Internet DRAFT - draft-ietf-radext-nai
draft-ietf-radext-nai
RADEXT Working Group DeKok, Alan
INTERNET-DRAFT FreeRADIUS
Obsoletes: 4282
Category: Standards Track
<draft-ietf-radext-nai-15.txt>
17 December 2014
The Network Access Identifier
draft-ietf-radext-nai-15
Abstract
In order to provide inter-domain authentication services, it is
necessary to have a standardized method that domains can use to
identify each other's users. This document defines the syntax for
the Network Access Identifier (NAI), the user identifier submitted by
the client prior to accessing resources. This document is a revised
version of RFC 4282, which addresses issues with international
character sets, as well as a number of other corrections to the
previous document.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on June 17, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
Appendix A - Changes from RFC4282 ............................ 3
1. Introduction ............................................. 4
1.1. Terminology ......................................... 6
1.2. Requirements Language ............................... 7
1.3. Purpose ............................................. 8
1.4. Motivation .......................................... 9
2. NAI Definition ........................................... 10
2.1. UTF-8 Syntax and Normalization ...................... 10
2.2. Formal Syntax ....................................... 11
2.3. NAI Length Considerations ........................... 11
2.4. Support for Username Privacy ........................ 12
2.5. International Character Sets ........................ 13
2.6. The Normalization Process ........................... 14
2.6.1. Issues with the Normalization Process .......... 15
2.7. Use in Other Protocols .............................. 16
2.8. Using the NAI format for other identifiers .......... 17
3. Routing inside of AAA Systems ............................ 18
3.1. Compatibility with Email Usernames .................. 19
3.2. Compatibility with DNS .............................. 19
3.3. Realm Construction .................................. 20
3.3.1. Historical Practices ........................... 21
3.4. Examples ............................................ 22
4. Security Considerations .................................. 23
4.1. Correlation of Identities over Time and Protocols ... 23
4.2. Multiple Identifiers ................................ 23
5. Administration of Names .................................. 24
6. IANA Considerations ...................................... 25
7. References ............................................... 25
7.1. Normative References ................................ 25
7.2. Informative References .............................. 26
Appendix A - Changes from RFC4282 ............................ 29
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1. Introduction
Considerable interest exists for a set of features that fit within
the general category of inter-domain authentication, or "roaming
capability" for network access, including dialup Internet users,
Virtual Private Network (VPN) usage, wireless LAN authentication, and
other applications.
By "inter-domain authentication", this document refers to situations
where a user has authentication credentials at one "home" domain, but
is able to present them at a second "visited" domain to access
certain services at the visited domain. The two domains generally
have a pre-existing relationship, so that the credentials can be
passed from the visited domain to the home domain for verification.
The home domain typically responds with a permit / deny response,
which may also include authorization parameters which the visited
domain is expected to enforce on the user.
That is, the "roaming" scenario involves a user visiting, or
"roaming" to a non-home domain, and requesting the use of services at
that visted domain.
Interested parties have included the following:
* Regional Internet Service Providers (ISPs) operating within a
particular state or province, looking to combine their efforts
with those of other regional providers to offer dialup service
over a wider area.
* Telecommunications companies who wish to combine their
operations with those of one or more companies in another areas or
nations, in order to offer more comprehensive network access
service in areas where there is no native service. e.g. In
another country.
* Wireless LAN hotspots providing service to one or more ISPs.
* Businesses desiring to offer their employees a comprehensive
package of dialup services on a global basis. Those services may
include Internet access as well as secure access to corporate
intranets via a VPN, enabled by tunneling protocols such as the
Point-to-Point Tunneling Protocol (PPTP) [RFC2637], the Layer 2
Forwarding (L2F) protocol [RFC2341], the Layer 2 Tunneling
Protocol (L2TP) [RFC2661], and the IPsec tunnel mode [RFC4301].
* Other protocols which are interested in leveraging the users
credentials in order to take advantage of an existing
authentication framework.
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In order to enhance the interoperability of these services, it is
necessary to have a standardized method for identifying users. This
document defines syntax for the Network Access Identifier (NAI).
Examples of implementations that use the NAI, and descriptions of its
semantics, can be found in [RFC2194].
When the NAI was defined for network access, it had the side effect
of defining an identifier which could be used in non-AAA systems.
Some non-AAA systems defined identifiers which were compatible with
the NAI, and deployments used the NAI. This process simplified the
management of credentials, by re-using the same credential in
multiple situations. Protocols that re-use the same credential or
the same identifier format can benefit from this management
simplicity. The alternative is to have protocol-specific credentials
or identifier formats, which increases cost to both the user and the
administrator.
There are privacy implications to using one identifier across
multiple protocols. See Section 2.7 and Section 4 for further
discussion of this topic.
The goal of this document is to define the format of an identifier
which can be used in many protocols. A protocol may transport an
encoded version of the NAI (e.g. '.' as %2E). However, the
definition of the NAI is protocol independent. The goal of this
document is to encourage the wide-spread adoption of the NAI format.
This adoption will decrease work required to leverage identification
and authentication in other protocols. It will also decrease the
complexity of non-AAA systems for end users and administrators.
This document only suggests that the NAI format be used, but does not
require such use. Many protocols already define their own identifier
formats. Some of these are incompatible with the NAI, while others
allow the NAI in addition to non-NAI identifiers. The definition of
the NAI in this document has no requirements on protocol
specifications, implementations, or deployments.
However, this document suggests that using one standard identifier
format is preferable to using multiple incompatible identifier
formats. Where identifiers need to be used in new protocols and/or
specifications, it is RECOMMENDED that the format of the NAI be used.
