sense whether this is still necessary? If
not, would be nice to delete; if still necessary, might be nice to collect
the guidance on site-locals into one place, to ensure that it's consistent.
Throughout the document - there are many recommendations that point out IPv6
resistence to scanning attacks as part of the recommendation. I'm wondering
if some of these could be removed (since there is an explicit section on
resistance to scanning attacks in the document).
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.
Spencer/Editorial: I struggled with the wording of the last two sentences,
just because it's so central to the point of the document. I'd suggest
something like "While a number of security risks are associated with
uncontrolled forwarding of ICMPv6 messages, ICMPv6 is essential to the
functioning of IPv6. This indicates that current filtering strategies
designed for the corresponding protocol ICMP in IPv4 networks should be
revisited, because these strategies are accommodating a useful auxiliary
protocol that may not be required for correct functioning." But this
paragraph doesn't seem as clear as the corresponding text in Section 1.
1. Introduction
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:
Spencer/Editorial: The following text is really dense. Is it possible to
provide any hiearchy/abstraction, even if it's only a list of the 15 or so
functions, without message detail, followed by the full version of the list
appearing below? If it's possible to combine functions (for example,
grouping all of the discovery functions into one category, followed by all
end-to-end signaling functions, followed by MIPv6 ...), that would be
better. A table would be better. Other proposals would be better.
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 [RFC4443].
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 [RFC4443].
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].[[anchor2: [RFC Editor: Please update references to
RFC2461 when the new version of RFC2461 is published.] --Authors]]
o If stateless auto-configuration of hosts is enabled, communicating
prefixes and other configuration information (including the link
MTU and suggested hop limit default) from routers to hosts. Uses
RS and RA [RFC2462]. [[anchor3: [RFC Editor: Please update
references to RFC2462 when the new version of RFC2462 is
published.] --Authors]]
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.
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 [RFC4286].
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.
2.3. Network Topology and Address Scopes
Local communications will use link-local addresses in many cases but
may also use global unicast addresses for example when configuring
Spencer/Editorial: this sentence may be correct as written but isn't easy to
read clearly - suggest "may also use global unicast addresses when
configuring global addresses, for example".
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
Spencer/Editorial: "will also be"
acting as a router so that all the messages that might be used in
configuring a router interface need to be accepted and generated.
This type of communication establishment messages should not be
passed through a firewall as they are normally intended for use
within a link.
3. Security Considerations
Firewalls will normally be concerned to monitor ICMPv6 to control the
Spencer/Editorial: "Firewalls will normally be used to monitor" - my
firewall seems extremely unconcerned most of the time, even when dropping
packets :-)
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
Spencer/Editorial: is this "passed to on-link"? But it seems like odd
English usage. "passed onto a specific link", perhaps?
link it might be possible to invalidate legitimate addresses or
disable interfaces.
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.
Spencer/Editorial: Is this saying, roughly, "Spurious Renumbering messages
can lead to the disruption of a site. Although Renumbering messages are
required to be authenticated with IPsec, so that it is difficult to carry
out such attacks in practice, they should not be allowed through a
firewall."?
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.
Spencer/More-than-Editorial: First "sentence" is a fragment - I'm not sure
that the paragraph is a complete thought, because I don't understand how the
second sentence is related to the first sentence (fragment), so I'm not
proposing an editorial change. (Just because you need something to happen
doesn't mean "there's no problem when it happens, so go ahead and pass the
traffic without filtering.") Someone smart, please provide suggested text
:-)
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 it might be possible
to carry out a covert conversation through the payload of ICMPv6
error messages or tunnel inappropriate encapsulated IP packets in
ICMPv6 error messages. This problem can be alleviated by filtering
ICMPv6 errors using a deep packet inspection mechanism to ensure that
the packet carried as a payload is associated with legitimate traffic
to or from the protected network.
