Internet DRAFT - draft-mudric-6man-lcs
draft-mudric-6man-lcs
6MAN Working Group D. Mudric
Internet-Draft Ciena
Updates: RFC5014, RFC6724 (if approved) A. Petrescu
Intended status: Standards Track CEA, LIST
Expires: May 18, 2021 November 14, 2020
Least-Common Scope Communications
draft-mudric-6man-lcs-02
Abstract
This draft formulates a security problem statement. The problem
arises when a Host uses its Global Unicast Address (GUA) to
communicate with another Host situated on the same link.
To address this problem, we suggest to select and use addresses of a
least scope that are common.
Status of This Memo
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
3. Least Common Scope Communications . . . . . . . . . . . . . . 3
4. LL Address Resolution . . . . . . . . . . . . . . . . . . . . 3
5. Sending algorithm with LL Address . . . . . . . . . . . . . . 7
6. Other Issues with LL Address Resolution . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . 9
Appendix A. ChangeLog . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Problem Statement
Sockets listening on a global addresses are exposed to attacks.
RFC6724 Rule 8 selects a candidate address with the smallest scope.
Applications don't always have LL candidate address. They usually
have a GUA address. If GUA is on a local link, an application will
open a socket using GUA. To avoid using GUA on the local link, a
sender needs to find a destination LL address. Currently SASA
algorithm (RFC 6724 "Default Address Selection for Internet Protocol
Version 6 (IPv6)") cannot use the smallest common scope, given
destination GUA.
For security reasons, hosts should use an address with the smallest
scope. To avoid these attacks, the host should use LL or ULA
addresses.
These security reasons, in more detail, are described next. There is
a security problem when a Host uses (one of) its Global Unicast
Address(es) (GUA) to communicate to another Host situated on the same
link. The problem appears even if that second Host uses its link-
local address (LL) for this communication.
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The problem is that the Host that uses the GUA to actively
communicate with another Host situated on the same link opens a
globally reachable entry point in its operating system kernel. This
entry point appears when the GUA is assigned to a socket structure.
Were that address an LL, and not a GUA, that entry would not be
globally reachable.
To realize communications between Hosts on the same link, it is
sufficient to rather use LL addresses on both Hosts.
When a Host uses a GUA to communicate to another Host situated on the
same link, it unnecessarily becomes an easy attack target. The
attacker might be situated anywhere in the Internet (globally).
3. Least Common Scope Communications
It is recommended that a Host that needs to communicate with another
Host that is situated in a particular scope, to use addresses of same
scope, or of the least common scope.
For example, two Hosts situated on the same link should ideally use
LL addresses to communicate to each other. An interpretation
suggests that, given GUA and ULA, a least common 'scope' is the ULA
scope (even though, formally, both ULA and GUA are of same global
scope). But the global unicast addresses (GUAs) should not be used
for two Hosts on the same link: the global scope is unnecessarily
large; it unnecessarily opens doors to attacks.
4. LL Address Resolution
The operation of resolving an LL address (LL address resolution) is
to find the link-local address that is assigned to the same interface
as a GUA (or an ULA). This operation can be realized in several
manners.
In one manner, the pair [GUA or ULA address; LL address] is stored in
a distributed file such as the Active Directory or the DNS. The
resolution operation is to query that file to find the LL address
that corresponds to a GUA or ULA address. There are some issues to
be considered. For example, typically the LL address is not assigned
neither by DHCPv6 nor by RA (it is self formed by a Host when the
interface is put up by using a universally known prefix "fe80::/10")
then how would DNS get that LL address? Another example is: how to
query DNS to request the LL address corresponding to an AAAA entry?
(it is known how to query DNS to obtain the AAAA of an FQDN, but not
the LL of an AAAA).
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In another manner, the operation of resolving a link-local address
(LL address resolution) is performed within the context of selecting
source and destination addresses within a Host. In that context, the
following steps occur:
1. Given multiple destination addresses, the DASA selects GUA and
ULA destination. The term 'DASA' designates the Destination
Address Selection Algorithm.
2. The LL address resolution operation is performed for these GUA
and ULA.
3. The GUA and the LL addresses are given as input to the SASA.
The term 'SASA' stands for Source Address Selection Algorithm.
The SASA selects LL.
To facilitate LL communication on the local link, given a destination
GUA or ULA:
o Prior to SASA, a host needs to check if a destination is ON-LINK
o for ON-LINK destination, a host needs to resolve the GUA or ULA
destination address into a destination host LL address,
o a socket needs to open a port for the source LL address, and
o send packets to the destination LL address.
If both GUA and ULA destinations are known, and ULA destination is
not on the link, SASA SHOULD use ULA address.
For the purposes of this document, Link Local (LL) address resolution
is the process through which a host determines the Link Local address
of a neighbor which is on the local subnet, given only neighbor's GUA
or ULA IPv6 address (this 'address resolution' term is different than
typical 'ND' term, or than the RFC4861 'address resolution' term
which resolves an IP address into a MAC address). LL address
resolution is performed only on addresses that are determined to be
on-link and for which the sender does not know the corresponding Link
Local address. Once the target LL address is learned, the
communication sockets use LL addresses and are not exposed to
security attacks.