That is, the interpretation of the identifier is context-specific,
while the format of the identifier remains the same. These issues
are discussed in more detail in Section 2.8, below.
The recommendation for a standard identifier format is not a
recommendation that each user have one universal identifier. In
contrast, this document allows for the use of multiple identifiers,
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and recommends the use of anonymous identifiers where those
identifiers are publicly visible.
This document is a revised version of [RFC4282], which originally
defined internationalized NAIs. Differences and enhancements
compared to that document are listed in Appendix A.
1.1. Terminology
This document frequently uses the following terms:
"Local" or "localized" text
Text which is either in non-UTF-8, or in non-normalized form. The
character set, encoding, and locale are (in general) unknown to
Authentication, Authorization, and Accounting (AAA) network
protocols. The client which "knows" the locale may have a
different concept of this text than other AAA entities, which do
not know the same locale.
Network Access Identifier
The Network Access Identifier (NAI) is a common format for user
identifiers submitted by a client during authentication. The
purpose of the NAI is to allow a user to be associated with an
account name, as well as to assist in the routing of the
authentication request across multiple domains. Please note that
the NAI may not necessarily be the same as the user's email
address or the user identifier submitted in an application layer
authentication.
Network Access Server
The Network Access Server (NAS) is the device that clients connect
to in order to get access to the network. In PPTP terminology,
this is referred to as the PPTP Access Concentrator (PAC), and in
L2TP terminology, it is referred to as the L2TP Access
Concentrator (LAC). In IEEE 802.11, it is referred to as an
Access Point.
Roaming Capability
Roaming capability can be loosely defined as the ability to use
any one of multiple Internet Service Providers (ISPs), while
maintaining a formal, customer-vendor relationship with only one.
Examples of cases where roaming capability might be required
include ISP "confederations" and ISP-provided corporate network
access support.
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Normalization or Canonicalization
These terms are defined in [RFC6365] Section 4. Those definitions
are incorporated here by reference.
Locale
This term is defined in [RFC6365] Section 8. Those definitions
are incorporated here by reference.
Tunneling Service
A tunneling service is any network service enabled by tunneling
protocols such as PPTP, L2F, L2TP, and IPsec tunnel mode. One
example of a tunneling service is secure access to corporate
intranets via a Virtual Private Network (VPN).
1.2. Requirements Language
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].
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1.3. Purpose
As described in [RFC2194], there are a number of providers offering
network access services, and essentially all Internet Service
Providers are involved in roaming consortia.
In order to be able to offer roaming capability, one of the
requirements is to be able to identify the user's home authentication
server. For use in roaming, this function is accomplished via the
Network Access Identifier (NAI) submitted by the user to the NAS in
the initial network authentication. It is also expected that NASes
will use the NAI as part of the process of opening a new tunnel, in
order to determine the tunnel endpoint.
This document suggests that other protocols can take advantage of the
NAI format. Many protocols include authentication capabilities,
including defining their own identifier formats. These identifiers
can then end up being transported in AAA protocols, so that the
originating protocols can leverage AAA for user authentication.
There is therefore a need for a definition of a user identifier which
can be used in multiple protocols.
While the NAI is defined herein, it should be noted that existing
protocols and deployments do not always use it. AAA systems MUST
therefore be able to handle user identifiers which are not in the NAI
format. The process by which that is done is outside of the scope of
this document.
Non-AAA systems can accept user identifiers in forms other than the
NAI. This specification does not forbid that practice. It only
codifies the format and interpretation of the NAI. This document
cannot change existing protocols or practices. It can, however,
suggest that using a consistent form for a user identifier is of a
benefit to the community.
This document does not make any protocol-specific definitions for an
identifier format, and it does not make changes to any existing
protocol. Instead, it defines a protocol-independent form for the
NAI. It is hoped that the NAI is a user identifier which can be used
in multiple protocols.
Using a common identifier format implifies protocols requiring
authentication, as they no longer need to specify protocol-specific
format for user identifiers. It increases security, as it multiple
identifier formats allow attackers to make contradictory claims
without being detected (see Section 4.2 for further discussion of
this topic). It simplifies deployments, as a user can have one
identifier in multiple contexts, which allows them to be uniquely
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identified, so long as that identifier is itself protected against
unauthorized access.
In short, having a standard is better than having no standard at all.
1.4. Motivation
The changes from [RFC4282] are listed in detail in Appendix A.
However, some additional discussion is appropriate to motivate those
changes.
The motivation to revise [RFC4282] began with internationalization
concerns raised in the context of [EDUROAM]. Section 2.1 of
[RFC4282] defines ABNF for realms which limits the realm grammar to
English letters, digits, and the hyphen "-" character. The intent
appears to have been to encode, compare, and transport realms with
the Punycode [RFC3492] encoding form as described in [RFC5891] There
are a number of problems with this approach:
* The [RFC4282] ABNF is not aligned with internationalization of DNS.
* The requirement in [RFC4282] Section 2.1 that realms are ASCII
conflicts with the Extensible Authentication Protocol (EAP)
defined in [RFC3748], and RADIUS, which are both 8-bit clean, and
which both recommend the use of UTF-8 for identitifiers.
* [RFC4282] Section 2.4 required mappings that are
language-specific, and which are nearly impossible for
intermediate nodes to perform correctly without information about
that language.
* [RFC4282] Section 2.4 requires normalization of user names,
which may conflict with local system or administrative
requirements.
* The recommendations in RFC4282] Section 2.4 for treatment of
bidirectional characters have proven to be unworkable.
* The prohibition against use of unassigned code points in
RFC4282] Section 2.4 effectively prohibits support for new
scripts.