Spencer/More-than-Editorial: Forgive my ignorance here, but are most
firewalls doing stateful deep inspection tracking for ICMP/ICMPv6 traffic
today? If not, this seems to need more discussion about why the additional
state is the right way to go.
4. Filtering Recommendations
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.
Spencer/More-than-Editorial: why "usually"? ("Why would you pass messages
you didn't have to pass?")
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.
Spencer/Editorial: "Messages that may be dropped, although they would
normally be dropped for other reasons anyway", and I think the parenthetical
should inclide both the e.g. and the "or the protocol specification" clause,
if I get the point being made.
4.1. Common Considerations
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 limit in the IPv6 header is set to 255 on
transmission. On reception the hop limit is required to be still
255 which would not be the case if the packet had passed through a
firewall/router.
Spencer/Editorial: second sentence "Receivers verify that the hop limit is
still 255, to ensure that the packet has not 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.
Spencer/Editorial: There really are "illegal packets", but I don't think
that's what you're talking about. "wish to drop adminstratively-prohibited
packets"? This term is used elsewhere in the specification, too.
In general, the scopes of source and destination addresses of ICMPv6
messages should be matched, and packets with mismatched addresses
should be dropped if they attempt to transit a router. However some
of the address configuration messages carried locally on a link may
legitimately have mismatched addresses. Node implementations need to
avoid over-zealous filtering of these messages delivered locally on a
link.
Spencer/More-than-Editorial: I know the specification is targeted as an
Informational RFC, but this recommendation seems awefully hand-wavish even
for Informational ("you should drop them unless doing so is over-zealous") -
is there better guidance than "turn it off and see if the phone rings"?
4.2.3. Traffic that May be Dropped but will be Caught Anyway
Spencer/Editorial: this phrase, which appears in several places in the
document, seemed odd to me every time I read it. Perhaps "Traffic that will
be dropped anyway for other reasons, so no special attention required"? The
text describing this is fine - the thing being described just seems odd.
4.2.4. Traffic for which a Dropping Policy Should be Defined
The message which the experimental Seamoby protocols are using will
Spencer/Editorial: This is actually probably correct and consistent but
still looks like a number disagreement "the message are". Maybe "The message
type used by the experimental Seamoby protocols will ..."?
be expected to have to cross site boundaries. Administrators should
Spencer/Editorial: "will be expected to cross site boundaries in normal
operation."?
determine if they need to support these experiments and otherwise
messages of this type should be dropped:
o Seamoby Experimental (Type 150)
4.3.1. Traffic that Must NOT be Dropped
Spencer/Editorial: "NOT" sure why the "NOT" is emphasized here, and it looks
like 2119 language gone bad :-)
A.2. Packet Too Big Error Message
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.
Spencer/More-than-Editorial: Having had experience with wireless links that
had a smaller MTU than the "guaranteeed minimum" in IPv4, I'm not entirely
comfortable with this guidance ("if you don't care about communicating with
people who violate the spec, drop the packets, and hope there's no GOOD
reason why someone is violating the spec"). At a minimum, there's a balance
between making the network more secure by not passing unnecessary packets
and making the path more fragile - maybe that's guidance that would be
helpful?
A.8. Redirect Messages
ICMPv6 Redirect Messages(Type 137) are used on the local link to
indicate that nodes are actually link-local and communications need
not go via a router, or to indicate a more appropriate first hop
router. Although they can be used to make communications more
efficient, they are not essential to the establishment of
communications and may be a security vulnerability, particularly if a
link is not physically secured. Conformant nodes are required to
provide configuration controls which suppress the generation of
Redirect 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.
Spencer/More-than-Editorial: should be something like "wireless link with no
link-level authentication/encryption is particularly hazardous", I think -
the problem isn't that the link is wireless, the problem is that the link is
open to eavesdropping and packet injection.
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
Spencer/Editorial: "They must traverse site-boundary firewalls in order for
Mobile IPv6 to function"?
protected by the use of IPsec authentication.
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