Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers
draft-ietf-6man-rfc6874bis-01
| Document | Type | Active Internet-Draft (6man WG) | |
|---|---|---|---|
| Authors | Brian E. Carpenter , Stuart Cheshire , Bob Hinden | ||
| Last updated | 2022-04-27 (Latest revision 2022-04-07) | ||
| Replaces | draft-carpenter-6man-rfc6874bis | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
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| Send notices to | (None) |
draft-ietf-6man-rfc6874bis-01
6MAN B. Carpenter
Internet-Draft Univ. of Auckland
Obsoletes: 6874 (if approved) S. Cheshire
Updates: 3986, 3987 (if approved) Apple Inc.
Intended status: Standards Track R. Hinden
Expires: 7 October 2022 Check Point Software
5 April 2022
Representing IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers
draft-ietf-6man-rfc6874bis-01
Abstract
This document describes how the zone identifier of an IPv6 scoped
address, defined as <zone_id> in the IPv6 Scoped Address Architecture
(RFC 4007), can be represented in a literal IPv6 address and in a
Uniform Resource Identifier that includes such a literal address. It
updates the URI Generic Syntax and Internationalized Resource
Identifier specifications (RFC 3986, RFC 3987) accordingly, and
obsoletes RFC 6874.
Discussion Venue
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the 6MAN mailing list
(ipv6@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ipv6/
(https://mailarchive.ietf.org/arch/browse/ipv6/).
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 7 October 2022.
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Copyright Notice
Copyright (c) 2022 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Issues with Implementing RFC 6874 . . . . . . . . . . . . . . 4
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 5
4. URI Parsers . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Options Considered . . . . . . . . . . . . . . . . . 10
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Uniform Resource Identifier (URI) syntax specification [RFC3986]
defined how a literal IPv6 address can be represented in the "host"
part of a URI. Two months later, the IPv6 Scoped Address
Architecture specification [RFC4007] extended the text representation
of limited-scope IPv6 addresses such that a zone identifier may be
concatenated to a literal address, for purposes described in that
specification. Zone identifiers are especially useful in contexts in
which literal addresses are typically used, for example, during fault
diagnosis, when it may be essential to specify which interface is
used for sending to a link-local address. It should be noted that
zone identifiers have purely local meaning within the node in which
they are defined, usually being the same as IPv6 interface names.
They are completely meaningless for any other node. Today, they are
meaningful only when attached to link-local addresses, but it is
possible that other uses might be defined in the future.
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The IPv6 Scoped Address Architecture specification [RFC4007] does not
specify how zone identifiers are to be represented in URIs.
Practical experience has shown that this feature is useful or
necessary, in multiple use cases:
1. A web browser may be used for simple debugging actions involving
link-local addresses on a host with more than one active link
interface.
2. A web browser must sometimes be used to configure or reconfigure
a device which only has a link local address and whose only
configuration tool is a web server, again in a host with more
than one active link interface.
3. The Apple and open-source CUPS printing mechanism [CUPS]
[OP-CUPS] uses an HTTP-based protocol [RFC3510][RFC7472] to
establish link-local relationships, so requires the specification
of the relevant interface.
4. The Microsoft Web Services for Devices (WSD) virtual printer port
mechanism can generate an IPv6 Link Local URL in which the zone
identifier is present and necessary, but is not recognized by any
current browser.
It should be noted that whereas some operating systems and network
APIs support a default zone identifier as described in [RFC4007],
others do not, and for them an appropriate URI syntax is particularly
important.
In the past, some browser versions directly accepted the IPv6 Scoped
Address syntax [RFC4007] for scoped IPv6 addresses embedded in URIs,
i.e., they were coded to interpret a "%" sign following the literal
address as introducing a zone identifier [RFC4007], instead of
introducing two hexadecimal characters representing some percent-
encoded octet [RFC3986]. Clearly, interpreting the "%" sign as
introducing a zone identifier is very convenient for users, although
it is not supported by the URI syntax in [RFC3986] or the
Internationalized Resource Identifier (IRI) syntax in [RFC3987].
Therefore, this document updates RFC 3986 and RFC 3987 by adding
syntax to allow a zone identifier to be included in a literal IPv6
address within a URI.