* No Authentication, Authorization, and Accounting (AAA)
client, proxy, or server has implemented any of the requirements
in [RFC4282] Section 2.4, among other sections.
With international roaming growing in popularity, it is important for
these issues to be corrected in order to provide robust and inter-
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operable network services.
Furthermore, this document was motivated by a desire to codify
existing practice related to the use of the NAI format and to
encourage widespread use of the format.
2. NAI Definition
2.1. UTF-8 Syntax and Normalization
UTF-8 characters can be defined in terms of octets using the
following ABNF [RFC5234], taken from [RFC3629]:
UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail /
%xE1-EC 2(UTF8-tail) /
%xED %x80-9F UTF8-tail /
%xEE-EF 2(UTF8-tail)
UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) /
%xF1-F3 3( UTF8-tail ) /
%xF4 %x80-8F 2( UTF8-tail )
UTF8-tail = %x80-BF
These are normatively defined in [RFC3629], but are repeated in this
document for reasons of convenience.
See [RFC5198] and section 2.6 of this specification for a discussion
of normalization. Strings which are not in Normal Form Composed (NFC)
are not valid NAIs and SHOULD NOT be treated as such.
Implementations which expect to receive a NAI, but which instead
receive non-normalised (but otherwise valid) UTF-8 strings instead
SHOULD attempt to create a local version of the NAI, which is
normalized from the input identifier. This local version can then be
used for local processing. This local version of the identifier MUST
NOT be used outside of the local context.
Where protocols carry identifiers which are expected to be
transported over an AAA protocol, it is RECOMMENDED that the
identifiers be in NAI format. Where the identifiers are not in the
NAI format, it is up to the AAA systems to discover this, and to
process them. This document does not suggest how that is done.
However, existing practice indicates that it is possible.
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As internationalized domain names become more widely used, existing
practices are likely to become inadequate. This document therefore
defines the NAI, which is a user identifier format that can correctly
deal with internationalized identifiers.
2.2. Formal Syntax
The grammar for the NAI is given below, described in Augmented
Backus-Naur Form (ABNF) as documented in [RFC5234].
nai = utf8-username
nai =/ "@" utf8-realm
nai =/ utf8-username "@" utf8-realm
utf8-username = dot-string
dot-string = string *("." string)
string = 1*utf8-atext
utf8-atext = ALPHA / DIGIT /
"!" / "#" /
"$" / "%" /
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~" /
UTF8-xtra-char
utf8-realm = 1*( label "." ) label
label = utf8-rtext *(ldh-str)
ldh-str = *( utf8-rtext / "-" ) utf8-rtext
utf8-rtext = ALPHA / DIGIT / UTF8-xtra-char
2.3. NAI Length Considerations
Devices handling NAIs MUST support an NAI length of at least 72
octets. Devices SHOULD support an NAI length of 253 octets.
However, the following implementation issues should be considered:
* NAI octet length constraints may impose a more severe constraint
on the number of UTF-8 characters.
* NAIs are often transported in the User-Name attribute of the
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Remote Authentication Dial-In User Service (RADIUS) protocol.
Unfortunately, RFC 2865 [RFC2865], Section 5.1, states that "the
ability to handle at least 63 octets is recommended." As a
result, it may not be possible to transfer NAIs beyond 63 octets
through all devices. In addition, since only a single User-Name
attribute may be included in a RADIUS message and the maximum
attribute length is 253 octets, RADIUS is unable to support NAI
lengths beyond 253 octets.
* NAIs can also be transported in the User-Name attribute of
Diameter [RFC6733], which supports content lengths up to 2^24 - 9
octets. As a result, NAIs processed only by Diameter nodes can be
very long. However, an NAI transported over Diameter may
eventually be translated to RADIUS, in which case the above
limitations will apply.
* NAIs may be transported in other protocols. Each protocol
can have its own limitations on maximum NAI length.
The above criteria should permit the widest use, and widest possible
inter-operability of the NAI.
2.4. Support for Username Privacy
Interpretation of the username part of the NAI depends on the realm
in question. Therefore, the utf8-username portion SHOULD be treated
as opaque data when processed by nodes that are not a part of the
home domain for that realm.
That is, the only domain which is capable of interpreting the meaning
of the utf8-username portion of the NAI is the home domain. Any
third-party domains cannot form any conclusions about the
utf8-username, and cannot decode it into sub-fields. For example, it
may be used as "firstname.lastname", or it may be entirely digits, or
it may be a random hex identifier. There is simply no way (and no
reason) for any other domain to interpret the utf8-username field as
having any meaning whatsoever.
In some situations, NAIs are used together with a separate
authentication method that can transfer the username part in a more
secure manner to increase privacy. In this case, NAIs MAY be
provided in an abbreviated form by omitting the username part.
Omitting the username part is RECOMMENDED over using a fixed username
part, such as "anonymous", since including a fixed username part is
ambiguous as to whether or not the NAI refers to a single user.
However, current practice is to use the username "anonymous" instead
of omitting the username part. This behavior is also permitted.
The most common use-case of omitting or obfuscating the username part
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is with TLS-based EAP methods such as TTLS [RFC5281]. Those methods
allow for an "outer" identifier, which is typically an anonymous
"@realm". This outer identifier allows the authentication request to
be routed from a visited domain to a home domain. At the same time,
the username part is kept confidential from the visited network. The
protocol provides for an "inner" authentication exchange, in which a
full identifier is used to authenticate a user.
That scenario offers the best of both worlds. An anonymous NAI can
be used to route authentication to the home domain, and the home
domain has sufficient information to identify and authenticate users.
