| Rfc | 7542 |
|---|
| Title | The Network Access Identifier |
|---|
| Author | A. DeKok |
|---|
| Date | May 2015 |
|---|
| Format: | TXT,
HTML |
|---|
| Obsoletes | RFC4282 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) A. DeKok
Request for Comments: 7542 FreeRADIUS
Obsoletes: 4282 May 2015
Category: Standards Track
ISSN: 2070-1721
The Network Access Identifier
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. It addresses issues with international
character sets and makes a number of other corrections to RFC 4282.
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/rfc7542.
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
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Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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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.
Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................6
1.2. Requirements Language ......................................7
1.3. Purpose ....................................................7
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 ....................................20
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 ......................................24
5. Administration of Names ........................................25
6. References .....................................................26
6.1. Normative References ......................................26
6.2. Informative References ....................................26
Appendix A. Changes from RFC 4282 .................................29
Acknowledgments ...................................................30
Author's Address ..................................................30
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 that 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 visited 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 other 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 that are interested in leveraging the users'
credentials in order to take advantage of an existing
authentication framework.
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 that could be used in non-AAA systems.
Some non-AAA systems defined identifiers that were compatible with
the NAI, and deployments used the NAI. This process simplified the
management of credentials, by reusing the same credential in multiple
situations. Protocols that reuse the same credential or the same
identifier format can benefit from this simplified management. 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 Sections 2.7 and 4 for further discussion of
this topic.
The goal of this document is to define the format of an identifier
that 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 widespread adoption of the NAI format.
This adoption will decrease the 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; it 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
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
"Local" or "localized" text is text that is in either non-UTF-8 or
non-normalized form. The character set, encoding, and locale are
(in general) unknown to Authentication, Authorization, and
Accounting (AAA) network protocols. The client that "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.
Normalization or Canonicalization
These terms are defined in Section 4 of [RFC6365]; those
definitions are incorporated here by reference.
Locale
This term is defined in [RFC6365], Section 8; that definition is
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].
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 that
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 that 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
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 that can be used
in multiple protocols.
Using a common identifier format simplifies protocols requiring
authentication, as they no longer need to specify a protocol-specific
format for user identifiers. It increases security, as 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
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 and 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 Section 2.1 of [RFC4282] that realms are ASCII
conflicts with the Extensible Authentication Protocol (EAP) as
defined in [RFC3748], and RADIUS, which are both 8-bit clean, and
which both recommend the use of UTF-8 for identifiers.
* Section 2.4 of [RFC4282] required mappings that are language
specific and that are nearly impossible for intermediate nodes to
perform correctly without information about that language.
* Section 2.4 of [RFC4282] requires normalization of usernames,
which may conflict with local system or administrative
requirements.
* The recommendations in Section 2.4 of [RFC4282] for treatment of
bidirectional characters have proven to be unworkable.
* The prohibition of the use of unassigned code points in
Section 2.4 of [RFC4282] effectively prohibits support for new
scripts.
* No Authentication, Authorization, and Accounting (AAA) client,
proxy, or server has implemented any of the requirements in
Section 2.4 of [RFC4282], among other sections.
With international roaming growing in popularity, it is important for
these issues to be corrected in order to provide robust and
interoperable 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 that are not Normal Form Composed (NFC)
are not valid NAIs and SHOULD NOT be treated as such.
Implementations that expect to receive an NAI but that instead
receive non-normalized (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 that are expected to be transported
over a 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.
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
Remote Authentication Dial-In User Service (RADIUS) protocol.
Unfortunately, [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
interoperability 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 that 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 subfields. 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
is with TLS-based EAP methods such as Tunneled Transport Layer
Security (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 authentication methods that
allow for "inner" and "outer" exchanges. Those protocols are limited
to using an identifier that 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.
Similar 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 is 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 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 that 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 that 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 user's 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 a 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
into the RADIUS User-Name attribute is problematic. This practice
can result in a AAA proxy encountering non-UTF-8 characters within
what it expects to be an NAI. An example of this requirement is
Section 2.1 of [RFC3579], 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 Section 2.4 of [RFC4282], 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 that 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 that 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:
* Edge systems used today generally do not normalize the NAI.
* Therefore, AAA systems SHOULD attempt to normalize the NAI.
The suggestions above contradict the suggestions 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 a 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 user's 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 are 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.
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
unescaped (i.e., utf8-clean) form.
2.8. Using the NAI Format for Other Identifiers
As discussed in Section 1, above, it 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 that are to be in a
well-known format and that will 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,
administration, 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 that 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 defined by 3GPP ensures that the identifier is
globally unique, as it is based on 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.
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 that 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 that are similar
in format to the NAI but that 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 dynamic DNS discovery (i.e.,
look up a record in the "utf8-realm" domain). This list is not
exhaustive and may 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 is handled as above. For example, 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
email.