For LL address resolution, 'L' flag is added to NS message. The
Target-Address, TA, field in the NS message contains the address of
the target of the solicitation (e.g., a host GUA or ULA address).
The 'L' flag is added to Neighbor Solicitation Message, for LL
address request
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
If L bit is set, either LL address assigned to the interface from
which this message is sent or (if Duplicate Address
Detection is in progress
[ADDRCONF rfc4861]) the
unspecified address.
Destination Address
Either the solicited-node multicast address
corresponding to the target GUA or ULA address, or the target
GUA or ULA address.
ICMP Fields:
L Link Local flag. When set, the L-bit indicates that
the sender is requesting Link Local address from the target.
Figure 1: NS with 'L' bit
After receiving the Neighbor Solicitation message, the target returns
its Link Local address in the Target Link-Local Address Option in a
unicast Neighbor Advertisement, NA, message.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|S|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
If NS L bit is set, LL address of the same GUA target
interface is provided
Possible options:
Target Link-Local address
The Link Local address of the same GUA target, the sender of NA.
This option MUST be included if NS L bit is set and LL
is available.
Type 4 (Target Link Local address)
Length 16 bytes
Link Local Address: e.g. fe80:0:0:0:aa:bb:cc:dd
Receivers MUST silently ignore this option if they do not recognize it
and continue processing the message.
Figure 2: NA for LL address resolution
The request for comments number 5014 [RFC5014], which treats about
socket APIs, needs to be updated to use the given destination GUA or
ULA addresses for ON-LINK determination, prior to SASA address
selection; it also needs to be be updated to specify to send packets
using LL address while talking to ON-LINK destinations.
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5. Sending algorithm with LL Address
A sender application can choose to use LL for on-link communication.
That request can be passed via a socket API to ND. ND should set NS
'L' bit to indicate the LL address resolution is required and use of
LL for the on-link communication, if a destination host returns it.
If a destination host is listening on GUA only for a particular
application, and this algorithm is supported, the host should disable
LL address resolution by not returning LL address in NA. By default,
the LL address resolution should be disabled. Otherwise, a sender
would send a packet to destination LL address and there is no socket
listening on that address. LL address resolution should be enabled
when all socket APIs are ready to support LL sockets (open one socket
for GUA and one for LL and after LL address resolution, NA with LL is
returned, close GUA socket) or all sockets are bound to ANY address.
The process starts with an application requesting a socket to send a
packet to GUA destination. First step is a destination address
selection and the sequence goes to the LL address resolution, step 4:
1st: A sending application should have an option to request LL vs.
GUA communication, when opening a socket to GUA destination, that
might be on a local link. Socket API should have this option and
use it to initiate LL address resolution.
2nd: Host should choose destination address, if multiple GUA and
ULA are provided
3rd: Host should choose a source address, for the selected
destination address
4th: Host should choose a next hop, and outgoing interface, based
on the source address prefix
5th: If a destination is on-link, the host should resolve
destination GUA into destination LL. Step 5 is further broken
down into:
5.1st: Sender creates a neighbour cache entry for GUA.
5.2nd: Sender sends NS, with L bit set, to GUA.
5.3rd: Sender receives NA with link-layer and LL addresses
5.4th: Sender updates GUA cache entry with the link-layer
address
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5.5th: Sender creates a neighbour cache entry for destination
LL address and sets the destination link-layer address of the
destination host
6th: Sender transmits a packet to link-layer address of the
destination host, using destination host LL address as IPv6
packet destination address
7th: Application sending to GUA should obtain the SASA address
(which is now LL address) for the further negotiations (e.g.
SIP needs to negotiate media addresses by sending re-INVITE).
8th: Sender closes the socket listening on GUA and opens a
socket listening on LL.
6. Other Issues with LL Address Resolution
If the Host 'switches' the destination address of an ongoing flow,
between the GUA and the LL, there might interruptions in
communications. The 'switching' behaviour depends on the
application. Some applications (e.g. a particular application using
the SIP protocol) the destination address is selected prior to
opening the socket dedicated to streaming the media data. In such an
application, a hard outage (e.g. interface down), might involve the
creation of a new socket, and thus interruptions in media streaming.
The question of maintaining an ongoing communication upon 'switching'
between a GUA and an LL destination address is valid, for certain
applications.
Multiple DNS aspects, for the resolution operation. Which LL address
corresponds to a GUA?. How would DNS get that LL address?
7. Security Considerations
Security
8. IANA Considerations
IANA
9. Contributors
Contributors.
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10. Acknowledgements
Mark Smith, Eduard Vasilenko.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
DOI 10.17487/RFC5014, September 2007,
<https://www.rfc-editor.org/info/rfc5014>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
Appendix A. ChangeLog
The changes are listed in reverse chronological order, most recent
changes appearing at the top of the list.
-00: initial version, with Dusan's comments.
Authors' Addresses
Dusan Mudric
Ciena
,
Canada
Phone:
+1-613-670-2425
Email:
dmudric@ciena.com
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Alexandre Petrescu
CEA, LIST
CEA Saclay
Gif-sur-Yvette
,
Ile-de-France
91190
France
Phone:
+33169089223
Email:
Alexandre.Petrescu@cea.fr
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