However, some protocols do not support authenticate methods which
allow for "inner" and "outer" exchanges. Those protocols are limited
to using an identifier which is publicly visible. It is therefore
RECOMMENDED that such protocols use ephemeral identifiers. We
recognize that this practice is not currently used, and will likely
be difficult to implement.
Similarly to the anonymous user, there may be situations where
portions of the realm are sensitive. For those situations, it is
RECOMMENDED that the sensitive portion of the realm also be omitted.
e.g. To use "@example.com" instead of "@sensitive.example.com", or
"anonymous@sensitive.example.com". The home domain is authoritative
for users in all subdomains, and can (if necessary) route the
authentication request to the appropriate subsystem within the home
domain.
For roaming purposes, it is typically necessary to locate the
appropriate backend authentication server for the given NAI before
the authentication conversation can proceed. As a result,
authentication routing is impossible unless the realm portion is
available, and in a well-known format.
2.5. International Character Sets
This specification allows both international usernames and realms.
International usernames are based on the use of Unicode characters,
encoded as UTF-8. Internationalization of the username portion of
the NAI is based on the "Internationalized Email Headers" [RFC6532]
extensions to the "local-part" portion of email addresses [RFC5322].
In order to ensure a canonical representation, characters of the
realm portion in an NAI MUST match the ABNF in this specification as
well as the requirements specified in [RFC5891]. In practice, these
requirements consist of the following item:
* Realms MUST be of the form that can be registered as a
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Fully Qualified Domain Name (FQDN) within the DNS.
This list is significantly shorter and simpler than the list in
Section 2.4 of [RFC4282]. The form suggested in [RFC4282] depended
on intermediate nodes performing canonicalizations based on
insufficient information, which meant that the form was not
canonical.
Specifying the realm requirement as above means that the requirements
depend on specifications that are referenced here, rather than copied
here. This allows the realm definition to be updated when the
referenced documents change, without requiring a revision of this
specification.
One caveat on the above recommendation is the issues noted in
[RFC6912]. That document notes that there are additional
restrictions around DNS registration which forbid some code points
from being valid in a DNS U-label. These restrictions cannot be
expressed algorithmically.
For this specification, that caveat means the following. Realms not
matching the above ABNF are not valid NAIs. However, some realms
which do match the ABNF are still invalid NAIs. That is, matching
the ABNF is a necessary, but not sufficient, requirement for an NAI.
In general, the above requirement means following the requirements
specified in [RFC5891].
2.6. The Normalization Process
Conversion to Unicode as well as normalization SHOULD be performed by
edge systems (e.g. laptops, desktops, smart phones, etc.) that take
"local" text as input. These edge systems are best suited to
determine the users intent, and can best convert from "local" text to
a normalized form.
Other AAA systems such as proxies do not have access to locale and
character set information that is available to edge systems.
Therefore, they may not always be able to convert local input to
Unicode.
That is, all processing of NAIs from "local" character sets and
locales to UTF-8 SHOULD be performed by edge systems, prior to the
NAIs entering the AAA system. Inside of an AAA system, NAIs are sent
over the wire in their canonical form, and this canonical form is
used for all NAI and/or realm comparisons.
Copying of localized text into fields that can subsequently be placed
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into the RADIUS User-Name attribute is problematic. This practice
can result in a AAA proxy encountering non-UTF8 characters within
what it expects to be an NAI. An example of this requirement is
[RFC3579] Section 2.1, which states:
the NAS MUST copy the contents of the Type-Data field of the
EAP-Response/Identity received from the peer into the User-Name
attribute
As a result, AAA proxies expect the contents of the EAP-
Response/Identity sent by an EAP supplicant to consist of UTF-8
characters, not localized text. Using localized text in AAA username
or identity fields means that realm routing becomes difficult or
impossible.
In contrast to [RFC4282] Section 2.4, AAA systems are now expected to
perform NAI comparisons, matching, and AAA routing based on the NAI
as it is received. This specification provides a canonical
representation, ensures that intermediate AAA systems such as proxies
are not required to perform translations, and can be expected to work
through AAA systems that are unaware of international character sets.
In an ideal world, the following requirements would be widely
implemented:
* Edge systems using "localized" text SHOULD normalize the NAI
prior to it being used as an identifier in an authentication
protocol.
* AAA systems SHOULD NOT normalize the NAI, as they may not have
sufficient information to perform the normalization.
There are issues with this approach, however.
2.6.1. Issues with the Normalization Process
The requirements in the preceding section are not implemented today.
For example, most EAP implementations use a user identifier which is
passed to them from some other local system. This identifier is
treated as an opaque blob, and is placed as-is into the EAP Identity
field. Any subsequent system which receives that identifier is
assumed to be able to understand and process it.
This opaque blob unfortunately can contain localized text, which
means that the AAA systems have to process that text.
These limitations have the following theoretical and practical
implications.
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* edge systems used today generally do not normalize the NAI
* Therefore AAA systems SHOUD attempt to normalize the NAI
The suggestion in the above sentence contradicts the suggestion in
the previous section. This is the reality of imperfect protocols.
Where the user identifier can be normalized, or determined to be in
normal form, the normal form MUST be used as the NAI. In all other
circumstances, the user identifier MUST NOT be treated as an NAI.
That data is still, however, a user identifier. AAA systems MUST NOT
fail authentication simply because the user identifier is not an NAI.
That is, when the realm portion of the NAI is not recognized by an
AAA server, it SHOULD try to normalize the NAI into NFC form. That
normalized form can then be used to see if the realm matches a known
realm. If no match is found, the original form of the NAI SHOULD be
used in all subsequent processing.
The AAA server may also convert realms to punycode, and perform all
realm comparisons on the resulting punycode strings. This conversion
follows the recommendations above, but may have different operational
effects and failure modes.
