Internet Engineering Task Force M. Smith
Internet-Draft October 19, 2015
Intended status: Informational
Expires: April 21, 2016
How to use IPv6 Link-Local Addresses in Applications
draft-smith-ipv6-link-locals-apps-00
Abstract
IPv6 Link-Local addresses can be used by applications. Doing so when
possible will provide robustness and security benefits to the
application. This memo describes the properties of Link-Local
addresses and the benefits and limitations of using them in
applications. It then describes how to use them in applications that
use the Sockets API.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on April 21, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Link-Local Prefix and Address Attributes . . . . . . . . . . 2
3. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Using Link-Locals with the Sockets API . . . . . . . . . . . 5
5.1. sin6_scope_id . . . . . . . . . . . . . . . . . . . . . . 5
5.2. getaddrinfo() . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. Change Log [RFC Editor please remove] . . . . . . . . . . . . 7
9. Informative References . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
IPv6 Link-Local addresses can be used by applications. Doing so when
possible will provide robustness and security benefits to the
application. This memo describes the properties of Link-Local
addresses and the benefits and limitations of using them in
applications. It then describes how to use them in applications that
use the Sockets API.
2. Link-Local Prefix and Address Attributes
o The Link-Local unicast prefix is fe80::/10 [RFC4291].
o All IPv6 interfaces are required to have at least one Link-Local
unicast address [RFC4291].
o Link-Local addresses have link-local scope, uniquely identifying
interfaces only within a single link [RFC4007].
o Across multiple links attached to the same node there may be
duplicate Link-Local addresses. These duplicate Link-Local
addresses are disambiguated using scope zone information, or more
simply, zone information, which describes on which specific link
the chosen Link-Local address exists [RFC4007].
o Link-Local addresses have infinite preferred and valid lifetimes,
meaning they never expire or age out [RFC4862].
o A specific Link-Local address is constantly present on an
interface (due to its infinite preferred and valid lifetime)
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unless actively replaced by an event such as the interface being
reinitialised or the interace being attached to a different link
(e.g., attached to a different wireless network access point)
[RFC4862].
o The Link-Local prefix is therefore present on all IPv6 enabled
links [RFC4291], regardless of the presence of a router and router
advertisements.
o By default, a host treats the Link-Local prefix as on-link
[RFC5942].
o Hosts use IPv6 Stateless Address Autoconfiguration (SLAAC)
[RFC4862] to form a Link-Local address when an interface is
enabled [RFC6434].
o Routers must be able to use IPv6 SLAAC [RFC4862] to form a Link-
Local address when an interface is enabled [RFC6434].
o A Link-Local unicast address is formed by combining fe80::/10 with
a link-specific format interface identifier (IID), positioned in
the lower part of the IPv6 address [RFC4862].
o As most IIDs are 64 bits in length [RFC4291][RFC7421], the Link-
Local prefix on a link is most commonly fe80::/64.
o Link-Local packets can be forwarded by routers, but only back onto
the same link they came from [RFC4007][RFC4291].
o By default, during default address selection, Link-Local addresses
are preferred as IPv6 source and/or destination addresses over all
other address types, with exception to the loopback address
[RFC6724].
o The Link-Local prefix is not advertised in IPv6 Router
Advertisement Prefix Information Options [RFC4861].
o
3. Benefits
There are a number of benefits to using Link-Local addresses in
applications.
o As all interfaces are required to have Link-Local addresses and
therefore the Link-Local prefix is always present on a link,
application connectivity is independent of the presence of a
router. This means that connectivity can be established without
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the presence of a router, or will not be impacted by the failure
of a router, unless the router is part of the forwarding path
between the Link-Local address application end-points.
o As Link-Local addresses have infinite preferred and valid
lifetimes, and presist on an interface unless actively replaced,
including across transient link failures, application connections
will persist for as long as the application needs them to unless a
period of packet loss occurs which the application's transport
protocol cannot tolerate. This is in contrast to the use of
Unique Local Addresses (ULA) [RFC4193] or Globally Unique
Addresses (GUA) [RFC4291] for application end-points, whose
lifetimes may expire if not refreshed, causing the application's
connection end-points to become invalid and therefore unavailable,
despite the link continuing to provide connectivity between the
involved hosts.
