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: 6 January 2023                             Check Point Software
                                                             5 July 2022

   Representing IPv6 Zone Identifiers in Address Literals and Uniform
                          Resource Identifiers


   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

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 6 January 2023.

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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.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Options Considered . . . . . . . . . . . . . . . . .  10
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  12
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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 various use cases, including the following:

   1.  A web browser may be used for simple debugging actions involving
       link-local addresses on a host with more than one active link

   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

   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",
   "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
       [RFC9110].  This requirement has been dropped from the present

   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

   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

   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 an example
   of this, since the zone identifier is of local significance only.
   Such a URI cannot be correctly interpreted outside the host to which
   it applies.

   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.  An IPv6 address
   literal never contains percent-encodings.  In terms of Section 2.4 of
   [RFC3986], the "%" character preceding a zone identifier is acting as
   a delimiter, not as data.  Any code handling percent-encoding or
   percent-decoding must be aware of this.

   As noted above, URIs including a zone identifier have no meaning
   outside the originating HTTP client node.  However, in some use
   cases, such as CUPS, 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 usually
   require it to be entered in a browser's input dialogue box.  However,
   URIs including a zone identifier might occur in HTML documents.  For
   example, a diagnostic HTML page might be tailored for a particular
   host.  Because of such usage, it is appropriate for browsers to treat
   such URIs in the same way whether they are entered in the dialogue
   box or encountered in an HTML document.

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.

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   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.

   It is conceivable that this format could be misused to probe a local
   network configuration.  In particular, a script included in an HTML
   web page could originate HTTP messages intended to determine if a
   particular link-local address is valid, for example to discover and
   misuse the address of the first-hop router.  However, such attacks
   are already possible, by probing IPv4 addresses, routeable IPv6
   addresses or link-local addresses without a zone identifier.  Indeed,
   with a zone identifier present, the attacker's job is harder because
   they must also guess the zone identifier itself; the zone identifier
   increases the search space compared to guessing only the interface
   identifier.  Zone identifiers vary widely between operating systems;
   in some cases they are simple integers or conventional names such as
   "eth0" but in other cases they contain arbitrary characters derived
   from MAC addresses.  In any case, an attacker must discover them
   before probing any link-local addresses.  This argues against the
   recommendation of [RFC4007] to support a default zone identifier.

   It should be noted that if a node uses an interface identifier in the
   outdated Modified EUI format [RFC4291] for its link-local address,
   the search space for an attacker is very significantly reduced, as
   discussed in Section of [RFC7707].  The resultant
   recommendations of [RFC8064] apply to all nodes, including routers,
   since they ensure that the search space for an attacker is of size
   2**64, which is impracticably large.

   Nevertheless, even a Modified EUI link-local address is significantly
   harder to guess than typical IPv4 addresses for devices such as home
   routers, which are often included in published documentation.

6.  IANA Considerations

   This document makes no request of IANA.

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,

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   [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,

   [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,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,

   [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/>.

              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,

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   [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,

   [RFC3510]  Herriot, R. and I. McDonald, "Internet Printing
              Protocol/1.1: IPP URL Scheme", RFC 3510,
              DOI 10.17487/RFC3510, April 2003,

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

   [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,

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

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.

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   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:


       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:


       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:


       Advantage: allows use of browser.

       Disadvantage: ugly and redundant; doesn't allow simple cut and

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   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 [RFC3986]:


       Advantage: allows use of browser; consistent with general URI

       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-02, 2022-07-05:

      -  Improve discussion of URLs in HTML documents

      -  Discuss scripting attack and Modified EUI IIDs

      -  Several editorial clarifications

      -  Some nits fixed

   *  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

   *  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.

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      -  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

Appendix C.  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, David Farmer, Stephen Farrell,
   Brian Haberman, Ted Hardie, Philip Homburg, Tatuya Jinmei, Yves
   Lafon, Barry Leiba, Ted Lemon, Ben Maddison, Radia Perlman, Tom
   Petch, Michael Richardson, Tomoyuki Sahara, Juergen Schoenwaelder,
   Nico Schottelius, Dave Thaler, Martin Thomson, Philipp S.  Tiesel,
   Ole Troan, Shang Ye, and others.

Authors' Addresses

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   Brian Carpenter
   School of Computer Science
   University of Auckland
   PB 92019
   Auckland 1142
   New Zealand
   Email: brian.e.carpenter@gmail.com

   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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