2.7. Use in Other Protocols
As noted earlier, the NAI format can be used in other, non-AAA
protocols. It is RECOMMENDED that the definition given here be used
unchanged. Using other definitions for user identifiers may hinder
interoperability, along with the users ability to authenticate
successfully. It is RECOMMENDED that protocols requiring the use of
a user identifier use the NAI format.
This document cannot require other protocols to use the NAI format
for user identifiers. Their needs are unknown, and at this time
unknowable. This document suggests that interoperability and inter-
domain authentication is useful, and should be encouraged.
Where a protocol is 8-bit clean, it can likely transport the NAI as-
is, without further modification.
Where a protocol is not 8-bit clean, it cannot transport the NAI as-
is. Instead, this document presumes that a protocol-specific
transport layer takes care of encoding the NAI on input to the
protocol, and decoding it when the NAI exits the protocol. The
encoded or escaped version of the NAI is not a valid NAI, and MUST
NOT be presented to the AAA system.
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For example, HTTP carries user identifiers, but escapes the '.'
character as "%2E" (among others). When HTTP is used to transport
the NAI "fred@example.com", the data as transported will be in the
form "fred@example%2Ecom". That data exists only within HTTP, and
has no relevance to any AAA system.
Any comparison, validation, or use of the NAI MUST be done on its un-
escaped (i.e. utf8-clean) form.
2.8. Using the NAI format for other identifiers
As discussed in Section 1, above, is RECOMMENDED that the NAI format
be used as the standard format for user identifiers. This section
discusses that use in more detail.
It is often useful to create new identifiers for use in specific
contexts. These identifiers may have a number of different
properties, most of which are unimportant to this document. The goal
of this document is to create identifiers which are to be in a well-
known format, and to have namespaces. The NAI format fits these
requirements.
One example of such use is the "private user identity", which is an
identifier defined by the 3rd-Generation Partnership Project (3GPP).
That identifier is used to uniquely identify the user to the network.
The identifier is used for authorization, authentication, accounting,
administation, etc. The "private user identity" is globally unique,
and is defined by the home network operator. The format of the
identifier is explicitly the NAI, as stated by Section 13.3 of
[3GPP]:
The private user identity shall take the form of an NAI, and shall
have the form username@realm as specified in clause 2.1 of IETF
RFC 4282
For 3GPP, the "username" portion is a unique identifier which is
derived from device-specific information. The "realm" portion is
composed of information about the home network, followed by the base
string "3gppnetwork.org". e.g.
234150999999999@ims.mnc015.mcc234.3gppnetwork.org.
This format as defiend by 3GPP ensures that the identifier is
globally unique, as it is based off of the "3gppnetwork.org" domain.
It ensures that the "realm" portion is specific to a particular home
network (or organization), via the "ims.mnc015.mcc234" prefix to the
realm. Finally, it ensures that the "username" portion follows a
well-known format.
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This document suggests that the NAI format be used for all new
specifications and/or protocols where a user identifier is required.
Where the username portions need to be created with subfields, a
well-known and documented method as has been done with 3GPP is
preferred to ad-hoc methods.
3. Routing inside of AAA Systems
Many AAA systems use the "utf8-realm" portion of the NAI to route
requests within a AAA proxy network. The semantics of this operation
involves a logical AAA routing table, where the "utf8-realm" portion
acts as a key, and the values stored in the table are one or more
"next hop" AAA servers.
Intermediate nodes MUST use the "utf8-realm" portion of the NAI
without modification to perform this lookup. As noted earlier,
intermediate nodes may not have access to the same locale information
as the system which injected the NAI into the AAA routing systems.
Therefore, almost all "case insensitive" comparisons can be wrong.
Where the "utf8-realm" is entirely ASCII, current AAA systems
sometimes perform case-insensitive matching on realms. This method
MAY be continued, as it has been shown to work in practice.
Many existing non-AAA systems have user identifiers which are similar
in format to the NAI, but which are not compliant with this
specification. For example, they may use non-NFC form, or they may
have multiple "@" characters in the user identifier. Intermediate
nodes SHOULD normalize non-NFC identifiers to NFC, prior to looking
up the "utf8-realm" in the logical routing table. Intermediate nodes
MUST NOT modify the identifiers that they forward. The data as
entered by the user is inviolate.
The "utf8-realm" provisioned in the logical AAA routing table SHOULD
be provisioned to the proxy prior to it receiving any AAA traffic.
The "utf8-realm" SHOULD be supplied by the "next hop" or "home"
system that also supplies the routing information necessary for
packets to reach the next hop.
This "next hop" information may be any of, or all of, the following
information: IP address; port; RADIUS shared secret; TLS certificate;
DNS host name; or instruction to use dyanmic DNS discovery (i.e. look
up a record in the "utf8-realm" domain). This list is not
exhaustive, and my be extended by future specifications.
It is RECOMMENDED to use the entirety of the "utf8-realm" for the
routing decisions. However, AAA systems MAY use a portion of the
"utf8-realm" portion, so long as that portion is a valid
"utf8-realm", and that portion is handled as above. For example,
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routing "fred@example.com" to a "com" destination is forbidden,
because "com" is not a valid "utf8-realm". However, routing
"fred@sales.example.com" to the "example.com" destination is
permissible.
Another reason to forbid the use of a single label (e.g.
"fred@sales") is that many non-AAA systems treat a single label as
being a local identifier within their realm. That is, a user logging
in as "fred@sales" to a domain "example.com", would be treated as if
the NAI was instead "fred@sales.example.com". Permitting the use of
a single label would mean changing the interpretation and meaning of
a single label, which cannot be done.