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It should be noted that in contexts other than a user interface, a
zone identifier is mapped into a numeric zone index or interface
number. The MIB textual convention InetZoneIndex [RFC4001] and the
socket interface [RFC3493] define this as a 32-bit unsigned integer.
The mapping between the human-readable zone identifier string and the
numeric value is a host-specific function that varies between
operating systems. The present document is concerned only with the
human-readable string.
Several alternative solutions were considered while this document was
developed. Appendix A briefly describes the various options and
their advantages and disadvantages.
This document obsoletes its predecessor [RFC6874] by greatly
simplifying its recommendations and requirements for URI parsers.
Its effect on the formal URI syntax [RFC3986] is different from that
of RFC 6874.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Issues with Implementing RFC 6874
Several issues prevented RFC 6874 being implemented in browsers:
1. There was some disagreement with requiring percent-encoding of
the "%" sign preceding a zone identifier. This requirement is
dropped in the present document.
2. The requirement to delete any zone identifier before emitting a
URI from the host in an HTTP message was considered both too
complex to implement and in violation of normal HTTP practice
[RFC7230]. This requirement has been dropped from the present
document.
3. The suggestion to pragmatically allow a bare "%" sign when this
would be unambiguous was considered both too complex to implement
and confusing for users. This suggestion has been dropped from
the present document since it is now irrelevant.
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3. Specification
According to IPv6 Scoped Address syntax [RFC4007], a zone identifier
is attached to the textual representation of an IPv6 address by
concatenating "%" followed by <zone_id>, where <zone_id> is a string
identifying the zone of the address. However, the IPv6 Scoped
Address Architecture specification gives no precise definition of the
character set allowed in <zone_id>. There are no rules or de facto
standards for this. For example, the first Ethernet interface in a
host might be called %0, %1, %25, %en1, %eth0, or whatever the
implementer happened to choose.
In a URI, a literal IPv6 address is always embedded between "[" and
"]". This document specifies how a zone identifier can be appended
to the address. The URI syntax defined by [RFC3986] does not allow
the presence of a percent ("%") character within an IPv6 address
literal. For this reason, it is backwards compatible to allow the
use of "%" within an IPv6 address literal as a delimiter only, such
that the scoped address fe80::abcd%en1 would appear in a URI as
http://[fe80::abcd%en1] or https://[fe80::abcd%en1].
This use of "%" as a delimiter applies only within an IPv6 address
literal, and is irrelevant to and exempt from the percent-encoding
mechanism [RFC3986].
A zone identifier MUST contain only ASCII characters classified as
"unreserved" for use in URIs [RFC3986]. This excludes characters
such as "]" or even "%" that would complicate parsing. For the
avoidance of doubt, note that a zone identifier consisting of "25" or
starting with "25" is valid and is used in some operating systems. A
parser MUST NOT apply percent decoding to the IPv6 address literal in
a URI, including cases such as http://[fe80::abcd%25] and
http://[fe80::abcd%25xy].
If an operating system uses any other characters in zone or interface
identifiers that are not in the "unreserved" character set, they
cannot be used in a URI.
We now present the corresponding formal syntax.
The URI syntax specification [RFC3986] formally defines the IPv6
literal format in ABNF [RFC5234] by the following rule:
IP-literal = "[" ( IPv6address / IPvFuture ) "]"
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To provide support for a zone identifier, the existing syntax of
IPv6address is retained, and a zone identifier may be added
optionally to any literal address. This syntax allows flexibility
for unknown future uses. The rule quoted above from [RFC3986] is
replaced by three rules:
IP-literal = "[" ( IPv6address / IPv6addrz / IPvFuture ) "]"
ZoneID = 1*( unreserved )
IPv6addrz = IPv6address "%" ZoneID
This change also applies to [RFC3987].
This syntax fills the gap that is described at the end of
Section 11.7 of the IPv6 Scoped Address Architecture specification
[RFC4007]. It replaces and obsoletes the syntax in Section 2 of
[RFC6874].
The established rules for textual representation of IPv6 addresses
[RFC5952] SHOULD be applied in producing URIs.
The URI syntax specification [RFC3986] states that URIs have a global
scope, but that in some cases their interpretation depends on the
end-user's context. URIs including a zone identifier are to be
interpreted only in the context of the host at which they originate,
since the zone identifier is of local significance only.
