This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 4534, EID 4560
Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 7622 &yet
Obsoletes: 6122 September 2015
Category: Standards Track
ISSN: 2070-1721
Extensible Messaging and Presence Protocol (XMPP): Address Format
Abstract
This document defines the address format for the Extensible Messaging
and Presence Protocol (XMPP), including support for code points
outside the ASCII range. This document obsoletes RFC 6122.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7622.
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
Provisions Relating to IETF Documents
(http://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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Addresses .......................................................3
3.1. Fundamentals ...............................................3
3.2. Domainpart .................................................5
3.3. Localpart ..................................................7
3.4. Resourcepart ...............................................8
3.5. Examples ...................................................9
4. Enforcement in JIDs and JID Parts ..............................13
5. Internationalization Considerations ............................15
6. IANA Considerations ............................................16
6.1. Stringprep Profiles Registry ..............................16
7. Security Considerations ........................................16
7.1. Reuse of PRECIS ...........................................16
7.2. Reuse of Unicode ..........................................16
7.3. Address Spoofing ..........................................16
8. Conformance Requirements .......................................19
9. References .....................................................21
9.1. Normative References ......................................21
9.2. Informative References ....................................22
Appendix A. Differences from RFC 6122 .............................26
Acknowledgements ..................................................27
Author's Address ..................................................27
1. Introduction
The Extensible Messaging and Presence Protocol (XMPP) [RFC6120] is an
application profile of the Extensible Markup Language [XML] for
streaming XML data in close to real time between any two or more
network-aware entities. The address format for XMPP entities was
originally developed in the Jabber open-source community in 1999,
first described by [XEP-0029] in 2002, and then defined canonically
by [RFC3920] in 2004 and [RFC6122] in 2011.
As specified in RFCs 3920 and 6122, the XMPP address format used the
"stringprep" technology for preparation and comparison of non-ASCII
characters [RFC3454]. Following the movement of internationalized
domain names away from stringprep, this document defines the XMPP
address format in a way that no longer depends on stringprep (see the
Preparation, Enforcement, and Comparison of Internationalized Strings
(PRECIS) problem statement [RFC6885]). Instead, this document builds
upon the internationalization framework defined by the IETF's PRECIS
working group [RFC7564].
Although every attempt has been made to ensure that the characters
allowed in Jabber Identifiers (JIDs) under stringprep are still
allowed and handled in the same way under PRECIS, there is no
guarantee of strict backward compatibility because of changes in
Unicode and the fact that PRECIS handling is based on Unicode
properties, not a hardcoded table of characters. Because it is
possible that previously valid JIDs might no longer be valid (or
previously invalid JIDs might now be valid), operators of XMPP
services are advised to perform careful testing before migrating
accounts and other data (see Section 6 of [RFC7613] for guidance).
This document obsoletes RFC 6122.
2. Terminology
Many important terms used in this document are defined in [RFC7564],
[RFC5890], [RFC6120], [RFC6365], and [Unicode].
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
[RFC2119].
3. Addresses
3.1. Fundamentals
An XMPP entity is anything that can communicate using XMPP. For
historical reasons, the network address of an XMPP entity is called a
JID. A valid JID is a string of Unicode code points [Unicode],
encoded using UTF-8 [RFC3629], and structured as an ordered sequence
of localpart, domainpart, and resourcepart, where the first two parts
are demarcated by the '@' character used as a separator and the last
two parts are similarly demarcated by the '/' character (e.g.,
<juliet@example.com/balcony>).
The syntax for a JID is defined as follows, using the Augmented
Backus-Naur Form (ABNF) as specified in [RFC5234].
jid = [ localpart "@" ] domainpart [ "/" resourcepart ]
localpart = 1*1023(userbyte)
;
; a "userbyte" is a byte used to represent a
; UTF-8 encoded Unicode code point that can be
; contained in a string that conforms to the
; UsernameCaseMapped profile of the PRECIS
; IdentifierClass defined in RFC 7613
;
domainpart = IP-literal / IPv4address / ifqdn
;
; the "IPv4address" and "IP-literal" rules are
; defined in RFCs 3986 and 6874, respectively,
; and the first-match-wins (a.k.a. "greedy")
; algorithm described in Appendix B of RFC 3986
; applies to the matching process
;
ifqdn = 1*1023(domainbyte)
;
; a "domainbyte" is a byte used to represent a
; UTF-8 encoded Unicode code point that can be
; contained in a string that conforms to RFC 5890
;
resourcepart = 1*1023(opaquebyte)
;
; an "opaquebyte" is a byte used to represent a
; UTF-8 encoded Unicode code point that can be
; contained in a string that conforms to the
; OpaqueString profile of the PRECIS
; FreeformClass defined in RFC 7613
;
All JIDs are based on the foregoing structure. However, note that
the formal syntax provided above does not capture all of the rules
and restrictions that apply to JIDs, which are described below.
Each allowable portion of a JID (localpart, domainpart, and
resourcepart) is 1 to 1023 octets in length, resulting in a maximum
total size (including the '@' and '/' separators) of 3071 octets.
Implementation Note: The length limits on JIDs and parts of JIDs
are based on octets (bytes), not characters. UTF-8 encoding can
result in more than one octet per character.