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 Section 2.5 of [RFC4282], this document states 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 UTF-8
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
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" encompasses both "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 Section 2 of [RFC6055], resolver Application Programming
Interfaces (APIs) are not necessarily DNS specific, so conversion to
Punycode needs to be done carefully:
Applications that 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 Internationalized Domain Names for Applications (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.
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 environments, 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 namespace 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
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 -- shown here 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
is NOT RECOMMENDED 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 [RFC2865]
[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 mechanisms 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 the 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 Sections 2.7 and 2.8 for using the NAI in
other protocols have implications for privacy. Any attacker who is
capable of observing traffic containing the NAI can track the user
and can correlate his activity across time and across multiple
protocols. The authentication credentials therefore SHOULD be
transported over channels that 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
malicious observer. However, provisioning and management of the
identifiers may be difficult to do in practice -- a likely reason 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
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 that permit anonymous
"outer" identifiers require that the "inner" domain be the same as,
or a subdomain 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 Microsoft CHAP (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 a 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
publicly use an NAI realm without first owning the corresponding
FQDN. Private use of unowned NAI realms within an administrative
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. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003,
<http://www.rfc-editor.org/info/rfc3629>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008,
<http://www.rfc-editor.org/info/rfc5198>.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008, <http://www.rfc-editor.org/info/rfc5234>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010,
<http://www.rfc-editor.org/info/rfc5891>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011, <http://www.rfc-editor.org/info/rfc6365>.
6.2. Informative References
[RFC2194] Aboba, B., Lu, J., Alsop, J., Ding, J., and W. Wang,
"Review of Roaming Implementations", RFC 2194,
September 1997, <http://www.rfc-editor.org/info/rfc2194>.
[RFC2341] Valencia, A., Littlewood, M., and T. Kolar, "Cisco
Layer Two Forwarding (Protocol) "L2F"", RFC 2341,
May 1998, <http://www.rfc-editor.org/info/rfc2341>.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, October 1998,
<http://www.rfc-editor.org/info/rfc2433>.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
W., and G. Zorn, "Point-to-Point Tunneling Protocol
(PPTP)", RFC 2637, July 1999,
<http://www.rfc-editor.org/info/rfc2637>.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999,
<http://www.rfc-editor.org/info/rfc2661>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000,
<http://www.rfc-editor.org/info/rfc2866>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003,
<http://www.rfc-editor.org/info/rfc3492>.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003,
<http://www.rfc-editor.org/info/rfc3579>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005,
<http://www.rfc-editor.org/info/rfc4282>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005,
<http://www.rfc-editor.org/info/rfc4301>.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008,
<http://www.rfc-editor.org/info/rfc5281>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008, <http://www.rfc-editor.org/info/rfc5322>.
[RFC5335] Yang, A., Ed., "Internationalized Email Headers",
RFC 5335, September 2008,
<http://www.rfc-editor.org/info/rfc5335>.
[RFC5729] Korhonen, J., Ed., Jones, M., Morand, L., and T. Tsou,
"Clarifications on the Routing of Diameter Requests Based
on the Username and the Realm", RFC 5729, December 2009,
<http://www.rfc-editor.org/info/rfc5729>.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
February 2011, <http://www.rfc-editor.org/info/rfc6055>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012,
<http://www.rfc-editor.org/info/rfc6532>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733, October 2012,
<http://www.rfc-editor.org/info/rfc6733>.
[RFC6912] Sullivan, A., Thaler, D., Klensin, J., and O. Kolkman,
"Principles for Unicode Code Point Inclusion in Labels in
the DNS", RFC 6912, April 2013,
<http://www.rfc-editor.org/info/rfc6912>.
[EDUROAM] "eduroam (EDUcation ROAMing)", <http://eduroam.org>.
[3GPP] 3GPP, "Numbering, addressing and Identification", 3GPP TS
23.003, Release 12, July 2014,
<ftp://ftp.3gpp.org/Specs/archive/23_series/23.003/>.
Appendix A. Changes from RFC 4282
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-UTF-8 characters (e.g., characters with the "high bit" set).
* The formal syntax in Section 2.1 of [RFC4282] has been updated to
allow UTF-8 in the "realm" portion of the NAI.
* The formal syntax in Section 2.1 of [RFC4282] 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 Section 2.1 of [RFC4282] and
the "utf8-addr-spec" defined in Section 4.4 of [RFC5335].
* 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
of [RFC4282] 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 "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 widespread
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.
* The "Examples" section has been updated to remove the instance of
the IDN being converted to ASCII. This behavior is now forbidden.
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.
Author's Address
Alan DeKok
The FreeRADIUS Server Project
EMail: aland@freeradius.org