The IPv6 Scoped Address Architecture specification [RFC4007] offers
guidance on how the zone identifier affects interface/address
selection inside the IPv6 stack. Note that the behaviour of an IPv6
stack, if it is passed a non-null zone index for an address other
than link-local, is undefined.
In cases where the RFC 6874 encoding is currently used between
specific software components rather than between a browser and a web
server, such usage MAY continue indefinitely.
4. URI Parsers
This section discusses how URI parsers, such as those embedded in web
browsers, might handle this syntax extension. Unfortunately, there
is no formal distinction between the syntax allowed in a browser's
input dialogue box and the syntax allowed in URIs. For this reason,
no normative statements are made in this section.
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In practice, although parsers respect the established syntax, they
are coded pragmatically rather than being formally syntax-driven.
Typically, IP address literals are handled by an explicit code path.
Parsers have been inconsistent in providing for zone identifiers.
Most have no support, but there have been examples of ad hoc support.
For example, some versions of Firefox allowed the use of a zone
identifier preceded by a bare "%" character, but this feature was
removed for consistency with established syntax [RFC3986]. As
another example, some versions of Internet Explorer allowed use of a
zone identifier preceded by a "%" character encoded as "%25", still
beyond the syntax allowed by the established rules [RFC3986]. This
syntax extension is in fact used internally in the Windows operating
system and some of its APIs.
It is desirable for all URI parsers to recognise a zone identifier
according to the syntax defined in Section 3. Any code handling
percent-encoding or percent-decoding must be aware that the "%"
character preceding the zone identifier is never itself percent-
encoded, as specified by ABNF above. In terms of Section 2.4 of
[RFC3986], this "%" character is acting as a delimiter, not as data.
URIs including a zone identifier have no meaning outside the
originating HTTP client node. However, in some use cases, such as
CUPS mentioned above, the host address embedded in the URI will be
reflected back to the client, using exactly the representation of the
zone identifier that the client sent.
The various use cases for the zone identifier syntax will cause it to
be entered in a browser's input dialogue box. Thus, URIs including a
zone identifier are unlikely to occur in HTML documents. However, if
they do (for example, in a diagnostic script coded in HTML), it would
be appropriate to treat them exactly as above.
5. Security Considerations
The security considerations from the URI syntax specification
[RFC3986] and the IPv6 Scoped Address Architecture specification
[RFC4007] apply. In particular, this URI format creates a specific
pathway by which a deceitful zone index might be communicated, as
mentioned in the final security consideration of the Scoped Address
Architecture specification.
However, this format is only meaningful for link-local addresses
under prefix fe80::/10. It is not necessary for web browsers to
verify this, or to validate the zone identifier, because the
operating system will do so when the address is passed to the socket
API, and return an error code if the zone identifier is invalid.
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It is conceivable that this format could be misused to probe a local
network configuration in some way. However, that would only be
possible for an attacker that had already gained sufficient control
of a host to originate HTTP messages. Such an attacker could more
easily probe using basic mechanisms such as the "ping" command.
6. Acknowledgements
The lack of this format was first pointed out by Margaret Wasserman
and later by Kerry Lynn. A previous draft document by Bill Fenner
and Martin Dürst [LITERAL-ZONE] discussed this topic but was not
finalised. Michael Sweet and Andrew Cady explained some of the
difficulties caused by RFC 6874. The ABNF syntax proposed above was
drafted by Andrew Cady.
Valuable comments and contributions were made by Karl Auer, Carsten
Bormann, Benoit Claise, Martin Dürst, Stephen Farrell, Brian
Haberman, Ted Hardie, Philip Homburg, Tatuya Jinmei, Yves Lafon,
Barry Leiba, Ben Maddison, Radia Perlman, Tom Petch, Michael
Richardson, Tomoyuki Sahara, Juergen Schoenwaelder, Nico Schottelius,
Dave Thaler, Martin Thomson, Ole Troan, Shang Ye, and others.
7. References
7.1. 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>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
DOI 10.17487/RFC4007, March 2005,
<https://www.rfc-editor.org/info/rfc4007>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[CUPS] Apple, "Apple CUPS", 2022, <https://www.cups.org/>.