Implementation Note: When dividing a JID into its component parts,
an implementation needs to match the separator characters '@' and
'/' before applying any transformation algorithms, which might
decompose certain Unicode code points to the separator characters.
Implementation Note: Reuse of the IP-literal rule from [RFC6874]
implies that IPv6 addresses are enclosed within square brackets
(i.e., beginning with '[' and ending with ']'), which was not the
case with the definition of the XMPP address format in [RFC3920]
but which was changed in [RFC6122]. Also note that the IP-literal
rule was updated between [RFC3986] and [RFC6874] to optionally add
a zone identifier to any literal address.
This document defines the native format for JIDs; see [RFC5122] for
information about the representation of a JID as a Uniform Resource
Identifier (URI) [RFC3986] or Internationalized Resource Identifier
(IRI) [RFC3987] and the extraction of a JID from an XMPP URI or IRI.
3.2. Domainpart
The domainpart of a JID is the portion that remains once the
following parsing steps are taken:
1. Remove any portion from the first '/' character to the end of the
string (if there is a '/' character present).
2. Remove any portion from the beginning of the string to the first
'@' character (if there is an '@' character present).
This parsing order is important, as illustrated by example 15 in
Section 3.5.
The domainpart is the primary identifier and is the only REQUIRED
element of a JID (a mere domainpart is a valid JID). Typically,
a domainpart identifies the "home" server to which clients connect
for XML routing and data management functionality. However, it is
not necessary for an XMPP domainpart to identify an entity that
provides core XMPP server functionality (e.g., a domainpart can
identify an entity such as a multi-user chat service [XEP-0045], a
publish-subscribe service [XEP-0060], or a user directory).
The domainpart for every XMPP service MUST be a fully qualified
domain name (FQDN), an IPv4 address, an IPv6 address, or an
unqualified hostname (i.e., a text label that is resolvable on a
local network).
Informational Note: The term "fully qualified domain name" is not
well defined. In [RFC1034], it is also called an absolute domain
name, and the two terms are associated in [RFC1535]. The earliest
use of the term can be found in [RFC1123]. References to those
older specifications ought not to be construed as limiting the
characters of a fully qualified domain name to the ASCII range;
for example, [RFC5890] mentions that a fully qualified domain name
can contain one or more U-labels.
Interoperability Note: Domainparts that are IP addresses might not
be accepted by other services for the purpose of server-to-server
communication, and domainparts that are unqualified hostnames
cannot be used on public networks because they are resolvable only
on a local network.
If the domainpart includes a final character considered to be a label
separator (dot) by [RFC1034], this character MUST be stripped from
the domainpart before the JID of which it is a part is used for the
purpose of routing an XML stanza, comparing against another JID, or
constructing an XMPP URI or IRI [RFC5122]. In particular, such a
character MUST be stripped before any other canonicalization steps
are taken.
In general, the content of a domainpart is an Internationalized
Domain Name (IDN) as described in the specifications for
Internationalized Domain Names in Applications (commonly called
"IDNA2008"), and a domainpart is an "IDNA-aware domain name slot" as
defined in [RFC5890].
After any and all normalization, conversion, and mapping of code
points as well as encoding of the string as UTF-8, a domainpart MUST
NOT be zero octets in length and MUST NOT be more than 1023 octets in
length. (Naturally, the length limits of [RFC1034] apply, and
nothing in this document is to be interpreted as overriding those
more fundamental limits.)
Detailed rules and considerations for preparation, enforcement, and
comparison are provided in the following sections.
3.2.1. Preparation
An entity that prepares a string for inclusion in an XMPP domainpart
slot MUST ensure that the string consists only of Unicode code points
that are allowed in NR-LDH labels or U-labels as defined in
[RFC5890]. This implies that the string MUST NOT include A-labels as
defined in [RFC5890]; each A-label MUST be converted to a U-label
during preparation of a string for inclusion in a domainpart slot.
In addition, the string MUST be encoded as UTF-8 [RFC3629].
3.2.2. Enforcement
An entity that performs enforcement in XMPP domainpart slots MUST
prepare a string as described in Section 3.2.1 and MUST also apply
the normalization, case-mapping, and width-mapping rules defined in
[RFC5895].
EID 4534 (Verified) is as follows:Section: 3.2.2
Original Text:
An entity that performs enforcement in XMPP domainpart slots MUST
prepare a string as described in Section 3.2.1 and MUST also apply
the normalization, case-mapping, and width-mapping rules defined in
[RFC5892].
Corrected Text:
An entity that performs enforcement in XMPP domainpart slots MUST
prepare a string as described in Section 3.2.1 and MUST also apply
the normalization, case-mapping, and width-mapping rules defined in
[RFC5895].
Notes:
RFC 5892 is just a list of codepoints; the rules to which it is referring (and that are referred to in the ABNF and other places in the document) are in RFC 5895.
Informational Note: The order in which the rules are applied for
IDNA2008 (see [RFC5892] and [RFC5895]) is different from the order
for localparts and resourceparts as described under Sections 3.3
and 3.4.
3.2.3. Comparison
An entity that performs comparison of two strings before or after
their inclusion in XMPP domainpart slots MUST prepare each string as
specified in Section 3.2.1 and then enforce the normalization,
case-mapping, and width-mapping rules specified in Section 3.2.2.