o Assuming an application only uses Link-Local addresses, due to
Link-Local traffic being limited to a single link, the potential
set of application attackers is limited to the set of nodes
attached to the same link. This set of link-attached nodes will
likely be significanlty smaller than sets of nodes with either ULA
or GUA addresses that could reach the application from off-link
locations. The likely close geographic proximity of nodes
attached to the same link would also provide a disincentive to
attack applications that are only using Link-Local addresses.
4. Limitations
There are also a number of limitations to using Link-Local addresses
in applications.
o Link-Local addresses cannot be used for application connections
that span multiple links.
o On some types of links, such as "Private VLANs" [RFC5517], a host
may not be able to reach another host's Link-Local address. This
is due to the hub-and-spoke forwarding constraints imposed by the
link on some nodes, and the default on-link assumption for the
Link-Local prefix [RFC5942]. An application could try to overcome
this limitation by attempting to connect to the target host's ULA
or GUA addresses if known, as full reachability might be available
for these address types over these types of networks.
[I-D.smith-6man-link-locals-off-link] proposes a method which
would restore Link-Local address reachability over these types of
links, while retaining hub-and-spoke forwarding constraints at the
link-layer.
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o A Link-Local address is ambiguous, as by itself it does not
inherently identify the link it is a member of, unlike ULA or GUA
addresses. Consequently, applications have to be prepared to
handle Link-Local zone information, which specifies on which link
a Link-Local address is present. This may include providing user
interface elements to allow an application user to specify per
Link-Local address zone information.
o When providing a Link-Local address to an application peer for use
as a referral, it is best to ensure that the application peer is a
member of the same link. This should then ensure that the
application peer receives both the Link-Local address and correct
zone information, implicit from the ingress interface the
application peer received the referral over. For example,
Multicast DNS achieves this by using link-local scope multicast
destination addresses [RFC6762]. A Link-Local address should not
be supplied to an off-link application peer for use as a referral
unless the application knows the peer will have the correct zone
information for the address, or can cope successfully with Link-
Local address connection failure or securely with successful
connection to an incorrect node (e.g., via some form of secure
node or application level authentication).
5. Using Link-Locals with the Sockets API
5.1. sin6_scope_id
The 'sockaddr_in6' structure is used by the Sockets API [RFC3493] to
hold IPv6 socket address data.
Within the 'sockaddr_in6' structure, the unsigned 32 bit
'sin6_scope_id' integer field identifies the scope or zone for the
IPv6 address carried in the 'sin6_addr' sub-structure.
In the case of Link-Local addresses, the 'sin6_scope_id' field
identifies the interface where the Link-Local address is located.
In the Sockets API, interfaces are identfied using an unsigned
integer, known as an 'interface index'. The if_nametoindex() and
if_indextoname() functions [RFC3493] are used to convert from an
interface name to an interface index and vice-versa.
When using Link-Local addresses in a 'sockaddr_in6' structure, the
'sin6_scope_id' field is set to or carries the interface index for
the Link-Local address.
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5.2. getaddrinfo()
When a node and service are successfully looked up using the
getaddrinfo() call [RFC3493], a set of one or more 'addrinfo'
structures is returned. Two of the fields within the returned
'addrinfo' structure(s) are the 'ai_addr' pointer, pointing to a
'sockaddr' structure, and the corresponding 'ai_addrlen' field,
describing the size of the 'sockaddr' structure. These two fields
are used to describe the 'sockaddr' structure holding one of the
Internet addresses that corresponds to the node and service looked up
using the getaddrinfo() call.
In the case of an IPv6 Internet address, the 'ai_addr' pointer will
point to a 'sockaddr_in6' structure, which will include the
'sin6_scope_id' field, described previously.