[LITERAL-ZONE]
Fenner, B. and M. Dürst, "Formats for IPv6 Scope Zone
Identifiers in Literal Address Formats", Work in Progress,
October 2005.
[OP-CUPS] Sweet, M., "OpenPrinting CUPS", 2022,
<https://openprinting.github.io/cups/>.
[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,
<https://www.rfc-editor.org/info/rfc3493>.
[RFC3510] Herriot, R. and I. McDonald, "Internet Printing
Protocol/1.1: IPP URL Scheme", RFC 3510,
DOI 10.17487/RFC3510, April 2003,
<https://www.rfc-editor.org/info/rfc3510>.
[RFC4001] Daniele, M., Haberman, B., Routhier, S., and J.
Schoenwaelder, "Textual Conventions for Internet Network
Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,
<https://www.rfc-editor.org/info/rfc4001>.
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/info/rfc6874>.
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[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7472] McDonald, I. and M. Sweet, "Internet Printing Protocol
(IPP) over HTTPS Transport Binding and the 'ipps' URI
Scheme", RFC 7472, DOI 10.17487/RFC7472, March 2015,
<https://www.rfc-editor.org/info/rfc7472>.
Appendix A. Options Considered
The syntax defined above allows a zone identifier to be added to any
IPv6 address. The 6man WG discussed and rejected an alternative in
which the existing syntax of IPv6address would be extended by an
option to add the zone identifier only for the case of link-local
addresses. It was felt that the solution presented in this document
offers more flexibility for future uses and is more straightforward
to implement.
The various syntax options considered are now briefly described.
1. Leave the problem unsolved.
This would mean that per-interface diagnostics would still have
to be performed using ping or ping6:
ping fe80::abcd%en1
Advantage: works today.
Disadvantage: less convenient than using a browser. Leaves some
use cases unsatisfied.
2. Simply use the percent character:
http://[fe80::abcd%en1]
Advantage: allows use of browser; allows cut and paste.
Disadvantage: requires code changes to all URI parsers.
This is the option chosen for standardisation.
3. Simply use an alternative separator:
http://[fe80::abcd-en1]
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Advantage: allows use of browser; simple syntax.
Disadvantage: Requires all IPv6 address literal parsers and
generators to be updated in order to allow simple cut and paste;
inconsistent with existing tools and practice.
Note: The initial proposal for this choice was to use an
underscore as the separator, but it was noted that this becomes
effectively invisible when a user interface automatically
underlines URLs.
4. Simply use the "IPvFuture" syntax left open in RFC 3986:
http://[v6.fe80::abcd_en1]
Advantage: allows use of browser.
Disadvantage: ugly and redundant; doesn't allow simple cut and
paste.
5. Retain the percent character already specified for introducing
zone identifiers for IPv6 Scoped Addresses [RFC4007], and then
percent-encode it when it appears in a URI, according to the
already-established URI syntax rules [RFC 3986]:
http://[fe80::abcd%25en1]
Advantage: allows use of browser; consistent with general URI
syntax.
Disadvantage: somewhat ugly and confusing; doesn't allow simple
cut and paste.
Appendix B. Change log
This section is to be removed before publishing as an RFC.
* draft-ietf-6man-rfc6874bis-01, 2022-04-07:
- Extended use cases
- Clarified relationship with RFC3986 language
- Allow for legacy use of RFC6874 format
- Augmented security considerations
- Editorial and reference improvements
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* draft-ietf-6man-rfc6874bis-00, 2022-03-19:
- WG adoption
- Clarified security considerations
* draft-carpenter-6man-rfc6874bis-03, 2022-02-08:
- Changed to bare % signs.
- Added IRIs, RFC3987
- Editorial fixes
* draft-carpenter-6man-rfc6874bis-02, 2021-18-12:
- Give details of open issues
- Update authorship
- Editorial fixes
* draft-carpenter-6man-rfc6874bis-01, 2021-07-11:
- Added section on issues with RFC6874
- Removed suggested heuristic for bare % signs
- Editorial fixes
* draft-carpenter-6man-rfc6874bis-00, 2021-07-05:
- Initial version
Authors' Addresses
Brian Carpenter
School of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States of America
Email: cheshire@apple.com
Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
United States of America
Email: bob.hinden@gmail.com
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