The two strings are to be considered equivalent if they are an exact
octet-for-octet match (sometimes called "bit-string identity").
3.3. Localpart
The localpart of a JID is an optional identifier placed before the
domainpart and separated from the latter by the '@' character.
Typically, a localpart uniquely identifies the entity requesting and
using network access provided by a server (i.e., a local account),
although it can also represent other kinds of entities (e.g., a
chatroom associated with a multi-user chat service [XEP-0045]). The
entity represented by an XMPP localpart is addressed within the
context of a specific domain (i.e., <localpart@domainpart>).
The localpart of a JID MUST NOT be zero octets in length and MUST NOT
be more than 1023 octets in length. This rule is to be enforced
after any normalization and mapping of code points as well as
encoding of the string as UTF-8.
The localpart of a JID is an instance of the UsernameCaseMapped
profile of the PRECIS IdentifierClass, which is specified in
[RFC7613]. The rules and considerations provided in that
specification MUST be applied to XMPP localparts.
Implementation Note: XMPP uses the Simple Authentication and
Security Layer (SASL) [RFC4422] for authentication. At the time
of this writing, some SASL mechanisms use SASLprep [RFC4013] for
the handling of usernames and passwords; in the future, these SASL
mechanisms will likely transition to the use of PRECIS-based
handling rules as specified in [RFC7613]. For a detailed
discussion about the implications of that transition (including
the potential need to modify or remove certain characters in the
underlying account database), see both Section 6 and Appendix A
of [RFC7613].
3.3.1. Further Excluded Characters
In XMPP, the following characters are explicitly disallowed in XMPP
localparts, even though they are allowed by the IdentifierClass base
class and the UsernameCaseMapped profile:
" U+0022 (QUOTATION MARK)
& U+0026 (AMPERSAND)
' U+0027 (APOSTROPHE)
/ U+002F (SOLIDUS)
: U+003A (COLON)
< U+003C (LESS-THAN SIGN)
> U+003E (GREATER-THAN SIGN)
@ U+0040 (COMMERCIAL AT)
Implementation Note: An XMPP-specific method for escaping the
foregoing characters (along with U+0020, i.e., ASCII space) has
been defined in the JID Escaping specification [XEP-0106].
3.4. Resourcepart
The resourcepart of a JID is an optional identifier placed after the
domainpart and separated from the latter by the '/' character. A
resourcepart can modify either a <localpart@domainpart> address or a
mere <domainpart> address. Typically, a resourcepart uniquely
identifies a specific connection (e.g., a device or location) or
object (e.g., an occupant in a multi-user chatroom [XEP-0045])
belonging to the entity associated with an XMPP localpart at a domain
(i.e., <localpart@domainpart/resourcepart>).
XMPP entities SHOULD consider resourceparts to be opaque strings and
SHOULD NOT impute meaning to any given resourcepart. In particular:
o Use of the '/' character as a separator between the domainpart and
the resourcepart does not imply that XMPP addresses are
hierarchical in the way that, say, HTTP URIs are hierarchical (see
[RFC3986] for general discussion); thus, for example, an XMPP
address of the form <localpart@domainpart/foo/bar> does not
identify a resource "bar" that exists below a resource "foo" in a
hierarchy of resources associated with the entity
"localpart@domainpart".
o The '@' character is allowed in the resourcepart and is often used
in the "handle" shown in XMPP chatrooms [XEP-0045]. For example,
the JID <room@chat.example.com/user@host> describes an entity who
is an occupant of the room <room@chat.example.com> with a handle
of <user@host>. However, chatroom services do not necessarily
check such an asserted handle against the occupant's real JID.
The resourcepart of a JID MUST NOT be zero octets in length and MUST
NOT be more than 1023 octets in length. This rule is to be enforced
after any normalization and mapping of code points as well as
encoding of the string as UTF-8.
The resourcepart of a JID is an instance of the OpaqueString profile
of the PRECIS FreeformClass, which is specified in [RFC7613]. The
rules and considerations provided in that specification MUST be
applied to XMPP resourceparts.
3.4.1. Applicability to XMPP Extensions
In some contexts, it might be appropriate to apply more restrictive
rules to the preparation, enforcement, and comparison of XMPP
resourceparts. For example, in XMPP Multi-User Chat [XEP-0045] it
might be appropriate to apply the rules specified in
[PRECIS-Nickname]. However, the application of more restrictive
rules is out of scope for resourceparts in general and is properly
defined in specifications for the relevant XMPP extensions.
3.5. Examples
The following examples illustrate a small number of JIDs that are
consistent with the format defined above (note that the characters
"<" and ">" are used to delineate the actual JIDs and are not part of
the JIDs themselves).