3.1. Compatibility with Email Usernames
As proposed in this document, the Network Access Identifier is of the
form "user@realm". Please note that while the user portion of the NAI
is based on the "Internet Message Format" [RFC5322] "local-part"
portion of an email address as extended by "Internationalized Email
Headers" [RFC6532], it has been modified for the purposes of Section
2.2. It does not permit quoted text along with "folding" or "non-
folding" whitespace that is commonly used in email addresses. As
such, the NAI is not necessarily equivalent to usernames used in e-
mail.
However, it is a common practice to use email addresses as user
identifiers in AAA systems. The ABNF in Section 2.2 is defined to be
close to the "addr-spec" portion of [RFC5322] as extended by
[RFC6532], while still being compatible with [RFC4282].
In contrast to [RFC4282] Section 2.5, this document state that the
internationalization requirements for NAIs and email addresses are
substantially similar. The NAI and email identifiers may be the
same, and both need to be entered by the user and/or the operator
supplying network access to that user. There is therefore good
reason for the internationalization requirements to be similar.
3.2. Compatibility with DNS
The "utf8-realm" portion of the NAI is intended to be compatible with
Internationalized Domain Names (IDNs) [RFC5890]. As defined above,
the "utf8-realm" portion as transported within an 8-bit clean
protocol such as RADIUS and EAP can contain any valid UTF8 character.
There is therefore no reason for a NAS to convert the "utf8-realm"
portion of an NAI into Punycode encoding form [RFC3492] prior to
placing the NAI into a RADIUS User-Name attribute.
The NAI does not make a distinction between A-labels and U-labels, as
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those are terms specific to DNS. It is instead an IDNA-valid label,
as per the first item in Section 2.3.2.1 of [RFC5890]. As noted in
that section, the term "IDNA-valid label" encompases both of the
terms A-label and U-label.
When the realm portion of the NAI is used as the basis for name
resolution, it may be necessary to convert internationalized realm
names to Punycode [RFC3492] encoding form as described in [RFC5891].
As noted in [RFC6055] Section 2, resolver Application Programming
Interfaces (APIs) are not necessarily DNS-specific, so conversion to
Punycode needs to be done carefully:
Applications which convert an IDN to A-label form before calling (for
example) getaddrinfo() will result in name resolution failures if the
Punycode name is directly used in such protocols. Having libraries
or protocols to convert from A-labels to the encoding scheme defined
by the protocol (e.g., UTF-8) would require changes to APIs and/or
servers, which IDNA was intended to avoid.
As a result, applications SHOULD NOT assume that non-ASCII names are
resolvable using the public DNS and blindly convert them to A-labels
without knowledge of what protocol will be selected by the name
resolution library.
3.3. Realm Construction
The home realm usually appears in the "utf8-realm" portion of the
NAI, but in some cases a different realm can be used. This may be
useful, for instance, when the home realm is reachable only via
intermediate proxies.
Such usage may prevent interoperability unless the parties involved
have a mutual agreement that the usage is allowed. In particular,
NAIs MUST NOT use a different realm than the home realm unless the
sender has explicit knowledge that (a) the specified other realm is
available and (b) the other realm supports such usage. The sender
may determine the fulfillment of these conditions through a database,
dynamic discovery, or other means not specified here. Note that the
first condition is affected by roaming, as the availability of the
other realm may depend on the user's location or the desired
application.
The use of the home realm MUST be the default unless otherwise
configured.
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3.3.1. Historical Practices
Some AAA systems have historically used NAI modifications with
multiple "prefix" and "suffix" decorations to perform explicit
routing through multiple proxies inside of a AAA network.
In RADIUS based environment, the use of decorated NAI is NOT
RECOMMENDED for the following reasons:
* Using explicit routing paths is fragile, and is unresponsive to
changes in the network due to servers going up or down, or to
changing business relationships.
* There is no RADIUS routing protocol, meaning that routing paths
have to be communicated "out of band" to all intermediate AAA
nodes, and also to all edge systems (e.g. supplicants) expecting
to obtain network access.
* Using explicit routing paths requires thousands, if not
millions of edge systems to be updated with new path information
when a AAA routing path changes. This adds huge expense for
updates that would be better done at only a few AAA systems in the
network.
* Manual updates to RADIUS paths are expensive, time-consuming,
and prone to error.
* Creating compatible formats for the NAI is difficult
when locally-defined "prefixes" and "suffixes" conflict with
similar practices elsewhere in the network. These conflicts mean
that connecting two networks may be impossible in some cases, as
there is no way for packets to be routed properly in a way that
meets all requirements at all intermediate proxies.
* Leveraging the DNS name system for realm names establishes
a globally unique name space for realms.
In summary, network practices and capabilities have changed
significantly since NAIs were first overloaded to define AAA routes
through a network. While manually managed explicit path routing was
once useful, the time has come for better methods to be used.
Notwithstanding the above recommendations, the above practice is
widely used for Diameter routing [RFC5729]. The routes described
there are managed automatically, for both credential provisioning and
routing updates. Those routes also exist within a particular
framework (typically 3G), where membership is controlled and system
behavior is standardized. There are no known issues with using
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explicit routing in such an environment.
However, if decorated identifiers are used, such as:
homerealm.example.org!user@otherrealm.example.net
Then the part before the (non-escaped) '!' MUST be a "utf8-realm" as
defined in the ABNF in Section 2.2. When receiving such an
identifier, the "otherrealm.example.net" system MUST convert the
identifier to "user@homerealm.example.org" before forwarding the
request. The forwarding system MUST then apply normal AAA routing
for the transaction, based on the updated identifier.