Although traditional unicast DNS AAAA resource records are not
prohibited from holding Link-Local addresses [RFC3596], AAAA resource
records can only hold a 128 bit IPv6 address; the IPv6 scope
information for the address is not stored. In this case, a AAAA
resource record holding a Link-Local address is going to be ambiguous
by itself; the 'sin6_scope_id' field value will likely be zero,
indicating the "default" zone [RFC4007]. It will be necessary to
supply scope information separately from getaddrinfo() to
successfully use a Link-Local address stored in a unicast DNS AAAA
resource record. Note that [RFC4472] recommends against storing
Link-Local addresses in unicast DNS AAAA resource records.
However, multicast DNS [RFC6762] queries for hosts within the
".local" top-level domain can implicitly provide the scope for a
Link-Local address received in an AAAA resource record, as multicast
DNS queries for the ".local" top-level domain uses a Link-Local scope
multicast address (ff02::fb). This means that both the multicast DNS
client and responder must be attached to the same link. As a
multicast DNS responder is required to advertise all of its valid
addresses, including its Link-Local address(es), a Link-Local address
received in a multicast DNS response will be present on the link over
which the response was received. Consequently, a multicast DNS
client module operating behind the getaddrinfo() call can supply an
unambiguous Link-Local address scope value in the 'sin6_scope_id'
field of a 'sockaddr_in6' structure. Other Sockets API calls that
use the 'sockaddr_in6' structure should then work successfully with
Link-Local addresses that have been supplied via multicast DNS.
Note that if getaddrinfo() returns multiple 'addrinfo' structures,
they will be sorted as destination addresses in accordance with
[RFC6724]. By default, Link-Local 'addrinfo' structures will appear
before all other IPv6 address types, with exception to the IPv6
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loopback address (::1). As applications are normally advised to step
through the list in order, making successive connection attempts
until one is successful, the earlier appearance of Link-Local
addresses should ensure the attempted use of Link-Local addresses for
application connections occurs before connections to larger scope ULA
and GUA addresses are attempted. The "Happy Eyeballs" technique
[RFC6555] will instead cause IPv4 to be quickly used if available, if
a connection to a Link-Local address fails.
6. Security Considerations
Using Link-Local addresses, when possible, increases application
security by limiting the set of possible attackers to those located
on the same link as a possible victim. In many cases, these possible
same link attackers will be located geographically close to a
possible victim, which may further discourage attempted attacks.
7. Acknowledgements
Review and comments were provided by YOUR NAME HERE!
This memo was prepared using the xml2rfc tool.
8. Change Log [RFC Editor please remove]
draft-smith-ipv6-link-locals-apps-00, initial version, 2015-10-19
9. Informative References
[I-D.smith-6man-link-locals-off-link]
Smith, M., "Indicating Link-Local Unicast Destinations are
Off-Link", draft-smith-6man-link-locals-off-link-00 (work
in progress), August 2015.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, DOI 10.17487/RFC3493, February 2003,
<http://www.rfc-editor.org/info/rfc3493>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>.
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
DOI 10.17487/RFC4007, March 2005,
<http://www.rfc-editor.org/info/rfc4007>.
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[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<http://www.rfc-editor.org/info/rfc4193>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472,
DOI 10.17487/RFC4472, April 2006,
<http://www.rfc-editor.org/info/rfc4472>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
[RFC5517] HomChaudhuri, S. and M. Foschiano, "Cisco Systems' Private
VLANs: Scalable Security in a Multi-Client Environment",
RFC 5517, DOI 10.17487/RFC5517, February 2010,
<http://www.rfc-editor.org/info/rfc5517>.
[RFC5942] Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
Model: The Relationship between Links and Subnet
Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
<http://www.rfc-editor.org/info/rfc5942>.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, DOI 10.17487/RFC6434, December
2011, <http://www.rfc-editor.org/info/rfc6434>.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
2012, <http://www.rfc-editor.org/info/rfc6555>.
[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,
<http://www.rfc-editor.org/info/rfc6724>.
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[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<http://www.rfc-editor.org/info/rfc7421>.
Author's Address
Mark Smith
PO BOX 521
HEIDELBERG, VIC 3084
AU
Email: markzzzsmith@gmail.com
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