+----------------------------------+-------------------------------+
| # | JID | Notes |
+----------------------------------+-------------------------------+
| 1 | <juliet@example.com> | A "bare JID" |
+----------------------------------+-------------------------------+
| 2 | <juliet@example.com/foo> | A "full JID" |
+----------------------------------+-------------------------------+
| 3 | <juliet@example.com/foo bar> | Single space in resourcepart |
+----------------------------------+-------------------------------+
| 4 | <juliet@example.com/foo@bar> | "At" sign in resourcepart |
+----------------------------------+-------------------------------+
| 5 | <foo\20bar@example.com> | Single space in localpart, as |
| | | optionally escaped using the |
| | | XMPP JID Escaping extension |
+----------------------------------+-------------------------------+
| 6 | <fussball@example.com> | Another bare JID |
+----------------------------------+-------------------------------+
| 7 | <fußball@example.com> | The third character is LATIN |
| | | SMALL LETTER SHARP S (U+00DF) |
+----------------------------------+-------------------------------+
| 8 | <π@example.com> | A localpart of GREEK SMALL |
| | | LETTER PI (U+03C0) |
+----------------------------------+-------------------------------+
| 9 | <Σ@example.com/foo> | A localpart of GREEK CAPITAL |
| | | LETTER SIGMA (U+03A3) |
+----------------------------------+-------------------------------+
| 10| <σ@example.com/foo> | A localpart of GREEK SMALL |
| | | LETTER SIGMA (U+03C3) |
+----------------------------------+-------------------------------+
| 11| <ς@example.com/foo> | A localpart of GREEK SMALL |
| | | LETTER FINAL SIGMA (U+03C2) |
+----------------------------------+-------------------------------+
| 12| <king@example.com/♚>; | A resourcepart of the Unicode |
| | | character BLACK CHESS KING |
| | | (U+265A) |
+----------------------------------+-------------------------------+
| 13| <example.com> | A domainpart |
+----------------------------------+-------------------------------+
| 14| <example.com/foobar> | A domainpart and resourcepart |
+----------------------------------+-------------------------------+
| 15| <a.example.com/b@example.net>| A domainpart followed by a |
| | | resourcepart that contains an |
| | | "at" sign |
+----------------------------------+-------------------------------+
Table 1: A Sample of Legal JIDs
Several points are worth noting. Regarding examples 6 and 7:
although in German the character esszett (LATIN SMALL LETTER SHARP S
(U+00DF)) can mostly be used interchangeably with the two characters
"ss", the localparts in these examples are different, and (if
desired) a server would need to enforce a registration policy that
disallows one of them if the other is registered. Regarding examples
9, 10, and 11: case-mapping of GREEK CAPITAL LETTER SIGMA (U+03A3) to
lowercase (i.e., to GREEK SMALL LETTER SIGMA (U+03C3)) during
comparison would result in matching the JIDs in examples 9 and 10;
however, because the PRECIS mapping rules do not account for the
special status of GREEK SMALL LETTER FINAL SIGMA (U+03C2), the JIDs
in examples 9 and 11 or examples 10 and 11 would not be matched.
Regarding example 12: symbol characters such as BLACK CHESS KING
(U+265A) are allowed by the PRECIS FreeformClass and thus can be used
in resourceparts. Regarding examples 14 and 15: JIDs consisting of a
domainpart and resourcepart are rarely seen in the wild but are
allowed according to the XMPP address format. Example 15 illustrates
the need for careful extraction of the domainpart as described in
Section 3.2.
The following examples illustrate strings that are not JIDs because
they violate the format defined above.
+----------------------------------+-------------------------------+
| # | Non-JID string | Notes |
+----------------------------------+-------------------------------+
| 16| <"juliet"@example.com> | Quotation marks (U+0022) in |
| | | localpart |
+----------------------------------+-------------------------------+
| 17| <foo bar@example.com> | Space (U+0020) in localpart |
+----------------------------------+-------------------------------+
[nothing]
EID 4560 (Verified) is as follows:Section: 3.5
Original Text:
| 18| <juliet@example.com/ foo> | Leading space in resourcepart |
Corrected Text:
[nothing]
Notes:
Example 18 is wrong because a leading space is currently allowed by RFC 7622 (at least, it is not disallowed by the OpaqueString profile defined in RFC 7613). It is true that leading and trailing spaces are disallowed by the Nickname profile, but we do not reference that here. This example should be removed in any updates to RFC 7622.
+----------------------------------+-------------------------------+
| 19| <@example.com/> | Zero-length localpart and |
| | | resourcepart |
+----------------------------------+-------------------------------+
| 20| <henryⅣ@example.com> | The sixth character is ROMAN |
| | | NUMERAL FOUR (U+2163) |
+----------------------------------+-------------------------------+
| 21| <♚@example.com> | A localpart of BLACK CHESS |
| | | KING (U+265A) |
+----------------------------------+-------------------------------+
| 22| <juliet@> | A localpart without a |
| | | domainpart |
+----------------------------------+-------------------------------+
| 23| </foobar> | A resourcepart without a |
| | | domainpart |
+----------------------------------+-------------------------------+
Table 2: A Sample of Strings That Violate the JID Rules
Here again, several points are worth noting. Regarding example 17:
even though ASCII space (U+0020) is disallowed in the PRECIS
IdentifierClass, it can be escaped to "\20" in XMPP localparts by
using the JID Escaping rules defined in [XEP-0106], as illustrated by
example 5 in Table 1. Regarding example 20: the Unicode character
ROMAN NUMERAL FOUR (U+2163) has a compatibility equivalent of the
string formed of LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL
LETTER V (U+0056), but characters with compatibility equivalents are
not allowed in the PRECIS IdentifierClass. Regarding example 21:
symbol characters such as BLACK CHESS KING (U+265A) are not allowed
in the PRECIS IdentifierClass; however, both of the non-ASCII
characters in examples 20 and 21 are allowed in the PRECIS
FreeformClass and therefore in the XMPP resourcepart (as illustrated
for U+265A by example 12 in Table 1). Regarding examples 22 and 23:
the domainpart is required in a JID.