3.4. Examples
Examples of valid Network Access Identifiers include the following:
bob
joe@example.com
fred@foo-9.example.com
jack@3rd.depts.example.com
fred.smith@example.com
fred_smith@example.com
fred$@example.com
fred=?#$&*+-/^smith@example.com
nancy@eng.example.net
eng.example.net!nancy@example.net
eng%nancy@example.net
@privatecorp.example.net
\(user\)@example.net
An additional valid NAI is the following, given as a hex string, as
this document can only contain ASCII characters.
626f 6240 ceb4 cebf ceba ceb9 cebc ceae 2e63 6f6d
Examples of invalid Network Access Identifiers include the following:
fred@example
fred@example_9.com
fred@example.net@example.net
fred.@example.net
eng:nancy@example.net
eng;nancy@example.net
(user)@example.net
<nancy>@example.net
One example given in [RFC4282] is still permitted by the ABNF, but it
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is NOT RECOMMMENDED because of the use of the Punycode [RFC3492]
encoding form for what is now a valid UTF-8 string.
alice@xn--tmonesimerkki-bfbb.example.net
4. Security Considerations
Since an NAI reveals the home affiliation of a user, it may assist an
attacker in further probing the username space. Typically, this
problem is of most concern in protocols that transmit the username in
clear-text across the Internet, such as in RADIUS, described in
[RFC2865] and [RFC2866]. In order to prevent snooping of the
username, protocols may use confidentiality services provided by
protocols transporting them, such as RADIUS protected by IPsec
[RFC3579] or Diameter protected by TLS [RFC6733].
This specification adds the possibility of hiding the username part
in the NAI, by omitting it. As discussed in Section 2.4, this is
possible only when NAIs are used together with a separate
authentication method that can transfer the username in a secure
manner. In some cases, application-specific privacy mechanism have
also been used with NAIs. For instance, some EAP methods apply
method-specific pseudonyms in the username part of the NAI [RFC3748].
While neither of these approaches can protect the realm part, their
advantage over transport protection is that privacy of the username
is protected, even through intermediate nodes such as NASes.
4.1. Correlation of Identities over Time and Protocols
The recommendations in Section 2.7 and Section 2.8 for using the NAI
in other protocols has implications for privacy. Any attacker who is
capable of observing traffic containing the NAI can track the user,
and correlate his activity across time and across multiple protocols.
The authentication credentials therefore SHOULD be transported over
channels which permit private communications, or multiple identifiers
SHOULD be used, so that user tracking is impossible.
It is RECOMMENDED that user privacy be enhanced by configuring
multiple identifiers for one user. These identifiers can be changed
over time, in order to make user tracking more difficult for a
malicous observer. However, provisioning and management of the
identifiers may be difficult in to do in practice, which is likely
why multiple identifiers are rarely used today.
4.2. Multiple Identifiers
Section 1.3 states that multiple identifier formats allow attackers
to make contradictory claims without being detected. This statement
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deserves further discussion.
Section 2.4 discussed "inner" and "outer" identifiers in the context
of TTLS [RFC5281]. A close reading of that specification shows there
is no requirement that the inner and outer identifiers be in any way
related. That is, it is perfectly valid to use "@example.com" for an
outer identifier, and "user@example.org" as an inner identifier. The
authentication request will then be routed to "example.com", which
will likely be unable to authenticate "user@example.org".
Even worse, a misconfiguration of "example.com" means that it may in
turn proxy the inner authentication request to the "example.org"
domain. Such cross-domain authentication is highly problematic, and
there are few good reasons to allow it.
It is therefore RECOMMENDED that systems which permit anonymous
"outer" identifiers require that the "inner" domain be the same as,
or a sub-domain of the "outer" domain. An authentication request
using disparate realms is a security violation, and the request
SHOULD be rejected.
The situation gets worse when multiple protocols are involved. The
TTLS protocol permits MS-CHAP [RFC2433] to be carried inside of the
TLS tunnel. MS-CHAP defines its own identifier which is encapsulated
inside of the MS-CHAP exchange. That identifier is not required to
be any particular format, is not required to be in UTF-8, and in
practice, can be one of many unknown character sets. There is no way
in practice to determine which character set was used for that
identifier.
The result is that the "outer" EAP Identity carried by TTLS is likely
to not even share the same character set as the "inner" identifier
used by MS-CHAP. The two identifiers are entirely independent, and
fundamentally incomparable.
Such protocol design is NOT RECOMMENDED.
5. Administration of Names
In order to avoid creating any new administrative procedures,
administration of the NAI realm namespace piggybacks on the
administration of the DNS namespace.
NAI realm names are required to be unique, and the rights to use a
given NAI realm for roaming purposes are obtained coincident with
acquiring the rights to use a particular Fully Qualified Domain Name
(FQDN). Those wishing to use an NAI realm name should first acquire
the rights to use the corresponding FQDN. Administrators MUST NOT
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publicly use an NAI realm without first owning the corresponding
FQDN. Private use of unowned NAI realms within an administative
domain is allowed, though it is RECOMMENDED that example names be
used, such as "example.com".
Note that the use of an FQDN as the realm name does not require use
of the DNS for location of the authentication server. While Diameter
[RFC6733] supports the use of DNS for location of authentication
servers, existing RADIUS implementations typically use proxy
configuration files in order to locate authentication servers within
a domain and perform authentication routing. The implementations
described in [RFC2194] did not use DNS for location of the
authentication server within a domain. Similarly, existing
implementations have not found a need for dynamic routing protocols
or propagation of global routing information. Note also that there
is no requirement that the NAI represent a valid email address.
6. IANA Considerations
This document has no actions for IANA.