4. Enforcement in JIDs and JID Parts
Enforcement entails applying all of the rules specified in this
document. Enforcement of the XMPP address format rules is the
responsibility of XMPP servers. Although XMPP clients SHOULD prepare
complete JIDs and parts of JIDs in accordance with this document
before including them in protocol slots within XML streams, XMPP
servers MUST enforce the rules wherever possible and reject stanzas
and other XML elements that violate the rules (for stanzas, by
returning a <jid-malformed/> error to the sender as described in
Section 8.3.3.8 of [RFC6120]).
Entities that enforce the rules specified in this document are
encouraged to be liberal in what they accept by following this
procedure:
1. Where possible, map characters (e.g., through width mapping,
additional mapping, special mapping, case mapping, or
normalization) and accept the mapped string.
2. If mapping is not possible (e.g., because a character is
disallowed in the FreeformClass), reject the string and return a
<jid-malformed/> error.
Enforcement applies to complete JIDs and to parts of JIDs. To
facilitate implementation, this document defines the concepts of "JID
slot", "localpart slot", and "resourcepart slot" (similar to the
concept of a "domain name slot" for IDNA2008 as defined in
Section 2.3.2.6 of [RFC5890]):
JID Slot: An XML element or attribute explicitly designated in XMPP
or in XMPP extensions for carrying a complete JID.
Localpart Slot: An XML element or attribute explicitly designated
in XMPP or in XMPP extensions for carrying the localpart of a JID.
Resourcepart Slot: An XML element or attribute explicitly designated
in XMPP or in XMPP extensions for carrying the resourcepart of
a JID.
A server is responsible for enforcing the address format rules when
receiving protocol elements from clients where the server is expected
to handle such elements directly or to use them for purposes of
routing a stanza to another domain or delivering a stanza to a local
entity; two examples from [RFC6120] are the 'to' attribute on XML
stanzas (which is a JID slot used by XMPP servers for routing of
outbound stanzas) and the <resource/> child of the <bind/> element
(which is a resourcepart slot used by XMPP servers for binding of a
resource to an account for routing of stanzas between the server and
a particular client). An example from [RFC6121] is the 'jid'
attribute of the roster <item/> element.
A server is not responsible for enforcing the rules when the protocol
elements are intended for communication among other entities,
typically within the payload of a stanza that the server is merely
routing to another domain or delivering to a local entity. Two
examples are the 'initiator' attribute in the Jingle extension
[XEP-0166] (which is a JID slot used for client-to-client
coordination of multimedia sessions) and the 'nick' attribute in the
Multi-User Chat extension [XEP-0045] (which is a resourcepart slot
used for administrative purposes in the context of XMPP chatrooms).
In such cases, the entities involved SHOULD enforce the rules
themselves and not depend on the server to do so, and client
implementers need to understand that not enforcing the rules can lead
to a degraded user experience or to security vulnerabilities.
However, when an add-on service (e.g., a multi-user chat service)
handles a stanza directly, it ought to enforce the rules as well, as
defined in the relevant specification for that type of service.
This document does not provide an exhaustive list of JID slots,
localpart slots, or resourcepart slots. However, implementers of
core XMPP servers are advised to consider as JID slots at least the
following elements and attributes when they are handled directly or
used for purposes of routing to another domain or delivering to a
local entity:
o The 'from' and 'to' stream attributes and the 'from' and 'to'
stanza attributes [RFC6120].
o The 'jid' attribute of the roster <item/> element for contact list
management [RFC6121].
o The 'value' attribute of the <item/> element for Privacy Lists
[RFC3921] [XEP-0016] when the value of the 'type' attribute
is "jid".
o The 'jid' attribute of the <item/> element for Service Discovery
defined in [XEP-0030].
o The <value/> element for Data Forms [XEP-0004] when the 'type'
attribute is "jid-single" or "jid-multi".
o The 'jid' attribute of the <conference/> element for Bookmark
Storage [XEP-0048].
o The <JABBERID/> of the <vCard/> element for vCard 3.0 [XEP-0054]
and the <uri/> child of the <impp/> element for vCard 4.0
[XEP-0292] when the XML character data identifies an XMPP URI
[RFC5122].
o The 'from' attribute of the <delay/> element for Delayed Delivery
[XEP-0203].
o The 'jid' attribute of the <item/> element for the Blocking
Command [XEP-0191].
o The 'from' and 'to' attributes of the <result/> and <verify/>
elements for Server Dialback [XEP-0220].
o The 'from' and 'to' attributes of the <iq/>, <message/>, and
<presence/> elements for the Jabber Component Protocol [XEP-0114].
Developers of XMPP clients and specialized XMPP add-on services are
advised to check the appropriate specifications for JID slots,
localpart slots, and resourcepart slots in XMPP protocol extensions
such as Service Discovery [XEP-0030], Multi-User Chat [XEP-0045],
Publish-Subscribe [XEP-0060], SOCKS5 Bytestreams [XEP-0065], In-Band
Registration [XEP-0077], Roster Item Exchange [XEP-0144], and Jingle
[XEP-0166].
5. Internationalization Considerations
XMPP applications MUST support IDNA2008 for domainparts as described
under Section 3.2, the UsernameCaseMapped profile for localparts as
described under Section 3.3, and the OpaqueString profile for
resourceparts as described under Section 3.4. This enables XMPP
addresses to include a wide variety of characters outside the ASCII
range. Rules for enforcement of the XMPP address format are provided
in [RFC6120] and specifications for various XMPP extensions.
Interoperability Note: For backward compatibility, many existing
XMPP implementations and deployments support IDNA2003 [RFC3490]
for domainparts, and the stringprep [RFC3454] profiles Nodeprep
and Resourceprep [RFC3920] for localparts and resourceparts.
6. IANA Considerations
6.1. Stringprep Profiles Registry
The stringprep specification [RFC3454] did not provide for entries in
the "Stringprep Profiles" registry to have any state except "Current"
or "Not Current". Because this document obsoletes RFC 6122, which
registered the Nodeprep and Resourceprep profiles of stringprep, IANA
has marked those profiles as "Not Current" and cited this document as
an additional reference.
7. Security Considerations
7.1. Reuse of PRECIS
The security considerations described in [RFC7564] apply to the
IdentifierClass and FreeformClass base string classes used in this
document for XMPP localparts and resourceparts, respectively. The
security considerations described in [RFC5890] apply to
internationalized domain names, which are used here for XMPP
domainparts.
7.2. Reuse of Unicode
The security considerations described in [UTS39] apply to the use of
Unicode characters in XMPP addresses.
7.3. Address Spoofing
There are two forms of address spoofing: forging and mimicking.
7.3.1. Address Forging
In the context of XMPP technologies, address forging occurs when an
entity is able to generate an XML stanza whose 'from' address does
not correspond to the account credentials with which the entity
authenticated onto the network (or an authorization identity provided
during negotiation of SASL authentication [RFC4422] as described in
[RFC6120]). For example, address forging occurs if an entity that
authenticated as "juliet@im.example.com" is able to send XML stanzas
from "nurse@im.example.com" or "romeo@example.net".
Address forging is difficult in XMPP systems, given the requirement
for sending servers to stamp 'from' addresses and for receiving
servers to verify sending domains via server-to-server authentication
(see [RFC6120]). However, address forging is possible if:
o A poorly implemented server ignores the requirement for stamping
the 'from' address. This would enable any entity that
authenticated with the server to send stanzas from any
localpart@domainpart as long as the domainpart matches the sending
domain of the server.
o An actively malicious server generates stanzas on behalf of any
registered account at the domain or domains hosted at that server.
Therefore, an entity outside the security perimeter of a particular
server cannot reliably distinguish between JIDs of the form
<localpart@domainpart> at that server and thus can authenticate only
the domainpart of such JIDs with any level of assurance. This
specification does not define methods for discovering or
counteracting the kind of poorly implemented or rogue servers just
described. However, the end-to-end authentication or signing of XMPP
stanzas could help to mitigate this risk, because it would require
the rogue server to generate false credentials for signing or
encryption of each stanza, in addition to modifying 'from' addresses.
7.3.2. Address Mimicking
Address mimicking occurs when an entity provides legitimate
authentication credentials for, and sends XML stanzas from, an
account whose JID appears to a human user to be the same as another
JID. Because many characters are visually similar, it is relatively
easy to mimic JIDs in XMPP systems. As one simple example, the
localpart "ju1iet" (using the Arabic numeral one as the third
character) might appear the same as the localpart "juliet" (using
lowercase "L" as the third character).
As explained in [RFC5890], [RFC7564], [UTR36], and [UTS39], there is
no straightforward solution to the problem of visually similar
characters. Furthermore, IDNA and PRECIS technologies do not attempt
to define such a solution. As a result, XMPP domainparts,
localparts, and resourceparts could contain such characters, leading
to security vulnerabilities such as the following:
o A domainpart is always employed as one part of an entity's address
in XMPP. One common usage is as the address of a server or
server-side service, such as a multi-user chat service [XEP-0045].
The security of such services could be compromised based on
different interpretations of the internationalized domainpart; for
example, a user might authorize a malicious entity at a fake
server to view the user's presence information, or a user could
join chatrooms at a fake multi-user chat service.
o A localpart can be employed as one part of an entity's address in
XMPP. One common usage is as the username of an instant messaging
user; another is as the name of a multi-user chatroom; and many
other kinds of entities could use localparts as part of their
addresses. The security of such services could be compromised
based on different interpretations of the internationalized
localpart; for example, a user entering a single internationalized
localpart could access another user's account information, or a
user could gain access to a hidden or otherwise restricted
chatroom or service.
o A resourcepart can be employed as one part of an entity's address
in XMPP. One common usage is as the name for an instant messaging
user's connected resource; another is as the nickname of a user in
a multi-user chatroom; and many other kinds of entities could use
resourceparts as part of their addresses. The security of such
services could be compromised based on different interpretations
of the internationalized resourcepart; for example, two or more
confusable resources could be bound at the same time to the same
account (resulting in inconsistent authorization decisions in an
XMPP application that uses full JIDs), or a user could send a
private message to someone other than the intended recipient in a
multi-user chatroom.