7. References
7.1. Normative References
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March, 1997.
[RFC3629]
Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63,
RFC 3629, November 2003.
[RFC5198]
Klensin J., and Padlipsky M., "Unicode Format for Network
Interchange", RFC 5198, March 2008
[RFC5234]
Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[RFC5890]
Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 5890, August 2010
[RFC5891]
Klensin, J., "Internationalized Domain Names in Applications
(IDNA): Protocol", RFC 5891, August 2010
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[RFC6365]
Hoffman, P., and Klensin, J., "Terminology Used in
Internationalization in the IETF", RFC 6365, September 2011
7.2. Informative References
[RFC2194]
Aboba, B., Lu, J., Alsop, J., Ding, J., and W. Wang, "Review of
Roaming Implementations", RFC 2194, September 1997.
[RFC2341]
Valencia, A., Littlewood, M., and T. Kolar, "Cisco Layer Two
Forwarding (Protocol) "L2F"", RFC 2341, May 1998.
[RFC2433]
Zorn G., and Cobb, S. "Microsoft PPP CHAP Extensions", RFC 2433,
October 1998.
[RFC2637]
Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W., and G.
Zorn, "Point-to-Point Tunneling Protocol", RFC 2637, July 1999.
[RFC2661]
Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and B.
Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August
1999.
[RFC2865]
Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
[RFC2866]
Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC3492]
Costello, A., "Punycode: A Bootstring encoding of Unicode for
Internationalized Domain Names in Applications (IDNA)", RFC 3492,
March 2003.
[RFC3579]
Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In
User Service) Support For Extensible Authentication Protocol
(EAP)", RFC 3579, September 2003.
[RFC3748]
Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748,
June 2004.
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[RFC4282]
Aboba, B. et al., "The Network Access Identifier", RFC 4282,
December 2005.
[RFC4301]
Kent, S. and S. Keo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[RFC5281]
Funk, P., and Blake-Wilson, S., "Extensible Authentication Protocol
Tunneled Transport Layer Security Authenticated Protocol Version 0
(EAP-TTLSv0)", RFC 5281, August 2008.
[RFC5322]
Resnick, P. (Ed), "Internet Message Format", RFC 5322, October
2008.
[RFC5335]
Y. Abel, Ed., "Internationalized Email Headers", RFC 5335,
September 2008.
[RFC5729]
Korhohen, J. (Ed) et. al., "Clarifications on the Routing of
Diameter Requests Based on the Username and the Realm", RFC 5729,
December 2009
[RFC6055]
Thaler, D., et al, "IAB Thoughts on Encodings for Internationalized
Domain Names", RFC 6055, February 2011.
[RFC6532]
Yang, A., et al, "Internationalized Email Headers", RFC 6532,
February 2012.
[RFC6733]
V. Fajardo, Ed., et al, "Diameter Base Protocol", RFC 6733, October
2012.
[RFC6912]
Sullivan, A., et al, "Principles for Unicode Code Point Inclusion
in Labels in the DNS", RFC 6912, April 2013.
[EDUROAM]
http://eduroam.org, "eduroam (EDUcational ROAMing)"
[3GPP]
3GPP, "TS 23.003 Numbering, addressing, and Identification (Release
12)", July 2014,
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ftp://ftp.3gpp.org/Specs/archive/23_series/23.003/.
Acknowledgments
The initial text for this document was [RFC4282], which was then
heavily edited. The original authors of [RFC4282] were Bernard
Aboba, Mark A. Beadles, Jari Arkko, and Pasi Eronen.
The ABNF validator at http://www.apps.ietf.org/abnf.html was used to
verify the syntactic correctness of the ABNF in Section 2.
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Appendix A - Changes from RFC4282
This document contains the following updates with respect to the
previous NAI definition in RFC 4282 [RFC4282]:
* The formal syntax in Section 2.1 has been updated to forbid
non-UTF8 characters. e.g. characters with the "high bit" set.
* The formal syntax in Section 2.1 has been updated to allow
UTF-8 in the "realm" portion of the NAI.
* The formal syntax in [RFC4282] Section 2.1 applied to the
NAI after it was "internationalized" via the ToAscii function.
The contents of the NAI before it was "internationalized" were
left indeterminate. This document updates the formal syntax to
define an internationalized form of the NAI, and forbids the use
of the ToAscii function for NAI "internationalization".
* The grammar for the user and realm portion is based on a
combination
of the "nai" defined in [RFC4282] Section 2.1, and the "utf8-addr-
spec" defined in [RFC5335] Section 4.4.
* All use of the ToAscii function has been moved to normal
requirements on DNS implementations when realms are used as the
basis for DNS lookups. This involves no changes to the existing
DNS infrastructure.
* The discussions on internationalized character sets in Section 2.4
have been updated. The suggestion to use the ToAscii function for
realm comparisons has been removed. No AAA system has implemented
these suggestions, so this change should have no operational
impact.
* The section "Routing inside of AAA Systems" section is new in this
document. The concept of a "local AAA routing table" is also new,
although it accurately describes the functionality of wide-spread
implementations.
* The "Compatibility with EMail Usernames" and "Compatibility
with DNS" sections have been revised and updated. The Punycode
transformation is suggested to be used only when a realm name is
used for DNS lookups, and even then the function is only used by a
resolving API on the local system, and even then it is recommended
that only the home network perform this conversion.
* The "Realm Construction" section has been updated to note
that editing of the NAI is NOT RECOMMENDED.
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* The "Examples" section has been updated to remove the instance
of the IDN being converted to ASCII. This behavior is now
forbidden.
Authors' Addresses
Alan DeKok
The FreeRADIUS Server Project
Email: aland@freeradius.org
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