XMPP services and clients are strongly encouraged to define and
implement consistent policies regarding the registration, storage,
and presentation of visually similar characters in XMPP systems. In
particular, service providers and software implementers are strongly
encouraged to apply the policies recommended in [RFC7564].
8. Conformance Requirements
This section describes a protocol feature set that summarizes the
conformance requirements of this specification (similar feature sets
are provided for XMPP in [RFC6120] and [RFC6121]). The summary is
purely informational, and the conformance keywords of [RFC2119] as
used here are intended only to briefly describe the referenced
normative text from the body of this specification. This feature set
is appropriate for use in software certification, interoperability
testing, and implementation reports. For each feature, this section
provides the following information:
o A human-readable name
o An informational description
o A reference to the particular section of this document that
normatively defines the feature
o Whether the feature applies to the client role, the server role,
or both (where "N/A" signifies that the feature is not applicable
to the specified role)
o Whether the feature MUST or SHOULD be implemented, where the
capitalized terms are to be understood as described in [RFC2119]
The feature set specified here provides a basis for interoperability
testing and follows the spirit of a proposal made by Larry Masinter
within the IETF's NEWTRK working group in 2005 [INTEROP].
Feature: address-domain-length
Description: Ensure that the domainpart of an XMPP address is at
least one octet in length and at most 1023 octets in length, and
that it conforms to the underlying length limits of the DNS.
Section: Section 3.2
Roles: Server MUST, client SHOULD.
Feature: address-domain-prep
Description: Ensure that the domainpart of an XMPP address conforms
to IDNA2008, that it contains only NR-LDH labels and U-labels (not
A-labels), and that all uppercase and titlecase code points are
mapped to their lowercase equivalents.
Section: Section 3.2
Roles: Server MUST, client SHOULD.
Feature: address-localpart-length
Description: Ensure that the localpart of an XMPP address is at
least one octet in length and at most 1023 octets in length.
Section: Section 3.3
Roles: Server MUST, client SHOULD.
Feature: address-localpart-prep
Description: Ensure that the localpart of an XMPP address conforms
to the UsernameCaseMapped profile of the PRECIS IdentifierClass.
Section: Section 3.3
Roles: Server MUST, client SHOULD.
Feature: address-resource-length
Description: Ensure that the resourcepart of an XMPP address is at
least one octet in length and at most 1023 octets in length.
Section: Section 3.4
Roles: Server MUST, client SHOULD.
Feature: address-resource-prep
Description: Ensure that the resourcepart of an XMPP address
conforms to the OpaqueString profile of the PRECIS FreeformClass.
Section: Section 3.4
Roles: Server MUST, client SHOULD.
9. References
9.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
November 2003, <http://www.rfc-editor.org/info/rfc3629>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC5892, August 2010,
<http://www.rfc-editor.org/info/rfc5892>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<http://www.rfc-editor.org/info/rfc6365>.
[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, <http://www.rfc-editor.org/info/rfc6874>.
[RFC7564] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 7564, DOI 10.17487/RFC7564, May 2015,
<http://www.rfc-editor.org/info/rfc7564>.
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>.
[Unicode] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
[UTR36] Unicode Technical Report #36, "Unicode Security
Considerations", edited by Mark Davis and Michel Suignard,
<http://www.unicode.org/reports/tr36/>.
9.2. Informative References
[INTEROP] Masinter, L., "Formalizing IETF Interoperability
Reporting", Work in Progress,
draft-ietf-newtrk-interop-reports-00, October 2005.
[PRECIS-Nickname]
Saint-Andre, P., "Preparation, Enforcement, and Comparison
of Internationalized Strings Representing Nicknames", Work
in Progress, draft-ietf-precis-nickname-18, June 2015.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535,
DOI 10.17487/RFC1535, October 1993,
<http://www.rfc-editor.org/info/rfc1535>.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
DOI 10.17487/RFC3454, December 2002,
<http://www.rfc-editor.org/info/rfc3454>.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, DOI 10.17487/RFC3490, March 2003,
<http://www.rfc-editor.org/info/rfc3490>.
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC3920,
October 2004, <http://www.rfc-editor.org/info/rfc3920>.
[RFC3921] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 3921, DOI 10.17487/RFC3921, October 2004,
<http://www.rfc-editor.org/info/rfc3921>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <http://www.rfc-editor.org/info/rfc3987>.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, DOI 10.17487/RFC4013,
February 2005, <http://www.rfc-editor.org/info/rfc4013>.
[RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<http://www.rfc-editor.org/info/rfc4422>.
[RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)",
RFC 5122, DOI 10.17487/RFC5122, February 2008,
<http://www.rfc-editor.org/info/rfc5122>.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, DOI 10.17487/RFC5895, September 2010,
<http://www.rfc-editor.org/info/rfc5895>.
[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 6121, DOI 10.17487/RFC6121, March 2011,
<http://www.rfc-editor.org/info/rfc6121>.
[RFC6122] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", RFC 6122,
DOI 10.17487/RFC6122, March 2011,
<http://www.rfc-editor.org/info/rfc6122>.
[RFC6885] Blanchet, M. and A. Sullivan, "Stringprep Revision and
Problem Statement for the Preparation and Comparison of
Internationalized Strings (PRECIS)", RFC 6885,
DOI 10.17487/RFC6885, March 2013,
<http://www.rfc-editor.org/info/rfc6885>.
[UTS39] Unicode Technical Standard #39, "Unicode Security
Mechanisms", edited by Mark Davis and Michel Suignard,
<http://unicode.org/reports/tr39/>.
[XEP-0004] Eatmon, R., Hildebrand, J., Miller, J., Muldowney, T., and
P. Saint-Andre, "Data Forms", XSF XEP 0004, August 2007,
<http://xmpp.org/extensions/xep-0004.html>.
[XEP-0016] Millard, P. and P. Saint-Andre, "Privacy Lists",
XSF XEP 0016, February 2007,
<http://xmpp.org/extensions/xep-0016.html>.
[XEP-0029] Kaes, C., "Definition of Jabber Identifiers (JIDs)",
XSF XEP 0029, October 2003,
<http://xmpp.org/extensions/xep-0029.html>.
[XEP-0030] Hildebrand, J., Millard, P., Eatmon, R., and P.
Saint-Andre, "Service Discovery", XSF XEP 0030, June 2008,
<http://xmpp.org/extensions/xep-0030.html>.
[XEP-0045] Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
February 2012, <http://xmpp.org/extensions/xep-0045.html>.
[XEP-0048] Blackman, R., Millard, P., and P. Saint-Andre,
"Bookmarks", XSF XEP 0048, November 2007,
<http://xmpp.org/extensions/xep-0048.html>.
[XEP-0054] Saint-Andre, P., "vcard-temp", XSF XEP 0054, July 2008,
<http://xmpp.org/extensions/xep-0054.html>.
[XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer,
"Publish-Subscribe", XSF XEP 0060, July 2010,
<http://xmpp.org/extensions/xep-0060.html>.
[XEP-0065] Smith, D., Miller, M., Saint-Andre, P., and J. Karneges,
"SOCKS5 Bytestreams", XSF XEP 0065, April 2011,
<http://xmpp.org/extensions/xep-0065.html>.
[XEP-0077] Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
January 2012, <http://xmpp.org/extensions/xep-0077.html>.
[XEP-0106] Hildebrand, J. and P. Saint-Andre, "JID Escaping",
XSF XEP 0106, June 2007,
<http://xmpp.org/extensions/xep-0106.html>.
[XEP-0114] Saint-Andre, P., "Jabber Component Protocol",
XSF XEP 0114, January 2012,
<http://xmpp.org/extensions/xep-0114.html>.
[XEP-0144] Saint-Andre, P., "Roster Item Exchange", XSF XEP 0144,
August 2005, <http://xmpp.org/extensions/xep-0144.html>.
[XEP-0166] Ludwig, S., Beda, J., Saint-Andre, P., McQueen, R., Egan,
S., and J. Hildebrand, "Jingle", XSF XEP 0166,
December 2009, <http://xmpp.org/extensions/xep-0166.html>.
[XEP-0191] Saint-Andre, P., "Blocking Command", XSF XEP 0191,
July 2012, <http://xmpp.org/extensions/xep-0191.html>.
[XEP-0203] Saint-Andre, P., "Delayed Delivery", XSF XEP 0203,
September 2009,
<http://xmpp.org/extensions/xep-0203.html>.
[XEP-0220] Miller, J., Saint-Andre, P., and P. Hancke, "Server
Dialback", XSF XEP 0220, August 2014,
<http://xmpp.org/extensions/xep-0220.html>.
[XEP-0292] Saint-Andre, P. and S. Mizzi, "vCard4 Over XMPP",
XSF XEP 0292, September 2013,
<http://xmpp.org/extensions/xep-0292.html>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation
REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
Appendix A. Differences from RFC 6122
Based on consensus derived from working group discussion,
implementation and deployment experience, and formal interoperability
testing, the following substantive modifications were made from
RFC 6122.
o Changed domainpart preparation to use IDNA2008 (instead of
IDNA2003).
o Changed localpart preparation to use the UsernameCaseMapped
profile of the PRECIS IdentifierClass (instead of the Nodeprep
profile of stringprep).
o Changed resourcepart preparation to use the OpaqueString profile
of the PRECIS FreeformClass (instead of the Resourceprep profile
of stringprep).
o Specified that internationalized labels within domainparts must be
U-labels (instead of "should be" U-labels).
o Specified that fullwidth and halfwidth characters must be mapped
to their decomposition mappings (previously handled through the
use of Normalization Form KC).
o Specified the use of Unicode Normalization Form C (instead of
Unicode Normalization Form KC as specified in the Nodeprep and
Resourceprep profiles of stringprep).
o Specified that servers must enforce the address-formatting rules.
Acknowledgements
Thanks to Ben Campbell, Dave Cridland, Miguel Garcia, Joe Hildebrand,
Jonathan Lennox, Matt Miller, Florian Schmaus, Sam Whited, and
Florian Zeitz for their input during working group discussion.
Dan Romascanu completed a helpful review on behalf of the General
Area Review Team.
During IESG review, Alissa Cooper, Brian Haberman, and Barry Leiba
provided comments that led to improvements in the document.
Thanks also to Matt Miller in his role as document shepherd, Joe
Hildebrand in his role as working group chair, and Ben Campbell in
his role as sponsoring Area Director.
The author wishes to acknowledge Cisco Systems, Inc., for employing
him during his work on earlier draft versions of this document.
Author's Address
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/